Extrasolar planetsExtrasolar planets

Astronomy 9601

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Topics to be coveredTopics to be covered

• 12 1 Physics and sizes12.1 Physics and sizes• 12.2 Detecting extrasolar planets

12 3 Ob ti f l t• 12.3 Observations of exoplanets• 12.4 Exoplanet statistics• 12.5 Planets and Life

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What is a planet? What is a star?What is a planet? What is a star?

• The composition of Jupiter closelyThe composition of Jupiter closely resembles that of the Sun: who’s to say that Jupiter is not simply a “failed star” p p yrather than a planet?

• The discovery of low-mass binary stars y ywould be interesting, but (perhaps) not as exciting as discovering new “true” planets.

• Is there a natural boundary between planets and stars?

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Planets and brown dwarfsPlanets and brown dwarfs• A star of mass less than 8%

of the Sun (80x Jupiter’s mass) will never grow hot

Luminosity “bump” due to short-lived deuterium burning mass) will never grow hot

enough in its core to fuse hydrogen

• This is used as the boundary between true stars and very

Steady luminosity due to H burning

between true stars and very large gas planets

• Objects below this mass are called brown dwarfs

• The boundary between BD and planet is more controversial– some argue it should be g

based on formation– other choose 0.013 solar

masses=13 Mj as the boundary, as objects below this mass will never reach even deuterium fusion

Nelson et al., 1986, AJ, 311, 2264

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Pulsar planetsPulsar planets• In 1992, Wolszczan and Frail 

announced the discovery of a multi‐planet planetary system around the millisecond pulsar PSR 1257+12 (an earlier announcement had been 

Artist’s conception of the planet orbiting pulsar PSR B1257+12

retracted).• These were the first two extrasolar 

l t fi d t b di dplanets confirmed to be discovered, and thus the first multi‐planet extrasolar planetary system discovered, and the first pulsar planets discovered

• However these objects are not in• However, these objects are not in planetary systems as we usually think of them 6

Worlds Beyond Our SunWorlds Beyond Our Sun• In 1995 a team of Swiss

astronomers disco eredastronomers discovered the first planet (in a non-pulsar system) outside

l t bitiour solar system, orbiting a sun-like star called 51 Pegasi.

• Further discoveries bring the grand total of known extrasolar planets to 861 (as of March 2013) and counting.

Artist's rendition of the star 51 Pegasi and its planetary companion 51 Pegasi B.

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Unseen CompanionsUnseen Companions

• Curiously enough,Curiously enough, most extrasolar planets remain unseen

• They are usually detected by indirect means, though their effects on their parenteffects on their parent star.This artist's concept shows the

Neptune-sized extrasolar planet circling the star Gliese 436. g

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Obstacles to Direct Detection• Direct detection is the only way to tell what these planets

are made of and whether there's water or oxygen in their t hatmospheres.

• But most known exoplanets are impossible to see with current technology gy

• Two reasons why: – known exoplanets are too dim

Jupiter for example is more than a billion times fainter than• Jupiter, for example, is more than a billion times fainter than the Sun. However it could easily be seen at large distances except for…

– known exoplanets orbit too close to their parent stars– known exoplanets orbit too close to their parent stars• most known exoplanets have orbits smaller than that of

Mercury

"It's like trying to see a firefly next to a searchlight from across town."9

The “first confirmed” image of an exoplanet:

GQ Lupi & Planetary Companion

21 Mj, 100 AU orbit. Imaged by ESO’s VLT, then HST and Subaru confirmed (early Apr 2005) 10

Detection methods: Astrometryy• oldest method,

used since 1943used since 1943• the wobble

induced in the plane of skyplane-of-sky motion of the star by planets is measured bymeasured by accurately observing its position over timeposition over time

• 1 detection

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Astrometry• STEPS (Stellar Planet Survey)

detected periodic proper motion of VB 10 a nearby brown dwarfof VB 10, a nearby brown dwarf.

• VB 10b is approximately 6 Jupiter masses, with a period of 9

hmonths. • No sign of planet when examined

with other techniques: busted! q

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Astrometry:Difficultiesy•Example: The Sun

bbl b b t itwobbles by about its diameter, mostly due to Jupiter. •At 30 light-years, this would produce an apparent motion of lessapparent motion of less than 1 milliarcsecond.• Typical good ground-based observingbased observing conditions produce positions with accuracies

A t ti f S f 30 lbelow but around 1 arc-second.

Apparent motion of Sun from 30 ly

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Detection methods: Pulsar planetsDetection methods: Pulsar planets

• Pulsar planets are planets that are found orbiting pulsarsp p g p– Pulsars are rapidly rotating neutron stars.

