Using radio observations, astronomers may be able to detect and characterize exoplanets by observing radio emissions from their magnetospheres. Planetary magnetospheres are theorized to produce bursts of decametric radiation through interactions with the solar wind that distort magnetic field lines and accelerate charged particles. While difficult to detect over a host star's emissions, observation of planetary radio signals could provide information on properties like the presence, size, and composition of a magnetosphere, as well as any orbiting satellites. The LOFAR radio telescope may enable the first detections of exoplanetary radio emissions within the next decade.

1. Using decametric and X-ray radiation to identify and
characterise Exoplanets
X-rays
Transit Method
Background
This iswhenthe planetmovesinfront of the star, temporarilyblockingthe light
fromthe star,whichdecreasesitsmagnitude forashort space of time.Todetect
these drops in magnitude we can study the stars light curve. If the drop in
magnitude is consistent then it tells us that the object is in a constant orbit
arounditsparentstar.Thismeansthatthisobjectcouldpotentiallybe an exoplanet.[Image
source: http://astronomyonline.org/Exoplanets/AmateurDetection.asp]
Secondary Transit
A secondarytransitis whenthe planetmovesbehindthe star.The planet'sdayside iseclipsed bythe
star. During the primary transit, the day side of the exoplanet is directedaway fromthe observer. As
the planet proceeds around its orbit a gradually increasing fraction of the day side becomes visible.
This allows direct measurements of the exoplanet's radiation.This secondary transit makes it easier
forustosee thermal radiationandreflectedlightasthe exoplanetdisappearsbehindthestarandthen
reappears again.
Limitations
 The orbithas tobe in the same plane as the observer’sviewingpointandthe exoplanet'sstar.
 The transit method also suffers from a high rate of false detections. For instance, if the star
that youare observinghassunspotspresentthenthese sunspotscanalsodecrease the star's
magnitude. This means that if a single drop in magnitude is observed, it requires additional
confirmation as this could just be a sunspot.
 The gravitational pullof anothermassiveobjectcanaffectthe regularityof the orbital period,
whichcouldbe mistakenasafalsedetection.Howeverthiscanalsobe anadvantageasitcould
hint towards the presence of another planet or that of a moon.
Transittimingvariationiswhenthe periodbetweenaplanet’sorbitchangesdue tothe gravitational
effectof anothermassive object.[see diagram]
[image source:http://www.sciencemag.org/content/330/6000/51/F5.full]

2. X-rayPhotometry
Background
Photometry is a technique where you measure the flux in brightness of an astronomical object.
Photometrycouldbe used in detectingthe x-rayscatteringandfluorescence whenthe planetenters
its secondary eclipse and hence detecting an increase in x-ray intensity and being able to detect a
planet.
Primaryandsecondarytransits have the capabilityof tellingusthe radiusof the planetand the make-
up of itsouteratmosphere.Forthe primaryeclipsetransitingmethod,the dipinbrightnesscantellus
the size of the objectaspassesinfrontof thestar.Forthesecondaryeclipse method,wecouldpossibly
finda wayto “image”the planetanddetectthe shape of itfrom the reflections,ashasbeendone by
observations of the day side of Jupiter.
Primary eclipse transits of an exoplanet have already been observed once before on the planet HD
189733b. It wasseenthatthe transitdepthof the planetwas6-8% ratherthan2.41% whenthe same
planetwasobservedwithvisiblelight. Thisisdue tothe atmosphere of the planetscatteringsome of
the x-rayscausingittoappearopaque.Thismeansyoucan potentiallydiscoverexoplanetsmore easily
due to a greater dip in the light curve during transit.
Secondaryeclipse transitinghasneverbeenusedbefore andwoulddependonwhetherthe intensity
of the planetisnegligible compared tothe intensityof x-raysfromthe star. For our sun, the amount
of x-ray flux seems to average at around 10-8
watts per m2
. The x-ray flux on Jupiter for its aurora is
between 1 and 4 gigawatts which would calculate to 6.5x10-20
watts per m2
however, the x-ray
emissionswouldbe more concentratednearthe polesmeaningthe intensitywouldbe greaterhere,
and thisisdisregardingthe scatteringof the sun’sx-raysoff Jupiter’satmosphere. Thereforealthough
direct images may not be possible,it may be possible to detect an increase in brightness during a
secondary eclipse.
Potential for X-ray spectroscopy
X-ray spectroscopycanbe carried out by studyingcharge exchange.10
Charge exchange emissioncan
occur when ions interact with neutral atoms or molecules from which one or more electrons are
transferred to the ion into an excited state. During the de-excitationof the electron a cascade of
photons are emitted. Charge exchange results in photon emissions at energies characteristic of the
ionsinvolved.A lineemissionspectracanthenbe carriedouttoidentifythe ions.8
The ionsthatresult
in the emission of X-rays include highly-charged oxygen, carbon, and neon, which are all present in
low percentages in the solar wind but also sulphur and oxygen also originate in the Jovian
magnetosphere and large Jupiter-like planets. The identification of sulphur would indicate the
presence of a volcanic moon such as Io orbiting Jupiter.
The ionsare carriedalongmagneticfieldlinesandprecipitate soX-rayswill be emittednearthe poles.
If an exoplanetdepictsthese characteristicswe will be able todetermine the presence of amagnetic
fieldandanexomoonif sulphurispresent.Howeverwe cannotknowthe orbitaldistanceof the moon
as we cannotmap the originof the x-rayson tothe surface of the planet.Thisisbecause the planetis
too far away for it to be spatially resolved.

