Nasa finds strange alien planet hiding outside our solar system

Very small and very cool

Nasa has found a strange alien planet sitting outside of our solar system.

The world is cool and small: two characteristics that make it a very strange thing indeed for scientists to find.

The little planet known as HD 21749b is the third planet to be found by TESS, Nasa's new planet-hunting telescope. It goes around its star on a long orbit, taking a relatively leisurely 36 days.

Despite that proximity, it is quite cool at around around 300 degrees Fahrenheit.

As such it could be a very important way for scientists to find out about such relatively little and temperate worlds.

“It’s the coolest small planet that we know of around a star this bright,” says Diana Dragomir, a postdoc in MIT’s Kavli Institute for Astrophysics and Space Research, who led the new discovery.

“We know a lot about atmospheres of hot planets, but because it’s very hard to find small planets that orbit farther from their stars, and are therefore cooler, we haven’t been able to learn much about these smaller, cooler planets. But here we were lucky, and caught this one, and can now study it in more detail.”

Even despite being called small by the scientists who found it, the world is still huge compared to us: it is about three times our size, and 23 times as massive. That puts it in the category of a "sub-Neptune", and means that it would be the first roughly Earth-sized planets to have been found by TESS.

But it is unlikely to look much like our own planet. It's probably made of gas like Neptune or Uranus, though something far more dense than their atmospheres.

“We think this planet wouldn’t be as gaseous as Neptune or Uranus, which are mostly hydrogen and really puffy,” Dragomir says. “The planet likely has a density of water, or a thick atmosphere.”