The Habitability of Planets March 28, 2020 1 Background The presence of liquid water is considered to be a prerequisite for life as we know it, which makes looking for water a practical way to begin our search for life beyond Earth. For water to exist on the surface of a planet, the planet must have the right temperature on its surface. The main driving force behind the surface temperature of any planet is the light it receives from its parent star. Around every star there is a region where the planet will receive just the right amount of light to give it temperatures that are conducive to liquid water - this region is call the star’s Habitable Zone. The orbit of the Earth currently falls within the Habitable Zone of our Sun. 2 The Habitability of the Earth To begin, load up the Habitable Zone simulator written by the University of Nebraska by entering the following URL in the address bar of your web browsers: http://astro.unl.edu/naap/habitablezones/animations/stellarHabitableZone.html The flash simulator will show you a visual diagram of the solar system in the top panel, a set of simulation settings in the middle panel, and a timeline of the habitability of the Earth in the bottom panel. To run the simulation, click the run in the bottom panel. This button immediately becomes a pause button which will allow you to pause the simulation at any time. To restart the simulation, press the restart button at the very top of the simulation. The blue region marked on the diagram is the Habitable Zone around our Sun. Notice how there is both an inner edge and an outer edge - the planets interior to the habitable zone are too hot to support liquid water, while the planets exterior to it are too cold. 1) The simulation is currently set to zero-age - this is the Solar System as it was when it first formed, 5 billion years ago. Which planets were in the Habitable Zone at this time? 1 http://astro.unl.edu/naap/habitablezones/animations/stellarHabitableZone.html 2) Press the start button and watch the Habitable Zone change with time. Pause the simulation when it reaches an age of 5 billion years (you can keep track of the time by looking at the timeline marker in the bottom panel). This is the Solar System as it is today - which planets are in the Habitable Zone now? 3) Allow the simulation to run until the Earth is no longer in the Habitable Zone. At what age does this happen? How long from now until this happens? You can use the timeline bar in the bottom panel to determine your answers. . 4) After the Earth is no longer within the Habitable Zone, what do you think the condi- tions on Earth will be like? 5) Resume the simulation and let it run until the end. Which planets other than the Earth fell within the Habitable Zone at any point during the Sun’s life? 6) If you had to choose planets of our Solar System for future colonization based on their future habitability, which would you choose, and why? 3 The Habitability Different Kinds .

1. The Habitability of Planets
March 28, 2020
1 Background
The presence of liquid water is considered to be a prerequisite
for life as we know it, which
makes looking for water a practical way to begin our search for
life beyond Earth. For water
to exist on the surface of a planet, the planet must have the right
temperature on its surface.
The main driving force behind the surface temperature of any
planet is the light it receives
from its parent star. Around every star there is a region where
the planet will receive just
the right amount of light to give it temperatures that are
conducive to liquid water - this
region is call the star’s Habitable Zone. The orbit of the Earth
currently falls within the
Habitable Zone of our Sun.
2 The Habitability of the Earth
To begin, load up the Habitable Zone simulator written by the
University of Nebraska by
entering the following URL in the address bar of your web
browsers:
http://astro.unl.edu/naap/habitablezones/animations/stellarHabit
ableZone.html

2. The flash simulator will show you a visual diagram of the solar
system in the top panel,
a set of simulation settings in the middle panel, and a timeline
of the habitability of the
Earth in the bottom panel. To run the simulation, click the run
in the bottom panel. This
button immediately becomes a pause button which will allow
you to pause the simulation
at any time. To restart the simulation, press the restart button at
the very top of the
simulation.
The blue region marked on the diagram is the Habitable Zone
around our Sun. Notice
how there is both an inner edge and an outer edge - the planets
interior to the habitable
zone are too hot to support liquid water, while the planets
exterior to it are too cold.
1) The simulation is currently set to zero-age - this is the Solar
System as it was when
it first formed, 5 billion years ago. Which planets were in the
Habitable Zone at this time?
1
http://astro.unl.edu/naap/habitablezones/animations/stellarHabit
ableZone.html
2) Press the start button and watch the Habitable Zone change
with time. Pause the
simulation when it reaches an age of 5 billion years (you can
keep track of the time by looking
at the timeline marker in the bottom panel). This is the Solar
System as it is today - which

