II.

Occurrence and Formation of Extrasolar Planets

Astronomers did not develop reliable techniques to find extrasolar planets until the 1990s, but more than ten new solar systems were discovered within the first few years of searching. Many astronomers believe that almost every Sun-like star has a solar system at some point in its development.

Astronomers have found that most young stars are surrounded by disks composed of dust and gas. Some of these disks show evidence of comet-like objects. Some, such as the disk around the star Fomalhaut, show an empty area around the star, or a stripe of empty space in the disk. Astronomers believe that dust in this area could be in the process of condensing into a planet.

By studying our solar system, astronomers developed a theory of how solar systems form known as the core accretion theory. However, many of the extrasolar planets and disks of dust around other stars do not seem to conform to this theory. The theory states that a disk of dust and gas collects around a star as the star forms. Bits of dust in the disk collide and stick together, forming larger and larger chunks of rock and ice. Farther out from the star (where the temperature is cooler), the gases in the disk freeze, adding to the mass available to form these chunks.

The pieces of rock continue to collide, forming large objects called protoplanets. The protoplanets far from the star are far larger than those closer to the star because of the increased amount of frozen gas material available. Sometimes protoplanets crash together, breaking apart and starting the process of formation all over again. At some point during the last part of the planet formation process, the star goes through a stage in its own evolution in which it blasts away the free gas that remains in the inner solar system.

If the protoplanets in the outer solar system are large enough, their gravitational pull grabs this gas and pulls it in toward the protoplanet. These outer protoplanets then become gas-giant planets, with deep layers of dense gas covering their cores. The smaller inner planets lose any gas that surrounds them. Small planets, such as Earth, that have atmospheres develop them later, when volcanic activity releases gases from within the planet. The star settles into a long quiet period, and the protoplanets grow into planets and develop regular orbits.

Many of the solar systems that astronomers have discovered contain very large planets very close to their host star. Systems like these are probably easier to detect with current methods than systems that resemble our own, so they may seem more common than they really are. Still, the core accretion theory does not explain such systems. Several popular possible revisions to the theory exist. It may be common for very large planets to form far from the star and then be drawn in closer by the gravitational pull of the star. Another possibility is that some situations allow a very large planet to form very close to a star. However, the reported discovery in 2006 of a planet smaller than Neptune orbiting a star at a distance of about 2 astronomical units (AUs) provides support for the core accretion theory and suggests that the very large planets orbiting very close to their stars are anomalous rather than common.