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Home , Pulsar planet

3/25/2013

Topics to be covered

• 12.1 Physics and sizes • 12.2 Detecting extrasolar Extrasolar planets • 12.3 Observations of • 12.4 statistics Astronomy 9601 • 12.5 Planets and Life

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What is a ? What is a star? Planets and brown dwarfs • A star of less than 8% Luminosity “bump” due to short- of the Sun (80x Jupiter’s • The composition of Jupiter closely lived deuterium burning mass) will never grow hot Steady luminosity due to H burning enough in its core to fuse resembles that of the Sun: who’s to say hydrogen • This is used as the boundary that Jupiter is not simply a “failed star” between true stars and very rather than a planet? large gas planets • Object s b el ow thi s mass are • The discovery of low-mass binary stars called brown dwarfs • The boundary between BD would be interesting, but (perhaps) not as and planet is more exciting as discovering new “true” planets. controversial – some argue it should be based on formation • Is there a natural boundary between – other choose 0.013 solar =13 Mj as the planets and stars? boundary, as objects below this mass will never reach even deuterium fusion

3 4 Nelson et al., 1986, AJ, 311, 226

Pulsar planets • In 1992, Wolszczan and Frail announced the discovery of a multi‐

Artist’s conception of the planet planet around the orbiting PSR B1257+12 PSR 1257+12 (an earlier announcement had been retracted). • These were the first two extrasolar planets 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 planetary systems as we usually 5 think of them 6

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Worlds Beyond Our Sun Unseen Companions

• In 1995 a team of Swiss astronomers discovered • Curiously enough, the first planet (in a non- most extrasolar pulsar system) outside planets remain our solar system, orbiting unseen asuna sun-like star called 51 Pegasi. • They are usually • Further discoveries bring detected by indirect the grand total of known means, though their extrasolar planets to 861 effects on their parent (as of March 2013) and This artist's concept shows the counting. Neptune-sized extrasolar planet star. circling the star Gliese 436. Artist's rendition of the star 51 Pegasi and its planetary companion 51 Pegasi B. 7 8

Obstacles to Direct Detection The “first confirmed” image of an exoplanet: GQ Lupi & Planetary Companion • Direct detection is the only way to tell what these planets are made of and whether there's water or oxygen in their atmospheres. • 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." 21 Mj, 100 AU orbit. Imaged by ESO’s VLT, 9 then HST and Subaru confirmed (early Apr 2005) 10

Detection methods: Astrometry • STEPS (Stellar Planet Survey) • oldest method, detected periodic proper motion used since 1943 of VB 10, a nearby . • the wobble • VB 10b is approximately 6 Jupiter induced in the masses, with a period of 9 plane-of-sky months. motion of the star by planets is • No sign of planet when examined measured by with other techniques: busted! accurately observing its position over time • 1 detection

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Astrometry:Difficulties Detection methods: Pulsar planets

•Example: The Sun • Pulsar planets are planets that are found orbiting wobbles by about its – Pulsars are rapidly rotating neutron stars. diameter, mostly due to • Pulsar planets are discovered through radio pulsar Jupiter. timing measurements, to detect anomalies in the •At 30 light-years, this pulsation period. Any bodies orbiting the pulsar will woldprodceanould produce an cause regulhlar changes iitltiSilin its pulsation. Since pulsars apparent motion of less normally rotate at near-constant speed, any changes can than 1 milliarcsecond. easily be detected with the help of precise timing • Typical good ground- measurements. based observing • The first ever planets discovered around another star, conditions produce were discovered around a pulsar in 1992 by Wolszczan positions with accuracies and Frail around PSR 1257+12. Some uncertainty below but around 1 arc- Apparent motion of Sun from 30 ly initially surrounded this due to an earlier retraction of a planet detection around PSR 1829-10 second. 13 14

