How Common Are Planets Around Other Stars? Transiting Exoplanets Kailash C. Sahu Space Tel. Sci. Institute
Earth as viewed by Voyager Zodiacal cloud "Pale blue dot" Look again at that dot. That's here. That's home. That's us. On it everyone you love, everyone you know, everyone you ever heard of, every human being who ever was, lived out their lives...--on a mote of dust suspended in a sunbeam. -Carl Sagan (Pale Blue Dot)
Sky at Cerro Tololo, Chile Is "Pale blue dot" unique? Are there other planets out there around other stars?
Extrasolar Planet Discoveries Search for planets around other stars began more than100 years ago. The first extrasolar planet around a normal star, 51 Peg, was discovered in 1995 through RV technique. This has a period of 4.2 days. >300 exoplanets discovered so far, mostly within ~500 light years. They are Jupiter or Neptune-like planets, include multiple planet systems, and planets in ~habitable zones. They include ~50 transiting planets.
Extrasolar Planet Discoveries Transits Transiting planets are ideal for studying the properties of atmospheres. Sodium and hydrogen have been detected in the atmosphere of one such planet using HST. Spitzer observations have recently shown silicate and water emission.
Transit by the planet SPITZER has directly detected the transit by the planet, and has led to temperature mapping of the planet. 1.01 1.00 Relative Flux 0.99 0.98 0.97 a 1.003 Relative Flux 1.002 1.001 1.000 b 0.999-0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6 Orbital Phase
First Visible Light Image of an Extrasolar Planet by Hubble Discovered by Kalas et al. in 2008 using Hubble. Fomalhaut b is at ~119 astronomical units (AU) from the star 25 light years from Earth
What about Earth-like Planets? Microlensing is currently the only technique capable of detecting earth-mass planets at distances of several AU, and at large distances from us.
Microlensing Red:lens Green: source Blue: images
Chief Scientist for Australia Penny Sackett PLANET Collaboration (Probing Lensing Anomalies NETwork) Founded in 1995 by Penny Sackett & Kailash Sahu If every star has a Jupiter, 1/7 should should show planetary signal. Se we needed to monitor at least 7 events per year. So far, we have monitored > 200 events, including many binaries, and discovered one planet.
OGLE-2005-BLG-390 PLANET Observations: Extra peak: 9 August, 2005 Additional obs MOA/OGLE 5.5 Earth-mass planet at 2.3 AU from a star of 0.25 solar mass. Two more probable detections by other groups. Text Text Text Microlensing observations suggest that Earth-mass planets must be more abundant than Jovian planets at > 1 AU.
OGLE-2005-BLG-390 PLANET Observations: Extra peak: 9 August, 2005 Additional obs MOA/OGLE 5.5 Earth-mass planet at 2.3 AU from a star of 0.25 solar mass. Two more probable detections by other groups. Text Text Text Microlensing observations suggest that Earth-mass planets must be more abundant than Jovian planets. The 5 earth-mass (m sin i) planet around GL581, with a period of ~13 days, supports this suggestion.
What do we know so far... More than 300 extrasolar planets have been detected. The planet detections have been mostly confined to 500 light years. They suggest that about 6% of stars have Jupiter-like planets. Earth-like planets are more common.
An analogy... Zodiacal cloud "Pale blue dot" "Solar neighborhood" There are 100 billion stars in the Galaxy, stretched over 100,000 light years. Are planets equally common in the entire Galaxy? To explore this question, we should look at a very different part of the Galaxy.
The technique: transits Characteristic light curve shape Probability of detection is small. Same in V and I Must repeat No variation outside transit Depth must be consistent with planet
Probability of planet detection *Approximate numbers
SWEEPS Project Location of SWEEPS field Ideal for studying our key questions Monitored this field continuously for 7 days
SWEEPS Project Sagittarius Window Eclipsing Extrasolar Planet Search 202 x202 Continuous monitoring for 7 days 180,000 stars to V ~ 27. 245,000 stars to V ~ 30 265 I images 254 V images 339 sec Int. time No transits missed Deepest Galactic field
FINDING PLANETARY TRANSITS: An analogy with MIRACLE IN THE 34th STREET (Baltimore)
We discovered 16 Planet Candidates (the largest number from a single set of observations). Example light curves in Blue and Red Black: Model fit Includes two RVconfirmed planets. This program can detect Jupiter-like planets around small stars (M~0.44 solar mass).
