Newly-discovered exoplanets could once have supported life
NASA's Kepler mission has confirmed the first Earth-sized, rocky planets orbiting a star similar to our own, and says it's possible that they could once have harbored life.
While Kepler-20e and Kepler-20f are too close to their star for liquid water to exist on the surface, NASA says they were once further from their star, and therefore cooler.
"The primary goal of the Kepler mission is to find Earth-sized planets in the habitable zone," says Francois Fressin of the Harvard-Smithsonian Center for Astrophysics.
"This discovery demonstrates for the first time that Earth-size planets exist around other stars, and that we are able to detect them."
Both belong to a five-planet system called Kepler-20, approximately 1,000 light-years away in the constellation Lyra. The host star belongs to the same G-type class as our sun, although it is slightly smaller and cooler.
Kepler-20e is slightly smaller than Venus, measuring 0.87 times the radius of Earth. It orbits its parent star every 6.1 days. Kepler-20f is slightly larger than Earth, at 1.03 times its radius, with an orbital length of 19.6 days.
Kepler-20f, at 800 degrees Fahrenheit, is about the same temperature as Mercury, while the surface temperature of Kepler-20e, at more than 1,400 degrees Fahrenheit, would melt glass.
The Kepler-20 system includes three other planets that are larger than Earth but smaller than Neptune. Kepler-20b, the closest planet, Kepler-20c, the third planet, and Kepler-20d, the fifth planet, orbit their star every 3.7, 10.9 and 77.6 days. All five orbit very close to their star - closer than Mercury is to our sun.
The planets are arranged rather oddly. In our solar system, small, rocky worlds orbit close to the sun and large, gaseous worlds farther out. In comparison, the planets of Kepler-20 are organized in alternating size: large, small, large, small and large.
"The Kepler data are showing us some planetary systems have arrangements of planets very different from that seen in our solar system," says Jack Lissauer, planetary scientist and Kepler science team member.
"The analysis of Kepler data continue to reveal new insights about the diversity of planets and planetary systems within our galaxy."
Scientists are uncertain how the system evolved - but they don't think the planets formed in their existing locations. They theorize the planets formed farther from their star and then migrated inward, likely through interactions with the disk of material from which they originated. This allowed the worlds to maintain their regular spacing despite alternating sizes.
"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," says Natalie Batalha, Kepler deputy science team lead and professor of astronomy and physics at San Jose State University.
"We are on the edge of our seats knowing that Kepler's most anticipated discoveries are still to come."