Standing on the shore at dusk, you see four planets and an orange sun near the horizon. At night, dazzling ribbons of auroras snake across the sky. You are on Kepler-186f, the smallest known exoplanet thought capable of supporting life.
鈥淥f all the planets we鈥檝e seen, this is the one that reminds us most of home,鈥� says at NASA鈥檚 Ames Research Center in Moffett Field, California. Barclay and his colleagues spotted the planet in data from NASA鈥檚 Kepler space telescope. Only 10 per cent larger than Earth, it is the smallest world outside our solar system that is known to orbit in the habitable zone, the region around a star where liquid water can exist.
Astronomers have found other small planets in the habitable zones of their host stars. But until now, the ones closest in size to our home world were Gliese-667Cf and Gliese-667Ce, which are both 1.4 times the size of Earth and live in a triple star system. Models suggest that planets near this size could have rocky surfaces, but it is also possible that they are more like Neptune, with much smaller cores covered by puffy atmospheres.
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Evening stars
鈥淲hile these planets could well be rocky, we don鈥檛 really know. We don鈥檛 have anything that size in our solar system, so we don鈥檛 have a proxy,鈥� says Barclay. 鈥淏ut we have two Earth-sized planets around our star, Venus and Earth, and they鈥檙e both rocky.鈥�
Kepler-186f orbits a dim red dwarf, so even with clear skies it wouldn鈥檛 get much brighter there than an overcast day on Earth. If the planet has an atmosphere like ours, though, things would get much more vibrant at night. The star was already known to host four other planets slightly bigger than Earth. These worlds are too close to the star to be habitable, but they would shine in the skies of Kepler-186f around dawn or dusk, just as Venus and Mercury do from Earth.
Also, red dwarf stars are highly active, producing frequent flares and strong winds of radiation. And as Kepler-186f is slightly larger than Earth, if it has a similar internal structure, it would support a stronger magnetic field. That would spark interactions between any potential atmosphere and stellar radiation that could create frequent and spectacular auroras visible from large regions of the planet.
Confidence boost
鈥淚t sounds like a great planet to visit, if we could figure out how to travel there,鈥� says Sara Seager of the Massachusetts Institute of Technology.
The system is about 500 light years away, so there is probably no way to tell whether any life forms are enjoying the sky shows on this particular world. But finding another small planet in the habitable zone bodes well for planned missions that hope to probe exoplanet atmospheres and hunt for signs of life.
鈥淧ersonally I find it simply awesome that we live in a time when finding potentially habitable planets is common, and the method to find them is standardised,鈥� says Seager. 鈥淭his gives us confidence for the future.鈥�
Journal reference: Science, DOI: 10.1126/science.1249403
