Guest Editorial

We have another ‘habitable zone’ planet to talk about, one not much bigger than the Earth, but it’s probably also time to renew the caveat that using the word ‘habitable’ carries with it no guarantees. The working definition of habitable zone right now is that orbital distance within which liquid water might exist on the surface of a planet. Whether it actually does is just one of the questions. A second is whether or not we’re in fact dealing with a rocky terrestrial world.

So we the announcement of Kepler-186f with guarded enthusiasm for an exoplanet that looks interesting indeed. Five planets circle this star, an M-dwarf a great deal smaller and cooler than the Sun. Discovered by the Kepler space observatory, the planet presents us with transit information telling us that it is about 1.1 Earth radii, although we don’t yet know what the mass of this world is, and hence can’t make a definitive call on whether or not it is rocky. But Stephen Kane (San Francisco State), one of the researchers involved in today’s announcement, thinks we have reason to think that it is:

“What we’ve learned, just over the past few years, is that there is a definite transition which occurs around about 1.5 Earth radii,” said Kane. “What happens there is that for radii between 1.5 and 2 Earth radii, the planet becomes massive enough that it starts to accumulate a very thick hydrogen and helium atmosphere, so it starts to resemble the gas giants of our solar system rather than anything else that we see as terrestrial.”

Kepler-186f is thus well below the value where we would expect it to accumulate a thick hydrogen and helium envelope, causing Kane to add “there’s a very excellent chance that it does have a rocky surface like the Earth.” If that’s the case, then we have a planet on the outer edge of its star’s habitable zone, though one that may have a somewhat thicker atmosphere than Earth’s because of its somewhat larger size. Perhaps the surface can avoid freezing. In any case, this is what lead author Elisa Quintana (NASA Ames) calls “the first definitive Earth-sized planet found in the habitable zone around another star.” The work appeared in Science.

The discovery team used so-called ‘speckle imaging’ in obtaining its high resolution observations from the eight-meter Gemini North telescope on Mauna Kea as well as adaptive optics observations from the ten-meter Keck II telescope to rule out extraneous sources that could account for the Kepler data, concluding that the signal has to be that of a transiting planet. The speckle data allowed direct imaging of the system to within 400 million miles, confirming there were no other stellar-sized objects orbiting within this distance from the star. “The Keck and Gemini data are two key pieces of this puzzle,” adds Quintana. “Without these complementary observations we wouldn'’t have been able to confirm this Earth-sized planet.”

The new planet orbits its star once every 130 days, receiving about a third of the heat energy that Earth does from the Sun. The four inner planets — Kepler-186b, Kepler-186c, Kepler-186d, and Kepler-186e — are all too hot for life as we know it, with periods of 3, 7, 13 and 22 days.

The objection to Kepler-186f as a home for life rests on the dangers of orbiting an M-dwarf, a class of star prone to flare activity. Move a planet close enough to the star to be in its habitable zone and the assumption is that it’s also tidally locked, presenting the same side to the star throughout its orbit, with all the complications that brings to climate models. Neither of these factors are complete show-stoppers — some climate studies show that temperature extremes can be mitigated by winds or ocean currents — and in the case of Kepler-186f, we do have a world on the habitable zone’s outer edge, perhaps far enough out not to suffer tidal lock.

So it’s an interesting place, this new world, about 490 light years away in the constellation Cygnus and thus tantalizingly out of reach for atmospheric analysis even with instrumentation planned for the near future. The James Webb Space Telescope itself won’t be able to help us with that task. But it’s pleasing to note that Kepler-186f has been studied over a frequency range of 1 to 10 GHz looking for emissions, though none has so far been found. Getting a detectable signal here from this star would require a transmitter between 10 and 20 times as powerful as the planetary radar system at Arecibo. SETI keeps coming up empty, but good for us if, in addition to our other studies, we keep our ears open for a long-shot detection.

The paper is Quintana et al., “An Earth-Sized Planet in the Habitable Zone of a Cool Star,” Science Vol 344, No. 6181 (18 April 2014), pp. 277-280.

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