Science/Planets

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A planet is a body which has sufficient mass that its own gravity makes it round and that has cleared its neighbourhood of smaller objects. There is one underneath you right now.

Planets-scale.jpg

Formation

Current thinking is that planets are formed in a stellar accretion disk. A nebula collapses into a disk of gas and dust, and at its centre a protostar forms. The remaining material is called a protoplanetary disk.

Dust collides and begins to form larger masses through the process of accretion. Mass concentrates in areas orbiting the star and eventually collapses inwards under its own gravity, producing protoplanets.

When the protostar ignites and becomes a star, it emits a solar wind and enormous numbers of photons which evaporate and blow away much of the remaining dust. Protoplanets will remain, and may collide or be gravitationally captured by other protoplanets, becoming moons. Asteroids and small bodies will impact any planetary surfaces, causing them to become very hot. Denser materials within protoplanets move towards their cores, so differentiating the interior of the planet by mass.

Gas giant planets are sufficiently large and cold that they can hold on to an atmosphere of hydrogen and helium, often of far greater mass than the central, rocky core. They can only form where ices are possible, though they can then migrate inwards to become 'hot Jupiters'.

Terrestrial planets cannot hold on to a hydrogen atmosphere. Under a certain size or within a certain distance from the star, their atmospheres will escape by various mechanisms. Volcanism replaces this with an atmosphere of mainly nitrogen and carbon dioxide, as well as providing large quantities of water. On some planets, oxygenic life will arise and begin to oxygenate the atmosphere. An ozone layer will then form, which prevents the loss of the planet's oceans.

Factors affecting the planetary system include the mass of the accretion disk, stellar class, metallicity and whether the star system has more than one star.

Types

  • Terrestrial planets are primarily composed from rock and iron. In our Solar System, we have Mercury, Venus, Earth and Mars.
  • Gas giants are mainly gases (largely hydrogen and helium) by mass. They are much larger than terrestrial planets. Ice giants may be a subcategory, which have significantly lower masses, less hydrogen and helium, and more rock and ice. Jupiter, Saturn, Uranus and Neptune are our gas giants.

There are also dwarf planets, which are not true planets as they are not gravitationally dominant in their orbits.

Prevalence

Most star systems are believed to possess planets. Our detection methods mean that high-mass planets very close to their stars are the easiest to detect, so our sample is skewed, but planets of all kinds have been detected around other stars.