16 planetary candidates (circled) We also detected 165 eclipsing binaries. 125 of them show ellipsoidal variations caused by close binaries.
Properties of the Planet Candidates These are the farthest planets known to date (26,000 light years away). Host masses are 0.44 to 1.24 M (lowest mass host stars so far). 5 planets have periods of < 1.0 d, shorter than the shortest period previously known. (Ultra-Short-Period Planets, or USPPs).
ARE THEY REALLY PLANETS? False Positive Contributions Blending Grazing eclipses Low-mass stars
Blending Blending of an eclipsing binary with a brighter star can mimic planetary signal. We used the binary statistics to estimate that the blending fraction is less than one.
Grazing Eclipses Estimated contamination ~ 1.4 (conservative since some grazing incidences are clearly recognizable as such).
Low-Mass Stars Star BD Hot Planet Jupiter 1-100 MJup. objects have similar radii, so it is difficult to rule out brown-dwarfs or low-mass stars from transit signals. One out of six may be low-mass stars. In summary: at least ~half of the candidates must be genuine planets. RV measurements only can provide unambiguous confirmation.
Radial Velocity Observations 4 nights of observations with VLT + UVES June 21-25, 2004. Long nights, clear weather, superb seeing We could observe 2 that were bright and isolated enough
Radial Velocity observations with 8-meter VLT SWEEPS-04 V ~18.8 brightest candidate < 3.8 MJup SWEEPS-11 V ~19.6 Planet detected at 9.7 MJup
All planets with P < 12 days Blue: RV planets Green: Transits Red: SWEEPS SWEEPs sample extends to 0.45M, leading to USPPs. Closer-in planets might be evaporatively destroyed, or planets can migrate to and survive in close-in orbits only around such old and low-mass stars.
Artist s Impression USPPs were not detected before since not enough low-mass stars with suitable metallicity were monitored.
Frequency of Planets Taking into account: (i) the geometric probability, and (ii) the detection efficiency, we estimate that ~0.5% of the bulge stars are orbited by hot Jupiters. From (i) Planet frequency from the local sample, (ii) the metallicity~planet-frequency dependence, (iii) metallicity distribution of bulge stars, we predict 0.5% to have such planets. The Jovian planet frequency in the bulge is thus similar to that of the solar neighborhood (5%). Earth-like planets are expected to be more common.
Key implications The planet frequency at 2 very different parts of the Galaxy are similar. This gives us some indication that the planets may be equally abundant in the entire Galaxy. There are 100 billion stars in the Galaxy. So there may be 6 billion Jovian planets, and an even larger number of Earth-like planets in the Galaxy.
Beyond the Galaxy? UDF is the deepest image of the sky This tiny region of the sky contains ~3000 galaxies Universe may contain ~100 billion galaxies The bulge is similar to the most common type of galaxy, elliptical. So it seems reasonable to expect that planets may be equally common in the whole Universe.
Future Missions: I. Kepler Mission NASA s first mission capable of finding Earth-like planets Launch date: 2009 March 5 Kepler will continuously monitor 100,000 stars for 4 to 6 + years Expected to find many Earth-like planets around Sunlike stars
Future Missions: II. James Webb Space Telescope NASA s next large telescope 6.5 m segmented primary Launch: 2013 Atmospheric Transmission Spectrum (4 hr) for HD209458-like Kepler source using NIRSpec R~3000 grating. Simulations by J. Valenti, STScI. JWST can detect atmospheres and possible biomarkers from Earth-like planets.
More than 300 exoplanets have been discovered to date, including the first direct image of a planet. But they are all confined to the ~500 light years from the Sun. SUMMARY Planets are equally abundant in the Galactic bulge, and around low-mass stars. 6% of stars have Jovian planets. Earth-mass planets must be more common. Our SWEEPS project discovered 16 exoplanet candidates, first exoplanets discovered by HST. This represents the farthest sample at 26,000 light years, and the largest number of planets discovered from a single set of observations. This project led to the discovery of USPPs, which occur only around low-mass stars. Planets may be equally common in the whole Universe. Is life common in the Universe? Kepler and JWST may provide some insight.