NASA said its orbiting Kepler telescope has spotted two planets the size of Earth orbiting another star, although both are far too hot to sustain life. The discovery does, however, bring scientists a big step closer to finding an Earth-like planet that could harbor life.
The planets, called Kepler-20e and Kepler-20f, appear to be rocky, like Earth, but orbit too closely to their star for liquid water to persist on their surfaces, a team of scientists reported in the journal Nature.
"These new planets are significantly smaller than any planet found up till now orbiting a Sun-like star," said Francois Fressin of the Harvard-Smithsonian Center for Astrophysics, who led the study.
Astronomers were excited about the discovery because other so-called exoplanets found orbiting stars outside our own solar system have all been bigger than Earth. These two compare in size and structure to Venus and Earth, and suggest strongly that planet-hunters will eventually find planets that look more like Earth, perhaps even with liquid water on the surface.
"In the cosmic game of hide and seek, finding planets with just the right size and just the right temperature seems only a matter of time," said Natalie Batalha, a professor of astronomy and physics at San Jose State University who works on the Kepler team. "We are on the edge of our seats knowing that Kepler's most anticipated discoveries are still to come."
The key would be for planets to be in the "habitable zone" - a distance from the parent star that would allow temperatures compatible with life.
"The primary goal of the Kepler mission is to find Earth-sized planets in the habitable zone," Fressin said. "This discovery demonstrates for the first time that Earth-size planets exist around other stars, and that we are able to detect them."
The $591 million Kepler space telescope is watching about 170,000 stars along a stretch of the Milky Way - the galaxy that includes our own solar system. It finds planets by measuring changes in light coming from stars, which indicate that a planet has passed in front of the star as it orbits.
What's unusual about the Kepler system is the way the planets mix it up. In our own system, the small, rocky planets - Earth, Mercury, Venus, and Mars - are closer to the star and the big gassy planets, like Jupiter, are farther out. In the Kepler system, the big planets alternate with the smaller ones. "We were surprised to find this system of flip-flopping planets," said David Charbonneau of the Center for Astrophysics. "It's very different than our solar system."