• Pulsar planets are discovered through radio pulsar timing measurements to detect anomalies in thetiming measurements, to detect anomalies in the pulsation period. Any bodies orbiting the pulsar will cause regular changes in its pulsation. Since pulsars normally rotate at near-constant speed any changes cannormally rotate at near constant speed, any changes can easily be detected with the help of precise timing measurements.

• The first ever planets discovered around another star• The first ever planets discovered around another star, were discovered around a pulsar in 1992 by Wolszczan and Frail around PSR 1257+12. Some uncertainty initially surrounded this due to an earlier retraction of ainitially surrounded this due to an earlier retraction of a planet detection around PSR 1829-10

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PSR 1257+12• Pulsar located 2630 light years away• These were the first extrasolar planets ever discovered• These were the first extrasolar planets ever discovered• Pulsar mass 0.3 Msun, rotational period 0.0062 seconds

Mass (ME) a (AU) Period (days) e

First planet 0.020 0.19 25.26 0.0

Second planet 4.3 0.36 66.54 0.02

possible small fourth object has an upper mass limit of 0 2 M

Second planet 4.3 0.36 66.54 0.02

Third planet 3.9 0.46 98.21 0.025

– possible small fourth object has an upper mass limit of 0.2 MPlutoand an upper size of R < 1000km.

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5 of the 12 known pulsar planet systemsP l l t M O bit di t O bit i dPulsar planet Mass Orbit distance Orbit period

PSR B1620-26 c 2.5 Jupiters 23 AU 100yr

V391 Peg b 3.2 Jupiters 1.7 AU 1170 days

PSR 1257+12 a 0.02 Earths 0.19 AU 25 days

b 4.3 Earths 0.36 AU 66 days

c 3.9 Earths 0.46 AU 98 daysc 3.9 Earths 0.46 AU 98 days

d 0.0004 Earths 2.7 AU 3.5 years

QS Vir b 6.4 Jupiters 4.2 AU 7.9 years

HW Vi b 19 2 J it 16HW Vir b 19.2 Jupiters 16 years

c 8.5 Jupiters 332 days

•Since neutron stars are formed after the violent death of massive stars•Since neutron stars are formed after the violent death of massive stars (supernovae), it was not expected that planets could survive in such a system.•Its now thought that the planets are either the remnant cores of giantIts now thought that the planets are either the remnant cores of giant planets that were able to weather the supernova, or later accretion products of supernova debris. 16

Detection methods: TransitsDetection methods: Transits

• Planets observed at inclinations (measured with respect to the plane of the sky) near 90o will pass in front of (“transit”) their host stars dimming the light of the star( transit ) their host stars, dimming the light of the star. This may be detectable by high-precision photometry.

•Note that the planet is invisible, being unresolved, only p , g , ythe brightness variation in the star is seen.

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The Observational ChallengeThe Observational ChallengeThe fraction of stars expected to have transits is:

f f f ff = fs fMS fCEGP ptfs = fraction of stars that are single = 0.5fMS = fraction of those on the main sequence = 0.5f = fraction of those that have a close in planet = 0 01fCEGP = fraction of those that have a close-in planet = 0.01pt = fraction of those with an inclination to transit = 0.1

• Need to look at 4000 stars to find 1 that transitsNeed to look at 4000 stars to find 1 that transits.• Need to sample often compared to transit duration.• Need 1% accuracy for a 3s detection of a 2 hour transit.• Need to look on sky for at least 1 orbital period.y p

Requires 1,000,000 15-minute samples with 1% accuracy to detect one transit.with 1% accuracy to detect one transit.

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TransitsTransits

• Assuming– The whole planet passes in front of the star

A d i i li b d k i f th t li ibl– And ignoring limb darkening of the star as negligible• Then the depth of the eclipse is simply the ratio

of the planetary and stellar disk areas:of the planetary and stellar disk areas:2

2

2

⎟⎟⎠

⎞⎜⎜⎝

⎛==

ΔRR

RR

ff ppπ f = light flux

• We measure the change in brightness, and estimate the stellar radius from the spectral type

*2

**⎟⎠

⎜⎝ RRf π

estimate the stellar radius from the spectral type 19

TransitsTransits

Ad t• Advantages– Easy. Can be done with small, cheap telescopes

• WASP, STARE, numerous others – Possible to detect low mass planets, including “Earths”,

especially from space (Kepler mission, launched Mar 2009)

• Disadvantages– Probability of seeing a transit is low

• Need to observe many stars simultaneouslyy y– Easy to confuse with binary/triple systems– Needs radial velocity measurements for confirmation,

massesmasses• Has found 294 exoplanets in 238 systems so far

(March 2013) 20

• OGLE-TR-10: Konacki et al. 20040 57M 1 24R P 3 1d• 0.57Mj, 1.24Rj, P=3.1days

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Kepler (transits)Kepler (transits)

With a total of 95 mega-pixels of CCDs Kepler is capable of observing over 100,000 stars all at once and

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p g ,measuring their brightness to an accuracy of better than 1 part in 100,000.

