This document summarizes research on the habitable zones around stars and the habitability of exoplanets. It discusses how the habitable zone is defined as the region where liquid water could exist on a planet's surface. The inner and outer edges of the habitable zone are modeled based on factors like the runaway greenhouse effect and CO2 condensation limits. Current exoplanet detection methods are also overviewed. While over 20 potentially habitable exoplanets have been identified, confirming life remains difficult due to natural phenomena creating false biosignatures. However, an inhabited exoplanet may be confirmed in the near future as detection capabilities continue advancing rapidly.

1. Habitable Zones in the Galaxy: The Habitability of Exoplanets
Claire Sullivan
Supervisor-Tom Stallard
April 20, 2015
Abstract
This paper examines the habitability of exoplanets and how close we are to finding an
inhabited planet. The habitable zones around stars, regions which could contain liquid wa-
ter, are considered, as well as the current exoplanet detection capabilities. Other variables
which could affect habitability are then discussed including: stellar mass and evolution,
tidal locking and planetary composition. The current ability to detect life is also briefly
considered. It is concluded that this field is evolving rapidly with the number of confirmed
exoplanets soaring in the last 5 years. This increases the datasets with which to consider
the habitability variables. This has led to lists of between 21 and 51 potentially habitable
planets. Confirming if they are inhabited is still beyond current capabilities due to nat-
ural phenomena giving false-positive bio-signatures however an inhabited planet could be
confirmed in the near future.
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2. Contents
1 Introduction to Habitable Zones 3
2 Modelling the Habitable Zone 4
2.1 The Inner Edge of the Habitable Zone . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1.1 The Runaway Greenhouse Effect . . . . . . . . . . . . . . . . . . . . . . . 4
2.1.2 The Moist Greenhouse Limit . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1.3 Early Venus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.1.4 Dune-like Planets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.2 The Outer Edge of the Habitable Zone . . . . . . . . . . . . . . . . . . . . . . . . 6
2.2.1 The Maximum Greenhouse Limit . . . . . . . . . . . . . . . . . . . . . . . 6
2.2.2 The 1st CO2 Condensation Limit . . . . . . . . . . . . . . . . . . . . . . . 7
2.2.3 Early Mars . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.3 The Effect of Clouds at Habitable Zone Limits . . . . . . . . . . . . . . . . . . . 8
3 Exoplanets 8
3.1 Detecting Exoplanets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
3.1.1 Transit Method and Kepler . . . . . . . . . . . . . . . . . . . . . . . . . . 9
3.1.2 Radial Velocity Variations . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
3.1.3 Transit-Timing Variations (TTVs) . . . . . . . . . . . . . . . . . . . . . . 10
3.2 Detecting Exomoons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
3.3 Available Information about Exoplanets . . . . . . . . . . . . . . . . . . . . . . . 11
3.4 Exoplanet Databases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
3.4.1 The Planetary Habitability Laboratory . . . . . . . . . . . . . . . . . . . 11
3.4.2 The Habitable Zone Gallery . . . . . . . . . . . . . . . . . . . . . . . . . . 12
4 Other Variables which Affect Habitability 14
4.1 Continuously Habitable Zones . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
4.2 The Size of the Star . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
4.3 Tidal Heating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
4.4 Gas Giants and Exomoons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
4.5 Super-Earths/ Small M Class Stars . . . . . . . . . . . . . . . . . . . . . . . . . . 19
4.6 Planets in Eccentric Orbits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
4.7 The Galactic Habitable Zone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
5 Discussion 23
6 Conclusions 23
7 Extended Bibliography 25
7.1 Journals, Articles and Books . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
7.2 Websites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
2

3. 1 Introduction to Habitable Zones
When looking for habitable planets we first need to define what we class as habitable. This
could include planets which host complex life or if a planet could be habitable by humans. The
most common definition however, is planets which host a body of surface liquid water. This is
a basic requirement for all known life and so is a good criteria for habitability. Surface water is
specified as opposed to liquid water in general. Although subsurface water could provided an
environment for life it is undetectable remotely so can not be used as a search criteria.
Given this requirement the first step in the search for habitable worlds is to look at the
temperature range at which there could be liquid water. This gives a basic range of distances
from a star at which a planet or moon could have surface water. From there other variables can
be introduced to fine tune these distance limits. These are brought together to form a model
for the habitable zones around stars.
The inner edge of the habitable zone is affected by the loss of volatiles due to coronal mass
ejections from the sun, evaporation of water, and the effects of clouds on planetary albedo.
Tidal locking is also a consideration, especially for small M class stars who’s initial habitable
zone is very close to the star.
At the outer edge there are similar and opposite considerations. Clouds also play an im-
portant role here as CO2 clouds can have either a cooling or heating effect dependant on their
height and abundance. The size of the planet also plays and important role as larger planets can
better retain their atmospheres which would help with warming through the greenhouse effect.
These boundaries will be discussed in detail in section 2 but a visual representation of habitable
zone limits for different temperature stars can be seen in Figure 1. The effective stellar flux is
similar to an orbital distance scale since the flux from the star disperses in accordance with the
inverse square law. 1 effective stellar flux is equivalent to what is experienced at Earth’s orbit.
Figure 1: Different habitable zone boundaries for a range of star classes from M to F overlaid
with the positions of solar system planets and exoplanets (some unconfirmed) [Kasting et al.,
2013].
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4. Figure 2: The orbits of the planets within the
inner solar system in relation to the habitable
zone of the Sun [Kane and Gelino, 2012b]
The boundaries were originally calculated
by Kasting et al. [1993] to extend from a re-
cent Venus limit of 0.95AU to an early Mars
limit of 1.77AU. Others have made their own
calculations over the years including Menou
and Tabachnik [2003] with limits of 0.7-2.0AU
and Haghighipour and Kirste [2011] with
0.78-1.8AU. Kasting’s research team made
new estimates in 2013 including updated data
sets and improved climate models [Kopparapu
et al., 2013]. Their new limits in terms of or-
bital radius range from 0.75AU at the early
Venus limit to 1.77AU at the early Mars limit.
These are the optimistic limits. More conser-
vative limits are also considered in section 2.
It is always useful to consider our own
habitable zone in order to understand what
to look for in extrasolar systems. Kane and
Gelino [2012b] produced an illustration of the
Sun’s habitable zone in relations to the inner
solar system, Fig.[2]. They discuss how plan-
etary temperatures at the surface and upper atmosphere are extremely complex and can vary
hugely across a planet such as Mercury. Therefore finding the range of temperatures for which
liquid water can exist involves complex functions of composition, dynamics and climate which
can only be approximated to the first order.
2 Modelling the Habitable Zone
Although there are now many researchers focusing on modeling the habitable zone boundaries
[Menou and Tabachnik, 2003] [Haghighipour and Kirste, 2011] [Kopparapu et al., 2013], most
of the recent models stem back to a paper by Kasting et al. [1993]. In that paper Kasting and
his research group used a 1D climate model to derive the boundaries of the habitable zones
around stars for optimistic and pessimistic limits as well as limits based on observations of
our own solar system. The contents of that paper have become the default model used in this
field and papers since then have mainly tweaked the boundaries [Menou and Tabachnik, 2003]
[Haghighipour and Kirste, 2011] [Kopparapu et al., 2013] or imposed extra conditions [Barnes
et al., 2009] while retaining the same core.
2.1 The Inner Edge of the Habitable Zone
2.1.1 The Runaway Greenhouse Effect
One limit for the inner edge of the habitable zone is the point at which a runaway greenhouse
effect will be caused due to the planets surface temperature. Water has a critical temperature
at which it will evaporate. For pure H2O this is 647K for Earth [Kasting et al., 2013]. If
the surface temperature of a planet exceeds the critical temperature then whole oceans should
evaporate. For a planet with less water, the critical temperature would be lower.
The phrase ‘runaway greenhouse effect’ also refers to an atmosphere which absorbs more
solar flux than it expels thermal infra-red flux. This imbalance would lead to a sharp rise in
4

