Transcript Document
The Search for Extra-Solar Planets Prof Webster Cash Astrophysical & Planetary Sciences With thanks to Dr Martin Hendry http://www.astro.gla.ac.uk/users/martin/teaching/ THE BIG QUESTIONS • What is Reality? • What are we? • Are we alone? How do we even get a handle on these questions? 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? 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 The Basic Problem: Stars are very bright and their glare makes it difficult to see fainter objects near them 25 Aug 2008 5 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 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 m2 r2 mS r rS rP rS 1 mP e.g. ‘Jupiter’ at 30 l.y. mS 2.0 1030 kg mP 1.9 1027 kg rS S radians d 1.5 107 deg SIM Planet Quest Just Cancelled! The Sun’s “wobble”, mainly due to Jupiter, seen from 30 = width of a tennis ball in London light years away 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 sinusoidally 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 sinusoidally Can detect motion from shifts in spectral lines 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 e- Star Laboratory 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 1ms-1 51 Peg – the first new planet Discovered in 1995 Doppler amplitude v 55ms -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 Complications Elliptical orbits Complicates maths a bit, but otherwise straightforward radius semi-major axis Orbital plane inclined to line of sight We measure only vS sin i obs If i is unknown, then we obtain a lower limit to mP ( vS vS sin i obs as sin i 1 ) Multiple planet systems Again, complicated, but exciting opportunity (e.g. Upsilon Andromedae) Stellar pulsations Can confuse signal from planetary ‘wobble’ Change in brightness from a planetary transit Brightness Star Planet Time 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. 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. Background stars Lens’ gravity focuses the light of the background star on the Earth Gravitational lens So the background star briefly appears brighter 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. The shape of the curve tells about the mass and position of the object which does the lensing Time 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! 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: vS sin i 474ms-1 Right Now 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 Transit Detection by OGLE III program in 2003 But the Future is in Direct Imaging…. External Occulters • Let’s Resurrect an Old Idea – Spitzer (1962) appears to be the first • Just Keep the Starlight Out of the Telescope Occulter Diagram Planet Target Star NWD Starshade JWST 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 Fly the Telescope into the Shadow Dropping It In Note: No Outer Working Angle New Worlds Observer Simulated Solar System The First Image of Solar System Uranus Galaxies Zodiacal Light Jupiter Saturn Neptune 10 arcseconds Simulated Image of Earth Planet Finding with Starshades Five Random Systems from Raymond Database JWST The higher resolution of ATLAST brings weak signals out of the noise ATLAST Spectroscopy • R > 100 spectroscopy will distinguish terrestrial atmospheres from Jovian with modeling HO 2 O2 CH4 NH3 S. Seager TRUE PLANET IMAGING 3000 km 1000 km 300 km Earth Viewed at Improving Resolution 100 km Conclusion By 2025 By 2013 H2O O2 Demonstration Program 2010-2013 Study Planets with Small Starshade 2018 Full Up New Worlds Observer 2027 Planet Imager – 2035? Lectures Complete • • • • • Final Exam 1:30-4:00pm Wednesday 17th 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)