Artists impression of Alpha Centauri B. Credit: ESO/L. Calada/N. RisingerAstronomers have created the most accurate model of planet formation in binary star system systems to date.Researchers from the University of Cambridge, Max Planck Institute for Extraterrestrial Physics and the University of Cambridge have demonstrated how exoplanets within binary star systems, such as the 'Tatooine’ planets detected by NASA's Kepler space telescope, came into existence without being destroyed in chaotic birth environments.They looked at a binary system in which the smaller companion star orbits its parent star about once every 100 years. Alpha Centauri is an example of such system."A system such as this would be the equivalent of a Sun where Uranus' is, which would make our solar system very different," stated Dr. Roman Rafikov, co-author from Cambridge's Department of Applied Mathematics & Theoretical Physics.Rafikov and his co-author Dr. Kedron Silsbee from the Max Planck Institute for Extra-terrestrial Physics found that for planets to form in these systems, the planetesimalsplanetary building blocks which orbit around a young starneed to start off at least 10 kilometres in diameter, and the disc of dust and ice and gas surrounding the star within which the planets form needs to be relatively circular.Astronomy & Astrophysics published the research. It explains how these planets could have formed, and provides a detailed explanation for why they were discovered.The protoplanetary disc, which is primarily composed of hydrogen, helium and small particles of ices, dust, orbiting a young star, is thought to be the place where planet formation begins. Core accretion is the most current theory about how planets form. It states that dust particles stick together and eventually form larger solid bodies. The result could be similar to a rock-earth planet. If the planet becomes larger than Earth, its gravity will be strong enough to trap large amounts of gas from the disc. This can lead to the formation of a giant gas star like Jupiter.Rafikov stated that this theory is valid for planetary systems that are formed around one star. However, planet formation in binary systems can be more difficult because the companion star acts as a giant eggbeater, stimulating the protoplanetary disk.Silsbee said, "In a system that has a single star, the particles in the disk are moving at low velocity, so they stick together when they collide. This allows them to grow." The gravitational eggbeater effect of the companion star in binary systems causes the solid particles to collide at a much faster velocity because they are more dense. They collide and destroy one another.Exoplanets from many other planets have been detected in binary systems. The question now is how they got there. Some astronomers even suggest that these planets may have been floating in interstellar spaces and were sucked into the gravity of a binary.Rafikov, Silsbee performed a series simulations to solve the mystery. They created a mathematical model of planetary development in a binary using realistic physical inputs. This model also accounts for processes that are often overlooked like the gravitational effect that the gas disc has on the motions of planetesimals within the disk.Silsbee stated that the disc has been shown to directly impact planetesimals via gas drag. It acts like a wind. "A few years back, we realized that the disc's gravity also dramatically altered the dynamics of planetesimals. In some cases, this allowed planets to form despite the gravitational perturbations caused by the stellar companion.Rafikov stated that the model he's built combines this and other work to test the theories of planet formation.The model showed that planets can be formed in binary systems like Alpha Centauri. This is provided that the planetesimals are at least 10 kilometers across and the protoplanetary disc is nearly circular without any major irregularities. These conditions are satisfied and the planetesimals within certain areas of the disc move slowly enough that they stick together rather than destroying one another.These results support the notion that streaming instability is an integral part in planet formation. This is a group effect that involves many solid particles in the presence gas. It is capable of concentrating small dust grains into planetesimals. These would be able to survive most collisions.These results provide valuable insights into the formation of planets around binary and single stars as well as hydrodynamic simulations of protoplanetary disks in binaries. The model could be used in the future to explain the origins of the 'Tatooine’ planetsexoplanets that orbit both the binary components, about a dozen of them having been identified by NASA’s Kepler Space Telescope.Discover more Giant Planets at a large distance from star-like puzzles by astronomers
