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Mystery of how planets form in binary star systems solved by Cambridge and Max Planck researchers

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Astronomers have explained how planets form in binary star systems.

The researchers, from the University of Cambridge and the Max Planck Institute for Extra-terrestrial Physics, described how exoplanets in such systems - such as the ‘Tatooine’ planets spotted by NASA’s Kepler Space Telescope - are able to to develop without being destroyed in their chaotic birth environment.

An artist's impression of a planet in a binary star system
An artist's impression of a planet in a binary star system

They studied a binary system where the smaller companion star orbits the larger ‘parent’ star roughly once a century. Alpha Centauri - our nearest neighbour at just 4.37 light years from the Sun - is an example of one.

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

Planet formation is believed to begin in a protoplanetary disc – primarily made of hydrogen, helium and tiny particles of ices and dust – orbiting a young star. Our understanding of ‘core accretion’ suggests the dust particles stick to each other, gradually former ever larger solid bodies. If the process stops in time, a rocky Earth-like planet forms. If it grows bigger, its gravity traps huge amounts of gas from the disc adna gas giant like Jupiter is formed.

“This theory makes sense for planetary systems formed around a single star, but planet formation in binary systems is more complicated, because the companion star acts like a giant eggbeater, dynamically exciting the protoplanetary disc,” said Dr Rafikov.

Dr Kedron Silsbee, from the Max Planck Institute for Extra-terrestrial Physics, adds: “In a system with a single star the particles in the disc are moving at low velocities, so they easily stick together when they collide, allowing them to grow. But because of the gravitational ‘eggbeater’ effect of the companion star in a binary system, the solid particles there collide with each other at much higher velocity. So, when they collide, they destroy each other.”

The researchers discovered that planets can form in binary systems if the planetary building blocks, known as planetesimals, are at least 10km in diameter, and the protoplanetary disc is relatively circular.

Under these conditions, the planetesimals in certain parts of the disc move slowly enough relative to each other that they stick together instead of destroying each other.

The researchers used a series of simulations and developed a mathematical model of planetary growth in a binary system that uses realistic physical inputs and accounts for processes often overlooked, such as the gravitational effect of the gas disc on the motion of planetesimals within it.

The research supports the idea of a mechanism of planetesimal formation known as the streaming instability being an integral part of the planet formation process.

This collective effect involves many solid particles in the presence of gas, which concentrates pebble to boulder-sized dust grains to produce a few large planetesimals that would survive most collisions.

In future, the model could help explain the origin of the ‘Tatooine’ planets – exoplanets orbiting both components of a binary. About a dozen of these have been identified by NASA’s Kepler Space Telescope.

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