Kepler Orrery

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Detection methods: microlensingDetection methods: microlensing

• If the geometry g yis correct, a planet can actually produceactually produce a brightening (rather than a (dimming) of a background star (not the parent(not the parent star) through gravitational microlensing.

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First detection: OGLE 2003 BLG 2352003 BLG-235

A l i fAnalysis of the light curve reveals second object seco d objectin lens with .4% of mass of the other

• 17,000 light years away, in the constellation Sagittarius. • The planet, orbiting a red dwarf parent star, is most likely p , g p , y

one-and-a-half times bigger than Jupiter. • The planet and star are three times farther apart than

Earth and the Sun.• Together, they magnify a farther, background star some

24,000 light years away, near the Milky Way center. 25

MicrolensingMicrolensing• Microlensing has some g

disadvantages – model-dependent– only see the planet onceonly see the planet once

• However, it is the “best” technique for finding smaller planets farthersmaller planets, farther from their star – ie. more Earth-like planets

th RV t h i ( t)than RV technique (next)• 18 detections so far

(Mar/2013)OGLE 2005-BLG-390 (Artist’s impression): Five Earth mass planet on a 10 yr orbit around a red ( ) p ydwarf star. First (probably) icy exoplanet found (25 Jan 2006)

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Detection methods: radial velocityDetection methods: radial velocity• Most of the planets known to

date were discovered using thedate were discovered using the “Doppler shift” or “radial velocity” method.

• A planet's gravity pulls its host p g y pstar back and forth during its orbit. This causes the light we receive to be "blueshifted" and "redshifted"redshifted .

• Although the Doppler signals are enough to convince us that extrasolar planets exist, these exoplanets are not seen directly.

• (~502 detections as of March 2013)

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Stellar Doppler shiftsObserve the period P

22*3 PGMr =

Assume a circular orbit (i iti ll ) t fi d l t

24π

K(initially) to find planet velocity

rGMV /= P

From conservation of momentum determine M

rGMVp /*= P

momentum, determine Mp

pp VVMM /**= Assume a mass for the star (from spectral type) to compute Mp sin i (K = V*sin i)

(i = inclination of orbital plane to line of sight)

pp VKMiM /sin *=28

EccentricityEccentricity• By looking more closely at the shape of the y g y p

curve, the eccentricity of the planet’s orbit can be determined.

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51 Pegasi b

• First planet discovered around a sun-like star outside of the solar system

• Radial velocity method• Detection from regular velocityDetection from regular velocity

changes in the star's spectral lines of around 70 metres per second

• Semi-major axis 0 052 AU (circular)Semi major axis 0.052 AU (circular)• Orbital period 4.23077 d• Mass >0.468 ± 0.007 MJ

G t di th J it d it• Greater radius than Jupiter despite its lower mass

• Superheated 700 K atmosphere• It is the prototypical ”hot Jupiter”• Orbital migration to present position? Artist’s conception 30

Observational challengesObservational challenges• Requires high-

precisionprecision repeatable spectroscopic

tmeasurements of Doppler shifts to ~ 1m/sshifts to 1m/s accuracy

• Most sensitive to massive planets near the star (“hotthe star ( hot Jupiters”)

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Direct DetectionDirect Detection• To understand extrasolar planets,

we really need their lightwe really need their light• None of the radial velocity planets

can be imaged with current technologytechnology– Planet is too faint and too close to the star

• Solution: Remove the starlight ( d i i h(adaptive optics, coronagraphy, interferometry)

• To optimize the contrast between Above: Gliese 229B – brownTo optimize the contrast between planet and star, one observes red dwarfs, brown dwarfs & white dwarfs and chooses a wavelength

Above: Gliese 229B brown dwarf companion to nearby M dwarf

dwarfs, and chooses a wavelength band that favours the planet

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The Adaptive Optics DifferenceThe Adaptive Optics Difference

• Images of the planet Neptune from the W.M. Keck observatory in Hawaii. Keck comprises two telescopes, each with a primary mirror 10 m in diameter. Support staff have recently installed an AO system on Keck II.

