Hunting for habitable worlds, NASA's Kepler space telescope has unveiled two new planets, some 950 light-years away, that are the smallest yet detected, and the closest in size to Earth. In a paper published in Nature, scientists from MIT and elsewhere report that the planets - one just about Earth's size, and the other a bit smaller - likely have rocky compositions, similar to Earth, and orbit a star much like the sun. But that's where the similarities end.

Compared with Earth's leisurely 365-day orbit, the new planets practically whiz around their star in a matter of days or weeks. Their tight circuits, closer even than Mercury's orbit around our sun, make the planets extremely hot - likely too hot to sustain life. While either planet is far from Earth's twin, scientists say the discovery is a technological milestone.

"For the Kepler space telescope, it's extremely significant, because it proves we can reach down to Earth's size," says co-author Sara Seager, the Ellen Swallow Richards Professor of Planetary Science and Professor of Physics at MIT. "It's a massive accomplishment just to find anything at all like this."

The new planets, which orbit the star Kepler 20, are part of a five-planet system, and have been named Kepler 20e and Kepler 20f.

"This will hopefully open the floodgates to discovering more of these Earth-sized and sub-Earth-sized planets," says co-author Leslie Rogers, a physics graduate student at MIT. "Then we'll be able to determine some sort of context for how common are habitable Earth-sized planets."

Star lines
Seager and Rogers worked with an international team of researchers to analyze and validate data from the Kepler telescope. The space-based instrument monitors more than 100,000 stars on an ongoing basis for infinitesimal flickers that may be signs of a passing planet.

The telescope records light from each star over time, which is processed and then represented on a graph as a roughly horizontal line. Scientists analyze such light signatures for dips, or changes in light intensity, which may indicate a "transit," or a planet passing in front of a star.

In the case of Kepler 20e and 20f, the team made sure that the dips it observed were in fact caused by planetary bodies.

Led by Francois Fressin of the Harvard-Smithsonian Center for Astrophysics, the researchers laid out all possible alternative explanations, such as a "binary star" - two neighboring stars blending their light together.

Fressin then modeled what the light signal would look like in each of these scenarios, comparing each signal with the two signals observed by the Kepler telescope.

The group found that Kepler 20e, the smaller planet, was 3,400 times more likely to be a planet than any other possible scenario; Kepler 20f was 1,370 times likelier to be a planet. These odds, the team determined, were sufficiently large to confidently validate the objects as planets.

"These two new planets are part of pushing the envelope of the types of worlds that exist, types that scientists did not predict or even imagine only some years ago," says Ben Oppenheimer, associate curator and professor of astrophysics at the American Museum of Natural History, who was not part of this study.

"We can hope that there are similar types of planets that are far closer and waiting for us to study in greater detail."

Scorching hot
As part of the MIT contribution to the paper, Rogers and Seager explored all possible compositions for each planet. The team used light signatures from the Kepler telescope to first calculate the planets' radii.

Scientists can determine a planet's radius by observing the depth of the dip in the light signature: The deeper the dip, the bigger the radius. They can also estimate a planet's orbital period - the time it takes to circle its star - by observing the number of dips over a long period of time.

Fressin had previously calculated that Kepler 20e takes only six days to orbit, while Kepler 20f orbits in 20 days.

Knowing the rough size of each planet and its proximity to its star, the MIT team then calculated the planet's equilibrium temperature: the temperature in the upper layer of its atmosphere.

They found Kepler 20e, the smaller, closer planet, was an intense 1,400 degrees Fahrenheit; Kepler 20f was a less scorching 800 degrees. Such temperatures make liquid oceans highly unlikely, according to Rogers. However, Kepler 20f, being a bit further out in the planetary system, could potentially hold a water vapor atmosphere.

"If it initially formed from a mixture of rock and ice, and then migrated closer in to the star, then it could've potentially retained some of that water over giga-year timescales," Rogers says.

"It could also have a rocky composition like the Earth, but we don't know for sure. There's a range of possibilities."

"Every indication now points to an enormous ... population of these short-period Earth-sized planets," says Greg Laughlin, an assistant professor of astronomy and astrophysics at the University of California at Santa Cruz who was not involved in the study.

"As Kepler continues its observations, astronomers will also have the capability to detect Earth-sized planets on genuinely Earth-like orbits, which, of course, is especially exciting."