5. surface temperature which would quickly surpass the critical temperature meaning the two uses
of ‘runaway greenhouse effect’ are equivalent.
The runaway greenhouse process doesn’t last very long since the atmosphere becomes sat-
urated with H2O. Once in the upper atmosphere water can be easily lost into space, followed
by hydrogen. This loss process takes a few tens of millions of years [Kasting et al., 2013].
Figure 3: Plots from which the runaway greenhouse limit can be derived [Kasting et al., 1993]
The runaway greenhouse effect was derived by Kasting et al. [1993] by considering the
incident solar flux (FS) and the outgoing infra-red flux (FIR). Combining these using the relation
shown in Fig.[3] shows how the effective solar flux (Seff ) varies with surface temperature. Seff
at the critical temperature is found to be 1.41 for this solar system which converts to an orbital
radius of 0.84AU. The point at which FIR levels out is when the atmosphere becomes optically
thick to infra-red wavelengths. FIR rises again when the surface starts to emits in the visible
[Kasting et al., 1993]. The slight increase in FS between 200 and 400K is due to an increase of
water vapour in the atmosphere which absorbs solar radiation before the atmosphere becomes
too reflective and FS decreases and levels off [Kasting et al., 2013].
2.1.2 The Moist Greenhouse Limit
A second limit for the inner edge of the habitable zone is called the moist greenhouse limit.
This is placed at the point where the surface temperature first causes water to be absorbed into
the troposphere to the extent where the tropopause is stretched high enough so the water can
condense. At this point the water becomes broken down by photons though photodissociation
and lost into space [Kasting et al., 2013]. This limit assumes that water vapour can escape
into the upper atmosphere when it constitutes more than 20% of the inner atmosphere [Kasting
et al., 1993].
It is considered that clouds could help to cool the planet’s surface. This is unlikely with
present models as high, warming, cirrus clouds are expected to be produced quicker with rising
temperature than the lower, cooling, stratus clouds [Kasting et al., 2013].
The original models for the moist greenhouse limit are shown in Fig.[4]. On the left can be
seen how the height of the tropopause rises with increasing surface temperature (the tropopause
is the altitude for each surface temperature at which the water vapour mixing ratio instantly
stabilizes). At a surface temperature of 340K is a rapid rise in the height of the tropopause.
This represents the point where water vapour makes up 20% of the inner atmosphere as required
for the moist greenhouse limit. By finding the associated H2O mixing ratio (10−3) the Seff can
be found for our system to be 1.1. The associated orbital radius is 0.95AU.
5

6. Figure 4: Plots from which the moist greenhouse limit can be derived [Kasting et al., 1993].
Each line represents the surface temperature labelled.
2.1.3 Early Venus
Kasting et al. [2013] and Kasting et al. [1993] discuss how Venus appears to have lost all of it’s
surface water before it’s last resurfacing. This is estimated to have occurred ∼1 billion years
ago. Since stars get brighter throughout their main sequence, 1 billion years ago the Sun was
8% less bright. This indicates that the habitable zone would have been closer to the Sun. If
Venus was not habitable at that time then the inner limit of the current habitable zone should
be at least as far out as Venus’ orbit of 0.72AU plus 4% to account for the Sun’s increased
luminosity. This places the current recent Venus inner limit at 0.75AU.
2.1.4 Dune-like Planets
Abe et al. [2011] discusses the possibility of another limit where hot rocky planets with some
water deposits could remain habitable at the polar regions. These are named Dune-like planets
due to their resemblance to the planet of the same name in Frank Herbert’s classic sci-fi novel,
Dune. The limit for these planets is calculated to be at Seff = 1.7. This limit lies between
the early Venus and the runaway greenhouse limits. Kasting et al. [2013] think this type of
planet unlikely since the minimal rainfall would fall over dry regions of the surface as well as
the poles. Water would be absorbed into minerals like clays and could no longer circulate.
Without outgassing returning water into the system, the same process would dry up half of an
Earth ocean in 2.5 billion years [Kasting et al., 2013]. Dune-like planets would have less water
reserves and so the removal time would be significantly shorter.
2.2 The Outer Edge of the Habitable Zone
2.2.1 The Maximum Greenhouse Limit
A fairly optimistic outer edge boundary is the maximum greenhouse limit. This relies on the
maximum warming effect that CO2 clouds could have on the planet’s surface. This would
minimise the amount of solar flux needed to maintain a temperature of 273K as required for
liquid water. CO2 above this limit will increase Rayleigh scattering due to higher particle
densities and so increase the albedo of the planet. This reduces the solar flux reaching the
surface, lowering the temperature. Kasting et al. [1993] and Kasting et al. [2013] state this limit
to be 1.67AU (Seff = 0.36) for the Sun. This result is derived from the plots in Fig.[5].
6

7. Figure 5: Plots from which the maximum greenhouse limit can be derived [Kasting et al., 1993]
As with the runaway greenhouse limit, the incident solar flux (FS) and outgoing infra-red
flux (FIR) can be used to plot the effective solar flux (Seff ). Since FS and FIR act in opposite
directions there is a minimum in Seff at 0.36 which gives the stated maximum greenhouse limit
in terms of orbital distance for our system.
2.2.2 The 1st CO2 Condensation Limit
Figure 6: Deriving the 1st CO2 condensation
limit [Kasting et al., 1993]
The outer edge of the habitable zone is
strongly dependant of the condensation of
CO2. The condensation limit is defined as
the point at which the temperature is high
enough for CO2 to condense back out of
the atmosphere. Below this temperature the
carbonate-silicate cycle is effected and slowed
[Kasting et al., 2013]. Volcanically active ter-
restrial planets have large amounts of carbon
and CO2 in the atmosphere. Silicate weather-
ing extracts these and deposits them as sed-
iment on the sea floor. When the tempera-
ture decreases this carbonate-silicate cycle be-
comes slower and weathering becomes less fre-
quent leading to an optically thick CO2 atmo-
sphere. The condensation limit is calculated
by Kasting et al. [1993] and Kasting et al.
[2013] to be 1.37AU (Seff = 0.53) for the
Sun although it has been considered that the
carbonate-silicate cycle will stabilize the sur-
face temperature since the buildup of a thick CO2 atmosphere will contribute a large greenhouse
effect Williams et al. [1997]. This is discussed in section 4.1.
The value stated for the 1st CO2 condensation limit was derived by Kasting et al. [1993]
from noting that carbon dioxide condenses at a partial pressure of just over 1 bar. By focussing
in on this region of the right side plot of Figure 5, Fig.[6] is created. From this the associated
Seff , and therefore the 1st CO2 condensation limit in terms of orbital radius, can be found.
7

