Exoplanet 2M1207b

Giant exoplanets such as 2M1207b, seen orbiting its parent star, bear similarities to the objects called brown dwarfs. Credit: ESO

Planetary science

What big planets have in common with failed stars

‘Gas giant’ exoplanets have an uncanny resemblance to galactic oddities called brown dwarfs.

Giant planets probably share a fundamental property with slightly larger bodies called brown dwarfs.

In the past decade, astronomers have found several examples of big planet-like bodies — those measuring up to 13 Jupiter masses  — orbiting young stars. But researchers have struggled to determine whether they formed by coalescing from a disk of gas and dust swirling around the star — as the planets in our Solar System did — or by clumping together directly from a gaseous cloud, as brown dwarfs do. Brown dwarfs are not big enough to produce their own light, earning them the nickname ‘failed stars’.

A team led by Marta Bryan at the California Institute of Technology in Pasadena used the Keck II telescope in Hawaii to examine three planet-like objects, each less than 13 Jupiter masses, located far from their stars. The scientists compared the rotation rates of those objects with the spins of six brown dwarfs up to 20 Jupiter masses in size. All shared some characteristics, such as a stable rotation rate that has probably endured for billions of years.

The results show that the same physical processes could drive the rotation rates of both big planets and brown dwarfs, regardless of how they formed. That gives astronomers a new tool for exploring how gas accretes onto both types of object as they grow.