Some planets in other star systems appear to be even fluffier than the meringue atop a lemon pie. Such planets are called super-puffs. But some of them might not be as puffy as they look. Instead, they may be encircled by large sets of rings.

Astronomers have measured the size and mass of many exoplanets — planets that circle stars other than the Sun. They use those measurements to calculate a planet’s density. And they’ve found that quite a few of them are as light as balsa wood — and, in some cases, much lighter.

Some of the planets are especially close to their stars, so they’re quite hot. And as they heat up, their outer layers expand. That makes them a lot bigger.

But other super-puffs are harder to explain. And that’s where the rings come in.

Astronomers usually measure the size of a planet by watching it pass in front of its star, blocking some of its light. The amount the star fades reveals the size of the planet. In some cases, though, they may be seeing rings pass in front of the star as well. That would make the planet look much bigger than it really is. So some “super-puffs” may not be puffy at all.

Saturn — the planet with the biggest ring system in our own solar system — perches just above the Moon at first light tomorrow. It looks like a fairly bright star. The much brighter planet Jupiter stands off to their left. It will stand close to the Moon on Wednesday, and we’ll have more about that tomorrow.

Script by Damond Benningfield

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