8. 2.2.3 Early Mars
Much discussion has taken place with regards to the habitability of Mars. It is one of the topics
which is still able to capture the imagination of the public and is often mentioned in mainstream
media. Since the habitable zone moves outwards as the Sun gets older and brighter, if Mars was
once habitable, it would now lie further into the habitable zone. Due to its small size however,
Mars lost it’s volatiles and atmosphere a long time ago since it did not have the gravity to retain
them. It is therefore extremely unlikely to currently have life on its surface.
From looking at the valley network on the surface, Mars appears to have lost all of its surface
liquid water at or before 3.8 billion years ago when the sun was 25% less bright. Its location
is therefore classed as an outer limit for the habitable zone for that time. The increase in the
luminosity of the sun related to Mars orbit gives a current outer edge ‘Early Mars’ limit of
1.77Au (Seff = 0.32)[Kasting et al., 1993] [Kasting et al., 2013].
2.3 The Effect of Clouds at Habitable Zone Limits
Clouds of any type can dramatically effect the temperature on a planets surface. If a planet
has a thick atmosphere, the clouds are drawn to lower altitudes where an increase in Rayleigh
Scattering reduces the solar flux, cooling the surface. This effect also occurs when there is a
large abundance of clouds at any altitude which, when thick, will increase the planets albedo,
reducing the heat from the sun reaching the surface [Mischna et al., 2000].
For the outer edge of the habitable zone a more important effect of CO2 clouds is their
potential for heating the planets surface by reflecting and thereby trapping infra-red radiation
back onto the surface. Mischna et al. [2000] discusses how, at an optimal thickness and altitude
at the top of the atmosphere, the reflection of IR outweighs the increase in planetary albedo,
resulting in an increased surface temperature. This could increase the maximum distance for
the outer edge of the habitable zone. Kasting et al. [1993] and Kasting et al. [2013] agree that
this is possible however they describe how a bigger warming effect could be achieved from a
significant increase in greenhouse gasses such as hydrogen.
3 Exoplanets
3.1 Detecting Exoplanets
After regions where liquid water could exist were modelled, the search for habitable worlds
could become much more targeted. In addition to the accuracy of the search parameters, the
development of exoplanet detection technology has increases rapidly, especially over the last
five years. This can be seen from Figure 7, which shows the number of exoplanet detections per
year and the method that was used to find them. Along the top of the figure shows the exact
number of exoplanets confirmed per year.
Although most of the hype about exoplanet detections is focused on Kepler, the total number
of confirmed exoplanets 1523[6], minus the 996 discovered by Kepler[9] shows that 527 exoplanets
have been found from other sources. This indicates the improvement in ground-based instru-
ments which are starting to match and potentially exceed the sensitivity of Kepler.
8

9. Figure 7: The number of confirmed exoplanet detections per year colour coded to represent the
number of detections made by each method. The data for 2014 is up to September 23rd[8].
3.1.1 Transit Method and Kepler
The most prolific method for detecting exoplanets has been the transit method as shown in green
in Fig.[7]. This looks at the periodic dip in the luminosity of a star when a planet crosses in
front or behind it from the Earth’s viewpoint. Transit detections can be affected by sunspots.
These are more frequent and bigger on smaller M stars [Barnes et al., 2009]. Multiple data
points are therefore highly important to confirm planets found with this method. Conveniently
planets around M stars are likely to have smaller orbits and therefore transits would occur more
frequently. This could balance the uncertainty due to sunspots. The transit method has the
advantage of getting absorption and transmission data which provides interesting insights into
the atmospheres of exoplanets.
Kepler, a 0.95m aperture space telescope, was launched by NASA in 2009 [Lissauer et al.,
2014]. In the subsequent 5 years it has been searching through stars within its field of view
looking for transiting planets. At time of writing the Kepler mission website states that Kepler
has confirmed 996 exoplanets with an additional 3187 potential planets[9]. From Fig.[7] we can
see the increase in the amount of confirmed exoplanets from the transit method (green) since
Kepler began operations in May 2009. The dramatic increase in confirmed planets in the last
year alone, could be due to ground based research sifting through the large quantities of Kepler
data from the last 5 years to confirm older candidates.
3.1.2 Radial Velocity Variations
The second most successful detection technique is the largely ground based radial velocity
method. This observes the Doppler wobble of a star due to it being orbited by one of more
objects. The centre of mass of the system is offset from the centre of the star and the star orbits
around this point. In doing so it moves towards and away from Earth in it’s small orbit causing
red-shift and blue-shift. For small M class stars, radial velocity measurements can be strongly
affected by stellar activity. Surface fluctuations could erase evidence of terrestrial sized planets
9

10. [Barnes et al., 2009] so datasets need to be checked for reliability.
A key instrument in the search for radial velocity variations is the High Accuracy Radial-
Velocity Planet Searcher (HARPS) located in Chile. According to Pepe et al. [2011], a third
of all stars exhibit radial velocity variations which usually indicate super-Earths or ice giants.
HARPS alone found 100 planetary candidates in its first 8 years of operation including 3 super-
Earths around the star Gliese 581 [Pepe et al., 2011], which is one of the most promising systems
for finding a habitable planet and is discussed in section 4.5. Instruments like HARPS are crucial
for finding exoplanets since the best way to confirm the existence of a planet will always be to
find it using two different methods. So no matter how many candidates Kepler finds, methods
other than transits are just as important to this field of research.
3.1.3 Transit-Timing Variations (TTVs)
Now that the transit method has been shown by Kepler to be very effective, especially at
detecting large gas giant exoplanets, research is being done into how to optimise this method to
detect smaller, more earth-like, planets [Lissauer et al., 2014]. Variations in the time a transit
occurs or the duration for which a planet transits a star could imply exchanges of energy and
momentum between the planet and other bodies in the system. Oscillations in the semi-major
axis and eccentricities in the orbit could be caused [Haghighipour and Kirste, 2011].
Orbital resonance occurs when two orbiting bodies have regular gravitational influence on
each other due to orbits related by small ratios. These could be easier to detect due to a large
effect they have on the transit-timing variations [Haghighipour and Kirste, 2011]. As well as
the possibility of detecting smaller planets, this method could also detect other bodies which
do not transit the star from our observing position. Small planets detected this way would be
very hard to confirm using other methods.
Calculations indicate that a small planet from 1-10M⊕ in the habitable zone of an M class
star could produce detectable transit-timing variations on a transiting Jupiter sized planet with
an orbital period longer than 10 days [Haghighipour and Kirste, 2011]. This is based, however,
on Kepler’s optimal accuracy. Transit-timing variations as small as 20 seconds can be measured
but the precision can vary from 20 seconds to 100 minutes which is problematic for this method
[Ford et al., 2011]. New, more precise instruments will be required before this method can be
reliable.
3.2 Detecting Exomoons
A paper by Kipping et al. [2009] discusses the feasibility of detecting exomoons with current
instruments of Kepler level photometry. Using the transit-timing variations method on exist-
ing Kepler data could detect exomoons. If both timing and duration variations are observed
moons could be confirmed and their mass and orbital distances could be obtained. Ground
based instruments could better serve this purpose since they have fewer restrictions than space
telescopes such as data-download speeds.
It would be easier to find habitable zone planets and moons around small M stars since the
habitable zone is closer to the star where bodies will have a larger affect on the dip in luminosity
during transits. They will also have shorter orbits meaning more transits in a time period. This
does not mean that exomoons around M class stars are the best candidates for life only that
they are the easiest to find. For comparison a habitable moon around an F class star would
have a very large orbital period so it could take over 6 years to get 3 transits which is still a
non-ideal amount of information to draw conclusions from [Kipping et al., 2009].
Kipping et al. [2009] looked to our own system to consider what would be the best size
planet to detect habitable moons around. They considered Neptune, Saturn and Jupiter sized
10

