Extrasolar planets

Astronomy 9601

1 Topics to be covered

• 12.1 Physics and sizes • 12.2 Detecting extrasolar planets • 12. 3 Observa tions o f exop lane ts • 12.4 Exoplanet statistics • 12.5 Planets and Life

2 What is a planet? What is a star?

• The composition of Jupiter closely resembles that of the Sun: who’s to say that Jupppyiter is not simply a “failed star” rather than a planet? • The discovery of low-mass binary stars would be interesting, but (perhaps) not as exciting as discovering new “true” planets. • Is there a natural boundary between planets and stars?

3 Planets and brown dwarfs • A star of mass less than 8% Luminosity “bump” due to short- of the Sun (80x Jupiter’s lived deuterium burning mass) will never grow hot Steady luminosity due to H burning enough in its core to fuse hydrogen • This is used as the boundary 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 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

4 Nelson et al., 1986, AJ, 311, 226 5 Pulsar planets • In 1992, Wolszczan and Frail announced the discovery of a multi‐

Artist’s conception of the planet planet planetary system around the orbiting pulsar PSR B1257+12 millisecond pulsar PSR 1257+12 (an earlier announcement had been retracted). • These were the first two extrasolar pltlanets confirme d to be discovere d, and thus the first multi‐planet extrasolar planetary system discovered, and the first pulsar planets discovered • However, these objects are not in planetary systems as we usually think of them 6 Worlds Beyond Our Sun

• In 1995 a team of Swiss astronomers disco vered the first planet (in a non- pulsar system) outside our soltlar system, orbiti ng 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. 7 Unseen Companions

• Curiously enough, most extrasolar planets remain unseen • They are usually detected by indirect means, though their effects on their parent This artist's concept shows the Neptune-sized extrasolar planet star. circling the star Gliese 436.

8 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 athtmospheres. • But most known exoplanets are impossible to see with current technology • Two reasons why: – known exoplanets are too dim • 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 • 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: Astrometry

• oldest method, used since 1943 • the wobble induced in the plane-of-sky motion of the star by planets is measured by accurately observing its position over time • 1 detection

11 Astrometry • STEPS (Stellar Planet Survey) detected periodic proper motion of VB 10, a nearby brown dwarf . • VB 10b is approximately 6 Jupiter masses, with a period of 9 months. • No sign of planet when examined with other techniques: busted!

12 Astrometry:Difficulties

•Example: The Sun wobbles by a bou t its diameter, mostly due to Jupiter. •At 30 light-years, this would produce an apparent motion of less than 1 milliarcsecond. • Typical good ground- based observing conditions produce positions with accuracies below but around 1 arc- AttifSf30lApparent motion of Sun from 30 ly second. 13 Detection methods: Pulsar planets

• Pulsar planets are ppgplanets that are found orbiting pulsars – Pulsars are rapidly rotating neutron stars. • Pulsar planets are discovered through radio pulsar timing 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 can easily be detected with the help of precise timing measurements. • 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 a planet detection around PSR 1829-10 14 PSR 1257+12

• Pulsar located 2630 light years away • 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 Third planet 3.9 0.46 98.21 0.025

– possible small fourth object has an upper mass limit of 0. 2 MPluto and an upper size of R < 1000km.

15 5 of the 12 known pulsar planet systems

PlPulsar pltlanet Mass ObitditOrbit distance ObitOrbit peri od 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 days d 0.0004 Earths 2.7 AU 3.5 years QS Vir b 6.4 Jupiters 4.2 AU 7.9 years HW Vir b 19.2 JitJupiters 16 years c 8.5 Jupiters 332 days

•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 giant planets that were able to weather the supernova, or later accretion products of supernova debris. 16 Detection 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. This may be detectable by high-precision photometry. •Note that the pp,g,ylanet is invisible, being unresolved, only the brightness variation in the star is seen.

17 The Observational Challenge

The fraction of stars expected to have transits is:

f = fs fMS fCEGP pt fs = fraction of stars that are single = 0.5 fMS = fraction of those on the main sequence = 0.5 fCEGP = fraction of those that have a close -in planet =001= 0.01 pt = fraction of those with an inclination to transit = 0.1

• Need 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 skyyp for at least 1 orbital period.

