A trio of super-Earths found around a nearby star – one of which may be the densest ever discovered – could help astronomers to understand why some planets are rocky and others gassy.

Astronomers have found that planets with radii greater than around 1.7 times that of Earth tend to have densities that suggest they have a gassy envelope, whereas smaller ones tend to be rocky.

The triplet planets, found around the star GJ 9827, have radii of 1.64 (planet b), 1.29 (planet c) and 2.08 (planet d), meaning they span across this turn over point.

This unique spread of planets around one star could be used to study the processes that create rocky planets, such as photoevapouration where radiation from the star strips away the planetary gases.

The next task will be to determine the composition of these worlds via their densities, which requires knowing their masses.

Astronomers were able to determine the masses of the three planets very quickly, as GJ 9827 was one of the many bright stars that was being studied by the Planet Finding Spectrograph at the Carnegie Institution for Science.

“Usually if a transiting planet is detected, it takes months if not a year or more to gather enough observations to measure its mass.

Because GJ 9827 is a bright star, we happened to have in the catalogue of stars that Carnegie astronomers have been monitoring for planets since 2010,” says Johanna Teske from the Carnegie Institution for Science, who led the research.

The mass of planet b was found to be eight times that of Earth, making it one of the densest super-Earth’s yet discovered. This suggests it could be around 50 per cent iron.

The masses of c and d are thought to be around 2.5 and four times the mass of the Earth respectively, but these measurements are still quite uncertain.

“More observations are needed to pin down the compositions of these three planets,” says Sharon Wang.

“But they do seem like some of the best candidates to test our ideas about how super-Earths form and evolve, potentially using NASA’s upcoming James Webb Space Telescope.”


Ezzy Pearson is the News Editor of BBC Sky at Night Magazine. Her first book about the history of robotic planetary landers is out now from The History Press.