3. Possible complications include if the parent star’s own emission spectra lie within those of key
emissions of charged ions present in the planet’s magnetosphere. The star’s emissions could easily
overshadow the signal of the planet making it difficult to characterise.
Athena Telescope
Athena'smainobjectives will be tomaphot gas structuresanddetermine theirphysical properties.It
also is searching for supermassive Black Holes.
Athena studies the solar system by providing deeper insights to magnetospheres and exospheres.It
will add enormously to our understanding of the interactions of space plasmas and magnetic fields
and itwill enable surfacecompositionanalysisthroughfluorescencespectraof the Galileansatellites.
It can alsoestablishhowplanetaryexospheresrespondtothe interactionwiththe solarwind.Athena
can explorethe magneticinterplaybetweenstarsandplanetsinX-raysbyanalysingthe X-rayspectra.9
Currentlythe ACISinstrumentonChandracombinesthe twotechniques;itfocusesx-raysontoaCCD
to produce an image (photometry) and simultaneously measures the energy of each photon
(spectroscopy). However, ongoing missions, such as Chandra and XMM-Newton, cannot currently
distinguishbetween Xrayemissionsfrom sulphurionsand thatof carbonionswhereasAthenawillbe
able to.
The use of Athenawill allowustocarryout highsensitivityobservationsof X-rayfluorescence inmuch
greaterdepthandresolution.Athenawill equallybe able togive insightstounderstandingthe details
of the charge exchange process andthe different,maybe unexpected,spectral linesemitted enabling
us to possibly identify and characterise exoplanets in greater detail.
Radiowaves
Background:
Giventhe knownfeaturesof planetarymagnetosphereswithinourownSolarSystem(e.g.Jupiter),it
can be inferred that magnetospheres of exoplanets should produce bursts of decametric radiation.
Such bursts may be detectable from the Earth using ground-based radio telescopes.
Causes of emissions:
Planetary magnetospheres interact with the solar weather causing distortion of the magnetosphere
and accelerationof chargedparticles,bothof whichare theorisedascausesof decametricradiation.
Withregardsto distortion,the reconnectionof magneticfieldlines,inadditiontoanaccumulationof
repulsiveforces,mayproduce burstsof radiation.Studiesof Jupiter’sradioemissionssuggestthatthe
acceleration of electrons from the solar wind and material ejected from Io can result in bursts of
“cyclotron” radiation, alongside auroral radiation.

