An illustration of K2-33b. (Photo: NASA/JPL-Caltech)

Of all the far-away exoplanets that NASA has been finding with the Kepler telescope, some of the most important and fascinating are young planets, which can help scientists understand how planets form.

In a new Nature paper, a team of astronomers describes K2-33b, the youngest “fully formed” planet ever discovered. In a second paper, another team of scientists describe a young “hot Jupiter.”

Both planets orbit remarkably close to the their starts. K2-33b is 10 times closer to its star than Mercury is to ours—so close that its orbit is just 5 days long. Its age is estimated to be between 5 and 10 million years, which is nothing, for a planet.

The second young planet is orbiting a star that’s just 2 million years old, and again, it’s remarkably close in. It’s so close that it’s characterized as a “hot Jupiter”—which is basically what it sounds like, a planet like our Jupiter that’s closer to a star and, therefore, hotter.

The mystery of this planet is how it got so close to the star. As the Guardian explains, astronomers have previously thought a planet as big as this one could not form this close to a star—there’s just not enough material. Finding this planet adds evidence to the idea that planets like this one form further out and are pushed closer in.

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