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What is a Planet?
The problem: the term “planet” has been around for thousands
of years. Until recently, no one had ever bothered to formally
define what makes a planet a planet.
The International Astronomical Union (IAU) decided
in 2006 that a planet was a celestial body that:
1. Is in orbit around the Sun,
2. Has sufficient mass for its self-gravity to overcome rigid
bodily forces so that it assumes a hydrostatic equilibrium
(nearly round) shape, and
3. Has cleared the neighborhood around its orbit.
The planets of our solar system can be
divides into two groups:
I. The telluric planets are spherical bodies
with a crust of rock (Mercury, Venus,
Earth and Mars).
II. The gas giant (or “Jovian”) 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 & ices.
A dwarf planet is a celestial body that:
1. Is in orbit around the Sun,
2. Has sufficient mass for its self-gravity to
overcome rigid bodily forces so that it assumes
a hydrostatic equilibrium (nearly round) shape,
and
3. Has not cleared the neighborhood around its
orbit, and
4. Is not a satellite.
Dwarf planets include (but are not limited to) Pluto,
Ceres and Eris, others may be added after further study.
Satellites (moons) are celestial bodies orbiting
another celestial bodies of larger size.
Exoplanets (extrasolar planets) are planets
that are orbiting a star different from our
Sun or a stellar remnant.