11. planets. They concluded that Jupiters are the hardest to search around and Saturns the easiest.
This is due to Saturn’s low density allowing closer orbits and therefore deeper transit depths.
Also Saturn’s low mass means it is more significantly affected by moons making them easier to
detect than small moons around a larger body like Jupiter.
Instruments with the sensitivity of Kepler could not detect a Sun-Earth-Moon relationship
in other systems. This is due to the uncertainties in the measurements being greater than the
transit timing variations of ∼14 seconds and duration variations of ∼112 seconds that would be
generated by such a system.
Given that there are approximately 100,000 main sequence stars within Kepler’s field of
view, Kipping et al. [2009] believe that 25,000 of these would be within range for habitable
exomoon detections. This would detect moons down to 0.2 M⊕. A study across the whole
galactic plane with Kepler-level instruments could survey a million stars with the potential to
host habitable exomoons.
3.3 Available Information about Exoplanets
Kepler alone has discovered over 4000 planetary candidates[9]. From initial analysis approxi-
mately a dozen of these could be rocky, contain liquid water and be within the habitable zone
of their star Kasting et al. [2013].
Traub [2012] analysed data from Kepler’s first 136 days of operation. His statistics indicate
that 9% of planets with orbital periods of less than 42 days were terrestrial and the frequency of
these terrestrial planets within the habitable zones of FGK stars is η⊕ =34±14%. He concluded
that a third of FGK stars should have at least one terrestrial planet in their habitable zone. Pepe
et al. [2011] reached the same conclusion with their study using the ground based High Accuracy
Radial Velocity Planet Searcher (HARPS) in Chile. Other studies have found drastically lower
estimations using different conditions such as Catanzarite and Shao [2011] who find η⊕ = 1−3%.
They assumed orbital periods ≥42 days meaning they had less data points due to fewer observed
orbits. The true value is therefore likely to be closer to the values found by Pepe et al. [2011]
and Traub [2012].
3.4 Exoplanet Databases
Data about exoplanets is freely available and there are many online databases. The Extrasolar
Planets Encyclopaedia[5] is a catalogue of mainly confirmed exoplanets and is a good source for
detailed information and literature about specific exoplanets. The Exoplanet Data Explorer[6]
and The Open Exoplanet Catalog[8] provide tools for extracting and manipulating exoplanet
data to produce figures such as Figure 7.
The following two sections take a closer look at exoplanet databases which focus on poten-
tially habitable planets. They draw there catalogues both from the mentioned databases as well
as straight from the source for promising planetary candidates [Kane and Gelino, 2012a].
3.4.1 The Planetary Habitability Laboratory
The Planetary Habitability Laboratory (PHL) maintain a habitable exoplanet catalogue. Figure
8 shows how this catalogue looked in December 2013.
The were 12 planets considered potentially habitable at that time. In the subsequent 9
months the two planets in the system of Gliese 581 (d and g) were removed since they could
not be confirmed. Considering newly detected and confirmed planets a further 11 were added
to the catalogue, more than doubling the list within those 9 months. The resulting most recent
version of the list from 2nd September 2014 can be seen in Fig.[9].
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12. Figure 8: The PHL habitable exoplanet catalogue as it looked in December 2013[3].
Figure 9: The PHL habitable exoplanet catalogue as of September 2014[3].
The quick increase in potentially habitable planets in the last year appears to be a direct
result of the general increase in confirmed planet detections seen in Fig.[7]. Having said this
only 5 of the 11 newly added planets were discovered by Kepler so 6 came from ground-based
instruments. 3 of these, Gliese 682c and Gliese 180b and c were discovered by researchers at
the University of Hertfordshire[10]. This could reinforce the suggestion that the extreme rise in
the number of planetary detections is due to improved ground based detections as well as from
deeper analysis of Kepler data.
3.4.2 The Habitable Zone Gallery
Similar to the Planetary Habitability Laboratory, the Habitable Zone Gallery produce a cat-
alogue of potentially habitable planets. They, however, focus on the orbital eccentricities of
exoplanets and the percentage of the orbit which an exoplanet spends within the habitable zone
of it’s star [Kane and Gelino, 2012a]. Their list does not take into account other effects, dis-
cussed in section 4, resulting in a fairly extensive list of 51 potentially habitable planets which
spend 100% of their orbit within the habitable zone[4]. For further studies they relax this to
consider orbits which are more than 50% within their habitable zones. The effects of eccentric
orbits with relation to the habitable zone is discussed in section 4.6. Some figures produced by
the Habitable Zone Gallery can be seen below.
On the left of Figure 10 is a log scale of the number of confirmed exoplanets per year. The
12

13. Figure 10: Left: A log scale of the number of confirmed exoplanets detected per year colour-
coded with respect to the locations of exoplanets in relation to their habitable zones.
Right: The relation between planet mass (in units of Jupiter mass) and orbital period. The
larger the data point, the more time the planet spends within the habitable zone[4].
colours represent the location of the planet in relation to the habitable zone. The green numbers
above each year state the amount of exoplanets with ≥50% of their orbit within the habitable
zone. It is not surprising that most of the detected planets are closer to the star than the inner
limit of the habitable zone. This again is due to the easy detection of close planets due to their
large gravitational and dimming effects on their host stars.
The plot on the right of Figure 10 shows the relation between planet mass and orbital period.
The larger the dot the higher percentage of their orbit is within the habitable zone. Predictably
mass and orbital period increase relatively linearly. There is however a large dispersion of
masses at higher orbital periods. This is due to the variations in composition of gas and ice
giant planets with large gas envelopes and formation dependant cores.
Figure 11: Left: The relation of stellar mass (in units of solar mass) to the effective stellar flux
at the planets orbit, relative to that of Earth.
Right: The relation of planet radius (in units of Earth radii) to the effective stellar flux at the
planets orbit, relative to that of Earth[4].
Figure 11 shows more plots from the Habitable Zone Gallery. The left and right figure show
stellar mass and planet radius respectively, in relation to the effective stellar flux experienced at
the planets orbit. The left plot shows a distribution focusing on Sun-like G class stars of stellar
mass ∼1. Most detections are too close to the star to be within the habitable zone as explained
13

14. previously. It is reasonable to expect a more even spread of planets with lower effective stellar
flux with better detection sensitivity.
The right plot shows a subset of planets with a mass ≤20M⊕. This is approximately the
super-Earth range for terrestrial planets. This plot therefore rules out many of the gas and ice
giants present in the left plot which have lower stellar fluxes. Again the detection bias results
in a large selection of interior planets. It does however show potential since the data points are
close to reaching the crucial Seff =1 value. With near future technology more planets could be
found slightly further from their star and therefore within habitable stellar flux zones.
4 Other Variables which Affect Habitability
4.1 Continuously Habitable Zones
The boundaries of the habitable zone have to adapt to the evolution of the star so the outer edge
gets further away as the star gets larger and hotter later in its cycle. Only the main sequence of
stars is considered since the increase in luminosity occurs too rapidly in the subsequent phases
for planets to remain habitable. These changes in luminosity during the evolution of a star can
be seen in Figure 12.
Figure 12: The evolution of a stars luminosity [Kasting et al., 1993]
The definition of life is often considered as a self-contained chemical system capable of
evolving in the ways predicted by Charles Darwin [Kasting et al., 2013]. When considering
advanced, multicellular lifeforms, the planet needs to have enough time, billions of years, for
life to evolve. The continually habitable zone gives limits on the disc around a star which would
remain potentially habitable for the time required for the evolution of advanced lifeforms. Figure
13 shows the continuously habitable zone for the Sun.
The right plot of Fig.[13] does not allow for planets which start outside the outer edge of
the habitable zone and then becomes inside the zone as the star evolves. Kasting et al. [1993]
discuss how such planets may not be capable of harbouring life after an uninhabitable start.
Such a planet may have a highly reflective ice coating which would require a very large flux to
thaw. Under this condition the outer edge of the continuously habitable zone is fixed from the
start of the sun’s main sequence.
Earth’s carbonate-silicate cycle is perfect for an evolving Sun since it reacts to temperature
variations on the planet’s surface to maintain an equilibrium temperature. In a cool climate
14

