The brightest star with a known planet climbs high across the sky on winter nights. Pollux is the brighter of the “twin” stars of Gemini. It’s about a third of the way up the eastern sky as night falls, with the other twin, Castor, to its upper left.

Pollux looks so bright for a couple of reasons. First, it’s quite close — only about 34 light-years away. And second, it really is bright — about 40 times brighter than the Sun. That’s because it’s a giant — it’s many times bigger than the Sun.

While that’s good for skywatchers, it’s not so good for the planet, which has been named Thestias, for the mythological mother of Pollux.

Pollux became a giant at the end of its normal lifetime. It had consumed the nuclear fuel in its core, so its outer layers puffed outward, making the star much brighter.

Thestias is about half again as far from Pollux as Earth is from the Sun. For much of the planet’s life, that probably was a comfortable distance. As Pollux puffed up, though, Thestias got hotter. Any given area on the planet gets about 20 times more energy than a similar area on Earth receives from the Sun. That makes the planet much hotter than Earth.

Thestias is a ball of gas that’s a couple of times heavier than Jupiter, the giant of our own solar system. So as Pollux has expanded, the extra energy probably has started to blow away the planet’s atmosphere — whittling down the only known planet of this bright star.

We’ll talk about Castor tomorrow.

Script by Damond Benningfield

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