Giant planets known as hot Jupiters appear to be growing in size as they are heated by their stars over billions of years.
Hot Jupiters are a loosely defined class of gas giant exoplanets that orbit stars incredibly closely, reaching temperatures of more than 4000掳C. They can be up to twice as large as their cooler counterparts, such as Jupiter in our own solar system. In 2016, research suggested that hot Jupiters orbiting red giants 鈥� stars in an expanded latter stage of their life 鈥� could be , partially accounting for the difference.
Now, Daniel Thorngren at the University of Montreal and his colleagues have discovered that the process happens to more of these planets than we thought. Studying 312 hot Jupiters at different stages of their lives, they found evidence that they were growing in size by up to 20 per cent on average during their lifetimes, following a brief period of shrinking and cooling after their formation.
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鈥淧lanets do seem to be reinflating pretty rapidly,鈥� says Thorngren, who presented the research virtually at the 236th聽meeting of the American Astronomical Society earlier this month.
This process occurs when the star heats the planet from the inside out. This causes gas inside the planet to expand, increasing its radius. 鈥淭he gas gets less dense and that pushes out the radius of the planet,鈥� says Thorngren.
How the planet is heated from the inside isn鈥檛 entirely clear, but one possibility is that thermal tides caused by the gravitational pull of a star make the planet bulge, which might create friction that dissipates heat inside the planet. More modelling will be needed to understand this further, says Thorngren.
鈥淭he next thing we need to do is really update our general circulation models of hot Jupiters,鈥� he says.
