Transcript exoplanets
Extrasolar Planets What’s an Extrasolar Planet? •An extrasolar planet is a planet that goes around another star •A star is something that is hot enough to fuse hydrogen •More on this later •A brown dwarf is too small to be a star, and too large to be a planet •A planet is something that is large enough, but not too large •Lower limit – irrelevant, we can’t see them that small •Upper limit – about 13 times Jupiter’s mass Extrasolar Planets Found = 861, Systems Found = 677 Detection Methods Methods for finding them •Direct Imaging •Gravitational Microlensing •Astrometry •Pulsar Timing Dynamical •Other Timing Methods •Radial Velocity •Transit Method Direct Imaging Disadvantages •Almost impossible to see planet next to bright star •Must be done from space Advantages •Can estimate size of planet •Can study spectrum of planet Score = 32 Gravitational Microlensing The Technique •Einstein – Light is bent by gravity •Any object passing in front of a distant star can cause it to look brighter Foreground Star Distant Star Planet •By measuring brightening carefully, can tell if there is just a star or star plus planet Gravitational Microlensing Disadvantages •Requires lucky alignment •Tells little about the planet •Cannot be repeated Advantages •Can monitor many stars cheaply •Sensitive to small masses Score = 18 Dynamical methods •Astrometry •Pulsar Timing •Other Timing •Radial Velocity How they work •Planet orbits star •Star also goes in a small circle •Easier to detect motion of star than of planet Astrometry How it works Star moves left and right as viewed by us Advantages •Can see planets with large orbits Disadvantages •Insensitive •Massive planets •Large Orbits •Patience Score = 1 Pulsar Timing How it works •Pulsar sends out very regular signal •Pulsar is slightly nearer/farther from us •Pulses come slightly late/early Advantages Disadvantages •Very sensitive •Works only on Pulsars (weird stars) •Cheap •See low mass planets Score = 5 Other Timing How it works •There are other very regular signals from stars •Orbiting eclipsing binary stars (9) •Extreme horizontal branch stars (1) •Signals come slightly late/early Advantages •Can see planets with large orbits Disadvantages •Works only in very special circumstances (rare) Score = 10 Radial Velocity / Doppler Method How it works •Star moves towards and away from us •Alternating red-shift / blue shift Advantages •Currently most sensitive method Disadvantages •Massive planets •Small Fast Orbits •No observation of planet Score = 501 Transit Method How it works •Planet blocks light from star, making it dim •Star blocks light from planet Disadvantages •Requires Luck •False Positives (requires confirmation) Advantages •Works well from space •Measure spectrum of planet •Measure atmosphere of planet Score = 294 Spacecraft for the Transit Method COROT •In operation since 2007 •Should see planets ~ Earth size •17 discovered so far Kepler •In operation since 2009 •Should see planets < Earth size •15 so far •1000+ candidates found, more coming Latest Candidates from Kepler 01/07/2013 Earth-sized planets from Kepler What Have We Learned? •Many large planets – Jupiter size and greater •Planets as small as Earth and smaller •Observational bias favors finding large planets •Often find planets very close to star •Observational bias •Gas giants can live very near their stars •Orbits often highly eccentric Lessons from Extrasolar Planets? •We probably need to rethink our understanding of how stellar systems form •Planetary migration is often an important factor? •Giant planets form far away •Later move inwards •Eccentric and even retrograde planets are not rare •Planets are not uncommon •Certainly > 10% of nearby sun-like stars •Stay tuned