In a surprise find, astronomers have discovered a planet possibly covered with oceans of magma "right around the corner."

Even more exciting to scientists is its size: about the same as Mars's, which would make the new world the closest known planet smaller than Earth.

Video: NASA Visualization of New Planet UCF-1.01

Researchers using NASA's Spitzer Space Telescope essentially stumbled upon the new planet while studying a hot, Neptune-size planet called GJ 436b.

In their data, however, the team caught a faint signal of the new planet and named it UCF-1.01, after their institution, the University of Central Florida. Both worlds were detected by watching for regular dimming of their host star, indicating a planet transiting, or crossing in front of, the star.

Thirty-three light years away, "we have a sub-Earth-sized planet that's slightly larger than Mars and essentially right around the corner, at least on a cosmic scale," said Kevin Stevenson, a planetary scientist now at the University of Chicago, who led the study that revealed UCF-1.01.

"It's one of the nearest transiting planets, it's tiny, and it may not be alone."

It's a Small World After All

Large planets are easier to find, but they're generally gas giants that don't have a rocky surface or an atmosphere like Earth's. So scientists are scanning the skies for smaller worlds, which should be more likely to support life as we know it.

NASA's Kepler space observatory, in particular, is actively hunting for Earth-size planets around sunlike stars. In three years it has found more than 3,000 potential new worlds and confirmed dozens (find out how). But Kepler scans parts of the sky between 100 and 2,000 light-years away, giving it a blind spot closer to home.

Spitzer's infrared detectors can "see" planets within a hundred light-years of Earth, but the telescope is normally used to fine-tune our knowledge of farther-out planets that scientists already know about.

"We weren't looking for this planet, even though other researchers suspected it might be there," Stevenson said.

UCF-1.01 is about 5,200 miles (8,400 kilometers) wide, making about a quarter the volume of Earth. And with a year that lasts only 1.4 Earth days, the new planet's orbit takes UCF-1.01 searingly close to its star.

"The only thing separating the planet from the star is about seven times the distance between the Earth and the moon," said Stevenson, who noted the probably lacks an atmosphere.

"It could be a thousand degrees Fahrenheit [540 degrees Celsius]. That may be hot enough to make an ocean of molten rock."

Planetary Companion?

Until the mass of UCF-1.01 is known, the planet can't be confirmed by official standards. "But we know it's there," Stevenson said.

And it may have hidden company. The larger planet, GJ 436b, has an oblong orbit likely caused by the gravitational pull of another planet or planets. And, Stevenson explained, the small planet alone doesn't seem strong enough to explain the eccentric orbit of its big companion.

"At some point I'd like to go back and make new observations," Stevenson said. "There could be a third planet there, called UCF-1.02."