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Planets Around d Other Stars How to makke a planet How to mak ke a planet • Large, cool cloud of gass and dust – Gas makes the star, dus , st is necessary for planet y p formation – Dust is usually made of Dust is usually made off metals (Fe, Ni, Al), rocks f metals (Fe Ni Al) rocks (silicates) and ices (solid H2O, CH4, NH3) – Mostly H and He (these Mostly H and He (thesee two elements make up e two elements make up about 98% of our Solarr System) • Cloud begins to collapsse under its own self‐ gravity How to makke a planet How to mak ke a planet • Collapse causes ll – Increase in temperaturre (conservation of energy) – Increase in rate of rotattion (conservation of angular momentum) • Result is a rapidly rotatting disk of gas and dust – Again, dust means rock Again, dust means rockks, metals and ices ks, metals and ices – at sufficient distance frrom the parent star, hydrogen compounds (H hydrogen compounds ( ( 2O, CH O, CH4 NH3) are ) are important in formation n of giants (Jovian planets are far from the Sun)) How to makke a planet How to mak ke a planet • Accretion – Dust grains collide to fo g orm larger particles ‐‐> g p – Collide small particles tto form still larger particles ‐‐> – Collide large Collide large “boulders boulders” to form planetesimals to form planetesimals (asteroids, comets, Kuiper Belt Objects)‐‐> – collide planetesimals to ll d l l o form planets f l How to makke a planet How to mak ke a planet • • • • • Nebular H b l Hypothesis h i Solar Nebula Contraction into rotatting disk w/ hot center Dust grains accrete to Dust grains accrete to o form larger and larger o form larger and larger particles L Large particles sweep i l out more material to i l form planetesimals Planetesimals eventuaally collide to form p planets Basic prroblem: A good scientific theoryy has to be tested, but we only have one So y olar System and we y cannot go back in tim me. We needed to fin We needed to fin nd other planets! nd other planets! So we did and … Exoplanets • A planet that orbits a sstar other then the Sun • Also can be called an Also can be called an “eextrasolar e planet” planet • There are 1890 exoplan nets as of February 2015 – Many more than in ourr Solar System • First planet (similar to t p ( those in our Solar System) discovered in ~~1995 – Detection is very challe Detection is very challeenging! Exoplanets • Known Exoplanets characteristics – Largest ~50 Jupiter mas g p sses – Smallest ~1 Earth masss – Closest to parent star ~ Closest to parent star ~0.01 AU ~0 ~ 01 AU • Period less than 1 day – Furthest from parent st h f tar ~1000 AU • Very wide range! y g Examples of Sttellar Systems Examples of St tellar Systems • Gliese Gli 8 6 876 a) Parent star: – M3.5 V, T=3480 K, L=0.01 M3 5 V T 3480 K L 0 01 124 Lsun 124 L b) Gas Giant – M=2MJ,, a=0.208 AU c) Gas Giant – M=0.62 MJ, within orbit o of Gliese 876 b d) Super Earth – Inside orbits of both Gliese 876 b and c Examples of Sttellar Systems Examples of St tellar Systems • Gliese Gli 581 a) Parent star: – M3 V, T M3 V T=3480 3480 K, L K L=0 0.013 013 Lsun b) Gas Giant (Neptune‐siized) – M=16 ME, a = 0.04 AU c)) Rocky Planet R k Pl t – M=5 ME, a = 0.07 AU, witthin habitable zone (Temperature could be as low as ‐3 oC or as high as 500 oC, due to runaway greenhouse akin to Venus) greenhouse akin to Venu d) Super Earth – M=7 ME, a = 0.22 AU, witthin habitable zone e) Super Earth – M=1.9 ME, a=0.03 AU Upsilon An p dromedae Two planets are several times moree massive than Jupiter The third planet mass 75% that of Jupiter, is so close to the star that The third planet, mass 75% that of J Jupiter J is so close to the star that it completes a full orbit every 4.6 6 Earth days Why is it so hard to find planets around other d th r stars? t ? • Faint planet glimmer is lost Faint planet glimmer is lost in glare from parent star in glare from parent star – Planets are small, close to their parrent star, and shine by reflected starlight • Thought experiment: Thought experiment: – Imagine grain of rice an inch from aa 100 Watt light bulb. Someone standing at end of a long dark hall would seee only the light bulb, not the grain of rice. • Consider the case of Jupiterr and the Sun: – As seen from the nearest star, Alpha Centauri, Jupiter would appear a billionth