NASA's Kepler space telescope has found the first confirmed Earth-size planets orbiting another star, astronomers announced Tuesday, a major milestone in an ongoing project aimed at finding out how commonplace--or rare--Earth-like worlds may be across the cosmos.
In a solar system 1,000 light years away with at least five planets, the newly confirmed Earth-size worlds orbit too close to their star to support life. But proving the Kepler observatory can, in fact, spot worlds as small as Earth across the vast reaches of interstellar space gives astronomers confidence many more such planets are awaiting discovery among the 2,326 planet candidates found by the telescope to date.
"The first of these two planets has a diameter just 3 percent larger than the Earth, which makes it the closest object to the Earth in terms of size in the known universe," Francois Fressin of the Harvard-Smithsonian Center for Astrophysics, told reporters during a teleconference. "The second planet is 13 percent smaller than the Earth, with a diameter of around 7,000 miles. It is also smaller than Venus, and this is, in fact, the smallest planetary body ever discovered in orbit around an Earth-like star.
"Most importantly, it is the first time we've crossed the Earth-size threshold. In other words, December 2011 could be remembered as the first time humanity has been able to detect a planet of Earth-size or smaller around another star."
On Dec. 5, the Kepler team announced the discovery of a world twice the size of Earth orbiting in its star's habitable zone, where liquid water can exist, the first time a relatively Earth-size world had been found at the right distance to possibly support life.
Kepler-20e and 20f share their parent star with at least three other Neptune-class planets, all of them orbiting closer than Mercury orbits the sun. Kepler-20e completes a trip around the star every 6.1 days and is believed to have a temperature of around 1,400 degrees Fahrenheit, hot enough to melt glass. Kepler-20f completes a "year" every 19.6 days and has a surface temperature of around 800 degrees. The masses of the two planets, and thus their density and general composition, remain a mystery.
Perhaps more important than finding two confirmed Earth-size worlds, the Kepler-20 solar system is at odds with current theories about how planetary systems form.
"Today, we are announcing five planets all orbiting this star, Kepler-20," said David Charbonneau, professor of astronomy at Harvard University. "However, the architecture of that planetary system is crazy."
In Earth's solar system, he said, there are two major types of planets: small, rocky worlds close to the sun (Mercury, Venus, Earth, and Mars) and large gas giants farther from the sun (Jupiter, Saturn, Uranus, and Neptune).
"In Earth's solar system, these two different kinds of planets don't mingle, they remain neatly separated from each other," Charbonneau said. "In fact, all four of the rocky planets in the solar system lie relatively close to the sun and the four gas giants lie outside of this. This separation of rocky planets and gas giants in the solar system really drives our understanding of planet formation and is one of the key features that we try to explain when we try to understand the formation of the solar system."
In the case of Kepler-20, a Neptune-size gas giant is the innermost of the five known worlds, followed by Kepler-20e, another Neptune-like world, Kepler-20f, and then another gas giant.
"I really want to dare my fellow astronomers to try to explain how this system could have formed and I think it's fair to say I need help," Charbonneau said. "As a professor of astronomy, I'm actually teaching a course in the spring on planet formation and you can bet the freshman in my class are going to be quick to point out how the model of the formation of the solar system is deeply challenged by the discovery that's being presented today."
Kepler, equipped with a 95-megapixel digital camera, was launched from Cape Canaveral on March 6, 2009. The camera is aimed at a patch of sky in the northern constellation Cygnus the size of an outstretched hand that contains more than 4.5 million detectable stars.
Of that total, some 300,000 are believed to be the right age and to have the right composition and the proper brightness to host Earth-like planets. More than 156,000 of those, ranging from 600 to 3,000 light years away, will be actively monitored by Kepler over the life of its mission.
To find candidate planets, the spacecraft's camera monitors the brightness of target stars in the instrument's wide field of view, on the lookout for slight changes that might indicate a world passing between the star and the telescope. By studying the slight dimming, and by timing repeated cycles, computers can identify potential extra-solar worlds even though the planets themselves cannot be seen.
Before a candidate planet can be confirmed, the data must be reviewed by other astronomers and results compared to findings by other telescopes and satellites. As of today, Kepler has found 33 confirmed planets and 2,326 candidates. Astronomers expect many more confirmations as data analysis continues.
"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, Kepler deputy science team lead and professor of astronomy and physics at San Jose State University, said in a NASA statement. "We are on the edge of our seats knowing that Kepler's most anticipated discoveries are still to come."
Linda Elkins-Tanton, director of the Carnegie Institution's Department of Terrestrial Magnetism in Washington, D.C., told reporters the big question underlying the Kepler research is "are we alone in our universe? And so we ask, how are planets made, and when do they end up like the Earth, both in size and also in climate? This new Kepler finding has made a really big step in our understanding of these questions."
She said astronomers believed solar systems likely followed the model seen in Earth's system, with small, rocky worlds orbiting close to their parent star and large gas giants orbiting much farther away, beyond the so-called "ice line." Based on earlier observations that found multiple examples of Jupiter-class planets orbiting close to their parent stars, scientists believed such worlds could migrate inward over time, kicking out smaller planets closer in.
"But now, with this new Kepler finding comes a solar system that doesn't fit any mold we have," Elkins-Tanton said. "The planets alternate big and little and are all so close to their star, this system forces us to change our ideas about how the planets are formed and how they reach stable orbits and where, indeed, there could be Earth-size rocky planets."
Whether Earth's solar system is the exception or the rule remains to be seen.