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About Stars: Planets

Many stars, as our Sun, have planets. But these are difficult to discover for us, because planets don't shine by their own and therefore are very dim and close to a much brighter star. They are more easy to find when certain irregularities in the proper motion of the star or fluctuations of its luminosity caused by an eclipse are discovered. By those one or more partners and their quality can be implied.
The most planets that have been discovered thus are gaseous giants like our Jupiter or Saturn. These are bigger than terrestrial planets as Earth or Mars and therefore easier to find. But which kind of planet is more frequent can't be decided yet.

Planets evolve from the dust disks that surround many young stars. Small, solid particles collide and pack together, first to kilometer sized chunks, so-called planetesimals, later to protoplanets, whose gravitational force even can hold gas around them. So the biggest planets become gaseous giants by drawing in the interplanetary matter, to a large part hydrogen and helium.

Planets outside our solar system, orbiting other stars, are called exoplanets. These, like our planets, could have moons which we cannot discover yet.
Each star has a life zone in which planets would have the right temperature for life as we know. Gaseous giants are unsuitable for life but they could have moons with better conditions.

Example: HD 70642

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    An exoplanet
An exoplanet.
Graphic: NASA/JPL-Caltech/T. Pyle (SSC)

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