as bright as the Sun. ose to the Sun, only 4 arc sec away. – Jupiter would also be extremely clo • Si Since all other stars are fart ll th t f ther than Alpha Centauri, h th Al h C t i Jupiter would be even hardeer to detect from other stars Planets are very hard to o observe directly (by detecting their own light) detecting thei r own light) • Planets are too faint compa pared with their star – This brown dwarf star is barely visible ‐ and its star is faint • Planets shine by reflected liight – Planets close to parent stars aare brightest, but hardest to see Gravitational Motions • The Sun and Jupiter orbit around their common center of mass. • The Sun therefore wobbles around that center of mass with same period as Jupiter. Gravitational Motions • Th The Sun’s motion S ’ ti around the solar system’ss center of system center of mass depends on tugs from all the planets. l t • Astronomers around other stars that other stars that measured this motion could determine the masses and orbits of all the planets all the planets. Astrometric Motion • W We can detect d planets by measuring the change in a star’ss the change in a star position on sky. • These tiny motions These tiny motions are very difficult to measure (~ 0.001 measure ( 0 001 arcsecond). Doppler Te pp Technique q • M Measuring a star’s i ’ Doppler shift can tell us its motion toward us its motion toward and away from us. • Current techniques can measure motions can measure motions as small as 1‐2 m/s (walking speed!) (walking speed!). First Extrasolar Planet: 51 Pegasi P Insert TCP 6e Figure 13.4a unannotated • Doppler shifts of the star 51 Pegasi indirectly revealed a planet with 4-day orbital period. • This short period means that the planet has a small orbital distance – well within orbit of Mercury. Mercury • This was the first extrasolar planet discovered around a normal stars (1995). 51 Peg b (1995) Half the mass of Jupiter, yet y orbiting much closer to the Sun tha an Mercury! Other Extrasolar Planets • Doppler Doppler shift data tell us shift data tell us about a planet about a planet’ss mass and mass and the shape of its orbit. Planetaryyy signatures g Size depe ends on mass of planet Low mass, high mass Circular, eccentric Periodicity y depends on period of planet Shape depends on eccentricity of planet Small period, large period Changes in RV depend on mass, period and eccentricity of planet Dopple pp er shift • Look for periodic shift in st Look for periodic shift in sttar tar’ss spectrum spectrum – Does not depend on distan nce of star – Need massive planet near d l star he faster the orbital speed (of • the closer the planet, th b h l both planet and star) d ) – Need very good spectrum • Many planet detections • Incredibly hard measurem Incredibly hard measurem ments have now become ments have now become standard Limitations Limitations of Do of Do oppler technique oppler technique From ground‐based g observatories, detect shifts > 1‐2 m/sec The nearer to the star and The nearer to the star and the more massive the planet, the easier to planet, the easier to detect Radial velocity method doesn’t give all the orbital info ll th bit l i formation ti • Doppler Doppler shift only detects shift only detectss velocity along line of sightt – Can’t distinguish massive g planet (or brown dwarf!) in tilted orbit from less massive planet in edge on massive planet in edge‐on n orbit – They both have the same y line‐of‐sight velocity • The only way to resolve this ambiguity is to hi bi i i observe using another method Tran nsits • As planet moves across face of star, it blocks a y g tiny bit of starlight • Watch for periodic dimm ming of star Planet detected around the star HD209458 by transit metthod • Planet Planet is 70% mass of Jupiter, is 70% mass of Jupiter but orbits in just 3.5 days • So it is very close to its parent star • Many planets have been found this way found this way • Amateur astronomers have organized to watch for g fluctuations in star brightness – http://www.transitsearch.org/ Transits and Eclipses Transits and • A transit is when a planet ccrosses in front of a star. • Eclipse l is when a planet goe h l es behind a star b h d • From both, can learn aboutt atmosphere of planet (b (beginning and end of trans d d f sit, eclipse) l ) Transitin ng planets gp Orbital inclination must