a montage of planetary images taken by spacecraft managed by the Jet Propulsion Laboratory in Pasadena, CA. Included are (from top to bottom) images of Mercury, Venus, Earth (and Moon), Mars, Jupiter, Saturn, Uranus and Neptune. (Credit: NASA/JPL)

The research was aimed at finding the connection between the properties of the proto-stellar disk and those of its progeny, the planets. A set of 100 simulations, based on data from over 250 planetary systems and spanning 500 million years, was carried out, covering a range of parameters. The two main parameters were the initial disk mass and its viscosity, which determines the rate of gas accretion onto the central star. 

The simulations show that the proto-stellar disk pushes the planets towards the star. As the planets approach each other, their interaction often results in an elongation of their orbits. Occasionally the outcome of these interactions is the flinging of a planet to a different location in the system, or even into deep space. Eventually the gas in the disk is all consumed and the planetary system emerges.

Due to the complexity of the developing system, which includes the disk-planet and planet-planet interactions described, the simulations resulted in random systems. Nevertheless, two dominant cases were detected. 

In a disk with low mass and high viscosity, the gas in the disk is removed before a planet can form, resulting in a system that has only rocky, icy bodies. At the other end, in a disk with high mass and low viscosity, planets are formed but are pulled towards the center of the system and acquire highly elliptical orbits around the star. 

In the intermediate case, planets form but undergo only modest migration towards the star and their orbits don't become as elliptical. This seems to be the case of the solar system. The simulation showed that this case is realized in a small number of systems, meaning the solar system does not resemble most planetary systems. 

One of the scientists who conducted this research, Frederic A. Rasio from Northwestern University in Illinois, said: “We now better understand the process of planet formation and can explain the properties of the strange exoplanets we’ve observed. We also know that the solar system is special and understand at some level what makes it special. The solar system had to be born under just the right conditions to become this quiet place we see. The vast majority of other planetary systems didn’t have these special properties at birth and became something very different.” 

TFOT has reported on the discovery of three planets orbiting a sun 1,000 light years away from earth, on the discovery of a planetary system that may have four suns, and on sixteen extra-solar planet candidates discovered by the Hubble Space Telescope. TFOT also covered the finding of an earth-like planet outside the solar system, which may contain water. 

Further information on the new research, which was published in Science magazine, can be found in the Arxiv website (PDF).