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The Search for Extra-Solar Planets Dr Martin Hendry Dept of Physics and Astronomy Extra-Solar Planets One of the most active and exciting areas of astrophysics About 150 exoplanets discovered since 1995 Extra-Solar Planets One of the most active and exciting areas of astrophysics About 150 exoplanets discovered since 1995 What we are going to cover How can we detect extra-solar planets? What can we learn about them? 1. How can we detect extra-solar planets? Planets don’t shine by themselves; they just reflect light from their parent star Exoplanets are very faint 2nd problem: Angular separation of star and exoplanet is tiny Distance units Astronomical Unit = mean Earth-Sun distance 1A.U. 1.496 10 m 11 For interstellar distances: Light year 1 light year 9.46110 m 15 e.g. ‘Jupiter’ at 30 l.y. Star r Planet d 30 l.y. 2.8 10 m 17 r 5 A.U. 7.5 10 m 11 r d d 2.7 10 radians 6 4 1.5 10 deg Earth e.g. ‘Jupiter’ at 30 l.y. d 30 l.y. 2.8 10 m 17 r 5 A.U. 7.5 10 m 11 r d 2.7 10 radians 6 4 1.5 10 deg Exoplanets are ‘drowned out’ by their parent star. Impossible to image directly with current telescopes (~10m mirrors) Keck telescopes on Mauna Kea, Hawaii 1. How can we detect extra-solar planets? They cause their parent star to ‘wobble’, as they orbit their common centre of gravity 1. How can we detect extra-solar planets? They cause their parent star to ‘wobble’, as they orbit their common centre of gravity Johannes Kepler Isaac Newton 1. How can we detect extra-solar planets? They cause their parent star to ‘wobble’, as they orbit their common centre of gravity 1. How can we detect extra-solar planets? They cause their parent star to ‘wobble’, as they orbit their common centre of gravity Star + planet in circular orbit about centre of mass, to line of sight Star + planet in circular orbit about centre of mass, to line of sight Star + planet in circular orbit about centre of mass, to line of sight Can see star ‘wobble’, even when planet is unseen. But how large is the wobble?… Star + planet in circular orbit about centre of mass, to line of sight Can see star ‘wobble’, even when planet is unseen. But how large is the wobble?… Centre of mass condition m1r1 m2r2 mS r rS rP rS 1 mP Star + planet in circular orbit about centre of mass, to line of sight Can see star ‘wobble’, even when planet is unseen. But how large is the wobble?… Centre of mass condition m1r1 m2r2 e.g. ‘Jupiter’ at 30 l.y. mS 2.0 1030 kg mP 1.9 1027 kg rS S radians d 7 1.5 10 deg Detectable routinely with SIM (launch date 2009) but not currently The Sun’s “wobble”, mainly due to Jupiter, seen from 30 light years away = width of a 5p piece in Baghdad! Suppose line of sight is in orbital plane Direction to Earth Suppose line of sight is in orbital plane Star has a periodic motion towards and away from Earth – radial velocity varies. Direction to Earth Suppose line of sight is in orbital plane Detectable via the Doppler Effect Star has a periodic motion towards and away from Earth – radial velocity varies Can detect motion from shifts in spectral lines Star Laboratory Stellar spectra are observed using prisms or diffraction gratings, which disperse starlight into its constituent colours Stellar spectra are observed using prisms or diffraction gratings, which disperse starlight into its constituent colours Doppler formula Change in wavelength Radial velocity v 0 c Wavelength of light as measured in the laboratory Speed of light Stellar spectra are observed using prisms or diffraction gratings, which disperse starlight into its constituent colours Doppler formula Change in wavelength Radial velocity v 0 c Wavelength of light as measured in the laboratory Limits of current technology: 0 Speed of light 300 millionth v 1 ms -1 51 Peg – the first new planet Discovered in 1995 Doppler ‘wobble’ v 55 ms -1 51 Peg – the first new planet Discovered in 1995 Doppler ‘wobble’ v 55 ms -1 How do we deduce planet’s data from this curve? 2 G 2 / 3 vS m mP S T 1/ 3 We can observe these directly We can infer this from spectrum When we plot the temperature and luminosity of stars on a diagram most are found on the Main Sequence Surface temperature (K) 25000 106 10000 8000 6000 . 5000 4000 3000 . . . . .. . . . . .. . . .. .. . ... .. . .... .. . .. ..... ... . . .. . ... . . ... ........ . .. .... . .. . . ..... ...... ...... .. .. ..... . ...... . . ... . . . ... ... .. Deneb -10 Rigel Betelgeuse Antares Luminosity (Sun=1) Arcturus 102 Aldebaran Regulus Vega Procyon A Altair 10-2 Pollux Sun Procyon B O5 B0 +5 +10 Barnard’s Star Sirius B 10-4 0 Mira Sirius A 1 -5 A0 F0 G0 Spectral Type K0 M0 M8 +15 Absolute Magnitude 104 Stars on the Main Sequence turn hydrogen into helium. Stars like the Sun can do this for about ten billion years Main sequence stars obey an approximate mass– luminosity relation 5 L~m 4 We can, in turn, estimate the mass of a star from our estimate of its luminosity 3 L log10 L Sun 3.5 2 1 0 -1 0 0.5 m log10 m Sun 1.0 Summary: Doppler ‘Wobble’ method Stellar spectrum Stellar temperature Luminosity Velocity of stellar ‘wobble’ + Stellar mass Orbital radius Planet mass + Orbital period From Kepler’s Third Law In recent years a growing number of exoplanets have been detected via transits = temporary drop in brightness of parent star as the planet crosses the star’s disk along our line of sight. Transit of Mercury: May 7th 2003 Change in brightness from a planetary transit Brightness Star Planet Time Ignoring light from planet, and assuming star is uniformly bright: Total brightness during transit Total brightness outside transit e.g. B* R R B* R*2 2 * 2 P RP 1 RS 2 Sun: RSun 7.0 108 m Jupiter: RJup 7.2 107 m Brightness change of ~1% REarth 6.4 106 m Brightness change of ~0.008% Earth: What have we learned about exoplanets? Highly active, and rapidly changing, field Aug 2000: 29 exoplanets What have we learned about exoplanets? Highly active, and rapidly changing, field Aug 2000: 29 exoplanets Nov 2005: ~150 exoplanets What have we learned about exoplanets? Highly active, and rapidly changing, field Aug 2000: 29 exoplanets Up-to-date summary at http://www.exoplanets.org Now finding planets at larger orbital semimajor axis Nov 2005: ~150 exoplanets What have we learned about exoplanets? Discovery of many ‘Hot Jupiters’: Massive planets with orbits closer to their star than Mercury is to the Sun Very likely to be gas giants, but with surface temperatures of several thousand degrees. Mercury What have we learned about exoplanets? Discovery of many ‘Hot Jupiters’: Massive planets with orbits closer to their star than Mercury is to the Sun Very likely to be gas giants, but with surface temperatures of several thousand degrees. Mercury Artist’s impression of ‘Hot Jupiter’ orbiting HD195019 ‘Hot Jupiters’ produce Doppler wobbles of very large amplitude Looking to the Future 4. NASA: Terrestrial Planet Finder ESA: Darwin } ~ 2015 launch These missions plan to use interferometry to ‘blot out’ the light of the parent star, revealing Earth-mass planets Looking to the Future 4. NASA: Terrestrial Planet Finder ESA: Darwin } ~ 2015 launch Spectroscopy will search for signatures of life:Spectral lines of oxygen, water carbon dioxide in atmosphere? Simulated ‘Earth’ from 30 light years The Search for Extra-Solar Planets What (or who) will we find?…