Planets, Extrasolar

The term extrasolar planets or exoplanets stands for planets outside our solar system—that is, planets that orbit other stars, not our sun. Planets in our solar system are defined as objects with enough mass to be spherical and round by their own gravity and to be alone on their orbit around the sun; in other words, to be the dominant object in a particular orbit and not to be a moon or asteroid

Most exoplanets are discovered by observing the stellar motion around the common center of mass of the combined star-and-planet system, that is, by observing somehow the motion of the objects in orbit around each other. This is typically done by measuring precisely the periodic variation of certain values, such as radial velocity or brightness, with time. For example, the first extrasolar planets were found with this timing technique around a pulsating neutron star.

The recent definition of “planets of our solar system” by the International Astronomical Union deals mainly with the question of the minimum mass for an object to qualify as planet and excludes Pluto. This matter was raised by the fact that more and more objects similar to Pluto were discovered by larger and larger telescopes. The questions of maximum mass and formation of planets were left out in this new definition, possibly partly because there is not yet a consensus in the international community. For a discussion of extrasolar planets, however, the maximum mass is very important in order to classify an object as planet or nonplanet and to distinguish between planets and brown dwarfs.

Both planets and brown dwarfs are substellar objects in the sense that they are less massive than stars so that they cannot fuse normal hydrogen (as stars do to produce energy and to shine for a long time). Brown dwarfs, while they cannot fuse normal hydrogen (which has an atomic nucleus of just one proton), can burn deuterium (heavy hydrogen, which has an atomic nucleus of a proton plus a neutron) so that they are self-luminous for a few millions of years until the original deuterium content is depleted.

The upper mass limit of planets can be defined either through the lower mass limit for deuterium fusion, which is around 13 times the mass of Jupiter (depending slightly on the chemical composition) or by the mass range of the so-called brown dwarf desert (as discussed in the following paragraphs).

We will next discuss the different exoplanet discovery techniques by chronological order of success and thereby also discuss the properties of objects found so far.