Astronomy of extrasolar planetary systems. Methods and results of searches for planets around other stars

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1 Astronomy of extrasolar planetary systems Methods and results of searches for planets around other stars

2 Course layout - methods Introduction and history of searches for planets Doppler spectroscopy and classical astrometry Optical interferometry Transit photometry and coronography Gravitational microlensing Timing of pulsars and white dwarfs

3 Course layout - results, theories, interpretation Results of planet searches Theories of Solar System formation Theories of formation and evolution of protoplanetary disks Interpretation of the results of planetary searches so far Future of astronomy of extrasolar planets

4 How to define a planet? Stars burn hydrogen, brown dwarfs do the same to deuterium, planets radiate away their energy while contracting The observed distribution of planet masses shows a deficiency of masses larger than ~12-13 M jup (1 M jup =0.001 M sun ) A lower mass limit above which a star can burn deuterium is ~13 M jup This agreement between theory and observations suggests a planet definition of a body of mass < 13 M Jup

5 Classification of planets Gas giants In the Solar System: Jupiter and Saturn Have extended gas envelopes Have masses on the order of M jup Chemical composition different from that of the Sun Gas had to be present in large quantities at the time of gas giant formation Terrestrial Planets Prototypes: Venus, Earth, Mars These planets are dense and rocky They are located close to the Sun compared with the distant gas giants

6 A review of planet detection methods

7 Indirect detection methods: I Doppler spectroscopy Observed quantity: radial component of stellar velocity in the star s motion around the center of mass of the star - planet system So far, most of the extrasolar planets (>150) have been detected with this method Astrometry Observed quantity: stellar motion in the plane of the sky Very likely the most efficient method in the future (optical interferometers on the ground and in space: Keck, VLTI, SIM, GAIA, TPF)

8 Indirect detection methods: II Transit photometry Observable: decrease of stellar brightness, when planet moves across the stellar disk Condition of observability: planetary orbit must be (almost) perpendicular to the plane of the sky Several hot Jupiters have been detected using this method Earth-mass planets can possibly be detected with this method using space-based telescopes (COROT, Kepler) Gravitational microlensing One observes a light curve of the star, which is lensed by the gravitational field of a star located along the line-of-sight between the lensed star and the observer. The presence of a planet around the lensing star modifies the light curve of the background star An appropriate geometry occurs very infrequently (need to observe millions of stars) and the phenomenon is not repeatable The method is sensitive to Earth-mass planets

9 Indirect detection methods: III Pulsar timing This method uses pulsars as clocks (microsecond timing precision) The observed quantity is a change of pulse arrival time caused by the pulsar s motion around the center of mass of the system Precision of the pulsar clocks makes it possible to detect planets down to asteroid masses The first extrasolar planets have been detected with the aid of the timing method.

10 Direct methods: I Adaptive optics Jupiter observed from the distance of 10 pc would be 10 5 weaker than the Sun, separated from it by about one millisecond of arc So far, adaptive optics has made it possible to detect brown dwarfs in binary systems Direct detection of planets requires that the stellar light be efficiently blocked off

11 Direct detection methods: II Coronagraphy This method of blocking off the stellar light has been originally worked out by Lyot The method removes 99% of the stellar light, and it leaves out ~50% of light from the hypothetical planets Nulling interferometry This method removes the stellar light observed with an interferometer by placing a dark interference fringe on it (destructive interference) Both methods are very promising, especially, when used with orbital telescopes

12 A comparison of some search projects

13 Some history The earliest, well-known planet search episode was a long-term effort by van de Kamp to detect planets around the Barnard star It was only in the 1970s that Gatewood and Eichhorn have shown, that van de Kamp s detections were the results of instrumental errors Around the same time, reports have been published on possible detections of planets around the Crab pulsar and another pulsar, PSR B In both cases, the results have been convincingly shown to be due to irregularities in the pulsar rotation

14 More history 1983 was an important year in planet detection history. At that time, Smith and Terrile have detected the first circumstellar debris disk (ß Pictoris). It was a spectacular confirmation of the prediction based on the known structure of the Solar System: disks and then planets should be a natural byproduct of the stellar birth Another seriously considered possibility for planet detection was a confirmed detection of a companion to star HD by Latham et. al in However, that object appears to be a brown dwarf rather than a planet

15 First extrasolar planets Planets around a pulsar Discovered in1991 by Wolszczan using the pulse timing method. Initially, it was two planets with masses of ~4 Earth masses, orbiting the millisecond pulsar, PSR B Confirmed in 1994 by detecting gravitational perturbations between planets B and C caused by a mean motion resonance Currently, three terrestrial-mass planets and a large asteroid (~2 masses of Ceres) are known in this system The first planet around a Sun-like star Discovered in1995 by Mayor and Queloz using the Doppler spectroscopy method. This Jupiter-mass planet circles a solartype star, 51 Pegasi, once every 4.2 days. So far, >150 giant planets have been detected around other stars, including hot Neptune planets with masses <20 Earth masses