Kepler 186-f: Another Earth?

Today, Kepler 186-f is being announced as the first planet discovered orbiting another star that shares the same basic properties as Earth. This is a monumental discovery in science that fulfills one of the Kepler mission’s major goals and brings us further down the path towards even bigger discoveries. This is the Kepler candidate I listed seventh one week ago on a list of possible earthlike planets, and now it’s the first one to be announced as confirmed.

What prompted this news?

Kepler “sees” planets as the star they orbit dims slightly when planets pass in front of them, blocking a tiny fraction of its light. These signals are hard to pick out, so discoveries begin as candidates, when we think they might be real, and are called confirmed when further investigation indicates they are (almost) certainly real.

We already knew that this discovery, if it turned out to be real, looked pretty earthlike, which is why it was high on my list. The news indicates that it is indeed a real planet.

What do we know about it?

We know that its star, a red dwarf (class M), is smaller and cooler than the Sun (class G). We know that the planet orbits its star every 130 days and is one of at least five planets orbiting the star. It orbits the star at about 40% the Earth’s distance from the Sun, which is the same distance that Mercury is from our Sun. But because its star is much cooler than the Sun, the amount of heat that the planet receives is about the same as the Earth does.

The estimated size of the planet is just a bit bigger than the Earth. For now, however, there is considerable uncertainty in that estimate, so it may turn out to be significantly bigger or smaller.

We have no way at present of estimating what Kepler 186-f’s climate might be like. It could be much hotter or colder than Earth, lack the kind of atmosphere that Earth has, or be suffocating under a much thicker atmosphere. We won’t know until we gather more data about planets of this size and temperature how often they evolve to be more like Venus (too hot), Mars (too cold), Earth (just right), or something else.

Can a red dwarf star support an earthlike planet?

There is speculation that a red dwarf might be a bad place for an earth-sized planet to become earthlike in other ways, because the planet has to orbit close-in to get enough warmth. At that distance, the tides that the star causes on the planet might cause an excess of volcanic activity, and/or force the planet’s rotation to keep one side always facing the star in eternal day and the other side in eternal night. Either of those things could make the development of life difficult or even impossible. But, that is all speculative, and in any case, Kepler 186-f orbits out at a relatively long period of 130 days, which is longer than Mercury’s. Mercury rotates in synch with its orbit in an interesting way, but it doesn’t have that eternal day-night divide, nor does it have tidally-powered volcanoes, so Kepler 186-f may escape that fate.

When will we know more?

We probably know about as much about Kepler 186-f now as we knew about Mars four hundred years ago, before the invention of the telescope. If we could see Kepler 186-f through a telescope, we could learn a lot about it, but that’s a huge challenge because it’s 500 light years away, and so close to its star that our best telescopes couldn’t even separate it from the star, which is millions of times brighter than the planet.

That distance also means that even if a spacecraft left tomorrow to go visit it, going at 99% the speed of light, it would still take 1000 years for us to get the data from that mission, so that’s simply not an option.

A future telescope, superior to any that have ever yet been planned, might be able to give us more information, perhaps by studying the light from its entire system and subtracting the light we receive when Kepler 186-f is behind its star from the light we receive when it is not.

The good news, however, is that we expect many other stars to have planets like this, and many of those stars will be much closer to us than Kepler 186-f. By the time we have a telescope that could perform more detailed scientific studies of Kepler 186-f, we will almost certainly know about many more planets to look at, and return more detailed information about the ones that are closer than 500 light years.

This discovery, however, should excite interest in the construction of such telescopes, which have been proposed but not approved.

Could Kepler 186-f have intelligent life?

The prevalence of extraterrestrial intelligence is something we can only speculate about today, but efforts like the SETI project, which search for radio signals that an extraterrestrial civilization might be sending, probably have no better single target than Kepler 186-f. All we can do is point our radio telescopes that way, listen, and wait.