PSR 1257+12 5 of the 12 known pulsar planet systems Pulsar planet Mass Orbit distance Orbit period • Pulsar located 2630 light years away PSR B1620-26 c 2.5 Jupiters 23 AU 100yr V391 Peg b 3.2 Jupiters 1.7 AU 1170 days • These were the first extrasolar planets ever discovered PSR 1257+12 a 0.02 Earths 0.19 AU 25 days • Pulsar mass 0.3 Msun, rotational period 0.0062 seconds b 4.3 Earths 0.36 AU 66 days c 3.9 Earths 0.46 AU 98 days Mass (M )a (AU) Period (days) e E d 0.0004 Earths 2.7 AU 3.5 years Firs t plltanet 0.020 0190.19 25.26 000.0 QS Vir b 6.4 Jupiters 4.2 AU 7.9 years Second planet 4.3 0.36 66.54 0.02 HW Vir b 19.2 Jupiters 16 years Third planet 3.9 0.46 98.21 0.025 c 8.5 Jupiters 332 days

– possible small fourth object has an upper mass limit of 0.2 MPluto •Since neutron stars are formed after the violent death of massive stars and an upper size of R < 1000km. (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 , or later

15 products of supernova debris. 16

Detection methods: Transits 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 = 0.01 pt = fraction of those with an inclination to transit = 0.1

• Need to look at 4000 stars to find 1 that transits. • Planets observed at inclinations (measured with respect • Need to sample often compared to transit duration. to the plane of the sky) near 90o will pass in front of • Need 1% accuracy for a 3s detection of a 2 hour transit. (“transit”) their host stars, dimming the light of the star. • Need to look on sky for at least 1 . This may be detectable by high-precision photometry. Requires 1,000,000 15-minute samples •Note that the planet is invisible, being unresolved, only with 1% accuracy to detect one transit. the brightness variation in the star is seen.

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Transits Transits

• Assuming • Advantages – Easy. Can be done with small, cheap telescopes – The whole planet passes in front of the star • WASP, STARE, numerous others – And ignoring limb darkening of the star as negligible – Possible to detect low mass planets, including “Earths”, especially from space (Kepler mission, launched Mar • Then the dep th o f the ecli pse is s imp ly the ra tio 2009) of the planetary and stellar disk areas: • Disadvantages 2 2 f = light flux Δf πRp ⎛ Rp ⎞ – Probability of seeing a transit is low = = ⎜ ⎟ • Need to observe many stars simultaneously f 2 ⎜ R ⎟ * πR* ⎝ * ⎠ – Easy to confuse with binary/triple systems • We measure the change in brightness, and – Needs measurements for confirmation, estimate the stellar radius from the spectral type masses • Has found 294 exoplanets in 238 systems so far 19 (March 2013) 20

Kepler (transits)

• OGLE-TR-10: Konacki et al. 2004 With a total of 95 mega-pixels of CCDs Kepler is capable of observing over 100,000 stars all at once and • 0.57Mj, 1.24Rj, P=3.1days measuring their brightness to an accuracy of better than 1 part in 100,000. 21 22

Kepler Orrery Detection methods: microlensing

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

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First detection: OGLE 2003 BLG-235 Microlensing • Microlensing has some Analysis of the light disadvantages curve reveals – model-dependent second object – only see the planet once in lens with .4% of mass • However, it is the “best” of the other technique for finding smaller planets, farther • 17,000 light years away, in the constellation Sagittarius. from their star • The planet, orbiting a parent star, is most likely – ie. more Earth-like planets one-and-a-half times bigger than Jupiter. than RV technique (next) OGLE 2005-BLG-390 (Artist’s • The planet and star are three times farther apart than • 18 detections so far impression): Five Earth mass Earth and the Sun. (Mar/2013) planet on a 10 yr orbit around a red • Together, they magnify a farther, background star some dwarf star. First (probably) icy 24,000 light years away, near the Milky Way center. exoplanet found (25 Jan 2006) 25 26

