Astronomers show how planets form in binary systems without getting crushed

Astronomers have developed the most realistic model to date of the formation of planets in binary star systems.

Researchers from the University of Cambridge and the Max Planck Institute for Extra-terrestrial Physics have shown how exoplanets in binary star systems, such as the “Tatooine” planets spotted by NASA’s Kepler Space Telescope, came into being without being destroyed in their chaotic birth environment.

They studied a type of binary system in which the smaller companion star orbits the larger parent star about once every 100 years – our closest neighbor, Alpha Centauri, is an example of such a system.

“A system like this would be the equivalent of a second Sun where Uranus is located, which would have made our own solar system very different,” said co-author Dr Roman Rafikov of the Department of Applied Mathematics and of Theoretical Physics from Cambridge.

Rafikov and his co-author, Dr Kedron Silsbee of the Max Planck Institute for Alien Physics, discovered that in order for planets to form in these systems, planetesimals – planetary building blocks that orbit a young star – must start at least 10 kilometers in diameter, and the disk of dust, ice and gas surrounding the star in which the planets are formed must be relatively circular.

The research, published in Astronomy and astrophysics, takes the study of the formation of planets in binaries to a new level of realism and explains how such planets, a number of which have been detected, were able to form.

The planet’s formation is believed to begin in a protoplanetary disk, made up mostly of hydrogen, helium, and tiny particles of ice and dust, orbiting a young star. According to the current mainstream theory of how planets were formed, known as nucleus accretion, dust particles stick together, ultimately forming solid bodies that are bigger and bigger. If the process stops early, the result may be an Earth-like rocky planet. If the planet becomes larger than Earth, then its gravity is sufficient to trap a large amount of gas from the disk, leading to the formation of a gas giant like Jupiter.

“This theory makes sense for planetary systems formed around a single star, but the formation of planets in binary systems is more complicated, as the companion star acts like a giant egg beater, dynamically exciting the protoplanetary disk,” Rafikov said.

“In a system with a single star, the particles on the disk move at a low speed, so they easily stick to each other when they collide, allowing them to grow,” Silsbee said. “But because of the companion star’s ‘egg beater’ gravitational effect in a binary system, the solid particles there collide with each other at a much higher speed. collide, they destroy each other. “

Many exoplanets have been spotted in binary systems, so the question is how they got there. Some astronomers have even suggested that these planets may have been floating in interstellar space and been sucked in by the gravity of a binary, for example.

Rafikov and Silsbee performed a series of simulations to help solve this mystery. They developed a detailed mathematical model of planetary growth in a binary that uses realistic physical inputs and takes into account often overlooked processes, such as the gravitational effect of the gas disk on the movement of planetesimals within.

“The disc is known to directly affect planetesimals through gas drag, acting like a kind of wind,” Silsbee said. “A few years ago, we realized that in addition to gas drag, the gravity of the disk itself dramatically changes the dynamics of planetesimals, in some cases allowing planets to form even despite gravitational disturbances due to the stellar companion. “

“The model we built brings together this work, along with other previous work, to test the theories of planetary formation,” Rafikov said.

Their model revealed that planets can form in binary systems such as Alpha Centauri, provided the planetesimals start at least 10 kilometers in diameter and the protoplanetary disk itself is close to circular, with no major irregularities. When these conditions are right, the planetesimals in parts of the disk end up moving slowly enough relative to each other to stick together instead of destroying each other.

These findings support a particular mechanism of planetesimal formation, called flux instability, which is integral to the process of planet formation. This instability is a collective effect, involving many solid particles in the presence of gas, capable of concentrating pebble-sized dust grains on a boulder to produce a few large planetesimals, which would survive most collisions.

The results of this work provide important information for the theories of the formation of planets around binary and single stars, as well as for hydrodynamic simulations of protoplanetary disks in binaries. In the future, the model could also be used to explain the origin of the “Tatooine” planets – exoplanets orbiting the two components of a binary – of which about a dozen have been identified by NASA’s Kepler Space Telescope. .