Working Group on Extra-Solar Planets

Groupe de Travail sur les planètes extra-solaires

Members of Working Group

Chairman: Alan Boss
Members: Paul Butler, William Hubbard, Philip Ianna, Martin Kurster, Jack Lissauer, Michel Mayor, Karen Meech, Francois Mignard, Alan Penny, Andreas Quirrenbach, Jill Tarter, Alfred Vidal-Madjar


The working group is charged with acting as a focal point for research on extra solar planets and organizing IAU activities in the field, including reviewing techniques and possibly maintaining a list of identified planets. The details of the Terms of Reference are available.



February 28, 2001

Rather than try to construct a detailed definition of a planet which is designed to cover all future possibilities, the WGESP has agreed to restrict itself to developing a working definition applicable to the cases where there already are claimed detections, e.g., the radial velocity surveys of companions to (mostly) solar-type stars, and the imaging surveys for free-floating objects in young star clusters. As new claims are made in the future, the WGESP will weigh their individual merits and circumstances, and will try to fit the new objects into the WGESP definition of a "planet", revising this definition as necessary. This is a gradualist approach with an evolving definition, guided by the observations that will decide all in the end.

Emphasizing again that this is only a working definition, subject to change as we learn more about the census of low-mass companions, the WGESP has agreed to the following statements:

1) Objects orbiting around solar-type stars with true masses above the limiting mass for thermonuclear fusion of deuterium (currently calculated to be 13 Jupiter masses for objects of solar metallicity) are "brown dwarfs" (no matter how they formed) while objects with true masses below this limiting mass are "planets".

2) Free-floating objects in young star clusters (which presumably formed in the same manner as stars and have not been shown to be ejected from planetary systems) with masses below the limiting mass for thermonuclear fusion of deuterium are not "planets", but are "sub-brown dwarfs" (or whatever name is most appropriate).

These statements are a compromise between definitions based purely on the deuterium-burning mass or on the formation mechanism, and as such do not fully satisfy anyone on the WGESP. However, the WGESP agrees that these statements constitute the basis for a reasonable working definition of a "planet" at this time. We can expect this definition to evolve as our knowledge improves.

Note that these statements are restricted to extrasolar planets and are not intended to address the question of a possible lower mass limit for "planets" in our Solar System.