Detection methods: radial velocity Stellar Doppler shifts Observe the period P

• Most of the planets known to 3 GM* 2 date were discovered using the r = 2 P “Doppler shift” or “radial velocity” 4π method. • A planet's gravity pulls its host Assume a circular orbit K star back and forth during its (initially) to find planet orbit. This causes the light we velocity receive to be "blueshifted" and "redshifted". Vp = GM* / r P • Although the Doppler signals are enough to convince us that From conservation of extrasolar planets exist, these momentum, determine M exoplanets are not seen directly. p M = M V /V Assume a mass for the star (from spectral • (~502 detections as of March p * * p type) to compute M sin i (K = V sin i) 2013) p * (i = inclination of orbital M p sin i = M *K /Vp plane to line of sight) 27 28

Eccentricity 51 Pegasi b

• First planet discovered around a • By looking more closely at the shape of the sun-like star outside of the solar curve, the eccentricity of the planet’s orbit can system be determined. • Radial velocity method • Detection from regular velocity changes in the star's spectral lines of around70d 70 me tres per secon d • Semi-major axis 0.052 AU (circular) • Orbital period 4.23077 d • Mass >0.468 ± 0.007 MJ • Greater radius than Jupiter despite its lower mass • Superheated 700 K atmosphere • It is the prototypical ”” • Orbital migration to present position? 29 Artist’s conception 30

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Observational challenges Direct Detection • Requires high- • To understand extrasolar planets, precision we really need their light repeatable • None of the radial velocity planets spectroscopic can be imaged with current measurements technology of Doppler – PlPlanetanet isis too faintfaint aandnd too ccloselose to tthehe star shifts to ~ 1m/s • Solution: Remove the starlight accuracy (adaptive optics, coronagraphy, interferometry) • Most sensitive • To optimize the contrast between Above: Gliese 229B – brown to massive planet and star, one observes red dwarf companion to nearby M planets near dwarfs, brown dwarfs & the star (“hot dwarfs, and chooses a wavelength Jupiters”) band that favours the planet 31 32

The Adaptive Optics Difference Planet brightness vs age • 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 • Images of the planet Neptune from the W.M. Keck times fainter in the observatory in Hawaii. Keck comprises two telescopes, thermal IR each with a primary mirror 10 m in diameter. Support •Young Jupiters and hot 4 staff have recently installed an AO system on Keck II. Jupiters may be only 10 Solid lines Burrows 1997 models, dashed lines times fainter than their • The left-hand image is what you normally see using Burrows 2002 models stars in the IR Keck II. The right-hand image was taken after the AO Models assume evolution in isolation: no additional system was turned on. 33 heating source or reflection component 34

The “first confirmed” image of an exoplanet: The “first” image of an exoplanet GQ Lupi & Planetary Companion • 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. 21 M , 100 AU orbit. Imaged by ESO’s VLT, Odd orbit means only confirmed after common j 36 proper motion confirmed (mid-Apr 2005) 35 then HST and Subaru confirmed (early Apr 2005)

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Caution! Michael Perryman, 2012, 12, 928. • 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 applying models to Close, , 2005, 433, 286 luminosities, ages and distances may be under- estimated by > factor 2 37 38

Scorecard (Mar. 13, 2013): 861 Exotic systems: PSR B1620-26c

• Radial velocity: 501 planets in 389 systems • Transits: 294 planets in 238 systems. • Pulsar 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 40

Exotic systems: HD 209458b HD 209458b

„ Envelope of hydrogen, carbon and • Spectroscopic radial velocity studies first revealed the presence of a oxygen around the planet that planet around HD 209458 on November 5, 1999 reaches a temperature of 10,000 K • 1.7% drop in HD 209458's brightness was measured, which was „ The heavier carbon and oxygen later confirmed as being due to a transit. Each transit lasts about atoms are being blown off of the three hours, and about 1.5% of the star's face is covered by the planet by the extreme planet during the transit "hyyydrodynamic drag " created b y its • Semi-major axis 0.045 AU (circular) evaporating hydrogen atmosphere • Orbital period 3.52474541d „ The hydrogen tail streaming off of • Inclination 86.1 ±0.1° the planet is 200,000 kilometers • Mass 0.69 ±0.05 MJ long • Radius 1.32 ±0.05 RJ „ Measured by differential • Density 370 kg/m³ spectroscopy during transit by HST • Temperature 1,130 ±150 K in UV (Vidal-Madjar et al 2004) • Probably a gas giant

