Last week’s big astronomy news was the discovery of a planet nearly the same size as Earth. This new discovery, known as Kepler-186f, is well within its host star’s habitable region, so it could conceivably have liquid water on its surface. Since all life as we know it requires liquid water in some form, that’s a reasonable first criterion for looking for life elsewhere in the cosmos. The habitable zone, then, is the sweet spot of a star system: close enough to the star that all water doesn’t freeze, but not so close as to boil the water (or even the atmosphere) away. Update: since I inexplicably failed to say this earlier, the Kepler-186 system is about 500 light-years away from Earth.

I covered this story in my column for The Daily Beast:

Small red stars vastly outnumber their larger cousins, and the new exoplanet is orbiting one of those. That already means Kepler-186f isn’t quite Earth-like: its orbit is smaller than Mercury’s in the Solar System. However, because the star is less than half the diameter of the Sun, it emits a lot less light, meaning the planet only gets around one-third of the light Earth gets. However, it’s enough warmth to place it in the star’s habitable zone. [Read more…]

There are a couple of interesting details I didn’t have space for in that piece. Astronomers have found a number of exoplanets in the habitable zone; like Kepler-186f, those systems contain red dwarf stars. These stars are much smaller and fainter than the Sun, so the habitable zone is correspondingly smaller. The inner edge, for example, is close enough to the star that an exoplanet orbiting there would be tidally locked, presenting the same face to the star, much as Earth’s Moon does. That could be problematic for life or even liquid water, because the side closest to the star would have eternal day, while the opposite side would be night forever. However, Kepler-186f is farther out, meaning it should rotate at a more reasonable rate. That’s a point in its favor for habitability — at least based on what we know about Earth.

Of course, it’s far too early to say if there’s water of any form on Kepler-186f, much less liquid oceans. We don’t know if the planet is rocky or a mixture of rock and ice, or anything about its atmosphere (if it even has one). We can only speculate about life bathed in light from a red star. However, it’s still exciting, and that much closer to an Earth-like world.