SCIENTISTS have discovered a new planet outside our solar system that is the closest Earth-size planet ever discovered.

Planet UCF 1.01, is an "exoplanet" orbiting a red dwarf star called GJ 436, about two-thirds the size of Earth and only 33 light years distant. It appears as if it is covered in molten lava.

"Cosmically speaking, that's right around the corner," said discoverer Kevin Stevenson, though that works out to 194 trillion miles.

Exoplanets circle stars beyond our Sun, but only a handful smaller than Earth have been discovered, according to NASA. UCF 1.01 is about 5,200 miles in diameter, slightly larger than Mars, smaller than Venus.

It's so close to its star - GJ 436 is about half the size of our Sun - that it revolves around it once every 1.4 days (about 33 1/2 hours.) It's so hot - at least 1,000 degrees Fahrenheit - that it has no atmosphere and may have a molten surface.

The discovery will be detailed in an article to be published online this week by The Astrophysical Journal.

The news is expected to create international buzz among astronomers and physicists, both because of the way Stevenson and his colleagues found it, and because of where they were looking when they did, said Michael Werner, project scientist at NASA's Jet Propulsion Laboratory in California.

He and other scientists at UCF's Planetary Sciences Group were using the NASA Spitzer Space Telescope, an infrared space telescope launched into orbit around the sun from Cape Canaveral in August 2003. That telescope was designed and is used mainly to study planets and other space objects that already have been discovered, or to look into deep, deep space.

UCF 1.01 is the first planet actually discovered by Spitzer. Most have been discovered by the NASA's Kepler Space Telescope, which is designed for that sort of research.

And not many scientists have been looking for planets around red dwarfs, which are smaller and much dimmer than yellow stars such as the Earth's sun. That should change now that UCF 1.01 demonstrates red dwarfs can have solar systems, including Earth-size planets, Werner said.

"It was a serendipitous discovery," Stevenson said.

UCF 1.01 is in the constellation Leo, though even its star GJ 436 is too faint to be seen with the naked eye. The planet itself can't even be seen with Spitzer, or any other telescope.

Scientists generally can't see planets outside our solar system, so they look for them by looking at the light emitted by stars. When the infrared light streaming from the star dips by a small amount, it could mean a planet is passing by the star, casting a tiny shadow. When that starlight dips regularly, and predictably, that probably is caused by a planet.

Stevenson, who was a physics doctoral student until he was awarded his doctorate in May; UCF planetary sciences professor Joseph Harrington, the project's principal investigator; and UCF graduate student Nate Lust were looking at GJ 436 two years ago to gather data on a much-larger planet, about the size of Neptune, that already had been detected in orbit around it.

While watching the red dwarf, Stevenson and the others witnessed an unexplained dip in the starlight.

"That's kind of the 'Aha!' moment," Stevenson said.

They asked NASA to aim Spitzer at the star again for another look. They studied hundreds of archived hours of observation data from that star gathered previously by Spitzer and other telescopes.

"Several weeks later it came to the epiphany: well, maybe it's another planet," said Stevenson, who is now at the University of Chicago.

Stevenson cautioned that at this point UCF 1.01 only qualifies as a candidate planet. Scientists don't award full planetary status until they have enough data to determine the planet's mass, and Spitzer and the other telescopes can't do that yet with something as small as UCF 1.01.

There's something else in the data though. There's another unexplained dip. It has shown up only a couple of times, but it's promising, Stevenson said.

They've already named it: UCF 1.02.