Artist’s conception41 42

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Orbit size distribution Metallicity: Max about 6 AU • Since most planets The abundance of detected by elements heavier than RV, there are a He relative to the Sun lot of massive planets near their stars • This preponderance is a selection effect no doubt, but how do the • Overall, ~5% of solar-like stars have radial velocity–detected Jupiters ones we see form? • 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

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The problem: hot Jupiters Mass distribution Possible solution: • In our SS, the giant Additional problem: why do the planets stop their planets form far • Super-Jupiters (M>several migration before falling into the star? from the Sun as the core-accretion MJup) are not model requires that common they form a core (including a lot of • Implications ice) that reaches for planet 10-20 Earth formation masses before theories? they can accrete gas • Or only exist • However, many in numbers at large exoplanets large orbit very close to separation their star that haven’t • This is perhaps the yet been outstanding detected? problem in the study of extrasolar planets.

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Cumming (2004) • Length of surveys limits distances planets have Low-mass planets been found from stars. Normally one would like • Low-mass to observe a planet for at planets are not least one orbital period easily detected (for RV and transit methods) by RV • Earliest surveys started technique. 1989 • Smallest Jupiter • Jupiter (5 AU from Sun) (except for takes 12 yrs to orbit Sun pulsar planets) – would only just have is α Cen B b Lines are 50% and 99% been discovered detection thresholds for RV • Saturn takes 30 years (radial v) at surveys for 5 observations per - would possibly remain 0.00355 M ~ year for 3, 6 and 12 yrs. J undetected 47 1.1 ME 48

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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

What about Earth-like planets? 49 50

Location of the Habitable Zone Habitable zones around other stars

• In practice the location of the Habitable Zone depends • Brighter stars have on the details of the planet itself, and possibly the wider HZ’s further out, planet’s recent history while low-mass stars – an “ice ball” may be harder to warm up have narrow HZ’s • By examining the Earth’s climate under different huddled near them. received solar fluxes,,(q the (liquid water) HZ stretches • This makes the HZ from about 0.95 to 1.4 AU harder to hit for the • 0.99 to 1.7 AU: Kopparapu et al. (Feb 2013) (common) faint stars • High mass stars have Case Inner limit (AU) Outer limit (AU) shorter lifetimes: so Standard model 0.95 (0.99) 1.37 (1.70) their larger HZ’s might Mars‐sized planet 0.98 (1.035) 1.49 (1.72) be counteracted by HZs for two different luminosity stars. 10x Earth mass 0.91 (0.94) 1.29 (1.67) the fact they die Stars between 0.7 and 1.5 solar planet before life can evolve? masses might live long enough for life Kasting et al 1993 (Kopparapu et al. 2013) to develop and have HZs far enough 51 from the star. 52

Continuously Habitable Zone Habitable zones and biomarkers (CHZ) • Though many exoplanet systems are seen to contain “hot • Additionally, a star will Jupiters” near their stars, they could contain as-yet typically increase in undetected low-mass planets in their HZ – if they were not previously cleared out by migration luminosity throughout • Some HJ’s that are within the HZ could harbour moons its lifetime, moving with more Earth-like properties. the HZ. • So we find a planet with the same mass as Earth, and in • If the zone moves too the habitable zone: – How can we tell it harbours life? much, there is no • Search for biomarkers “continously” – Water habitable zone (CHZ) – Ozone – Albedo

53 Luminosity evolution of the Sun (Kasting et al 1993) 54

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The End

Earthshine spectrum with some features that might indicate life-bearing planets 56

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