Astronomers have found thousands of exoplanets, planets that orbit other stars, in the past several years. Most of these exoplanets tend to be at least a billion years old. Today, astronomers at Caltech announced they have located an exoplanet that's considerably younger, only about 5 to 10 million years old. Their discovery is reported in the most recent edition of Nature.

The new planet, K2-33b, is a gas giant, like Jupiter, but it orbits its host star much closer, 20 times closer than the distance from Earth to the Sun. That's about seven times smaller than the orbit of Mercury. It's around six times bigger than Earth, or about 50 percent bigger than Neptune, and it's about the same as Neptune in terms of mass. It orbits its star very quickly, making a full revolution once every 5 days.

A comparison between the K2-33 system and our own solar system. The newly discovered planet, K2-33b, orbits very close to its host star. NASA/JPL-Caltech

K2-33b's age, 5 million years, may seem like a long time to us, but compared to the Earth, which has been around for 4.6 billion years, K2-33b is basically an infant. To put it's age into perspective, while our first hominid ancestors were climbing out of the trees and spreading throughout the African continent, K2-33b was still forming from a protoplanetary disk. K2-33b is about as old as sloths, and younger than giraffes, bears, and kangaroos.

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K2-33b is important for studying how planets, especially large planets, form around their host stars. Many of the discovered exoplanets are what are known as "Hot Jupiters," large, Jupiter-sized planets that orbit very close to their host star. In our solar system, the large gas giants orbit far away from the Sun, but the prevalence of hot Jupiters suggests that this is uncommon. Most scientists currently believe that in other solar systems, these hot Jupiters initially form at the same distance as our gas giants and migrate in closer over time. The fact that K2-33b, a very young planet, can be found so close to its star at such a young age could mean that this migration happens very quickly. Alternatively, it might mean that it doesn't happen at all, and that hot Jupiters simply form close to the star to begin with.

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