be Orbital inclination must bee closely aligned to our line e closely aligned to our line‐ of‐sight This makes many addition This makes many additional interesting planetary al interesting planetary properties possible to meaasure size, mass, temperature and sp pectra Can test the atmospheric mode p els that have been developed for p planets in our solar system Some of these planets are subjeect to more exciting conditions than the ones in the Solar System: Small distance from star Extreme eccentricities NASA’s Kepler Space Mission to detect transitingg planets March 2009: NASA mission Keepler was launched Repeated images ~100,000 stars in the constellation Cygnus for transiting planet siignals Has found thousands of possible planets some have been confirrmed some have been confir not called a planet until follow‐up is executed NASA’s Kep pler Mission Determining the frequency of Earth-size E and larger planets in the habitable zon ne of Sun-like stars Kepler 10‐b • Earth‐sized planet • Very close to its parent star, so very parent star, so very hot Transit and d Doppler pp Measurementss Yield Density + 10b Size Dennsity Mass Volume = 8.8 g/cm3 Composition o off Kepler Kepler-10b 10b Gravitational Microlensing g • Background: Microlensing Background: Microlensing around a star (or black hole) around a star (or black hole) Needs N d almost l t perfect alignment between source and lens. Planet detection: ffine structure on microlensingg light curve g • A few planets have been p discovered this way Potentially very useful: • Potentially very useful: can detect planets at large distances from us large distances from us – Even farther away than transit method can – Much farther than radial velocity or astrometry can • Samples a different p p population of stars than other techniques Direct Imaging Direct I • Use planet Use planet’ss own light own light • Take image of it – Can Can reconstruct full orbit reconstruct full orbit by watching it go around • Can also obtain spectra p – Learn about physical conditions, atmosphere, maybe even presence of b f life • Jupiter Jupiter is a billion times is a billion times fainter than the Sun, in visible light! visible light! First Images of o Exoplanets: Exoplanets: HR 8799 So Solar l System S t Marois et al. 2008, Science Magazine First images of exopla g p nets: Fomalhaut Star location Neptune-sized orbit http://dps.aas.org/education/dpsdisc/ Hubble Space Telescope Hubble Space Telescope visible image of the star Fomalhaut (whose light was blocked) with a was blocked), with a dust belt similar to the Kuiper belt. Inset: Images taken ~2 years apart show a planet moving around the star. Old list of other planetary systems Characteristics of extra‐solar planets p • Much higher eccentriciity in most of their orbits than in our Solarr System y • Much higher fraction o of planets very close to their parent stars their parent stars • Many planets are “super‐Jupiters” (up to 10 times more massive than Jupiter) • Microlensing suggests Microlensing suggests systems like our own systems like our own may be around ~1/3 off all stars Eccentric O Orbits Page Many extrasolar planets are very close to pareent stars very close to pare ent stars • This is a selection effect (caused by detection method)) • But it does show that such planets l t exist! i t! • Our Solar System is very different (green points) Mercury is farther away from m Sun than many of the extra-solar planets are from their stars Patterns in ecceentricity Patterns in ecce • Most new planets are in elliptical orbits • Short period planets: Short period planets: – Very close to parent stars very low stars, very low eccentricity – Same process that S h moved planets close to star circularized their i l i d h i orbits Parent stars of exxtrasolar planets Parent stars of ex • Hi High in elements heavier hi l t h i than hydrogen and h li helium (usually > Sun) ( ll > S ) – reasonable: planets form from dust form from dust • Probability of finding a planet increases as heavy l ti h element content of parent star increases t t i 1.6 Pplanet ~ (NFe/ NH) What are theese planets? What are the ese planets? • Our Solar System has small rocky planets close to star, large gas giants further awaay – no experience of large massivve planets close to sun in our S l S t Solar System • Theory of