The smallest planet yet spied outside our solar system has been found orbiting a sunlike star about 560 light-years away, astronomers announced today. Known as Kepler-10b, the planet is just 1.4 times Earth's size and 4.6 times its mass.

The planet, found using NASA's Kepler spacecraft, is the first of the more than 500 known exoplanets that's definitively rocky—much like Earth, Mars, Venus, or Mercury—the study team says. Launched in March 2009, Kepler was designed to hunt for potentially habitable Earthlike planets.

Astronomers have been studying Kepler-10b since its discovery in 2009, when the team detected a periodic dimming of the host star as the planet passed in front of the star.

Finding such a small planet this way was no easy feat—seen from a similar distance, Earth passing in front of the sun would cause a 0.01 percent reduction in the star's brightness, said Natalie Batalha of San Jose State University, lead author of an upcoming paper describing the find.

"Imagine you have 10,000 light bulbs and you take one away. That's the change in brightness we're looking for," Batalha said today during a meeting of the American Astronomical Society in Seattle, Washington.

Still, after using Kepler and other instruments to precisely calculate the new planet's size, mass, and density, Batalha said, "we know without question that this is a rocky world."

Smallest Planet Has Density of Iron

Before figuring out the nature of Kepler-10b, the scientists looked at the host star's properties, as revealed by starquakes, acoustic disturbances that make the entire star ring like a bell.

"In the same way that we use a sonogram to probe an unborn fetus and earthquakes to probe the interior of the Earth, we use starquakes to probe the interior structure and properties of the star itself," Batalha said.

"A tiny star would yield different [vibration] frequencies than a large one, just as when you strum a violin you're going to get a different sound than when you strum a cello."

Using starquakes, Batalha and colleagues were able to accurately determine the size, mass, and age of the star, which in turn allowed them to make very fine-tuned estimates of the new planet's characteristics.

Astronomers carefully studied the tiny variations in starlight to determine Kepler-10b's size. The observations also revealed that the planet is very close to the star, orbiting once every 20 hours.

Using the W. M. Keck Observatory in Hawaii, the Kepler team made precise measurements of minute shifts in the wavelengths of the host star's light. This data showed how the star nodded back and forth in response to the planet's gravity, allowing the team to calculate the masses of both objects.

Based on this combined data, the team concludes that Kepler-10b must be a rocky world with an average density of 8.8 grams per cubic centimeter—about the same as a chunk of iron.

Rocky World is "Planetary Missing Link"

Being rocky, however, is no guarantee that a planet will be habitable. In the case of Kepler-10b, one side of the planet always faces the star, so that side would have a surface temperature of 2,500 degrees F (1,370 degrees C), Batalha estimates.

It's highly unlikely that such a world would retain an atmosphere, since the searing hot gases would rapidly escape into space.

Still, Kepler-10b is an enormously important find, said Geoffrey Marcy, a planet hunter at the University of California, Berkeley. Marcy, who is involved in the Kepler mission, was not a core member of Batalha's team.

"In astronomy, we've been discovering giant gas planets for 15 years. But the ultimate goal is to discover habitable worlds, like Earth," he said at the AAS meeting. (Related: "New Planet System Found—May Have Hidden 'Super Earth.'")

According to Marcy, Kepler-10b is the "planetary missing link."

"It's definitely not a gas giant like Jupiter. Nor is it habitable—it's too hot. This is a transitional planet somewhere between a gas giant and what we've been hoping to find."

One other possibly rocky planet, COROT-7b, might be even more Earthlike in size and mass, Batalha agreed. But its star is much more active with flares and other disturbances, making it difficult to nail down important parameters with the needed precision. (See "'Super Earth' May Really Be New Planet Type: Super-Io.")