NASA’s Kepler mission has discovered the first Earth-size planets orbiting a sun-like star outside our solar system.
The planets, called Kepler-20e and Kepler-20f, are too close to their star to be in the so-called habitable zone where liquid water could exist on a planet’s surface, but they are the smallest exoplanets ever confirmed around a star like our sun.
The find comes on the heels of Kepler’s first potentially Earthlike planet orbiting squarely within its star’s water-friendly “Goldilocks zone”—the region that’s not too hot and not too cold for liquid water to exist on a planet’s surface.
The discovery marks the next important milestone in the ultimate search for planets like Earth. The new planets are thought to be rocky. Kepler-20e is slightly smaller than Venus, measuring 0.87 times the radius of Earth.
Kepler-20f is a bit larger than Earth, measuring 1.03 times its radius. Both planets reside in a five-planet system called Kepler-20, approximately 1,000 light-years away in the constellation Lyra.
“The first of these planets has a diameter just 3% larger than the Earth, which makes it the closest object in terms of size in the known universe,” said Dr François Fressin of the Harvard-Smithsonian Centre for Astrophysics in Cambridge, Massachusetts, who led the team of researchers that found the planets.
“The second one is 13% smaller than the Earth. With a diameter around 7,000 miles, which is also smaller than Venus, it is in fact the smallest planetary body ever discovered in orbit of another sun-like star.”
“Most importantly, it is the first time we cross the Earth-size threshold. In other words, it is the first time that humanity has been able to detect a planet of Earth’s size or smaller around another sun.”
But both new extrasolar planets—or exoplanets—orbit their star much too closely to be within the habitable zone.
In fact, the entire Kepler-20 system is believed to contain at least five planets all orbiting their star within a distance smaller than that between Mercury and the sun.
This orbital distance makes the planets very hot. For instance, Kepler-20e is estimated to have an average surface temperature of 1,400ºF (760ºC), while Kepler-20f is a “cooler” 800ºF (427ºC).
“The primary goal of the Kepler mission is to find Earth-sized planets in the habitable zone,” said Francois Fressin. “This discovery demonstrates for the first time that Earth-size planets exist around other stars, and that we are able to detect them.”
Since it was launched in 2009, NASA’s planet-hunting Kepler telescope has found evidence of dozens of possible Earth-sized planets. But Fressin’s report is the first to provide confirmation, said Alan Boss of the Carnegie Institution for Science in Washington. He’s a member of the Kepler science team but not an author of the paper.
“The Kepler data are showing us some planetary systems have arrangements of planets very different from that seen in our solar system,” said Jack Lissauer, planetary scientist and Kepler science team member at NASA’s Ames Research Center in Moffett Field, Calif.
“The analysis of Kepler data continue to reveal new insights about the diversity of planets and planetary systems within our galaxy.”
Scientists are not certain how the system evolved but they do not 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.
Scientists have so far confirmed 33 planets that Kepler has discovered, but with more than 2,300 potential candidates waiting to be confirmed, the most exciting discoveries may lie ahead.
“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,” Natalie Batalha, a co-investigator for Kepler mission, said in a statement. “We are on the edge of our seats knowing that Kepler’s most anticipated discoveries are still to come.” [via NASA, Huffington Post, Guardian and National Geographic]