New planet small but tough

SEATTLE — A newly discovered planet beyond the solar system is not only the smallest extrasolar planet yet found but also the first confirmed to be made entirely of solid material. Discovered by NASA’s Kepler spacecraft and dubbed Kepler-10b, the body has a diameter only 40 percent larger than Earth’s.

Likely to be partially molten, the planet is too hot to contain liquid water or support life as known on Earth. But the planet’s mass and diameter are known to such high accuracy that the object “is the first unquestionably rocky planet that humanity has ever seen,” said Kepler team member Natalie Batalha of San José State University in California. She described the findings January 10 at the winter meeting of the American Astronomical Society. Scientists are especially interested in finding rocky planets because chemical reactions that form the building blocks of life may happen most readily on solid surfaces.

“The more rocky planets we can find, the better placed we will be to understand the subset that are in the habitable zone,” said theorist Rory Barnes of the University of Washington in Seattle. Astronomer Adam Burrows of Princeton University concurred, saying that the discovery bodes well for Kepler finding other such rocky objects with high precision.

The finding, said Kepler researcher Geoff Marcy of the University of California, Berkeley, ranks “as among the most profound scientific discoveries in human history.” Kepler-10b, he added, “will go into every textbook worldwide.”

Only a few months after the Kepler’s launch in March 2009, the craft recorded a periodic dip in light — a decrease of only 0.0152 percent — from a star called Kepler-10. The tiny dip suggested that the star has a planet not much bigger than Earth that whips about Kepler-10 in a 20-hour orbit.

Although the craft had soon recorded hundreds of such dimmings, Batalha’s team still had to measure the mass of the candidate planet and make sure that the mini-eclipses weren’t caused by some spurious background object in the same patch of sky.

For those measurements, the researchers turned to the large Keck I telescope atop Hawaii’s Mauna Kea. Keck observations revealed that Kepler-10, some 565 light-years from Earth, was indeed being tugged ever so slightly by a planet less than six times the mass of Earth. But to pin down the planet’s composition, the team needed to get much more precise values of the exoplanet’s mass and diameter.

And that required measuring the mass, size and age of the parent star more accurately than ever before. “Everything we measure [about the planet] is relative to the star,” noted Batalha.

To examine Kepler-10 more closely, the team began examining the star at one-minute intervals with the spacecraft (SN Online: 10/26/10). The researchers recorded tiny flickers in starlight that weren’t due to the planet’s passages but were generated by sound waves emanating deep within the star.

Analyzing the strength and frequency of those oscillations allowed the team to pin down the star’s properties and then the planet’s — a diameter 1.4 times that of Earth and a mass 4.56 times Earth’s — to within 2 to 6 percent. Those measurements reveal that Kepler-10b must be composed entirely of solid silicate and metal grains, Batalha said. The planet is likely to have a higher metal content than Earth, yielding a density similar to a chunk of iron. Temperatures there, the researchers calculate, could get nearly high enough to melt iron: Kepler-10b appears to be a scorched world, with oceans of lava on the side facing the star, where estimated surface temperatures reach up to 1,800 kelvins (more than 1,500° Celsius).

“I would call it a rocky planet, and I would say it is more like a super-Mercury” than a super-Earth, commented Diana Valencia of the Observatoire de la C´te d’Azur in Nice, France.

A small extrasolar planet previously detected by a European spacecraft called COROT is also likely to be rocky, but because its diameter and mass are not as well determined, the composition of the body is less certain (SN: 10/10/09, p. 8), Valencia noted.

The case for a rocky planet is much clearer for Kepler-10b, she said.

In early February, the Kepler team is expected to announce a trove of newly discovered exoplanets, including several super-Earths.