15. Figure 13: The continuously habitable zone for the Sun. (left) allowing the thawing of planets,
(right) not allowing thawing. Dashed lines represent the moist greenhouse and 1st CO2 con-
densation limits. The solid lines show the runaway and maximum greenhouse limits. The finely
dotted outer lines represent the recent Venus and early Mars limits [Kasting et al., 1993].
silicate weathering slows, increasing the amount of CO2 in the atmosphere, increasing the
greenhouse effect. In a warm climate weathering is sped up, extracting more CO2 from the
atmosphere, reducing the greenhouse affect [Williams et al., 1997]. If other planets also have
volcanism and silicate weathering to generate this process they would be well suited to evolving
stellar luminosity.
4.2 The Size of the Star
As well as age, the size of the star is critical. Figure 14 shows how the derivations of the
habitable zone limits are effected for a larger F class star and a smaller M class star.
The conclusions from these plots are that the optimistic habitable zone limits (runaway and
maximum greenhouse) extend by 30% for the larger F star and contract by 30% for the smaller
M star. The conservative limits (moist greenhouse and 1st CO2 condensation) are extended by
10% for the F star and contracted by the same amount for the M star [Kasting et al., 1993].
This implies that the habitable zone of larger stars is further out.
From the log distance scale in Fig.[15], it can also be observed that smaller stars have
wider habitable zones than sun-like stars. This is due to the albedo of planets being dependant
on the spectrum of their star. Because Rayleigh scattering is so important at the outer edge
of the habitable zone, due to the CO2 dense atmospheres, the dependence on spectrum is
more apparent at that edge. Kasting et al. [2013] observes that if the distribution of planets
around main sequence stars is similar to our system then the frequency of Earth-like planets
ηEarth ∼ 25% larger for M stars than G stars due to this widening of the habitable zone. They
find that measurements from different papers are converging on ηEarth = 0.4 − 0.5 for small
stars. ηEarth would be around 0.3-0.35 for Sun-like G class stars.
15

16. Figure 14: Deriving habitable zone limits for different sized stars. (left) inner edge (right) outer
edge [Kasting et al., 1993]
Figure 15: Moist greenhouse and maximum greenhouse habitable zone boundaries for a log
range of distances and stellar masses overlaid with the positions of solar system planets and
exoplanets (some unconfirmed) [Kasting et al., 2013].
16

17. 4.3 Tidal Heating
With a large focus being put on finding terrestrial planets around M class stars due to their
short orbital periods and easy detection, the effect of tidal heating on these planets needs to
be considered as it may affect their habitability. If the heating rates on an exoplanet are at all
similar to on Io then they could experience extreme volcanism driven by the tidal heating. On
Io this causes a resurfacing of the moon every million years which is a very small period on the
time scale of habitability [Barnes et al., 2009].
Kasting et al. [1993] considered the overlap between the habitable zone and the region inside
the tidal-lock radius. Figure 16 shows this relation. The region within the dashed lines is where
terrestrial planets should exist. It is encouraging for the search for another Earth that for
stars larger than late K, the entire habitable zone is within the region of terrestrial planets.
K and M stars do, however, have their habitable zones entirely within the tidal-lock radius
meaning all their potentially habitable planets would be tidally locked unless affected by strong
orbital resonance. Locked planets are likely to freeze water and volatiles on the dark hemisphere
rendering them uninhabitable Kasting et al. [1993].
Figure 16: The habitable zone, tidal-lock ra-
dius and region of terrestrial planets (between
dashed lines) for different classes of stars [Kast-
ing et al., 1993].
Alternately if the effect of tidal heating
is not enough to generate plate tectonics (as-
suming no internal heat) then CO2 is less
likely to be recycled through subduction in
the carbonate-silicate cycle thereby initiat-
ing a runaway greenhouse effect which would
purge the planet’s surface also rendering it un-
inhabitable [Barnes et al., 2009].
The confirmed exoplanet Gliese 581e has
a mass of 2M⊕ and could experience intense
tidal heating due to its very short orbit not
within its star’s habitable zone. The planets
eccentricity reaches 0.1. Barnes et al. [2009]
states that, with common tidal heating mod-
els, Gliese 581e could have 2 orders of magni-
tude more tidal heating than Jupiter’s moon
Io which is extremely volcanic and uninhabit-
able. A planet within a habitable zone could
experience similar tidal heating which could
drastically reduce its habitability. Alterna-
tively tidal heating could generate the heat
required to make a planet more habitable. This could be by generating plate tectonics in a
planet without internal heat. This could allow for long-lived stability in the climate of the
planet due to the generation of the carbonate-silicate cycle.
Tidal habitable zone limits were modeled by Barnes et al. [2009] which they overlaid on the
original habitable zone. They made a crucial assumption to ignore internal heating. This is
important since the main heat source on Earth is internal radiogenic heating. The subset of
exoplanets which their model could apply to may be very small, although smaller planets are
more likely to lack an internal heat source, in which case tidal heating would dominate. The
internal heating of exoplanets could be considered on a case by case basis in addition to tidal
heating to compare them to the derived tidal habitable zone limits. Other assumptions made
include 50% cloud cover, a semi-major axis within the habitable zone and a surface temperature
for planets in eccentric orbits which reflects the orbit-averaged stellar flux.
17

18. Figure 17: Tidal habitable limits overlaying the
original habitable zone for a 10M⊕ exoplanet.
Left yellow strip e=0.01 and right strip e=0.5
[Barnes et al., 2009]
They take the outer limit of the tidal
habitable zone to be the heat flux on Io
(hmax=2 Wm−2) since this causes a resur-
facing timescale of a million years which re-
sults in an uninhabitable environment. The
inner limit was set at the heat flux (hmin=0.04
Wm−2) at which geophysicists think Mars’
volcanic activity stopped, since this is consid-
ered essential for maintaining the carbonate-
silicate cycle which regulates climate. For
comparison the radiogenic heating on Earth
is hrad⊕=0.08 Wm−2 which is comfortably
within these limits.
The resulting tidal habitable zone can be
seen in Figure 17 for 2 orbital eccentricities:
e=0.01 (left) and e=0.5 (right). The green re-
gions show the overlap between the tidal hab-
itable zone and the original habitable zone.
The red lines represent transit depth (hori-
zontal) and transit probability (diagonal) but
are not important to this paper. The overlap
demonstrates the possibility of tidal heating allowing for the habitability of exoplanets that
would have otherwise not had the internal heat to generate plate tectonics. It is still unclear
how many exoplanets this would be applicable for.
The effects of tidal heating on planets within the habitable zone is an interesting point of
discussion and requires more research. Intense volcanism and swift resurfacing can be seen to
be issue from looking at Io and Gliese 581e so it is reasonable to assume habitable zone planets
will be affected. It could filter out many exoplants which would otherwise have been considered
habitable candidates.
4.4 Gas Giants and Exomoons
Gas giants are not uncommon in the lists of detected exoplanets however they are not often
considered when looking for habitable worlds. This is largely due to their lack of a solid or
liquid surface on which they could host life. Their moons however could be good candidates
[Scharf, 2006][Williams et al., 1997]. These would need to be large enough (≥0.12M⊕) to retain
a long-lasting atmosphere.
Scharf [2006] investigated whether 74 known extrasolar giant planets further than 0.6AU
from their star could host habitable moons. He found that 60% of these giant planets could
maintain moon systems up to ∼0.04AU wide. This is 3 times a wide as how far the Galilean
satellites extend from Jupiter. Scharf [2006] also finds that 15-27% of all known exoplanets
could harbour small, icy moons up to temperatures of 273K.
A large planet is likely to have a strong magnetic field which would bombard a moon within
the magnetosphere with energetic ions. The electron flux in Jupiter’s inner magnetosphere is
4×108 cm−2s−1. This is a thousand times more than the solar flux received on Mars. This
would likely strip the atmosphere off the moon in as little as 500 million years unless it also has
a strong magnetic field to counteract that of the planet [Williams et al., 1997].
The hill sphere of a Jovian planet is the region of orbital stability for moons. If a Jupiter-
sized planet orbited at 1AU from a Sun-like star, all of it’s moons would be within the tidal lock
18

