The hunt for Earth’s alien twin reached a new milestone with the discovery of a faraway planet that’s similar in size to our globe and has the right temperature to potentially support liquid water — and possibly life.
The planet, called Kepler-186f, is the first Earth-size sphere outside our solar system with an orbit around its star that is in the “Goldilocks zone” — the habitable range that is neither too hot nor too cold.
Researchers say the planet may meet what they believe are two basic requirements for life. Being close to Earth’s size increases the chances that the planet has a rocky, rather than gaseous, surface. And because it gets the right amount of stellar radiation, Kepler-186f is able 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 UCLA, 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 the exoplanets, 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, 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 passes between the telescope’s field of view and that star, it will block some 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 a 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,” 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 — when a planet orbits with the same side always facing its star — 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.
Quintana says that the planet is “more of an Earth cousin” than a twin — but experts seem to agree that further composition analysis of bodies like 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 Kipping. “Now we just need to find the closest one.”
In 2017, NASA will launch the Transiting Exoplanet Survey Satellite (TESS), which will use an array of wide-field cameras to identify nearby exoplanets for further mass, composition and atmospheric analysis. The James Webb Space Telescope. scheduled to be launched in 2018, will serve as the fine-toothed comb that measures the physical and chemical properties of those planets to assess the potential for life.
But some scientists argue that the transit method, which TESS will also use, has drawbacks. In some ways, it requires a happy accident to work — the planet has to be aligned edge-wise along the observer’s line of sight, passing between the observer and the star. A tighter orbit around the star or a larger planet-to-star size ratio can increase the probability of seeing the characteristic dip in light values.
But this leaves many exoplanets and star systems largely undetected. For instance, even Earth itself — given its distance from the Sun and comparatively small size — would likely not be easy to spot using the transit method from afar.
Kim is a freelance science journalist based in Philadelphia.