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We thought young gas giant planets would be large and low-density, but the gas giants around a star that is just 20 million years old don’t fit this model



Space



2 December 2021

Red Jupiter-like planet

An artist’s impression of a Jupiter-like exoplanet

Alexandr Yurtchenko/Alamy

Two Jupiter-like planets that orbit a young star are much smaller than expected, which may suggest we need to rethink our ideas of the early evolution of gas giant planets.

Our current understanding of giant planetary evolution predicts that these worlds start out as large, low-density objects. “We expect them to be like very giant, fluffy balls of gas,” says Alejandro Suárez Mascareño at the Institute of Astrophysics of the Canary Islands in Spain.

Then, over the course of a few hundred million years, the planets are expected to slowly contract until they reach their final size, roughly similar to the size of Jupiter or Saturn in our solar system. However, due to the difficulty in monitoring infant planetary systems, these predictions have remained untested until now.

A young star – just 20 million years old – known as V1298 Tau has given astronomers a rare window into the formation of gas giants. In the early 2010s, the Kepler space telescope observed that V1298 Tau is orbited by four gas giants. Suárez Mascareño and his colleagues followed up on the discovery by monitoring the star and its planets between April 2019 and April 2020.

Of the four planets, the researchers found that the two outermost planets – V1298 Tau b and V1298 Tau e – had features they hadn’t predicted. They are around 0.64 and 1.16 times the mass of Jupiter respectively, while their radii are 0.868 and 0.735 times that of Jupiter. This means the two planets are much smaller and denser than the researchers had expected, which suggests they contracted faster than our current ideas indicate.

As this is one of the first detailed studies of such a young planetary system, it is unclear whether these features are normal or strange, says Suárez Mascareño. It may be that our understanding of giant planet evolution is wrong. Alternatively, the planets may be unusual gas giants with cores that are abnormally massive, which would accelerate the contraction process.

“Our understanding of the early stages of planetary evolution and planetary systems evolution might actually be very limited,” says Suárez Mascareño. “Right now, this case contradicts our previous knowledge. But it’s one case – you cannot do a generalisation from one case.”

By collecting more data on more infant planetary systems, Suárez Mascareño hopes to shine a light on the formation of our own solar system.

Journal reference: Nature Astronomy, DOI: 10.1038/s41550-021-01533-7

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By CA