19. radius. It would only take a few billion years from their formation to become tidally locked to
the planet [Williams et al., 1997]. This could cause severe temperature fluctuations across the
moons surface as mentioned for planets in section 4.3. Moons of planets in strongly eccentric
orbits could also experience such effects.
Moons that form within the accretion disks of planets might receive different volatiles than
a planet formed in the accretion disk of a star. Williams et al. [1997] discuss the theory that
terrestrial planets receive volatiles from collisions with comets or carbonaceous asteroids. They
consider that if a gas giant can maintain it’s moons as it migrates, this could be the source of
volatiles on moons too, since comet bombardment is common during migration.
This could explain how Saturn’s moon Titan got it’s atmosphere. In contrast Jupiter’s moons
do not have atmospheres. Williams et al. [1997] think this could be due to the relative velocities
with which the comets would have hit the moons. Since Saturn has a lower gravitational
potential well, the impacts would have less velocity and therefore theoretically less of the moon’s
atmosphere would have been eroded by the impact. This suggests that moons orbiting larger
planets, like Jupiter, would be less likely to retain volatiles from comets.
Water-ice moons similar to Ganymede and Callisto can also be considered. These have
densities of 1.8 gcm−3 and 1.5 gcm−3 respectively and are likely to be more than half water-ice
[Scharf, 2006][Williams et al., 1997]. If brought to Earth’s orbital distance they would become
water planets/moons with ∼1000km of liquid water. They would be far from uninhabitable
but would rule our land-based lifeforms. Deep oceans on tidally locked moons could stabilize
climate effects over long time periods despite rapid temperature variation Williams et al. [1997].
Jupiter’s closest large moons, Io and Europa, are more Earth-like with densities of 3.5 gcm−3
and 3.0 gcm−3 respectively and are mostly rocky [Scharf, 2006]. The inner region of Jupiter’s
nebula during formation was hotter and so more rocky and less icy moons formed in this region.
This implies that moons closer to large planets should be more Earth-like in their composition.
If a moon of a similar composition to Europa were tidally locked it would experience extreme
seasonal temperature variations however if it had oceans, coastal regions could be stabilized
[Williams et al., 1997].
Williams et al. [1997] concludes that a Mars size moon with Ganymede’s magnetic field and
an orbital-resonance similar to Io would have the potential to remain habitable for billions of
years. A very similar conclusion is made by Scharf [2006].
4.5 Super-Earths/ Small M Class Stars
Super-Earths are a subset of exoplanets with radii between Earth and Neptune (∼3.8R⊕)[Lissauer
et al., 2014]. Small, lower temperature stars of M or late K class appear to host many Super-
Earths and are the focus of many of the current searches for habitable planets [Haghighipour
and Kirste, 2011][Kasting et al., 2013]. This is likely due to Kepler, since most of the planets
it has discovered are Super-Earths [Lissauer et al., 2014]. Due to their small mass, they have
a large radial velocity wobble when orbited by a body. There is also a greater reduction in
luminosity when a planet transits a low mass star. Both of these make identifying orbiting
planets easier than for larger stars.
In addition to making planets easier to detect, the low temperatures mean the habitable
zone moves closer to the star ∼0.1-0.2AU. Planets within a small habitable zone would therefore
have orbital periods in the range of 20-50 days [Haghighipour and Kirste, 2011]. Planets with
short orbits are much easier to confirm quickly with multiple detections compared to planets
with orbits of over a year.
Improved detection techniques have found many systems thought to have one super-Earth
may contain a lot more. For example the system around the star Gliese 581 is now believed
19

20. to contain 4-6 planets with radii between 1.7-7R⊕ although only 3 of these are confirmed[1]
[Haghighipour and Kirste, 2011].
Figure 18: The predicted planets around star Gliese 581, their relative sizes and positions with
respect to the habitable zone[2].
Figure 18 shows the habitable zone for Gliese 581. Multiple planets appear to be within the
zone although of the confirmed planets (Gl 581b, Gl 581c, Gl 581e)[1 only Gl 581c has potential
and then only for an optimistic habitable zone. By far the most promising planet in this system
for harbouring life is Gl 581g although it has not yet been confirmed.
Small planets can orbit very close to M stars while still being within the habitable zone. It
should be noted that this region can be a more hostile environment than any other part of a
habitable zone. Not only could they be affected by tidal heating but they are also subject to
solar flares and x-ray emissions from increases in solar activity [Barnes et al., 2009].
Weiss and Marcy [2014] analysed data from 65 exoplanets where the radius of the planet
Rp ≤ 4R⊕. Their aim was to extract information on the relationship between planetary mass
and radius. A distinct separation in the results was found at 1.5R⊕. Below this value the
density increases with radius. Above 1.5R⊕ the average density decreases with radius, Fig.[19].
This is inconsistent with rocky structures implying that larger planets have a large volume of
volatiles overlaying a solid core.
The densities of planets with Rp ≤ 1.5R⊕ is calculated by Weiss and Marcy [2014] to be
ρp = 2.43+3.39(
Rp
R⊕
)gcm−3. This is consistent with rocky composition. For larger planets there
is a scatter in the measured densities. The density of planets of 2R⊕ varied from less dense than
water to the density of Earth. This implies variations in how the planets formed for given radii,
resulting in different core sizes, compositions and the volume of volatiles. The small increase in
mass with a large increase in radius implies a substantial volume change from 3.4V⊕ to 64V⊕
for 4M⊕ to 10M⊕. This can only be explained if lightweight gases are present.
The database of knowledge about exoplanets relies on the ever increasing amount of available
data for each planet. Planets with large orbits i.e. ≥ 1 year have few data points and take a
long time to confirm. The study of Weiss and Marcy [2014] therefore only selected planets with
short orbits so as to have reliable data. Most planets used had orbits of ≤50 days.
Petigura et al. [2013] found from their analysis that exoplanets between the size of Earth
and Neptune, with 5-50 day orbits should be in the systems of 24% of stars in the galaxy.
Finding these sorts of exoplanets, especially knowing their masses and compositions is vital to
modelling the formation of Earth-like planets.
20

21. Figure 19: The radius-density and radius-mass relations calculated by Weiss and Marcy [2014]
4.6 Planets in Eccentric Orbits
Figure 20: The eccentric orbit of exoplanet
HD80606 which spends 40% of its 111 day or-
bit within the habitable zone [Kane and Gelino,
2012a].
Planets in eccentric orbits could still spend
a significant about of time within the hab-
itable zone of their star. Kane and Gelino
[2012b] discuss whether a planet should only
be considered as a habitable candidate if it
spends 100% of it’s orbit within the zone.
They consider examples such as the exoplanet
HD80606, Fig.[20]. It has a mass of 3.9Mj
and its eccentricity is 0.93. At periastron its
temperature reaches 1546K while it drops to
286K at apoastron. Due to it being within
the habitable zone at its apoastron, it spends
40% of its 111 day orbit within the HZ.
Kane and Gelino [2012b] discuss how or-
ganisms could be shielded from the heat dur-
ing a close approach to the star. They refer-
ence microfossils surviving re-entry to Earth
with 5cm of shielding from sedimentary rock.
Also lichens have be shown to survive space
conditions for at least 10 days. So micro-
bial life at least is more hardy than might
be assumed. Known terrestrial organisms can
withstand long periods of extreme conditions
in a vacuum, with exposure to highly ener-
getic ultra-violet cosmic radiation and cosmic rays. It is not, therefore, inconceivable that
similar organisms could survive periastron flash-heating with a minimal amount of rock shield-
ing.
4.7 The Galactic Habitable Zone
To this point the habitability of individual planets and the habitable zones around specific stars
has been considered. But what if a much larger scale were to be considered? Lineweaver et al.
21