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

18 Transits

• Assuming – The whole planet passes in front of the star – An d ignor ing lim b dar ken ing o f the s tar as neg lig ible • Then the depth of the eclipse is simply the ratio of the planetary and stellar disk areas: 2 2 Δf πR ⎛ R ⎞ f = light flux = p = ⎜ p ⎟ 2 ⎜ ⎟ f* πR* ⎝ R* ⎠ • We measure the change in brightness, and estimate the stellar radius from the spectral type

19 Transits

• Advan tages – 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 manyyy stars simultaneously – Easy to confuse with binary/triple systems – Needs radial velocity measurements for confirmation, masses • Has found 294 exoplanets in 238 systems so far (March 2013) 20 • OGLE-TR-10: Konacki et al. 2004

• 0. 57Mj, 1. 24Rj, P31dP=3.1days

21 Kepler (transits)

With a total of 95 mega-pixels of CCDs Kepler is cappg,able of observing over 100,000 stars all at once and measuring their brightness to an accuracy of better than 1 part in 100,000. 22 Kepler Orrery

23 Detection methods: microlensing

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

24 First detection: OGLE 2003 BLG-235

AliAnalysis o f the light curve reveals second object in lens with .4% of mass of the other

• 17,000 light years away, in the constellation Sagittarius. • The pp,lanet, orbiting a red dwarf parent star , is most likel 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 Microlensing

• Microlensing has some disadvantages – model-dependent – only see the planet once • However, it is the “best” technique for finding smaller planets, farther from their star – ie. more Earth-like planets than RV tec hn ique (nex t) OGLE 2005-BLG-390 (Artist’s • 18 detections so far impression): Five Earth mass (()Mar/2013) ppylanet on a 10 yr orbit around a red dwarf star. First (probably) icy exoplanet found (25 Jan 2006) 26 Detection methods: radial velocity

• Most of the planets known to date were discovered using the “Doppler shift” or “radial velocity” method. •A ppgyplanet's gravity pulls its host star back and forth during its orbit. This causes the light we receive to be "blueshifted" and "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)

27 Stellar Doppler shifts Observe the period P GM r 3 = * P2 4π 2

Assume a circular orbit K (iitill)tfidl(initially) to find plane t velocity

Vp = GM * / r P

From conservation of momentum, determine Mp Assume a mass for the star (from spectral M p = M *V* /Vp type) to compute Mp sin i (K = V*sin i)

(i = inclination of orbital M p sin i = M *K /Vp plane to line of sight) 28 Eccentricity

•Byyg looking more closel y at the sha pe of the curve, the eccentricity of the planet’s orbit can be determined.

29 51 Pegasi b

• First planet discovered around a sun-like star outside of the solar system • Radial velocity method • Detection from regular velocity changes in the star's spectral lines of around 70 metres per second • Semi-major axis 0. 052 AU (circular) • Orbital period 4.23077 d • Mass >0.468 ± 0.007 MJ • GtGreater radi dithJitditus 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 challenges • Requires high- precision repeatable spectroscopic measurements of Doppler shifts to ~ 1m/s accuracy • Most sensitive to massive planets near the star ( “hot Jupiters”) 31 Direct Detection • To understand extrasolar planets, we really need their light • None of the radial velocity planets can be imaged with current technology – Planet is too faint and too close to the star • Solution: Remove the starlight (d(adapt ive optics, coronagraphhy, interferometry) • To optimize the contrast between Above: Gliese 229B – brown planet and star, one observes red dwarf companion to nearby M dwarfs, brown dwarfs & white dwarf dwarfs, and chooses a wavelength band that favours the planet 32 The 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 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 time. •Hot Jupiters emit more strongly in the thermal IR than more distant gas giants. •Jupiter is 109 times fainter than the Sun in the visible, but only 106 times fainter in the thermal IR •YJidhYoung Jupiters and hot Jupiters may be only 104 Solid lines Burrows 1997 models, dashed lines times fainter than their Burrows 2002 models stars in the IR Models assume evolution in isolation: no additional heating source or reflection component 34 The “first” image of an exoplanet • 2M1207 parent “star” is a brown dwarf – 10Myr old (young) – in an association of newly formed stars • Planet

– mass =5Mj • determined from model of spectrum of companion= uncertainty!

– radius = 1.5 Rj – 41 AU from the star • Chauvin et al. 2004, A&A, 425, L29 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 dwarf • Brown dwarf companion • In this case, the mass could also be measured from direct observations of orbit over time • 2. 5x more massive than spectral models predict (90 MJ vs 36 MJ) • So the planet is “just” a brown dwarf / • Masses measured by appl yin g models to Close, Nature, 2005, 433, 286 luminosities, ages and distances may be under- estimated by > factor 2 37 Michael Perryman, 2012, Astrobiology 12, 928.

