on Extrasolar Planets
Defintion of a "Planet"
POSITION STATEMENT ON THE DEFINITION OF A "PLANET"
WORKING GROUP ON EXTRASOLAR PLANETS (WGESP)
OF THE INTERNATIONAL ASTRONOMICAL UNION
February 28, 2001
Modified: February 28, 2003
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
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
as we learn more about the census of low-mass companions, the WGESP
has agreed to the following statements:
1) Objects with true masses below the limiting mass for thermonuclear
fusion of deuterium (currently calculated to be 13 Jupiter masses for
objects of solar metallicity) that orbit stars or stellar remnants are
"planets" (no matter how they formed). The minimum mass/size required
for an extrasolar object to be considered a planet should be the same
that used in our Solar System.
2) Substellar objects with true masses above the limiting mass for
thermonuclear fusion of deuterium are "brown dwarfs", no matter how
formed nor where they are located.
3) Free-floating objects in young star clusters 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
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.