Gliese15Ab is a newly discovered planet roughly five times the mass of Earth. Researchers aren't sure what the planet's atmosphere is like, and it's close enough to its parent star to heat it past the boiling point of water. But the most interesting thing about Gliese 15Ab isn't how close it is to its star. It's how close it is to us.
Image Credit: NASA, ESA, and D. Aguilar (Harvard-Smithsonian Center for Astrophysics).
The interesting bit is that its host star, Gl 15A, is a mere 11.7 light years from Earth. It's one of the 20 closest stellar systems known, making GL 15Ab quite possibly the closest known exoplanet!
Gliese 15 is a binary star, two cool, dim red dwarfs orbiting each other. Red dwarfs are the most common type of star in the galaxy, but they're so intrinsically faint that not a single one is visible to the naked eye; you need a telescope to see them. The closest star to the Sun we know of, Proxima Centauri, is only 4.2 light years away and even then too faint to see without using at least good binoculars.
Gliese 15 A and B (as the two stars are called, or just Gl 15A and B for short) orbit each other at a distance of about 22 billion kilometers, which is five times the distance Neptune orbits the Sun, so they're pretty far apart. The planet discovered has a very tight orbit around the brighter of the two stars, Gl 15A, circling it a mere 11 million kilometers out. That's close. Even though the star itself is a dim bulb, the planet is so near to it that it's heated to at least the boiling point of water, and possibly hotter.
The biggest takeaway from this announcement doesn't even have to do with Gliese15Ab – at least not directly. It has to do with the fact that this planet is one of many, many confirmed and candidate exoplanets being discovered at an increasing rate. The Milky Way is thought to contain billions of Earth-like worlds, as many as 60 billion of which are believed to orbit red dwarfs like Gliese15A. Discoveries like this remind us that these unfathomably huge estimates are based in more than theory. The exoplanets are out there, waiting to be discovered. And some of them, like Gliese15Ab, may be hiding right in our back yard.