What is a Planet?

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The universe is composed of trillions of galaxies, each of which is comprised of billions of stars. Around these stars,Artists impression of HD 209458b, located 150 ly away.
any number of planets could be orbiting. Where stars are each massive balls of superheated plasma, converting
hydrogen into helium, planets are a much more varied class of being.

In our own solar system, two classes of planets exist. Close to the sun are the terrestrial planets; Mercury, Venus,
our own Earth, and Mars. They are all rocky bodies with limited atmospheres. Beyond are four more planets;
Jupiter, Saturn, Uranus, and Neptune. All four have dense atmospheres that surround massive cores. A major
question of planet formation was why these planets were like this.

It was eventually proposed that some stars formed with discs of material around them. As the baby star developed,
gravity would cause the material to lump into planets. In the inner region of a star system, the star would have already
swallowed the hydrogen, and there would also be less material to collect, creating smaller planets with small
atmospheres. In further orbits, more material and hydrogen would be available for collection and gas giants would
form.

Of course, not all planets stay in the same location. Some planets have been ejected from their solar systems, others
have had orbital decay, and now orbit their star at ridiculously close orbits, taking less than a week to rotate. But all
planets have a central core of heavy composition, and none of them convert hydrogen, lacking the mass.