Hubble's Latest Discovery Could Change Everything We Know About How Planets Grow

Not too long ago, NASA’s Kepler Space Telescope lost a second reaction wheel. It was a crippling setback, leaving the telescope unable to get a clear view of distant stars. People were devastated when the news broke, saying we’d never find another exoplanet.

But we have. Quite a few. And they are among the neater exoplanet finds.

A couple of weeks ago, the Hubble Space Telescope found compelling evidence of a planet forming 7.5 billion miles away from its star. That’s far. Not only is it the furthest planet we’ve ever found from a star, it’s far enough away to challenge existing theories about how planets form.

The planet in question is in orbit around a red dwarf star called TW Hydrae— itself a popular target for astronomers—which is located 176 light-years away, in the constellation Hydra the Sea Serpent. When looking at the star, Hubble found a mysterious gap, 1.9 billion miles wide, in the 41 billion-mile protoplanetary disk of gas and dust swirling around the star. The likeliest cause of the gap is a planet, a small, young one that is gravitationally sweeping up material.

Early estimates indicate the planet is small, somewhere between 6 and 28 times more massive than Earth, and moving slowly around its star. If this planet were in orbit around our Sun, it would be twice as far away as Pluto.

Scientists have estimated that a planet 7.5 billion miles from its star should take more than 2,000 million years to form. Orbiting so far from its star, a protoplanet would have less available material to siphon into itself.

But this theory doesn’t work when it comes to TW Hydrae. The star is only 8 million years old, so no planet should have had a chance to form in its orbit yet. The star is also a low mass star, just 55 percent as massive as our Sun. There’s also a dearth of larger material in the disk, something that planets usually need to form.

So how might this planet have formed if it’s too young to fit into the usual model? An alternative theory suggests a portion of a protoplanetary disk can become so unstable it collapses onto itself, possibly forming a planet in as little as a few thousand years. But for now that’s just a theory.

Another red dwarf has also turned out to be a planetary host. This week, an international team of scientists found three potentially habitable planets in orbit around Gliese 667C, a star 22 light-years from Earth.

They are all rocky super-Earths, planets less massive than Neptune but larger than the Earth, and they all orbit right in their star’s "habitable zone," the magic, temperate region where liquid water can exist on a planets’ surface. They’re the right size and in the right orbit to support life. They aren’t alone—there are six or seven planets in orbit around Gliese 667C, although they aren’t in the habitable zone.

What’s interesting about this discovery is Gliese 667C’s low mass. This is the first time three potentially habitable worlds have been found in orbit around a low mass star, and it's changing the way scientists hunt for exoplanets. Instead of sifting through data on ten stars looking for one planet, they can potentially study one star and find multiple planets.

To make it even more of a sci-fi fantasy, Gliese 667C is part of a three-star system. So the view from these potentially habitable worlds would be one with three suns in the sky. Because Gliese 667C is the faintest of the three stars, its stellar companions (Gliese 667A and Gliese 667B) would shine as bright as the full Moon at night and be visible during the day.

These latest finds of planets around red dwarfs are not simply interesting because they are exoplanets — they are also changing the way we think about exoplanets. So while Kepler’s mission might be coming to an end, the hunt for exoplanets is far from over.