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NOEL KING, HOST:

In a solar system nearby, astronomers have found a planet that's made almost entirely of iron. This alien world could help us understand our own solar system's innermost planet, Mercury.

Here's NPR's Nell Greenfieldboyce.

NELL GREENFIELDBOYCE, BYLINE: Thousands of planets have been discovered orbiting distant stars in recent years. Now scientists say they've detected an unusual planet located about 30 light-years away. It's smaller than Earth and orbits a red dwarf, whizzing all the way around this star every eight hours.

Kristine Lam is a researcher at the German Aerospace Center's Institute of Planetary Research. She says because this planet is so close to its star, it must be very, very hot.

KRISTINE LAM: It could be as hot as 1400 degrees Celsius.

GREENFIELDBOYCE: That's like 2500 degrees Fahrenheit.

LAM: So we can imagine this planet is probably, like, a molten planet, at a temperature where a lot of metals can start to melt.

GREENFIELDBOYCE: And this planet has a lot of metal. In the journal Science, Lam and her colleagues say it's probably mostly made of iron.

LAM: This planet is quite dense and has an interior similar to Mercury.

GREENFIELDBOYCE: That's intriguing because scientists have long wondered why Mercury seems to contain so much more iron than the other rocky planets - Venus, Earth and Mars.

Josh Winn is an astronomer at Princeton University. He says there's been speculation that some cosmic collision vaporized much of Mercury's rock, basically leaving its metal core behind. But if scientists find other close-in planets like this new one that have a lot of iron...

JOSH WINN: I think that story becomes even less plausible.

GREENFIELDBOYCE: And researchers may need to think of another way that close-in planets could end up with so much of this metal. He says this new planet orbits so close to its star from its surface, the star would look 30 times larger in the sky than our sun does.

WINN: So imagine swelling up the sun to 30 times its usual size. And you also have to color it red.

GREENFIELDBOYCE: He's fascinated by planets like this one, that orbit their stars in less than one Earth day. They're usually small and sometimes called hot Earths.

WINN: They're rare. Nobody expected that they would exist. And those are the kind of things I love because it's the weird ones that are often the most informative or revealing.

GREENFIELDBOYCE: Researchers should be able to learn more about this planetary oddball with NASA's James Webb Space Telescope. It's scheduled to launch later this month.

Nell Greenfieldboyce, NPR News.

(SOUNDBITE OF ERIC LAU'S "STAR TREKKING")