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What is an exoplanet?
The planets of the solar system
The two families of planets in the solar system
The solar system refers to the set of celestial
bodies bound by the gravitational attraction of the
Sun. Among these bodies, we can distinguish :
the five planets visible to the naked eye and
known since antiquity : Mercury, Venus, Mars,
Jupiter and Saturn;
two planets discovered more recently through
the use of a telescope : Uranus (1781) and
Neptune (1846);
and several dwarf planets, such as Ceres
(1801), Pluto (1930) and Eris (2003).
Altogether there is a total of eight planets (Earth
included) orbiting the Sun.
Copyright : NASA / JPL and Paris Observatory / UFE
The planets of the solar system can be divided into two groups, the telluric planets and the gas giant (or
"Jovian") planets. The telluric planets are spherical bodies with a crust of rock, and the gas giant planets
are spheres composed of gas and ice (Jupiter, Saturn, Uranus and Neptune). Pluto is a solid body, whose
composition consists of a variety of rocks and ices.
Contrary to the Sun and the stars, thermonuclear fusion reactions do not occur in planets (nor do they
occur in other bodies in the solar system). The planets are visible because they reflect sunlight.
Exoplanets : planets outside the solar system
An exoplanet (or extrasolar planet) is a planet orbiting a star different from the Sun (the "exo" prefix
means "outside" in Greek). Up until now, one has found mainly gas giant planets, which are easier to detect
than telluric planets. However, due to the increasing sensitivity of the detection methods, one already
begins to observe the first planets of sizes comparable to the Earth.
The definition of a planet
The recent discovery of numerous objects with
different properties leads one to question the way in
which to define a planet.
The name "planet" has historical origins. Up until
2006, it was arbitrary in its restriction to the nine
planets of the solar system. Pluto was among the
bodies called "planets" while Eris wasn't among them,
even though it is more massive than Pluto.
At the time of its discovery in 1801, Ceres was
not defined as a planet because it looked like
a star under a telescope. It is now considered
to be a large asteroid (diameter of about 933
km), within a family of small bodies mainly
located between Mars and Jupiter.
Pluto and Charon
Though Pluto has a similar appearance on
Copyright : NASA / HST
photographic plates, Pluto was first called a
planet in 1930, because it was thought massive enough to disturb Neptune's motion. It was soon
realized that this was not the case.
Recently, on has discovered that Pluto could also be considered one of the larger known objects of
the Kuiper belt, a family of small bodies located outside the orbit of Neptune.
The International Astronomical Union (IAU) decided in 2006 to change the definition of a planet in order
to solve the problems arising with Pluto and Eris, and to use a definition based on physical concepts instead
of being purely arbitrary. One now defines a planet as a celestial body that :
is in orbit around the Sun,
has sufficient mass for its self-gravity to overcome rigid bodily forces so that it assumes a hydrostatic
equilibrium (nearly round) shape, and
has cleared the neighbourhood around its orbit.
A dwarf planet is a celestial body that :
is in orbit around the Sun,
has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic
equilibrium (nearly round shape),
has not cleared the neighbourhood around its orbit, and
is not a satellite.
It is important to notice that, according to this definition, a dwarf planet is not a planet ! One can
also notice that this definition does not apply to exoplanets, because they do not orbit around the Sun. In
other words, according to this definition, an exoplanet (or extrasolar planet ) is not a planet despite
the fact that an exoplanet is often described as being a planet orbiting a different star than the Sun.
The IAU Working Group on Extrasolar Planets has not yet decided on a definitive definition of the word
exoplanet . Its "working definition" (which dates from 2003) is the following : an exoplanet
is an object with a true mass below the limiting mass for thermonuclear fusion of deuterium
(currently calculated to be 13 Jupiter masses for objects of solar metallicity),
is in orbit around a star or stellar remnant,
has a mass and/or size that is superior to the one used as a limit for a planet in our Solar System.
Unfortunately, the definition of a planet in the Solar System does not specify a limit in mass or size, so
this definition cannot be used precisely. One could hope for a more universal definition of the word "planet",
that could be applicable to bodies detected around other stars than our Sun. In fact, with current detection
methods, it is in general impossible to find out if an exoplanet is spherical or if it has cleared the
neighbourhood around its orbit.
The distinction between planets and stars has also been found to be rather vague since the discovery of
the brown dwarfs.