2. The Search for Extra-Solar Planets Dr Martin Hendry Dept of Physics and Astronomy

3. Extra-Solar Planets  One of the most active and exciting areas of astrophysics  About 150 exoplanets discovered since 1995

4. 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?

5. 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

6. 2nd problem: Angular separation of star and exoplanet is tiny Distance units Astronomical Unit = mean Earth-Sun distance For interstellar distances: Light year

7. Star Planet Earth e.g. ‘Jupiter’ at 30 l.y.

9. Exoplanets are ‘drowned out’ by their parent star. Impossible to image directly with current telescopes (~10m mirrors) Keck telescopes on Mauna Kea, Hawaii

10. 1. How can we detect extra-solar planets?  They cause their parent star to ‘wobble’, as they orbit their common centre of gravity

11. 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

12. 1. How can we detect extra-solar planets?  They cause their parent star to ‘wobble’, as they orbit their common centre of gravity

13. 1. How can we detect extra-solar planets?  They cause their parent star to ‘wobble’, as they orbit their common centre of gravity

14. Star + planet in circular orbit about centre of mass, to line of sight

16. Can see star ‘wobble’, even when planet is unseen. But how large is the wobble?…

17. 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

18. 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 e.g. ‘Jupiter’ at 30 l.y.

19. The Sun’s “wobble”, mainly due to Jupiter, seen from 30 light years away = width of a 5p piece in Baghdad! Detectable routinely with SIM (launch date 2009) but not currently

20. Suppose line of sight is in orbital plane Direction to Earth

21. Suppose line of sight is in orbital plane Star has a periodic motion towards and away from Earth – radial velocity varies.

22. Suppose line of sight is in orbital plane Star has a periodic motion towards and away from Earth – radial velocity varies Detectable via the Doppler Effect Can detect motion from shifts in spectral lines

23. Star Laboratory

24. Stellar spectra are observed using prisms or diffraction gratings, which disperse starlight into its constituent colours

25. Doppler formula Wavelength of light as measured in the laboratory Change in wavelength Radial velocity Speed of light

26. Limits of current technology: Stellar spectra are observed using prisms or diffraction gratings, which disperse starlight into its constituent colours Doppler formula Wavelength of light as measured in the laboratory Change in wavelength Radial velocity Speed of light

27. 51 Peg – the first new planet Discovered in 1995 Doppler ‘wobble’

28. 51 Peg – the first new planet Discovered in 1995 Doppler ‘wobble’ How do we deduce planet’s data from this curve? We can observe these directly We can infer this from spectrum

29. 25000 10000 8000 6000 5000 4000 3000 Surface temperature (K) O5 B0 A0 F0 G0 K0 M0 M8 Luminosity (Sun=1) Spectral Type 1 10 2 10 4 10 6 10 -2 10 -4 -10 -5 0 +5 +10 +15 Absolute Magnitude...................................................................................................................... Regulus Vega Sirius A AltairSun Sirius B Procyon B Barnard’s Star Procyon A....... Aldebaran Mira Pollux Arcturus Rigel Deneb Antares Betelgeuse Stars on the Main Sequence turn hydrogen into helium. Stars like the Sun can do this for about ten billion years When we plot the temperature and luminosity of stars on a diagram most are found on the Main Sequence

30. Main sequence stars obey an approximate mass– luminosity relation  We can, in turn, estimate the mass of a star from our estimate of its luminosity L log L Sun 10 00.51.0 0 1 2 3 4 5 m log m Sun 10 L ~ m 3.5

31. Summary: Doppler ‘Wobble’ method Stellar spectrum Velocity of stellar ‘wobble’ Stellar temperature Luminosity Orbital period + Orbital radius Planet mass From Kepler’s Third Law Stellar mass +

32. Transit of Mercury: May 7 th 2003 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.

34. Change in brightness from a planetary transit Brightness Time Star Planet

35. Ignoring light from planet, and assuming star is uniformly bright: Total brightness during transit e.g.Sun: Jupiter: Earth: Total brightness outside transit  Brightness change of ~1%  Brightness change of ~0.008%

36. What have we learned about exoplanets? Highly active, and rapidly changing, field Aug 2000: 29 exoplanets

37. What have we learned about exoplanets? Highly active, and rapidly changing, field Aug 2000: 29 exoplanets Nov 2005: ~150 exoplanets

38. What have we learned about exoplanets? Highly active, and rapidly changing, field Aug 2000: 29 exoplanets Nov 2005: ~150 exoplanets Up-to-date summary at http://www.exoplanets.org Now finding planets at larger orbital semimajor axis

39. 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

40. 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

41. 4. NASA: Terrestrial Planet Finder ESA: Darwin Looking to the Future } ~ 2015 launch These missions plan to use interferometry to ‘blot out’ the light of the parent star, revealing Earth-mass planets

43. 4. NASA: Terrestrial Planet Finder ESA: Darwin Looking to the Future } ~ 2015 launch Spectroscopy will search for signatures of life:- Spectral lines of oxygen, water carbon dioxide in atmosphere? Simulated ‘Earth’ from 30 light years

44. The Search for Extra-Solar Planets What (or who) will we find?…