SAN FRANCISCO — NASA’s Kepler observatory has located the most Earth-like planet yet discovered beyond the Solar System, but it is so hot that it streams material into space like a comet’s tail.

Called Kepler-10b, the planet is 1.4 times the size of Earth but orbits its star, called Kepler-10, 20 times closer than Mercury is to our Sun. Radiation from its star prevents Kepler-10B from having an atmosphere.

Kepler-10b is the smallest exoplanet yet discovered. The planet’s surface presents the same face to its star, just as the Moon does to Earth, and the surface is a 2500F molten ocean, hotter than a lava flow. It is so close that silicon and iron flecks are blasted from its surface into space.

The fact that Kepler-10B orbits its star in just 0.84 days was a big help for the Kepler science team to single it out from the 150,000 stars in the constellations Cygnus-Lyra that the observatory observes constantly, says Natalie Batalha, Kepler’s deputy science team leader at NASA’s Ames Research Center in California.

Launched in March 2009, the Kepler spacecraft uses the largest array of sensors in space to search for exoplanets. They work by detecting the faint dimming of a star’s light that occurs when a planet transits in front of it. Many of these transits take months to complete and must be triple-checked to distinguish them from sunspots.

But Kepler-10B’s orbital period is so fast that its dimming quickly stood out. It was picked up in May 2009, even as NASA was still performing instrument checkout, and by that August Kepler’s science team had enlisted the Keck Observatory in Hawaii to perform confirming observations of the planet’s mass and radius, which were completed in January 2010.

“This is unquestionably a rocky planet orbiting another star,” says Batalha, lead author of a paper with 54 worldwide scientists presented Jan. 10 to the American Astronomical Society meeting in Seattle. The Kepler-10B astroseismology team is led by Aarhus University in the Netherlands.

At 8.8 grams per cubic centimeter, Kepler-10B has a far greater density than any of the inner planets in our Solar System, which average about 5 grams. Batalha compares it to an iron dumbbell.

The star Kepler-10 spins slowly, has a weak magnetic field, few sunspots and is 11.9 billion years old — more than twice the Sun’s 4.5 billion years.

Batalha says when she first saw artist renderings of the planet she was reminded of the hypothetical planet Vulcan that 18th century scientists thought accounted for slight orbital variations of Mercury.

Einstein’s General Theory of Relativity has since explained that phenomenon, but like the Roman god for which it was named, Vulcan was thought to be glowing hot, which Kepler-10B appears to be.

Meanwhile, observations with Kepler have just resumed following an anomaly on Dec. 22, 2010 that caused the spacecraft to enter safe mode.

Noise in Kepler’s sun-sensor circuitry falsely signaled it was pointing its delicate sensors too close to the Sun, which caused an automatic shutdown. Engineers say circuitry workarounds will prevent any recurrence of the false alarm.