The closest is just ten light years -- 60 trillion miles -- away. That's the astronomer's equivalent of our own back yard.
It's a planet the size of our Jupiter, orbiting a sun like ours called Epsilon Eridani. Star Trek fans will know it as the home star of the fictional planet Vulcan.
Astronomer William Cochran says, "This is a star that's on our own block It's telling us that planets are very common around stars like the sun and there's an awful lot of them out there -- and finding one like the Earth is, I think, something that's going to happen soon."
The problem is, astronomers can only get a glimpse of the Epsilon Eridani solar system --not enough to see the mystery planet itself.
But as new satellite technologies come on line over the next few years, this is one of the first places they're going to take a closer look.
Scientists predict that after manned missions to the Moon and eventually to Mars this mysterious new planet, so new it doesn't have a name, could be next.