Brown Dwarfs and Planetary Systems
Sub-stellar companions can be divided in two classes, namely planets and brown dwarfs. There exist three major genesis indicators that can help classify sub-stellar objects as either brown dwarfs or planets: mass, shape and alignment of the orbit, and composition and thermal structure of the atmosphere. Mass alone cannot be decisive for the classification of a low-mass companion to a star - to date, theory has not been able to establish either a firm lower limit to the mass of a brown dwarf or the upper bound to the mass of a planet.
Correlations between eccentricity and the logarithm of orbital period for pre-main and main sequence binaries and for objects thought to be the result of accretion in a disc (such as the giant planets in the Solar System) are significantly different. Most of the candidate planets discovered so far by the radial velocity programmes appear to follow the (e, log P) relation for pre-main sequence binaries. Low eccentricity alone is not sufficient to classify a newly discovered low-mass companion as a planet, since low eccentricity is also expected for stellar companions orbiting close to the other star.
Brown Dwarfs in Binaries
An isolated brown dwarf is typically visible only at ages < 1 Gyr because of the rapidly fading luminosity (see the Stellar Astrophysics section for further information on Isolated Brown Dwarfs). The same constraint applies if we want to observe it as a resolved component of a binary system. In a binary system, the mass is conserved, and the gravitational effects on a main sequence secondary remain observable over much longer intervals. Gaia will have the power to investigate the mass-distribution of brown dwarf binaries with 1-30 year periods, through analysis of the astrometric orbits.
At present there are more than 100 candidate extra-solar planets, including the triple planet systems of Upsilon Andromedae and 55 Cancri. These have minimum masses of the order of 0.1 to 10 Jupiter mass. These discoveries have raised new questions in our understanding of the properties of planetary systems. Several of these candidate planets have characteristics that are hard to explain within the context of current theoretical models for the formation and evolution of planetary systems. Present theories are based on nearly circular orbits and giant planets formed at several AU from the central star, in contrast with the very short orbital periods and high eccentricities found for several candidate planets.
A better understanding of the conditions under which planetary systems form and of their general properties requires sensitivity to less massive planets, better characterization of known systems (mass and orbital elements) and complete samples of planets, with useful upper limits on Jupiter-mass planets up to several AU from the central star. Astrometric measurements good to 2-10 microarcsec, will contribute substantially to these tasks. Gaia's strength will be its discovery potential following from the astrometric monitoring of all of the several hundred thousand bright stars out to distances of about 200 pc.
The ability of Gaia to simultaneously and systematically determine planetary frequency and distribution of orbital parameters for the stellar mix in the Solar neighbourhood is a fundamental contribution that Gaia will uniquely provide, the only limitations being those intrinsic to the mission i.e. to the actual sensitivity of the Gaia measurements to planetary perturbations.
Gaia will play an important, though indirect, role in the quest for habitable Earth-size planets. These should be found around Solar type stars with a Jupiter-like planets orbiting at a distance of more than 3 AU, effectively protecting terrestrial planets from cometary impacts. Gaia measurements are particularly sensitive to such systems thanks to the mission duration and the favourable magnitude of Solar-type stars up to about 200 pc from the Sun (brighter than V = 12 mag). Gaia could provide us with a quantitative estimate of the likelihood that inhabitable Earths exist in the Solar neighbourhood.
Simulations indicate that Gaia will detect all existing Jupiter-mass planets within 50 pc and with periods between 1.5 and 9 years. The range of periods which can be probed narrows with increasing distance. There is good overlap with the range of periods probed by spectroscopy. Studies using a standard Galaxy model and planetary frequencies from recent radial velocity surveys yield estimates for the number of detections of Jupiter mass planets somewhere between 10 000 and 50 000, depending on details of the detection or orbital distribution hypotheses.
We know of at least two candidate extra-solar multi-planet systems (UpsAnd and 55 Cnc) at present. Establishing the frequency of such systems is important to further our understanding of planetary systems. The possible discovery of several multi-planet systems will also provide an understanding of whether the apparent regularity of the spacing of the planets in the Solar System is a common feature of planetary systems.
The knowledge of the parameters of the main planets of a multi-planet system is very important, as they affect dynamically all the remaining bodies by gravitational interactions. When these planets are known, it is possible to conduct stability analysis of the possible orbits for other smaller bodies in the system. In this way, it will be possible to assess the existence of stable zones for Earth-like planets. By contrast, if the habitable zone is largely chaotic due to the presence of the large planets, this will dismiss the possibility of finding planets similar to the Earth in the corresponding planetary system. Gaia's survey will probably find a number of such multi-planet systems thus identifying possible locations of Earth-like planets.
Extra-solar planetary occultations for planets with small orbital radius have been shown to be clearly evident in the Hipparcos epoch photometry data. Occultations lead to estimates of the orbital inclinations, hence masses, and the planet radius. It is expected that the Gaia photometric data base may contain several thousand such occultations.