What are planets made of?
Earth and the other three inner planets of our solar system (Mercury, Venus and Mars) are made of rock, containing common minerals like feldspars and metals like magnesium and aluminum. So is Pluto. The other planets are not solid. Jupiter, for instance, is made up mostly of trapped helium, hydrogen, and water. In our solar system, the four "gas giants" are Jupiter, Saturn, Neptune, and Uranus.Planets with their relative sizes to scale. The tiny bluish dot at the lower left is Earth. The tiny red dots to either side are Venus and Mars. Mercury and Pluto are pinpoints at opposite corners.Image from The Nine Planets, a Multimedia tour of the Solar System by Bill Arnett http://seds.lpl.arizona.edu/nineplanets/nineplanets/nineplanets.html
Scientists define planets as dark bodies that orbit around stars. Dark bodies are objects in space that do not release enough light to be visible to the eye. We can still see the planets (and our moon) in the night sky because light from the Sun reflects off of them.
20th century
| 1 Mercury  | 2 Venus  | 3 Earth  | 4 Mars  | 5 Jupiter  | 6 Saturn  | 7 Uranus  | 8 Neptune  |
In the 20th century, Pluto was discovered. After initial observations led to the belief that it was larger than Earth, the object was immediately accepted as the ninth planet. Further monitoring found the body was actually much smaller: in 1936, Ray Lyttleton suggested that Pluto may be an escaped satellite of Neptune, and Fred Whipplesuggested in 1964 that Pluto may be a comet. As it was still larger than all known asteroids and seemingly did not exist within a larger population, it kept its status until 2006.
| 1 Mercury | 2 Venus | 3 Earth | 4 Mars | 5 Jupiter | 6 Saturn | 7 Uranus | 8 Neptune | 9 Pluto  |
In 1992, astronomers Aleksander Wolszczan and Dale Frail announced the discovery of planets around a pulsar, PSR B1257+12. This discovery is generally considered to be the first definitive detection of a planetary system around another star. Then, on October 6, 1995, Michel Mayor and Didier Queloz of the Geneva Observatory announced the first definitive detection of an exoplanet orbiting an ordinary main-sequence star (51 Pegasi).
The discovery of extrasolar planets led to another ambiguity in defining a planet: the point at which a planet becomes a star. Many known extrasolar planets are many times the mass of Jupiter, approaching that of stellar objects known as brown dwarfs. Brown dwarfs are generally considered stars due to their ability to fuse deuterium, a heavier isotope of hydrogen. Although objects more massive than 75 times that of Jupiter fuse hydrogen, objects of only 13 Jupiter masses can fuse deuterium.
Deuterium is quite rare, and most brown dwarfs would have ceased fusing deuterium long before their discovery, making them effectively indistinguishable from supermassive planets.
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