The extremely short 10-day orbits of hot Jupiters may be explained by the planets being born among a high density of stars.
Hot Jupiters have long confused astrophysicists. As the name suggests, they are planets similar in size and appearance to Jupiter – but unlike Jupiter, they lie very close to their star, which means it takes them just a matter of days to complete one orbit.
Andrew Winter at Heidelberg University, Germany, and his colleagues used data from the to model how a planet’s birth environment might affect its subsequent mass and orbit.
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“This type of research was not possible until we got data from Gaia,” says Winter. The spacecraft aims to make the largest ever 3D map of space.
The researchers looked specifically at two types of environments where a planet is typically born: areas where there used to be lots of stars, called stellar clusters, and those where there used to only be a few stars.
“Stellar clusters typically disperse within a billion years, which is younger than most exoplanets,” says Winter. This means that most hot Jupiters today appear to exist outside stellar clusters.
The researchers therefore used positional data from Gaia to estimate which stars were more likely to have once belonged to a cluster. “You can make the argument that a star is likely to have occupied a stellar cluster when there are other stars that share a similar velocity,” says Winter.
When the researchers had reconstructed the ancient stellar clusters, it was clear that they were the birthplace of most hot Jupiters. The planets were twice as likely to be present among stellar clusters than not.
The excess of hot Jupiters in these environments could explain the planets’ extreme orbits, says Winter. Planets including hot Jupiters are born in discs that revolve around a single star. If that star belongs to a cluster, then thousands of planetary systems may evolve near each other.
“Being in a dense cluster environment means that stars close to one another could perturb the orbit of planets,” says Winter. Planets as big as Jupiter could be pushed into an eccentric orbit which is then more easily affected by the gravitational pull of its host star. This can ultimately make its orbit smaller until it becomes a hot Jupiter.
“This study suggests that the number of stars that form together has an influence on planet formation,” says Hannah Wakeford at the University of Bristol, UK. “This is not particularly surprising, but it might be the first time we have numbers and observations to back it up.”
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