38 Scorecard (Mar. 13, 2013): 861

• Radial velocity: 501 planets in 389 systems • Transits: 294 planets in 238 systems. • Pllt15lti12tPulsar planets: 15 planets in 12 systems • Microlensing: 18 planets in 16 systems • Direct imaging: 32 planets in 28 systems • Astrometry: 1 planet Past scorecards Apr 7 2006: 194 • (SETI: nil) Mar 13 2008: 278 Nov 25 2009: 404

Nov 7 2011: 697 39 Exotic systems: PSR B1620-26c

40 Exotic systems: HD 209458b

• Spectroscopic radial velocity studies first revealed the presence of a planet 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 the planet during the transit • Semi-major axis 0.045 AU (circular) • Orbital period 3. 52474541d • Inclination 86.1 ±0.1° • Mass 0.69 ±0.05 MJ • Radius 1321.32 ± 000.05 RJ • Density 370 kg/m³ • Temperature 1,130 ± 150 K • Probably a gas giant

Artist’s conception41 HD 209458b

„ Envelope of hydrogen, carbon and oxygen around the planet that reaches a temperature of 10,000 K „ The heavier carbon and oxygen atoms are being blown off of the planet by the extreme "hydrodynamic drag" created by its evaporating hydrogen atmosphere „ The hydrogen tail streaming off of the planet is 200,000 kilometers long „ Measured by differential spectroscopy during transit by HST in UV (Vidal-Madjj)ar et al 2004)

42 Metallicity:

The ab un dance o f elements heavier than He relative to the Sun

• 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 planets

43 Orbit size distribution Max about 6 AU • Since most planets detected by RV, there are a lot of massive planets near their stars •This pppreponderance is a selection effect no doubt, but how do the ones we see form?

44 The problem: hot Jupiters

PiblltiltPossible solution: planetary m itiigration • In our SS, the giant Additional problem: why do the planets stop their planets form far migration before falling into the star? from the Sun as the core-accretion model requires that they form a core (including a lot of ice) that reaches 10-20 Earth masses before they can accrete gas • However, many large exoplanets orbit very close to their star • This is perhaps the outstanding problem in the study of extrasolar planets.

45 Mass distribution • Super-Jupiters (M>several MJup) are not common

• Implications for planet formation theories?

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

46 Cumming (2004) • Length of surveys limits distances planets have been found from stars. Normally one would like to 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 have Lines are 50% and 99% been discovered detection thresholds for RV • Saturn takes 30 years surveys for 5 observations per -would poss ibly rema in year for 3, 6 and 12 yrs. undetected 47 Low-mass planets • Low-mass pltlanets are no t easily detected by RV technique. • Smallest (except for pulsar planets) is α Cen B b (radial v) at

0. 00355 MJ ~

1.1 ME 48 What about Earth-like planets? 49 Habitable zone

• For a planet to be Earth-like in the sense of having life, it likely must have a “moderate temperature” – liquid water – 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 vaguely defined as: “where liquid water can exist”) is called the “habitable zone” or HZ

50 Location of the Habitable Zone

• In practice the location of the Habitable Zone depends on the d e tail s o f the p lane t itse lf, an d poss ibly the planet’s recent history – an “ice ball” may be harder to warm up • BiithEth’litddifftBy 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: 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 0.91 (0.94) 1.29 (1.67) planet

Kasting et al 1993 (Kopparapu et al. 2013) 51 Habitable zones around other stars

• Brighter stars have wider HZ’s fur ther ou t, while low-mass stars have narrow HZ’s huddled near them. • This makes the HZ harder to hit for the (common) faint stars • High mass stars have shorter lifetimes: so their larger HZ ’s might be counteracted by HZs for two different luminosity stars. the fact they die Stars between 0.7 and 1.5 solar before life can evolve? masses might live long enough for life to develop and have HZs far enough from the star. 52 Continuously Habitable Zone (CHZ) • Additionally, a star will typically increase in luminosity throughout its lifetime, moving the HZ. • If the zone moves too much, there is no “continously” habitable zone (CHZ)

Luminosity evolution of the Sun (Kasting et al 1993) 53 Habi tabl e z on es an d bi om ark er s • Though many exoplanet systems are seen to contain “hot Juppyiters” near their stars, they could contain as-yet undetected 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 moons with more Earth-like properties. • So we find a planet with the same mass as Earth, and in the habitable zone: – How can we tell it harbours life? • Search for biomarkers – Water – Ozone – Albedo

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

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