giant planet form mation says they have to form outside “frost form outside frost line line” Page Hot Jupiters p How are “Hot Hot Ju upiters” formed? upiters Theory for our Solar System: Stellar wind from young Sun blew volatiles outwards Ice condensation at 5 AU where water-ice solidified Fast accretion of large icy planet (~10 MEarth) which then collected H/He atmosphere Gas giants Jupiter, Saturn just outside “frost line” Small rocky planets inside Slowly accreting icy planets in outer system (Uranus, Nept ne) Neptune) Extrasolar giant planets: Do they form in situ? looks impossible: too hot for i ices, ttoo little littl material t i l ffor rock k Do they form outside frost line and g inwards? migrate planet forms in gas/dust disc around star drag from remaining gas/dust causes it to spiral inwards Does scattering from other giant planets causes migration why does it stop? Interesting questions! Interesting • O Original theories of solar sy i i l h i f l ystem formation developed f i d l d when our own Solar System m was the only one – Mostly circular orbits M tl i l bit – Giant planets in outer solar ssystem, terrestrial planets inside • Many new Solar Systems a Many new Solar Systems are (in general) not like ours re (in general) not like ours • Needs new ideas • How to arrive at a new parradigm? – Use computer simulations to o develop ideas, test hypotheses, make predictions make predictions – Test predictions against obseerved young solar systems, disks Page Theories for how giant planets got so close to t l heir stars 1. Interactions between individual new planets and gaseous disk. “Migration”” 2. After gas disk cleared awaay, several giant planets in outer parts of solar system m were left – Three‐body gravitational intteractions between them – One giant planet got slung o outwards, a second was slung inwards and got “captured”” by the star in a close orbit – But why isn’t the close orbitt very elliptical? • Why didn’t Jupiter migrate inwards close to Sun? What have we learned so far? What have we learned so far? • Most stars have planets h l of some sort f y y ar to ours • Many systems not simila • A large fraction do have systems similar to ours though – Roughly 1/3 • M More work is needed to ki d d t understand new systems d t d t and to better refine how w many stars have systems similar to ours i il t – Also need more work to find Earth analogs What is a Habitaable Planet? What is a Habita able Planet? A good planet is: • N Not too big g • Not N too small • Not N too hot or too cold What does “habittable” mean • Right temperature • Liquid water • Free oxygen to breath • Light for warmth and perception • Radiation shield • Some form of asteroid/comet protection Things That Affect TTemperature Want temperature so you can ha Want temperature so you can haave liquid ave liquid water on the surface of the p planet 280° 260° 240° 220° Water boils -> 200° Temperature of star 180° 160° 160 Distance from the star 140° 120° p p or Shape of planet’s orbit: circular o elliptical p g e Planet’s atmosphere: greenhouse gases 100° 80° 60° 40° Water freezes -> 20° 20 0° -20° -40° The Habitable Zone for Stars of Various Masses The Habitable Zone (HZ) in grreen is the distance from a star where liquid water is expected d to exist on the planets surface. Summ mary • The The ~ 2000 planets we have 2000 planets we havee detected to date are only e detected to date are only a sub‐set of potential planeets out there • These new solar systems ha These new solar systems haave raised big questions ave raised big questions about how planets formed in general • Future search methods hav F t h th d h ve high probability of finding hi h b bilit f fi di more (and more varied) plaanets • It’s hard to find Earth‐like p planets – But prospect of finding Earth‐‐like planets is exciting! Page Suppose you found a st f d tar with the same mass h h as the Sun moving back and forth with a period of 16 months. What could you conclude? a. b. c c. d. The star has a planet orrbiting at less than 1 AU The star has a planet orrbiting at greater than 1 AU The star has a planet orrbiting at exactly 1 AU The star has a planet, but we do not have enough information to know its o orbital distance What do you think of the observation that other planets may exist in the habitable zo ones of other stars?