• The left-hand image is what you normally see using• The left-hand image is what you normally see using Keck II. The right-hand image was taken after the AO system was turned on. 33

Planet brightness vs age•Gas giant planets are hotter when they form, and cool over timeand cool over time. •Hot Jupiters emit more strongly in the thermal IR than more distant gasthan more distant gas giants.•Jupiter is 109 times fainter than the Sun in the visible, but only 106

times fainter in the thermal IRY J i d h

Solid lines Burrows 1997 models, dashed lines Burrows 2002 models

•Young Jupiters and hot Jupiters may be only 104

times fainter than their stars in the IRBurrows 2002 models

Models assume evolution in isolation: no additional heating source or reflection component

stars in the IR

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The “first” image of an exoplanetThe first image of an exoplanet• 2M1207 parent “star” is a

brown dwarfbrown dwarf – 10Myr old (young)– in an association of newly

formed stars• Planet

– mass =5Mj• determined from model of• determined from model of

spectrum of companion= uncertainty!

– radius = 1.5 Rjj– 41 AU from the star

• Chauvin et al. 2004, A&A, 425, L29 Imaged with NACO (an adaptive optics instrument), Imaged with NACO (an adaptive optics instrument)

on ESO’s Very Large Telescope (VLT) Sep 2004. Odd orbit means only confirmed after common

proper motion confirmed (mid-Apr 2005) 35

The “first confirmed” image of an exoplanet:

GQ Lupi & Planetary Companion

21 Mj, 100 AU orbit. Imaged by ESO’s VLT, then HST and Subaru confirmed (early Apr 2005) 36

Caution!• AB Dor: nearby, young (~50

million years, 15pc) red dwarfdwarf

• Brown dwarf companion• In this case, the mass could

also be measured from directalso be measured from direct observations of orbit over time

• 2 5x more massive than2.5x more massive than spectral models predict (90 MJ vs 36 MJ)

• So the planet is “just” aSo the planet is just a brown dwarf

• Masses measured by applying models to Close, Nature, 2005, 433, 286pp y gluminosities, ages and distances may be under-estimated by > factor 2 37

Michael Perryman, 2012, Astrobiology 12, 928.

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Scorecard (Mar. 13, 2013): 861Scorecard (Mar. 13, 2013): 861 

• Radial velocity: 501 planets in 389 systemsRadial velocity: 501 planets in 389 systems• Transits: 294 planets in 238 systems.

P l l t 15 l t i 12 t• Pulsar planets: 15 planets in 12 systems• Microlensing: 18 planets in 16 systems• Direct imaging: 32 planets in 28 systems• Astrometry: 1 planet Past scorecardsAstrometry: 1 planet• (SETI: nil)

Past scorecards Apr 7 2006: 194

Mar 13 2008: 278Nov 25 2009: 404Nov 7 2011: 697 39

Exotic systems: PSR B1620-26c

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Exotic systems: HD 209458b

• Spectroscopic radial velocity studies first revealed the presence of a planet around HD 209458 on November 5, 1999planet around HD 209458 on November 5, 1999

• 1.7% drop in HD 209458's brightness was measured, which was later confirmed as being due to a transit. Each transit lasts about three hours, and about 1.5% of the star's face is covered by thethree hours, and about 1.5% of the star s face is covered by the planet during the transit

• Semi-major axis 0.045 AU (circular)• Orbital period 3 52474541d• Orbital period 3.52474541d• Inclination 86.1 ± 0.1°• Mass 0.69 ± 0.05 MJ

1 32 0 0• Radius 1.32 ± 0.05 RJ• Density 370 kg/m³• Temperature 1,130 ± 150 K

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• Probably a gas giantArtist’s conception

HD 209458bEnvelope of hydrogen, carbon and oxygen around the planet that reaches a temperature of 10,000 Kreaches a temperature of 10,000 KThe heavier carbon and oxygen atoms are being blown off of the planet by the extremeplanet by the extreme "hydrodynamic drag" created by its evaporating hydrogen atmosphereThe hydrogen tail streaming off of the planet is 200,000 kilometers longMeasured by differential spectroscopy during transit by HST in UV (Vidal-Madjar et al 2004)( j )

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Metallicity:Metallicity:

Th b d fThe abundance of elements heavier than He relative to the Sun

Overall 5% of solar like stars have radial velocity detected Jupiters• Overall, ~5% of solar-like stars have radial velocity–detected Jupiters• But if we take metallicity into account:

– >20% of stars with 3x the metal content of the Sun have planets

– only ~3% of stars with 1/3rd of the Sun’s metallicity have planets43

Orbit size distribution

• Since most planets

Max about 6 AU

pdetected by RV, there are a lot of massive planets nearplanets near their stars

• This preponderance p pis a selection effect no doubt, but how do the ones we seeones we see form?

