The hunt for Earth's alien twin reached a new milestone with the discovery of a faraway exoplanet that's not much bigger than our own globe and is theoretically capable of retaining liquid water.
The planet is the first Earth-size sphere found outside our solar system that also resides in the "Goldilocks zone" -- the habitable range where the temperature is neither too hot nor too cold --in other words, the right conditions for life to potentially thrive.
The planet, called Kepler-186f, meets what researchers believe are two basic requirements for life. One is that its size is similar to Earth's, which increases the chance that it has a rocky, rather than gaseous, surface. The other is that it gets the right amount of stellar radiation to support liquid water, as opposed to ice or vapor.
"We don't fully understand what makes a planet habitable, so we look for what we know," said theoretical astrophysicist Brad Hansen of the University of California, Los Angeles, who was not involved in the finding. "The basic assumption is that you need to have a rocky surface to stand on and liquid water for life to use."
Using data gathered by NASA's Kepler space-based telescope, the team of astronomers discovered a group of five planets orbiting a star 500 light-years from Earth. The star, called Kepler-186, is a relatively cool red dwarf about half the size of our sun. Four of the planets venture extremely close to the star, making them too hot for liquid water -- and therefore, life as we know it. But the outermost planet soaks in just enough energy for surface water to stay liquid.
Last year, the Kepler spacecraft discovered three exoplanets, all larger than Earth, within the habitable zone of two different stars. One of these three, Kepler-62f, is 40 percent larger than Earth, and previously held the record for the habitable exoplanet that is closest to the size of our planet. The newly found Kepler-186f set a new record by being only 10 percent larger than Earth.
"We thought it was special when we first saw the little blip in the data," said study author and astronomer Elisa Quintana of the SETI Institute. The findings were published online Thursday in the journal Science.
To find Kepler-186f, Ms. Quintana and her colleagues sifted through the mounds of data gathered by the telescope as it scrutinized one patch of the sky continuously for four years, looking for signs of planets outside our solar system.
Because the telescope can't see exoplanets directly, astronomers use a technique called the transit method to infer their presence. The light intensity from a star will normally read as continuous and flat -- but if a planet happens to pass between the telescope's field of view and that star, it will block some of the light and show up as a dip in the data.
So Kepler-186f may be close to the Earth in size, but is it truly Earth's twin? Most likely, no. For one thing, the planet is colder than Earth. The amount of stellar energy it receives is only one-third of the energy that the Earth gets from the sun.
"This planet actually receives less warmth than Mars does," said astronomer David Kipping at the Harvard-Smithsonian Center for Astrophysics, who was not involved in the study.
Also, the transit method provides information about a planet's size -- but not about its mass. "Because you don't get the mass, you don't know if it's a big rock, or a small rock with a big, gaseous atmosphere," UCLA's Mr. Hansen said.
While a smaller radius does mean Kepler-186f has a higher probability of having a rocky rather than gaseous surface, scientists at this point can only speculate about its physical composition.
Tidal locking -- or a planet orbiting with the same side always facing its star -- also has not been ruled out for Kepler-186f. The absence of a day-night cycle wouldn't rule out life entirely, but it would make for a world very different from ours.
"It would have one sunny side, and the other would be permanently dark, meaning it wouldn't have an Earth-like climate," said atmospheric scientist James Kasting of Penn State University, also not a part of the Kepler team.
Ms. Quintana says the planet is "more of an Earth cousin" than a twin -- but experts seem to agree that further composition analysis of bodies such as Kepler-186f is a necessary next step to find out what these Earth-like exoplanets are really like.
"Earth-like planets are very common -- they're actually all over the place," said Harvard's Mr. Kipping, whose specialty is exomoons. "Now, we just need to find the closest one."
In 2017, NASA will launch the Transiting Exoplanet Survey Satellite, or TESS, which will use an array of wide-field cameras to identify nearby exoplanets for further mass, composition and atmospheric analysis. Then, when the James Webb Space Telescope is launched in 2018, it will serve as the fine-toothed comb that measures the physical and chemical properties of those planets to assess the potential for life.