D-5: The King

The following was originally published on the Stellaris forums.


Welcome to another episode of the astroknowledge series! We’re approaching the end now, and I’ve been saving the best for you.

The search for habitable Earthlike planets is full of hype. NASA and ESA are both guilty of feeding people’s imaginations with (often absurdly) overoptimistic portrayals of every new planet we discover. Some astronomers like Steve Vogt, the discoverer of Gliese 581g, are inspiringly optimistic and hopeful people who see only what could be. Scarcely a month seems to go by without a lurid “Most Earthlike Planet Ever” headline. For the rest of us, particularly for pessimists like myself, this often gets annoying.

Therefore, it is nice to be able to discuss a planet which is unquestionably as good as it seems. This is a planet which Guillermo Torres (a profoundly realistic man) rates as a good prospect, and evenAndrew LePage cannot find any issues with.

Let’s talk about Kepler 186f. In 2014 she was discovered by the transit method which means we know her radius but not her mass, along with five other closer planets, orbiting a small, cool red star. The optimists portrayed her as the very twin of Earth. The pessimists have tried very hard to disagree, but we can’t.

Kepler 186f is probably habitable.

Throughout this series I have been trying to paint the rosiest picture possible, saying “may” instead of “probably does not” whenever possible. Here, I don’t have to. Even I can breathe out a sigh and say that after nearly two years of intensive study, Kepler 186f is a greater than fifty percent bet. In fact, she’s so good that SETI have been listening intently for radio transmissions from her. (So far nothing.)

(Artist’s impression courtesy NASA)

Let’s talk about the king.

Kepler 186f has a radius 1.1 times that of Earth; if she’s Earthlike then her mass will be about 1.4 times that of Earth, meaning that her effective surface gravity will be about 17% stronger. I’m 70kg; on Kepler 186f I would move like someone who’s 82kg. This isn’t even that noticeable.

Kepler 186f is on the outer edge of the habitable zone, but is still inside what’s known as the “conservative habitable zone”, meaning that we don’t need to invent fanciful scenarios for how she’ll be warm enough. A reasonable level of carbon dioxide will be enough to put her above 0 C. This will allow liquid water on her surface and water vapour in her atmosphere.

The problem with planets close to their stars is that they’re often tidally locked. In the case of Kepler 186, the first five of her six known planets are almost certainly tidally locked. Kepler 186f, however, is probably not tidally locked yet.

(Tidal lock is a matter of time: unless a planet has one or more moons large enough to prevent it, or unless the planet or star dies first, the planet will eventually become tidally locked. The bigger the planet is and further out it is the longer it will take.)

What would it be like to live there?

She’ll probably be on the way to a tidal lock, meaning that her day/night cycle will be slow, with each “day” taking weeks to months. This will likely cause intense weather as temperatures on the day and night side seek to equalise.

On the plus side, we think that her orbit is extremely regular and she has hardly any axial tilt, so she’ll have extremely mild seasons if she has any at all.

It is not only possible, but probable – and it feels amazing to type that – that if there is water on Kepler 186f, humans will be able to live there. The only thing stopping us is that 182 parsecs (500 light years) is a long way.

The fact that we know of 5 other planets in the system means that there will probably be lots of other stuff orbiting: asteroids, comets, stuff like that. The planets around Kepler 186 are close enough to the star that moons will have to be very small, but they might still exist. Moons would act to prevent tidal locking and make her even more habitable.

Oh, and she’s probably within a billion years of Earth’s age, which means that if she had a biosphere, it may have developed to similar levels. If life exists outside of Earth, this is our best shot at finding it.

This is it. This is real. We should go there.

Hail to the king.


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