3. planets are in the Habitable Zone now?
3) Allow the simulation to run until the Earth is no longer in the
Habitable Zone. At
what age does this happen? How long from now until this
happens? You can use the timeline
bar in the bottom panel to determine your answers. .
4) After the Earth is no longer within the Habitable Zone, what
do you think the condi-
tions on Earth will be like?
5) Resume the simulation and let it run until the end. Which
planets other than the
Earth fell within the Habitable Zone at any point during the
Sun’s life?
6) If you had to choose planets of our Solar System for future
colonization based on their
future habitability, which would you choose, and why?
3 The Habitability Different Kinds of Stars
Now that you’ve simulated the Habitable Zone around our Sun,
we’ll run the same simulation
for other stars. Astronomers classify stars with letters, O, B, A,
F, G, K, and M. The O
stars are the hottest and brightest, while the M stars are the
dimmest and coolest. Every
kind of star has a Habitable Zone, but the brighter the star the
farther out the Habitable
Zone. Imagine putting and extra log on a campfire - the campers
all have back off a few feet
to maintain the same comfortable temperature.
But in order for complex life to have a chance to develop, a

4. planet must remain habitable
for an extended period of time. How long? We only have Earth
to use as an example, so we
really don’t know. For the purpose of this exercise, we’ll
assume that Earth is “typical” and
that planets around other stars mostly follow the timeline of
events on Earth shown below:
2
Billions of Years Ago Development Toward Complex Life
4.5 Earth forms
4.3-4.4 Earth cools, oceans form
3.8 first bacterial fossils
2.4 rise of oxygen in atmosphere
2.0 first complex cells
0.55 first complex animals (fossils in Clapp)
The next table shows several different kinds of stars. Notice
how they each have a different
mass - the mass of a star determines what kind of a star it is.
Reset the Habitable Zone
simulator with the reset button at top, and then adjust the star
mass with the initial star
mass slider bar in the middle panel. Notice how the Habitable
Zone immediately changes size.
Notice also that you can adjust the orbit of “Earth” by adjusting
the initial planet distance
slider bar in the middle panel. The units of distance from the
star are AU - astronomical
units, the distance of the Earth from the Sun. The Earth is one
AU from the Sun.
For each of the star types in the table below, find the planet

5. orbit that remains habitable
the longest. To do this you’ll need to run the simulation many
times for each star type,
each time adjusting the initial planet distance until you find a
distance that keeps the planet
habitable the longest. Record in the table 1) the size of this
orbit, in AU, 2) how long this
orbit remains habitable, 3) the most advanced type of life that
can develop during this time
frame, assuming the Earth’s timeline for life is typical.
Type Star Mass Longest Habitable Orbit Habitable Lifetime
Most Advanced Life
[Solar Masses] [Astronomical Units] [Billions of Years]
O 15.
B 5.0
A 2.0
F 1.3
G (Sun) 1.0
K 0.7
M 0.4
4 Tidal Locking
Unfortunately, for low-mass M type stars the habitable zone is
quite close to the star - so
close that planets in this zone are likely to be tidally locked.
This means that the same side
of the planet will always face the star, just as the same side of
the Earth’s moon always faces
the Earth. The simulator indicates that a planet is tidally locked
when it is split between
one brown and the other side being light gray. In this section
we’ll experiment with planets
around M type stars. Adjust the stars “initial stellar mass” to

6. 0.3 (30% of the Sun’s mass),
3
and adjust the “initial planet distance” until the planet is in the
star’s Habitable Zone. The
planet should switch to the tidally locked icon, even if the
planet is in the Habitable Zone.
1) What impact do you think tidal locking would have on the
prospect of life on this
planet?
2) Try adjusting the star’s mass. What is the lowest mass star
that would allow a non-
tidally locked planet in the Habitable Zone at the beginning of
the star’s life?
3) What is the lowest mass star that would allow a non-tidally
locked planet in the Hab-
itable Zone at any point during the star’s life?
5 Tying it All Together
1) Given what you’ve learned so far, what type of star is the
best place to look for life?
2) The Sun is a G-type star. What do you think the development
of life on planets
orbiting hotter types of stars would be like? What about cooler
types of stars? Do you think
that life in such conditions is even possible? Justify your
answer.

7. 3) If you were the director of a NASA program to search for life
beyond Earth, toward
which type of star would you direct your attention? Why?
Justify your answer using the
evidence above, and also any other lines of reasoning you like.
4
4) 76 percent of all stars are M-type; does your answer to the
above question change?
Why?
5
BackgroundThe Habitability of the EarthThe Habitability
Different Kinds of StarsTidal LockingTying it All Together