NASA’s Kepler mission has discovered a 5-planet system that includes a hot Mars and four super-Earths, two of which might host liquid water. These aren’t quite the Earth-like exoplanets Kepler’s been looking for, but they’re close.
In a system 1,200 light-years away, five small planets whizz around an orange star about two-thirds the size of our Sun. One of them is a hot Mars, a planet half the size of Earth that whips around the star every 12.4 days. Another two are hot super-Earths, completing orbits every 5.7 days and 18.2 days. But the final two are the ones that capture the imagination: half again the size of Earth and orbiting in the star’s habitable zone, these worlds could be the pale blue dots we’ve been looking for. Maybe.
“I'd be hesitant to call any of these worlds potentially 'Earth-like.'” says Caleb Scharf (Columbia University), an exoplanet expert not involved in the study. “But their discovery is definitely leading us closer and closer to places that might represent alien, but nonetheless similar, environments to our own.”
The planets Kepler-62e and Kepler-62f are 60% and 40% bigger than Earth, respectively, and their orbits last 122 days and 267 days, William Borucki (NASA Ames Research Center) and his team report this week in the journal Science. But the team is careful to point out that planets so small are difficult to characterize — even pinning down their masses will be a challenge.
To find these planets, Kepler has been snapping pictures of more than 150,000 stars roughly every 30 minutes for more than four years now, looking for the periodic dips in brightness that could signal a planet passing in front of its star. Kepler-62 is one of these, a hydrogen-fusing K2 star almost twice as old as the Sun.
Kepler’s automatic pipeline searches the hundreds of thousands of stars for brightness variations, and this pipeline discovered four of the five exoplanets. But the fifth, Kepler-62f, required a more hands-on approach. The planet has only made three transits so far, and one of these transits came too close to a data-transfer break for the pipeline’s liking. A more careful look by coauthors Eric Agol and Brian Lee (University of Washington) identified the third transit, upgrading the signal to planet status.
The holy grail of exoplanet searches is finding an Earth-sized planet in a star’s habitable zone, but that’s more easily defined in words — “the region around a star where a rocky planet with an atmosphere could host liquid water on its surface” — than in practice. Borucki’s team used two approaches to measure the habitable zone. The first assumes any planets are rocky with carbon dioxide-water vapor atmospheres. Under these strict assumptions, Kepler-62f would receive enough stellar flux to keep liquid water on its surface with the help of the greenhouse effects, but Kepler-62e would be too hot.
A more liberal approach defines the habitable zone by calculating the solar flux Venus and Mars received when they were still able to host liquid water. This definition allows Kepler-62e to sneak into the warm end of the habitable zone.
Even if we take the latter tack, are the planets actually habitable? Their masses are too low to nail down via wiggles in the star’s position or variations in the transits’ timing, so with just their sizes we can only speculate about what they’re made of. Still, by not finding these other signals, the astronomers can limit Kepler-62e to 36 Earth masses and Kepler-62f to 35 Earth masses. Those limits are enough to confirm that the bodies are planets.
The two planets’ diameters, combined with their distance from the star and the star’s age, suggest both worlds have likely lost any thick, Jupiter-esque gas envelope that might have enshrouded them in their early years. So, depending on their exact masses, the planets are either waterworlds or rocky.
“If they're Earth-mass, they'll be strange water-rich worlds,” Scharf speculates. “It's not inconceivable that they could harbor a liquid water ocean close to the surface, but this would presumably have to be a very deep ocean, turning to increasingly pressurized phases of ice at depth.”
Even if the planets are many times Earth’s mass, they could be still rocky instead of watery, Scharf says. But at such large masses, the rocky planets would be decidedly different from Earth, possibly very geologically active. Then again, since Kepler-62 is 7 billion years old, there’s also a chance the planets could have simmered down by now.
There’s still a chance of getting a better answer someday. By continuing to measure transits over many years, the team could one day determine exact masses by measuring the gravitational effect of interplanetary tugs. With Kepler scheduled to fly at least until 2016 and possibly beyond, there’s a chance this method could one day provide better answers about these planets’ nature.
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