Planetary System - Origin and Evolution

Origin and Evolution

Early theories of planetary systems were based on knowledge of the formation and evolution of the Solar System. Planetary systems are generally believed to form as part of the same process which results in star formation. Some early theories involved another star passing extremely close to the star, drawing material out from it which then coalesced to form the planets. However, the probability of such a near collision is now known to be far too low to make this a viable model. Accepted theories today argue that a protoplanetary disk forms by gravitational collapse of a molecular cloud and then evolves into a planetary system by collisions and gravitational capture.

Some planetary systems may form differently, however. Planets orbiting pulsars—stars which emit periodic bursts of electromagnetic radiation—have been discovered by the slight variations they cause in the timing of these bursts. Pulsars are formed in violent supernova explosions, and a normal planetary system could not possibly survive such a blast—planets would either evaporate, be pushed off of their orbits by the masses of gas from the exploding star, or the sudden loss of most of the mass of the central star would see them escape the gravitational hold of the star. One theory is that existing stellar companions were almost entirely evaporated by the supernova blast, leaving behind planet-sized bodies. Alternatively, planets may somehow form in the accretion disk surrounding pulsars.

The formation of planetary systems has also been found to be linked to stellar classification. Stars are composed mainly of the light elements hydrogen and helium. They also contain a small proportion of heavier elements such as iron, and this fraction is referred to as a star's metallicity. Stars of higher metallicity are much more likely to form and retain planetary systems, and the planets within them have tend to be more massive than those of lower-metallicity stars. It has also been shown that stars with planets are more likely to be deficient in lithium. Recent observations by the Spitzer Space Telescope indicate that planetary formation does not occur around other stars in the vicinity of an O class star due to the photoevaporation effect.

Models of the Solar System point to material being ejected during the formation and early evolution as the system stabilised. Similar interactions are thought to be common to many planetary systems. The orbits of large bodies are being established, they may migrate and cause planetary body collisions. Others may be sufficiently perturbed to be ejected completely from the system. These ejected bodies are known as rogue planets and may retain their natural satellites. Systems such as these in interstellar space, known as planetars may orbit the galaxy directly and although difficult to detect are now thought to be extremely common.