NASA’s planet-hunting spacecraft Kepler has spotted the youngest planet ever found outside our Solar System. Dubbed K2-33b, this exoplanet is about 5 to 10 million years old. That may seem pretty damn old for the likes of you and me, but in the timeline of the Universe, this world is a mere infant. The Universe is thought to be about 14 billion years old, while Earth has been around for the last 4.5 billion years. Finding an exoplanet this young is pretty rare, and astronomers hope it can teach us more about planetary formation.

This world is a mere infant

The Kepler spacecraft was able to pick up K2-33b’s signal by studying the star that the exoplanet orbits. When an exoplanet passes in front of its host star, the star’s brightness dims slightly — but sometimes, that’s just enough for Kepler to observe from space. These light dips can be used to determine if a planet is present around a star, as well as how big the world is and the period of its orbit. Thanks to Kepler’s measurements, and follow-up observations from the W. M. Keck Observatory on Mauna Kea, Hawaii, astronomers found that K2-33b is a bit larger than Neptune, with an orbit of just five Earth days. That means the planet is extremely close to its host star, about 10 times closer than Mercury is to our Sun.

K2-33b’s close orbit is a bit baffling for scientists. Astronomers have found many big exoplanets like this one that also orbit super-close to their stars — but no one is really sure how that’s possible. "After the first discoveries of massive exoplanets on close orbits about 20 years ago, it was immediately suggested that they could absolutely not have formed there," Trevor David of Caltech in Pasadena, who co-authored a paper about K2-33b in the journal Nature, said in a statement. That’s because the temperature would have been way too hot for the planet to maintain its shape during planetary formation. So the prevailing idea was that these large planets must have migrated from a much more distant orbit to the close ones they’re in now. But that process was thought to take hundreds of millions of years — way longer than K2-33b has been around.

So astronomers have come up with two theories: either K2-33b migrated to its position over a course of just hundreds of thousands of years, or it formed right where it is now. If it’s the latter, that would open up new possibilities about how some massive planets form. "The question we are answering is: did those planets take a long time to get into those hot orbits, or could they have been there from a very early stage?" said David. "We are saying, at least in this one case, that they can indeed be there at a very early stage."

Rocky Earth-sized exoplanet orbiting nearby star