22. [2004] modelled a simplistic habitable zone for our galaxy. Their model simplifies to:
PGHZ = SFR × Pmetals × Pevol × PSN . (1)
It considers 4 prerequisites for complex life: a host star, enough heavy elements to form
terrestrial planets, time for significant evolution of life and outside the range of supernovae
radiation. The presence of a host star is represented in the model by the star formation rate
(SFR). This expresses that the more stars are present the more potentially habitable worlds
could exist.
To form a terrestrial planet there needs to be an abundance of elements heavier than hydro-
gen and helium. There is an upper limit however. Lineweaver et al. [2004] discuss how analysis
of exoplanets shows a correlation between large close-orbiting planets and high metallicity. The
boundaries of metallicity are therefore set as enough to form a terrestrial planet but not enough
that giant planets could migrate towards the star, potentially destroying Earth-mass planets in
their path. The probability of a star harbouring terrestrial planets Pmetals is calculated from
the space-time distribution of metals in the galaxy.
Pevol is determined by the time required for life to evolve. This is set at 4±1 billion years
for complex life since this is how long it took on Earth. This could be easily altered for less
complex life. Pevol is calculated from from the cumulative integral of a normal distribution of
mean 4 billion years and dispersion 1 billion years.
The consideration of supernovas is important since they can trigger shock waves and all
sorts of radiation such as cosmic rays, gamma-rays and x-rays. All of these could be fatal to
planets close by. Using the risk of supernova damage on Earth as a basis, it is modelled that
if the risk doubles, the probability of complex life surviving is half. There is a probability of
complete survival PSN =1 if the risk is half the value on Earth. The chance of survival is 0 for
4 times the risk value on Earth.
Combining these requirements produces a plot for the habitable zone of the Milky Way,
Fig.[21].
Figure 21: The galactic habitable zone (GHZ) [Lineweaver et al., 2004]
PGHZ represents the relative amount of planetary systems suitable for harbouring complex
life as a function of space and time. The solid white lines encompass 68% (inner) and 95%
22

23. (outer) of these systems. This inner limit represents less than 10% of all stars that ever existed
in the Milky Way. The galactic habitable zone lies between the galactic bulge, which would
contain too high a density of stars, and the barren outer galaxy with a lack of heavy elements.
After its initiation around 8 billion years ago, the galactic habitable zone widened as metallicity
spread outwards and the rate of supernovae reduced.
5 Discussion
The eventual aim is to detect life on other worlds. The most promising way to do this would
be to remotely detect life signatures in the atmospheres of exoplanets. This could potentially
be done by detecting thermodynamic disequilibrium in the atmospheric spectra as discussed by
Kasting et al. [2013]. This occurs when two chemicals co-exist when they should react with
each other. This is caused by a constant cycle of chemical processes on the surface, associated
with life. There are problems with this method since sometimes natural phenomena can explain
the chemical imbalance. For instance CO and H2 could be observed when they would naturally
combine to make CH4 and H2O. Their presence could however, be explained by large impacts
or photolysis of CO2 [Kasting et al., 2013]. We would have to be very careful to be able to
identify false positives with these detections.
Kasting et al. [2013] also discusses how life could exist within underground reservoirs. This
would likely be undetectable by remote methods since there is no evidence that noticeable
changes in the atmosphere would be produced from subsurface life. The best chance of con-
firming this possibility is through in situ measurements on Mars and Europa. This was also
considered by Ehlmann et al. [2011] in a discussion of the subsurface clay minerals discovered
on Mars.
Future searches for habitable planets and studies into their properties will require more
advanced telescopes. Kasting et al. [2013] states that for a 95% chance of finding at least one
Earth candidate, a telescope of at least 4m diameter would be required. They go on to suggest
that the best option for future study would be to focus on the pessimistic habitable zone, from
the moist greenhouse to the maximum greenhouse limits. This would require a bigger telescope
however an instrument with these detection capabilities could easily be reassigned to extend
the search to wider limits including Dune-like and H2 rich, strong greenhouse, planets. This
could not be said of a telescope made with the optimistic limits in mind.
6 Conclusions
It was considered how to find a habitable planet. The habitable zones around stars were derived
for optimistic and pessimistic limits using the original paper by Kasting et al. [1993]. This gives
the disk around a star in which liquid water could exist.
Searches have be made for planets within this region. 1523 exoplanets have now been
confirmed[6], 996 from the Kepler space telescope alone[9]. The Habitable Zone Gallery compiled
a list of 51 exoplanets with 100% of their orbit within the habitable zones of their stars[4].
Additional considerations were reviewed for habitability. It was found that the habitable
zones of smaller stars are at smaller orbital radii allowing for easier exoplanet detection due to
planets having a larger effect on their host star. As stars evolve through their main sequence,
the luminosity increases causing the habitable zone limits to extend outwards over time. Planets
which start outside of the habitable zone and then become included as the star evolves were
mentioned. It is possible that they could never be habitable due to a potentially highly reflective
surface. This would significantly reduce the habitable zone for complex life which needs billions
23

24. of years to evolve.
Tidal locking was also examined and was found to be very important due to a bias in
detections giving a lot of planets close to their stars. Tidal locking would generally lead to an
uninhabitable planet due to freezing of water and volatiles on the dark hemisphere. For small
planets and exomoons however, it could provide the heating required to make them habitable
by generating plate tectonics.
With these additional constraints in mind the Planetary Habitability Laboratory[3] produced
a list of potentially habitable planets which currently contains 21 candidates. The next step
is to consider these planets individually and try to identify if they are inhabited. Detecting
life was contemplated and found to be very difficult with current atmospheric analysis due to
false-positive bio-signatures being caused by natural phenomena.
References
Yutaka Abe, Ayako Abe-Ouchi, Norman H Sleep, and Kevin J Zahnle. Habitable zone limits
for dry planets. Astrobiology, 11(5):443–460, 2011.
Rory Barnes, Brian Jackson, Richard Greenberg, and Sean N Raymond. Tidal limits to plane-
tary habitability. The Astrophysical Journal Letters, 700(1):L30, 2009.
Joseph Catanzarite and Michael Shao. The occurrence rate of earth analog planets orbiting
sun-like stars. The Astrophysical Journal, 738(2):151, 2011.
Bethany L Ehlmann, John F Mustard, Scott L Murchie, Jean-Pierre Bibring, Alain Meunier,
Abigail A Fraeman, and Yves Langevin. Subsurface water and clay mineral formation during
the early history of mars. Nature, 479(7371):53–60, 2011.
Eric B Ford, Jason F Rowe, Daniel C Fabrycky, Joshua A Carter, Matthew J Holman, Jack J
Lissauer, Darin Ragozzine, Jason H Steffen, Natalie M Batalha, William J Borucki, et al.
Transit timing observations from kepler. i. statistical analysis of the first four months. The
Astrophysical Journal Supplement Series, 197(1):2, 2011.
Nader Haghighipour and Sabrina Kirste. On the detection of (habitable) super-earths around
low-mass stars using kepler and transit timing variation method. Celestial Mechanics and
Dynamical Astronomy, 111(1-2):267–284, 2011.
Stephen R Kane and Dawn M Gelino. The habitable zone gallery. Publications of the Astro-
nomical Society of the Pacific, 124(914):323–328, 2012a.
Stephen R Kane and Dawn M Gelino. The habitable zone and extreme planetary orbits. As-
trobiology, 12(10):940–945, 2012b.
James Kasting. How to find a habitable planet. Princeton University Press, 2010.
James F Kasting and Chester E Harman. Extrasolar planets: Inner edge of the habitable zone.
Nature, 504(7479):221–223, 2013.
James F Kasting, Daniel P Whitmire, and Ray T Reynolds. Habitable zones around main
sequence stars. Icarus, 101(1):108–128, 1993.
James F Kasting, Ravikumar Kopparapu, Ramses M Ramirez, and Chester E Harman. Remote
life-detection criteria, habitable zone boundaries, and the frequency of earth-like planets
around m and late k stars. Proceedings of the National Academy of Sciences, page 201309107,
2013.
24