4. Detection:
Radio emissions from planetary magnetospheres will be characterised by being highly polarised and
correlated with emissions from the host star’s magnetosphere in bursts. However, given that such
emissions will be so small compared to those of the star they will be difficult to distinguish. Stellar
activitymayproduce similaremissions andsoplanetarydetectionwouldbe nullifiedtherefore nonew
planets have been detected by this method. Magnetospheres will emit on a limited range of
frequencies,accordingtotheirsize andshape.For example Jupiter’semissionshave afrequency~40
MHz.
Observation:
Given the difficulties involved, it may prove more productive to use radio observations to study
(potentially habitable) exoplanets alreadyknownto exist,as it will be more immediately possible to
distinguishradioemissionsof the planetfromthose of the star1
.Radioobservationof browndwarves
has alreadybeensuccessful2
andthuscanbe seenas a steptowardsplanetaryobservation.Planetary
information which can be provided includes:-
 Presence of a magnetosphere.
 Size and strength of the magnetosphere3
– From the frequency of the radiation one can
determine the flux density, which in turn can be used to infer the magnetosphere’s
properties.
 Plasma environment around the planet – How the magnetic field appears to interact with
stellaractivityandthuscause the flux (andthereforeradiosignal)tofluctuate will determine
the local plasma environment (e.g. radiation belts).
 Presence of satellites – As satellites move through their host planet’smagnetosphere,they
may induce a magnetic moment, producing additional radio emissions and potential drops
that may even be separately detectable (e.g. Io produces notable fluctuations in Jupiter’s
magnetosphere).
 Rotationrate – The magneticmomentof planetsinthe Solar Systemhasbeenfoundtobe a
function of their rotation rate and mass.
 Composition4
–Magnetospheresare generatedbyadynamoeffectof acirculatingconductive
medium.Whatthismediumiscanbe inferredbystudyingthe magnetosphere,andfromthis
False-colour maps of radioemissions around the planets
of the Solar System.
The frequency of the emissions is unique to the
characteristics of a planet’s magnetosphere, as shown
with this comparison of Solar System planets.

5. infer the compositionof the planet as a whole. Gas giant planets (e.g. Saturn) will have a
medium of metallic hydrogen; “ocean planets” or “ice giants” (e.g. Uranus, GJ 1214 b) will
have superfluid volatiles; and rocky planets (e.g. Earth) will have molten metal.
Implications for habitability:
 Rotationrate5
–If aplanetistidally-lockedtoitshoststarorrotates veryslowly(asistheorised
to be the case withplanetsrecentlydiscoveredinthe habitable zone of reddwarf stars) then
thispresentschallengesforhabitabilityintermsof anerraticclimatepronetolong-termstorm
events.
 Satellites–Earth-sizedmoons(orevensmall moonsgivensufficienttidal heating) mayprove
to be equally viable and more numerous habitats for life than exoplanets.
 Magnetosphere – The shielding provided by a magnetosphere may protect organisms from
genetic damage and help to retain water and an atmosphere.
 Composition – Rocky planets are the most likely planets to be able to support life.
 Albedo – Incident charged particles not deflectedby the magnetosphere encourage cloud
formation, thus increasing albedo and reducing surface temperature.
Instrument specification:
Earth’sionosphere reflectsradiationatfrequencies≤10MHz6
,therefore,if habitableplanetsare taken
to have analogous magnetospheres, they should also emit at similar frequencies. However, Earth’s
atmosphere has a high opacity at these frequencies. Also, the frequency is so low that instruments
would require extremely high resolution to distinguish the emissions of an Earth-analogue
magnetosphere. We therefore propose the use of a large-scale, ground-based radio telescope array
for the following reasons:-
 Large numbers of exoplanets can be observed simultaneously
 Cheaper and easier to maintain than a space-based telescope
 Strong precedent for land-based radio telescopy
 Range of orientations provided my individual antennae
 Easier to increase resolution by simply increasing antenna spacing
 Sufficiently large surface area overcomes the problem of high atmospheric opacity7
.
Perhaps the strongest advantage is that exoplanet observation could take place alongside other
projects, reducing the need for specialist development and thus greatly reducing cost and
development. A suitable candidate may be the Square Kilometre Array (SKA) which is under
construction in South Africa and Australia would be able to detect frequencies down to 8MHz and
have adequate resolution,andwouldbe able to observe exoplanetsaspart of itswiderprogramme.
A suitable test target may be the known exoplanet Tau Bootes b.
Artist’s impression of the completed SKA at night.Data on the decametric radio emissions of
Jupiter from the Ulysses mission.

6. 1 www.sr.bham.ac.uk/exoplanets/esp_radio.php
2 www.tauceti.caltech.edu/cs18/
3 https://skaoffice.atlassian/wiki/philippe-zarka/PoS-exopla-AASKA.pdf
4 www.physics.irfu.se/Publications/Theses/Danielsson:MSc:2007.pdf
5 www.ece.vt.edu/swe/lwa/memo/lwa0013.pdf
6 journees-radio.scienceconf.org/conference/journees-radio/program/Jeudi_S1_3_Zarka.pdf
7 kiss.caltech.edu/workshops/magnetic2013/presentations/hallinan2.pdf
8 XMM-NEWTON Spectroscopy of Jupiter, G Branduardi-Raymont
9 The Hot and Energetic Universe- Solar System and Exoplanets,G Branduardi-Raymont
10 http://www2.le.ac.uk/departments/physics/research/xroa/astrophysics-1/SWCX