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The problem: hot JupitersP ibl l ti l t i ti • In our SS, the giant

planets form far from the Sun as the core-accretion

Possible solution: planetary migrationAdditional problem: why do the planets stop their migration before falling into the star?

model requires that they form a core (including a lot of ice) that reaches )10-20 Earth masses before they can accrete gasg

• However, many large exoplanets orbit very close to their startheir star

• This is perhaps the outstanding problem in the study of extrasolarstudy of extrasolar planets.

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Mass distribution • Super-Jupiters

(M>several MJup) are not commoncommon

• Implications for planetfor planet formation theories?

Or only exist• Or only exist in numbers at large separation that haven’tthat haven t yet been detected?

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Cumming (2004)• Length of surveys limits distances planets have pbeen found from stars. Normally one would like to observe a planet for atto observe a planet for at least one orbital period (for RV and transit methods)• Earliest surveys started 1989

Jupiter • Jupiter (5 AU from Sun) takes 12 yrs to orbit Sun

would only just haveLines are 50% and 99% detection thresholds for RV surveys for 5 observations per

– would only just have been discovered• Saturn takes 30 years

ld ibl isurveys for 5 observations per year for 3, 6 and 12 yrs.

- would possibly remain undetected

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Low-mass planetsLow mass planets• Low-mass

l t tplanets are not easily detected by RVby RV technique.

• SmallestSmallest (except for pulsar planets) is α Cen B b (radial v) at 0 00355 M0.00355 MJ ~ 1.1 ME 48

What about Earth-like planets? 49

Habitable zoneHabitable zone• For a planet to be Earth-like in the sense of having life, it p g

likely must have a “moderate temperature”– liquid water

organic molecules stable– organic molecules stable– energy available

• Ignoring geothermal heat, this likely means an appropriate distance from its parent star

The “appropriate” region (which may be as simply and(which may be as simply and vaguely defined as: “where liquid water can exist”) is called the “habitable zone” orcalled the habitable zone or HZ

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Location of the Habitable ZoneLocation of the Habitable Zone• In practice the location of the Habitable Zone depends

th d t il f th l t it lf d ibl thon the details of the planet itself, and possibly the planet’s recent history – an “ice ball” may be harder to warm up

B i i th E th’ li t d diff t• By examining the Earth’s climate under different received solar fluxes, the (liquid water) HZ stretches from about 0.95 to 1.4 AU 0 99 to 1 7 AU Kopparap et al (Feb 2013)• 0.99 to 1.7 AU: Kopparapu et al. (Feb 2013)

Case Inner limit (AU) Outer limit (AU)

Standard model 0.95 (0.99) 1.37  (1.70)

Mars‐sized planet 0.98  (1.035) 1.49  (1.72)

10x Earth mass planet

0.91  (0.94) 1.29  (1.67)

Kasting et al 1993 (Kopparapu et al. 2013)

p

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Habitable zones around other starsHabitable zones around other stars• Brighter stars have

id HZ’ f th twider HZ’s further out, while low-mass stars have narrow HZ’s huddled near themhuddled near them.

• This makes the HZ harder to hit for the (common) faint stars(common) faint stars

• High mass stars have shorter lifetimes: so their larger HZ’s mighttheir larger HZ s might be counteracted by the fact they die before life can

HZs for two different luminosity stars. Stars between 0.7 and 1.5 solar masses might live long enough for lifeevolve? masses might live long enough for life to develop and have HZs far enough from the star. 52

Continuously Habitable Zone (CHZ)(CHZ)

• Additionally, a star willAdditionally, a star will typically increase in luminosity throughout its lifetime, moving the HZ.

• If the zone moves too much, there is no “continously”continously habitable zone (CHZ)

Luminosity evolution of the Sun (Kasting et al 1993) 53

Habitable zones and biomarkersab tab e o es a d b o a e s• Though many exoplanet systems are seen to contain “hot

Jupiters” near their stars, they could contain as-yet p y yundetected low-mass planets in their HZ– if they were not previously cleared out by migration

• Some HJ’s that are within the HZ could harbour moonsSome HJ s that are within the HZ could harbour moons with more Earth-like properties.

• So we find a planet with the same mass as Earth, and in the habitable zone:the habitable zone:– How can we tell it harbours life?

• Search for biomarkers– Water– Ozone– Albedo

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<snip>

Earthshine spectrum with some features that might indicate life-bearing planets

The End

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