25. David M Kipping, Stephen J Fossey, and Giammarco Campanella. On the detectability of
habitable exomoons with kepler-class photometry. Monthly Notices of the Royal Astronomical
Society, 400(1):398–405, 2009.
Ravi Kumar Kopparapu, Ramses Ramirez, James F Kasting, Vincent Eymet, Tyler D Robin-
son, Suvrath Mahadevan, Ryan C Terrien, Shawn Domagal-Goldman, Victoria Meadows,
and Rohit Deshpande. Habitable zones around main-sequence stars: new estimates. The
Astrophysical Journal, 765(2):131, 2013.
Charles H Lineweaver, Yeshe Fenner, and Brad K Gibson. The galactic habitable zone and the
age distribution of complex life in the milky way. Science, 303(5654):59–62, 2004.
Jack J Lissauer, Rebekah I Dawson, and Scott Tremaine. Advances in exoplanet science from
kepler. Nature, 513(7518):336–344, 2014.
Kristen Menou and Serge Tabachnik. Dynamical habitability of known extrasolar planetary
systems. The Astrophysical Journal, 583(1):473, 2003.
Michael A Mischna, James F Kasting, Alex Pavlov, and Richard Freedman. Influence of carbon
dioxide clouds on early martian climate. Icarus, 145(2):546–554, 2000.
F Pepe, Christophe Lovis, Damien Segransan, W Benz, F Bouchy, Xavier Dumusque, Michel
Mayor, Didier Queloz, NC Santos, and St´ephane Udry. The harps search for earth-like planets
in the habitable zone: I–very low-mass planets around hd20794, hd85512 and hd192310. arXiv
preprint arXiv:1108.3447, 2011.
Erik A Petigura, Geoffrey W Marcy, and Andrew W Howard. A plateau in the planet population
below twice the size of earth. The Astrophysical Journal, 770(1):69, 2013.
Caleb A Scharf. The potential for tidally heated icy and temperate moons around exoplanets.
The Astrophysical Journal, 648(2):1196, 2006.
Wesley A Traub. Terrestrial, habitable-zone exoplanet frequency from kepler. The Astrophysical
Journal, 745(1):20, 2012.
Lauren M Weiss and Geoffrey W Marcy. The mass-radius relation for 65 exoplanets smaller
than 4 earth radii. The Astrophysical Journal Letters, 783(1):L6, 2014.
Darren M Williams, James F Kasting, and Richard A Wade. Habitable moons around extrasolar
giant planets. 1997.
7 Extended Bibliography
7.1 Journals, Articles and Books
Descriptions of the information received from each reference:
[Abe et al., 2011] - Discussion of Dune-like desert planets with habitable regions at the poles
[Barnes et al., 2009]- Model of a tidal habitable zone (assuming no internal heat) which
overlays on the original habitable zone. Also general discussion of the effects of tidal heating
on exoplanets.
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26. [Catanzarite and Shao, 2011] - Extrapolations of Kepler data to estimate the frequency of
Earth like exoplanets.
[Ehlmann et al., 2011] - Interesting considerations of the subsurface water on Mars detected
from the distribution of clays around impact craters on the surface
[Ford et al., 2011] - Considers the timing accuracy of the Kepler data
[Haghighipour and Kirste, 2011] - Considerations of super-Earths around small M stars.
Multiple planet systems are discussed and their possible detection from TTVs and orbital
resonance.
[Kane and Gelino, 2012a] - Explanations of the Habitable Zone Gallery website and how
models were made to generate their figures. Also explains how they source the data to create a
dataset of potentially habitable planets.
[Kane and Gelino, 2012b] - Discussion of the habitability of planets in eccentric orbits which
spend some time in the habitable zone. They use comparisons to terrestrial organisms which
can survive extreme environments with some screening.
[Kasting et al., 1993] - Complete derivations of the habitable zone boundaries and how these
change with time and stellar mass. Also includes a discussion about the effects of tidal locking,
especially on smaller stars who’s habitable zones lie entirely within the tidal-lock limit.
[Kasting, 2010] - A general overview of the whole field in this book written by the lead
author of the paper which originally modelled the habitable zone.
[Kasting and Harman, 2013] - New considerations for the inner edge of the habitable zone
using updated climate models
[Kasting et al., 2013] - Physical explanations of the habitable zone limits. Also a discussion of
detecting life from bio-signatures in the atmospheres of exoplanets. They consider the frequency
of Earth-like planets and the effect of a stars spectrum on Rayleigh scattering leading to wider
habitable zones for smaller stars.
[Kipping et al., 2009] - A discussion of the possibility of detecting habitable exomoons using
current Kepler-class photometry particularly through the transit-timing variations method.
[Kopparapu et al., 2013] - An update to the Kasting et al. [1993] paper using new climate
models and updated assumptions.
[Lineweaver et al., 2004] - A model of the galactic habitable zone is produced using 4
prerequisites of life including: the presence of a host star, enough metals to form a terrestrial
planet, time to evolve complex life and the absence of nearby dangerous supernovae.
[Lissauer et al., 2014] - A discussion of Kepler’s properties and achievements including
considerations of how Kepler has impacted on the field of exoplanet science.
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27. [Menou and Tabachnik, 2003] - Discussed tweaking the habitable zone boundaries using
exoplanet data and discussing the boundaries used in other papers.
[Mischna et al., 2000] - The affect of CO2 clouds on the outer edge of the habitable zone
with the focus on early Mars.
[Pepe et al., 2011] - Consideration of the achievements of the ground-based High Accuracy
Radial-Velocity Planet Searcher (HARPS)
[Petigura et al., 2013] - Analysis of Kepler data to find the frequency of super-Earths,
specifically around solar-type stars.
[Scharf, 2006] - A discussion of the likelihood of habitable exomoons around large planets.
74 giant exoplanets are considered to see if they could be possible hosts for detectable exomoons.
[Traub, 2012] - Analysis of Kepler data from its first 136 days of operation to calculate the
frequency of Earth-like exoplanets around FGK stars.
[Weiss and Marcy, 2014] - Analysis of 65 exoplanets smaller than 4R⊕ leading to the re-
lationship between radius and density for small super-Earths. This pointed out the different
trends for ≤1.5R⊕ and 1.5-4R⊕.
[Williams et al., 1997] - A discussion of the properties and habitability of exomoons with
comparisons to the Galilean moons of Jupiter.
7.2 Websites
1. Gliese 581 planets Exoplanet.eu http://exoplanet.eu/catalog/?f=’Gl%20581’+in+name
Accessed 17/11/2014
A direct link to information about the Gliese 581 system to identify which planets in the
system have been confirmed.
2. Gliese 581 Habitable Zone-ESO http://pttu.hq.eso.org/blogs/posts/view/79724/
Accessed 17/11/2014
A diagram of the system of Gliese 581 with relation to it’s habitable zone and the Sun’s
system.
3. Planetary Habitability Laboratory http://phl.upr.edu/ Accessed 05/12/2014
A catalogue of potentially habitable exoplanets including confirmed and potential planets.
4. The Habitable Zone Gallery http://hzgallery.org/ Accessed 05/12/2014
A catalogue of potentially habitable exoplanets with a focus on the percentage of the orbit
an exoplanet spends within the habitable zone. Contains many figures and diagrams of
the systems of each planet. It includes confirmed and potential planets.
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28. 5. The Extrasolar Planets Encyclopaedia exoplanet.eu/ Accessed 05/12/2014
A catalogue of data mainly about confirmed exoplanets. Each exoplanet has links to
related literature.
6. The Exoplanet Data Explorer http://exoplanets.org/ Accessed 05/12/2014
A database of exoplanet information as well as tools for manipulating the data.
7. NASA Planet Quest http://planetquest.jpl.nasa.gov/ Accessed 05/12/2014
Mainly press-releases about exciting exoplanet discoveries with a focus on habitable plan-
etary candidates.
8. Open Exoplanet Catalog http://www.openexoplanetcatalogue.com/ Accessed 06/12/2014
A database of exoplanet information as well as tools for manipulating the data.
9. Kepler Mission Website http://kepler.nasa.gov/ Accessed 06/12/2014
Lists Kepler’s confirmed exoplanets and planetary candidates as well as information about
the mission.
10. PHL University of Hertfordshire Discoveries http://phl.upr.edu/press-releases/multiple-HZ
Accessed 06/12/2014
Mentions the discovery of 4 potentially habitable exoplanets by researchers at the Univer-
sity of Hertfordshire.
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