Exoplanets: Extremely dense world with an enormous iron core has an 8-hour year

The exoplanet GJ 367b

The exoplanet GJ 367b orbits its star in an extremely short time

SPP 1992 (Patricia Klein)

An extraordinarily dense exoplanet seemingly made mostly of iron has joined a group called the ultrashort-period planets that orbit so close to their stars that their years are less than one Earth day long.

Kristine Lam, now at German Aerospace Center in Berlin, Germany, and her colleagues found this weird world, called GJ 367b, using the Transiting Exoplanet Survey Satellite. They then performed follow-up observations using the High Accuracy Radial velocity Planet Searcher at La Silla Observatory in Chile.

They found that the exoplanet has a radius about 72 per cent of Earth’s and a mass just over half our planet’s mass. With those measurements, they were able to predict its interior structure – it probably has an iron core that takes up about 86 per cent of its radius, similar to Mercury’s strangely huge iron core.

“It turns out this is one of the densest planets among the known exoplanets, and the densest and smallest of the ultrashort-period planets,” says Lam. It circles its star once every 7.7 hours and the side of the planet that faces the star reaches temperatures above 1400°C.

This is much hotter than the dayside of Mercury and close to the melting point of iron. That could help explain why so much of the planet is taken up by its core, when most other relatively small planets have thick, rocky mantles and crusts.

“GJ 367b is so close to its star that the heat from the star could have boiled off the rest of the planet over time,” says Lam. “This could be the remains of a gaseous planet smaller than Neptune.”

It will take more observations to be sure, but the star’s relative brightness and the planet’s close orbit will make ongoing study easy, says Lam. The researchers also found hints of a second planet in the system, which will require follow-up observations to confirm.

Journal reference: Science, DOI: 10.1126/science.aay3253

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