The Search for Extra-Solar Planets

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

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

3. 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?

4. 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

5. 2nd problem: Angular separation of star and exoplanet is tiny Distance units Astronomical Unit = mean Earth-Sun distance For interstellar distances: Light year

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

7. e.g. ‘Jupiter’ at 30 l.y.

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

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

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

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

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

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

15. 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?…

16. Centre of mass condition 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?…

17. e.g. ‘Jupiter’ at 30 l.y. Centre of mass condition 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?…

18. 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!

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

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

22. Star Laboratory

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

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

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

26. 51 Peg – the first new planet Discovered in 1995 Doppler ‘wobble’

27. 51 Peg – the first new planet Discovered in 1995 Doppler ‘wobble’ How do we deduce planet’s data from this curve? We can infer this from spectrum We can observe these directly

28. When we plot thetemperature and luminosity of stars on a diagram most are found on the Main Sequence Surface temperature (K) 25000 10000 8000 6000 5000 4000 3000 . . . 106 . -10 . Deneb . . . . Rigel . . . . Betelgeuse . . . . . Antares 104 . -5 . . . . . . . . . . . . . Arcturus . . . Aldebaran . . . . . . . . . . . . Regulus . . . . . . . 102 Vega . . . . . 0 . . . Mira Sirius A . . . Stars on the Main Sequenceturn hydrogen into helium. Stars like the Sun can do this for about ten billion years . . . Pollux . Procyon A . . . . Luminosity (Sun=1) . . Altair Sun Absolute Magnitude 1 . . . +5 . . . . . . . . . . . . . . . 10-2 . +10 . . . . . . . . . . . . . . . . Barnard’s Star . . . . Sirius B . . . . 10-4 . +15 . . Procyon B . . . . O5 B0 A0 F0 G0 K0 M0 M8 Spectral Type

29. 5 3.5 L ~ m 4 3 2 1 m log log mSun LSun 10 10 0 -1 0 0.5 1.0 L 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

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

31. 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

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

34. 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%

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

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

37. 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

38. 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.

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 ‘Hot Jupiters’ produce Doppler wobbles of very large amplitude Artist’s impression of ‘Hot Jupiter’ orbiting HD195019

40. Looking to the Future } 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

42. Looking to the Future } 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

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