Searching for Other Worlds

Transcription

1 Searching for Other Worlds Lecture 32 1 In-Class Question What is the Greenhouse effect? a) Optical light from the Sun is reflected into space while infrared light passes through the atmosphere and heats the Earth b) A green home saves you money c) Optical light from the Sun passes through the atmosphere and most infrared radiation cannot escape because of H 2 0 and CO 2 in the atmosphere d) Plants reflect green light

2 Lecture Topics How and where to look for exosolar planets Techniques Direct detection Stellar Wiggle Doppler Spectroscopy Success! Planets found Brightness variations Are exosolar planets habitable? 3 How do we find other planets? Methods are analogous to looking for binary stars Four Possible Methods Direct observation Reflected light from star Intrinsic infrared radiation Measure wiggles on the sky Doppler spectroscopy Search for brightness variations

3 Where to look? Nearby stars are the best because: Planets are brighter. The angular separation between planets and the star are larger. Stars are brighter Doppler shifts easier to measure Motions on the sky easier to measure 5 Nearby stars (d < 10 pc) Spectral Mass N total N single Class (M sun ) A F G K M <

4 Main-Seq. stars with m v < 10 Class r max (pc) # of M-S Stars M5 4.4 ~ 5 K ~ 750 G5 95 > 11,000 F5 208 ~ r max = maximum distance a star of this class can be seen if m v < Direct observation Very difficult because the star is very bright and the planet faint. Hubble Space Telescope can detect objects to m v ~ 29. see Jupiter to ~ 30 pc (near G2 star) with a separation of only 0.16!

5 0 Relative energy distributions Sun Log (Flux) Earth Jupiter Uranus Visible Infrared Wavelength (microns) 9 The IR can be better Planets are much cooler than the stars so their spectrum peaks in the infrared. Why not look in the infrared instead of in the visual? It s difficult. Angular resolution of telescopes not as good in the infrared. ( = / D ) Can see much fainter objects in the visual

6 Exoplanet Imaged?! Star 2M1207A Brown Dwarf Distance = 59pc 13 mag at J (1.25 m) Planet Data: (2M1207b) M p = 5 M J (~1500 M earth ) distance = 55 AU 100 times fainter than star First discovered in April 2004 Follow-up observations (February & March 2005) confirm it s a companion to the star. 11 Exoplanet Imaged?! 1RXS J Star similar to Sun Only 5 Myr old! 500 lyr away from us Confirmation First discovered in September 2008 Follow-up in July 2010 Planet 330 AU from star T = 1800 ± 200 K (young) M ~ 8 M Jup

7 Exoplanet Imaged?! While imaging the debris disk around Fomalhaut, an A4V, astronomers found a planet 120 AU from the star The three times the distance of Pluto from the Sun Stellar Reflex Motion As the planet moves around the star, the star has a reflex motion. 1/4 orbit later Orbit of Star

8 Orbital Motions Motion of the star is greatly exaggerated. Planet Star The star will wiggle back and forth (stellar wiggle) and move towards and away from us (Doppler shift) 15 Reflex motion The reflex motion can cause the star to wiggle on the sky. the stellar spectrum to show periodic Doppler shifts. These effects get larger as the planet to star mass ratio increases large planets better less massive stars better

9 Stellar wiggle Star would drift in a straight line on the sky. But an unseen companion causes the star to wiggle. Due to stellar reflex motion. Drift w/ planet Drift w/o planet 17 Stellar wiggle size If the sun were at 10 pc, the wiggle is: Planet Period Wiggle Amplitude (years) (arcsec) Jupiter x10-3 Uranus x Less massive stars display a larger motion but will have longer periods arcsec possible with Keck No wiggle detection of Exosolar Planets yet

10 Doppler shift Star moving towards us higher pitch bluer light Star moving away from us lower pitch redder light Astronomers can detect motions of about 5 meters per second (a person running). 19 Binary star simulation

11 51 Pegasi First Exosolar Planet Found via Doppler Spectroscopy 1995 M. Mayor and D. Queloz (Geneva Observatory) G5 main-sequence star, 42 lyr away 70 m/sec Doppler shift, P = 4.2 days Planet parameters Orbital period is very short (4.23 days!) 0.05 AU from star 0.6 M jupiter ( ~ 7 x Earth s diameter) T ~ 1300 K 21 Velocity Curve for 51 Peg The first exoplanet detection: 51 Peg b Now many, many more Most early detections are planets

12 A Few Examples of Exoplanets Doppler Detections The number of verified extrasolar planetary systems now detected is well over 500 and counting. Technique Limitations Only a lower limit to the mass! Preferentially finds high mass planets Preferentially finds close in planets

13 Brightness variations Transits: (need edge-on orbital plane) - Planets pass in front of the star. - Last a few hours, variation < 0.1% This technique has now been used very effectively by the NASA space mission Kepler Three are also many ground based efforts 25 Eclipse Light Curve The light from the star is dimmed by the passage the planet in front of it Get information on planet radius and composition of its atmosphere

14 NASA Kepler Mission Mission designed to find transiting planets Continuously surveys 100,000 main-seq. stars Launched March 7, 2009 into Earth-trailing orbit Mission lifetime is 3.5 yrs, extendible to > 6 yrs Detectors: 95 mega pixels (21 modules each with two 2200x1024 pixel CCDs) Has found over 2321 exoplanet candidates! 77 follow-up confirmed planets See their home page for updates 27 Kepler multi-planet systems 32-14

15 Kepler multi-planet systems Brightness variations (cont d) Gravitational Microlensing Star amplifies light of a background star Planet modifies the light curve One time event, need statistical studies This technique has results in a few (tentative) detections

16 In-Class Question How have extrasolar planets been detected? a) direct observation b) eclipsing (brightness variations) c) motion on the sky d) Doppler spectroscopy e) a, b and d 31 Are these planets habitable? In general, they are much more massive than the earth. And not the correct temperature. 14 / L Tplanet 12 / d L = luminosity of star, d = planet-star distance

17 100 Habitability of y Exosolar p Planets Habitable Zone 10 M p sin i (M Jup ) V E J S U N M M Equivalent Solar System Orbital Distance (AU) 20-Nov Planets detected: 843 Technique Radial velocity Planetary Systems 382 Number of Planets 491 Multiple Planet Systems 80 Transiting Microlensing Imaging Timing See:

18 Conclusions Many Exosolar planets are now known Over 800 exoplanets and counting NASA's Kepler mission has over a thousand more candidates (to be confirmed) Many systems have multiple planets Doppler spectroscopy and transits are the dominant tools for finding exoplanets Some planets are in the habitable zone But they are planets much more massive than the Earth, more like Jupiter than the Earth Detecting Earth-like planets around stars like the Sun is a very hard problem