2. The Search for Extra-Solar Planets With thanks to Dr Martin Hendry http://www.astro.gla.ac.uk/users/martin/teaching / Prof Webster Cash Astrophysical & Planetary Sciences

3. THE BIG QUESTIONS What is Reality? What are we? Are we alone? How do we even get a handle on these questions?

4. Extra-Solar Planets  One of the most active and exciting areas of astrophysics  Nearly 4000 exoplanets discovered since 1995 Some important questions o How common are planets? o How did planets form? o Can we find Earth-like planets? o Do they harbor life?

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 Earth is about 10Billion times fainter than the Sun

6. 25 Aug 20085 The Basic Problem: Stars are very bright and their glare makes it difficult to see fainter objects near them

7. 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

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

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

11. 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.

12. The Sun’s “wobble”, mainly due to Jupiter, seen from 30 light years away = width of a tennis ball in London SIM Planet Quest Just Cancelled!

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

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

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

16. Absorption e - Electron absorbs photon of the precise energy required to jump to higher level. Light of this energy (wavelength) is missing from the continuous spectrum from a cool gas

17. Star Laboratory

18. 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

19. 51 Peg – the first new planet Discovered in 1995 Doppler amplitude How do we deduce planet’s data from this curve? We can observe these directly We can infer this from spectrum

20. Complications  Elliptical orbits Complicates maths a bit, but otherwise straightforward radius semi-major axis  Orbital plane inclined to line of sight We measure only If is unknown, then we obtain a lower limit to ( as )  Multiple planet systems Again, complicated, but exciting opportunity (e.g. Upsilon Andromedae)  Stellar pulsations Can confuse signal from planetary ‘wobble’

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

23. Another method for finding planets is gravitational lensing The physics behind this method is based on Einstein’s General Theory of Relativity, which predicts that gravity bends light, because gravity causes spacetime to be curved. This was one of the first experiments to test GR: Arthur Eddington’s 1919 observations of a total solar eclipse.

24. Another method for finding planets is gravitational lensing If some massive object passes between us and a background light source, it can bend and focus the light from the source, producing multiple, distorted images.

25. Background stars Gravitational lens Lens’ gravity focuses the light of the background star on the Earth So the background star briefly appears brighter

26. Even if the multiple images are too close together to be resolved separately, they will still make the background source appear (temporarily) brighter. We call this case gravitational microlensing. We can plot a light curve showing how the brightness of the background source changes with time. Time The shape of the curve tells about the mass and position of the object which does the lensing

27. Even if the multiple images are too close together to be resolved separately, they will still make the background source appear (temporarily) brighter. We call this case gravitational microlensing. We can plot a light curve showing how the brightness of the background source changes with time. If the lensing star has a planet which also passes exactly between us and the background source, then the light curve will show a second peak. Even low mass planets can produce a high peak (but for a short time, and we only observe it once…) Could in principle detect Earth mass planets!

28. 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 e.g. Tau Boo:

29. 1. The Doppler wobble technique will not be sensitive enough to detect Earth-type planets (i.e. Earth mass at 1 A.U.), but will continue to detect more massive planets 2. The ‘position wobble’ (astrometry) technique will detect Earth-type planets – Space Interferometry Mission after 2010 (done with HST in Dec 2002 for a 2 x Jupiter-mass planet) 3. The Kepler mission (launch 2008?) will detect transits of Earth-type planets, by observing the brightness dip of stars Right Now

30. Transit Detection by OGLE III program in 2003

31. But the Future is in Direct Imaging….

32. External Occulters Let’s Resurrect an Old Idea – Spitzer (1962) appears to be the first Just Keep the Starlight Out of the Telescope

33. Occulter Diagram  Telescope big enough to collect enough light from planet  Occulter big enough to block star –Want low transmission on axis and high transmission off axis  Telescope far enough back to have a properly small IWA  No outer working angle: View entire system at once NWD Starshade JWST Target Star Planet

34. Fly the Telescope into the Shadow

35. Dropping It In Note: No Outer Working Angle

36. New Worlds Observer

37. Simulated Solar System

38. The First Image of Solar System Jupiter Saturn Uranus Neptune Zodiacal Light Galaxies 10 arcseconds

39. Simulated Image of Earth

40. Planet Finding with Starshades Five Random Systems from Raymond Database The higher resolution of ATLAST brings weak signals out of the noise ATLAST JWST

41. Spectroscopy R > 100 spectroscopy will distinguish terrestrial atmospheres from Jovian with modeling O2O2 H2OH2O CH 4 NH 3 S. Seager

42. Earth Viewed at Improving Resolution 100 km 300 km3000 km1000 km TRUE PLANET IMAGING

43. Conclusion By 2025 O2O2 H2OH2O By 2013 Demonstration Program 2010-2013 Study Planets with Small Starshade 2018 Full Up New Worlds Observer 2027 Planet Imager – 2035?

44. Lectures Complete Final Exam 1:30-4:00pm Wednesday 17 th Here. Just like the mid-terms except twice as long Covers everything (comprehensive) A bit extra on last four lectures One or two longer essays Review Session by Josh Monday 5:30-6:30 here I will do office hours 12:00-1:30 Wednesday for last minute questions (Duane F913)