Warning: I wrote the page below in 2001. Many dscoveries occurred since then! I keep the page for archive puprose.
This comparison between theoretical models and observations will at last break with the limitations of theoretical scenarii about the formation and history of planetary systems based entirely on the data observed in our own solar system.
This field was open by Epikouros in 300 B. C. but it is only 23 centuries later, with the development of the instruments sensibility, that it became a real research field.
The situation only evolved a few years ago with the discovery of three telluric mass objects in 1993 (Wolzczan, 1994) and four Jupiter mass objects around solar type stars in 1995/96 (Mayor, 1995 and Marcy, 1996).
The first three planets which were discovered around a pulsar, are untypical because they formed in " post-mortem " conditions, which are very different from the ones that prevail in circumstellar discs around young stars but which are also unfitted to an eventual life development.
The other four are, from this point of view, able to help understanding the development of planetary systems by further constraining the models. All the discovered planets to date are closer to the star than Jupiter is to the Sun. This causes serious problems to current models of the gaseous mass giant planets development. We notice, in particular, the pretty frequent presence (in more than 5 % of the cases) of giant planets at only about ten stellar radii, the "hot Jupiters". This makes us think that the detection of a large number of Jupiters will help a better comprehension of their origin and role in the appearance and evolution of a planetary system.
After the discovery of Jupiters around solar type stars, the next major step is clearly the discovery of telluric objects (complete planets or giant gaseous ones embryo) at an evolution state concomitant or previous to the solar system’s one. Apart from their astrophysical significance, they mean a lot from the exobiology point of view which is dedicated to develop in the future. We are living a fascinating time where the answers to these questions are about to be found thanks to observations.
The detection of a telluric planet is a major challenge and is expected as the next big step in astronomy. It belongs to a " step-by-step " approach which tries to successively search for:
Their detection around solar type stars is very difficult because of their small masses. Possible detection methods are: coronography, astrometry, radial velocity, timing, gravitational amplification, occultations.
Before the achievement of ambitious spatial projects (interferometers, coronographs), there are only 2 possible methods on a short or mid term for the telluric planets detection, the gravitational amplification and the occultations. As emphasizes in the Townes Report (Comments on the Blue Ribbon Panel on ExNPS, 1996), gravitational amplifications can not allow to determine the statistical frequency of planets at several kpc from the Sun (galactic bulb), while we would need to detect earths, located a few pc from the sun, in order to subsequently study them in a more detailed way. The occultations method or transit method, allows on the contrary to precisely determine the orbital period and the size of the planet. Concerning close enough planets, it would enable to study their atmosphere chemical composition thanks to the differential absorption of the star radiance. Yet this method needs a high precision photometry (10-3 to 10-4) and a continuous observation during a long period (several months). Today, these two requirements can not be simulnateously satisfied by observations from the ground.
In order to estimate the number of possible transits due to telluric planets, we have to estimate the number of solar type stars escorted a priori with at least one planet. Concerning Jupiter-like planets, this number seems to be of a few % (Marcy 1996). Qualitatively, it is in agreement with Zuckermann’s observations (1995) which claimed that about 10 % of young stars have got an hydrogen cloud. Around the others, the stellar wind has dispersed the hydrogen, preventing Jupiters formation.
Regarding the telluric planets, their formation from Planetesimals is not inhibited by this stellar wind mechanism because it can’t disperse the asteroïd like bodies. Since 50 % of the young stars have a dust disc (Beckwith, 1993), it is reasonable to a priori assume that half of the stars have got telluric planets and that 20% of them have got planets whose radius is superior to the earth’s one. Our quantitative estimates are based upon this hypothesis.
We would also like to emphasize that the quantitative estimates shouldn’t be based upon the solar system taken as an example. The planetary systems already found to date have got orbits which are closer than in the solar system (Mayor and Queloz 1995, Marcy and Butler 1996), or the telluric planets mass are superior to the earth ones (Wolszczan, 1994).
We should notice that ESA and NASA are thinking of very ambitious programs to try to reach the research stage for inhabitable planets and signs of biologic activities. The discovery of telluric planets by current techniques would set an "existence theorem" and would give to this mission a strategic significance in these programs.
The main objective of the French COROS program, for example, is to detect extrasolar planet that resemble the Earth. The photometric method that COROS will be using, particularly well adapted to the telluric planets can also detect giant extra-solar planets (detectable by spectroscopy from the ground) and determine their albedo, increasing the scientific return. Thus it is important to know the frequency of the appearances of these planets in the planetary systems as well as their role in the subsequent system evolution. COROS should also confirm that the detections of "close Jupiters" like 51 Peg are not artifacts of the spectroscopic method as recently suggested (Gray 1997).
To "See" an extra solar planet located around a star close to the Sun, is as difficult as finding a firefly located at 40 kilometers of a car light directed at the observer, the whole being 80 000 kilometers away from the observer (1 fifth of the Earth-Moon distance).
In other words, one has to locate an object 10 million times less shiny than its neighbor, located at 0.1 second of angle of its neighbor, and distant from the observer as a nearby star is from the Sun.
The most used method until now, which allowed already the discovery of several dozens planets are indirect methods: planets are detected by measuring their gravitational impact on the nearby star, This method does not allow to "see" the planet, it allows to detect that there is a planet or several planet and to get an indication on its or their minimal weight.
The occultations of a star by one of its planet in orbit around her are observed.
The observation strategy is to photometricaly keep watch over stars for several revolution periods of the searched planets. 3 periods minimum are needed to obtain a firm detection.
The motion of a planet around its parent star can modulate the stellar luminosity observed at the Earth. The detection probability is 100 %in this case, while it is only 1 or at best 10 % with the occultations method. Yet, this detection method is only optimal for planets close to their star. It is more efficient than the occultations method for orbital distances less than to 0,2 UA.
The "InfraRed Space Interferometry Mission" DARWIN (IRSI or DARWIN for short) is a cornerstone candidate in the ESA Horizon 2000+ science plan. The goals for this mission is for the first time to detect terrestrial planets in orbit around other stars than our Sun and to allow, also for the first time, high spatial resolution imaging in the approximately 6-30 µm wavelength region. Selection for a launch after 2009 on cost, science and technology grounds will be made around 2000.
Goals are:
An artist view of the Darwin project's Infrared Interferometry telescopes in space.
