Extrasolar Planets Years After the First Discovery

The first detection of an extrasolar planet orbiting a solar-type star, that was announced as such, took place in 1995. Some 13 years after this major discovery, more than three hundred planets have been discovered, primarily by the method used to detect 51 Peg b - radial-velocity determinations - but also by the transit method, which is beginning to bear fruit. These detection methods are 'indirect' (we do not 'see' the planet, but instead deduce its presence from the effect that it has on its central star). They do, however, allow us (from knowledge of the characteristics of the central star) to estimate certain planetary parameters, in particular:

• the mass of the body (or, more precisely, the product m.sin(i), where i is the angle at which the system is observed (i = 90° if the system is seen edge-on)

• the semi-major axis of the orbit (deduced from Kepler's Third Law - cf. Appendix)

• the eccentricity of the orbit

• the possibility that the system may be multiple

• the radius of the planet when the system is observed simultaneously by radialvelocity measurements and in transit.

In this chapter we shall discuss these planets, and determine, from a statistical point of view, the preliminary information that these objects provide. All the graphs and statistical representations shown in this chapter have been prepared from the list of objects available on the site: exoplanet.eu, which is maintained by J. Schneider.

Figure 3.1 shows a histogram plot of the annual totals of planetary discoveries. It will be seen that the annual totals have been practically constant since about 2000, which indicates that the detection methods currently employed have got into their stride. It will undoubtedly be necessary to implement new methods to refine our knowledge of the properties of the objects that have already been detected, and to discover others with different characteristics, in particular, less massive bodies.

M. Ollivier et al., Planetary Systems. Astronomy and Astrophysics Library, DOI 978-3-540-75748-1 _3, © Springer-Verlag Berlin Heidelberg 2009

Fig. 3.1 Annual totals of detected exoplanets. It is obvious that the yearly total has increased a lot since 2007 after five years where it was more or less constant

3.1 Exoplanets and Exoplanetary Systems

By the beginning of September 20081, 306 planets have been detected, including those around the pulsar PSR 1257 + 12. They fall into 262 planetary systems, of which 31 are multiple systems that may include as many as five planets (the 55-Cancri system, for example). Most of these planets are giant planets, because the methods used to detect them (mainly radial-velocity measurements and/or transit measurements from the ground) are method which are particularly sensitive to this kind of object. However, microlensing and pulsar timing methods as well as radialvelocity measurements around low mass stars have enabled several objects of a few Earth-masses to be detected close to their parent star. The current statistical picture will therefore be rendered complete only when it has become possible to identify planets with lower masses. The arrival of satellites for the observation of transits from space (primarily COROT and KEPLER), should enable us to achieve this goal. Given the length of time that observations have been made (some ten years at most), it has only been possible to confirm the existence of planets with periods that do not exceed a few years (again, up to about ten years).

3.2 The Mass-Distribution of Exoplanets

Searching for companion bodies by the measurement of radial velocities (i.e., the measurement of the Doppler shift in the spectrum of the parent star), is not specific to the detection of exoplanets. It enables us to identify all types of multiple system,

1 The date on which these numbers were updated was 1 September 2008

and thus companions, whether they are stellar (double or multiple stars, known as 'spectroscopic' doubles), sub-stellar (brown dwarfs), or planetary. By measuring M.sin(i) (where i is the angle at which the system is viewed; with i = 90° the system is viewed side-on), we obtain a lower limit for the mass of the companion. Before concentrating specifically on exoplanets, it is interesting to examine the distribution of the masses of companions, both stars and planets. Figure 3.2 shows this distribution for some one hundred objects.

It is immediately obvious that the mass distribution is bimodal, with planets (M < 0.01 M0) on the left2, and stars (M < 0.08 M0) on the right. Between the two there are very few objects. This zone, known since the 1980s, is called the 'brown dwarf desert'. It was revealed by the first programmes devoted to the systematic search for sub-stellar companions, carried out on a few tens of objects, and which indicated the relative non-existence of this type of object (Campbell et al., 1988, Marcy and Benitz, 1989). Current programmes for monitoring radial velocities have only allowed the identification of about 20 brown-dwarf candidates (where the value of M.sin(i) that has been measured lies within the range of brown-dwarf masses).

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