D-34: The Dark World

The following piece was originally published on the Stellaris forums.


Today we’ll be talking about a planet that’s dear to many people and holds a special place in the history of humanity. I’m talking of course about Gliese 667Cc.

To the naked eye, Gliese 667 appears to be a single barely-visible star. When humanity started to use better telescopes it became apparent that it’s actually three dim stars close together. In 1957 Wilhelm Gliese named a thousand star or star systems after himself, and the 667th of these was this system (hence the name). It was thought to be unremarkable: a triple-star system quite close to Earth, consisting of two yellow K-class stars and one red M-class.

Gliese 667A and B are K-class stars only slightly smaller than our Sun, although they appear to be far dimmer because they contain large amounts of non-hydrogen elements. They’re slightly smaller than the Sun, and orbit one another very closely in a manner which physicists would describe as “very pretty” and non-physicists would describe as “horrible fiery chaotic death oh my god why did we come here.” Gliese 667C, meanwhile, is a reddish M-class star which stays a sensible distance from them.

Some decades after Wilhelm Gliese’s little exercise in narcissism, humanity started to search for exoplanets. Because Gliese 667 is quite close to earth we can get a lot of data from it, even if the stars are quite dim. In 2011, the European Southern Observatory’s HARPS group (who search for planets) announced that Gliese 667C had planets. The second planet, named Gliese 667Cc (and designated “Pilf” by Randall Munroe of XKCD) looked very interesting.

Gliese 667Cc is a big planet, about four times the mass of earth. This is within the range required to hold onto an oxygen-nitrogen atmosphere like humans breathe. Intriguingly, it also has a stable orbit quite close to its dim star, meaning that it’s surface temperature is probably warmer than earth but not too hot for liquid water. We call this the Goldilocks zone: not too heavy, not too light, not too hot, not too cold.

Just because it could have an atmosphere and water doesn’t mean it does have them, of course. However, in astrophysics, “not explicitly impossible” is very good news.


(Image courtesy Cornell)

Could life exist there?

Plausibly, yes. It was the first possibly inhabitable earth-sized exoplanet ever discovered, and even today it’s the third most inhabitable exoplanet we’ve come across. Gliese 667Cc is very exciting.

What would it be like to live there?

I am not a biologist, so this is speculation.

For a start, it’d be dark. All three Gliese 667 stars are dim for their class, and Gliese 667C is M-class which don’t get that bright anyway. What light it would get would mostly be towards the red end of the spectrum. If you have a dimmer switch in your house, then turn it down to about 20% and add a red filter: that’s what Gliese 667Cc would be like.

On the other hand, there is a lot of infrared light. This means that species adapted to life on Gliese 667Cc might use infrared vision instead of normal “visual spectrum” vision.

If you haven’t played with infrared vision, one of the best things about it is that your body, like all hot objects, emits infrared light. This means that hot objects like humans are extremely visible even in pitch blackness. It also means that you can use your own body as a lamp: objects near you will reflect your infrared light and you can see them with infrared goggles. Infrared can also pass through some barriers, including interior walls, so you would have limited “x-ray vision.” The downside of infrared is that you can’t see “colour” with it: you can’t tell white from black, and you can’t read a newspaper unless the ink is specially formulated.

This means that the inhabitants of Gliese 667Cc might be very sensitive to temperature, probably using subtle variations of it in the same way as we use colour. They may well keep their vehicles and habitations pitch black, since they don’t need visible light to navigate. On the other hand, they might not care as much about “colour” in the way we do. Their homes may be tasteless garish mixes of shades with no consistency, much as our homes would be tasteless garish mixes of temperatures with no consistency.

Infrared adaption is useful for cave-dwelling life, including artificial underground farming. The surface would always be more comfortable than underground, but people who became adapted to Gliese 667Cc might find it easy to go into tunnels or bunkers for long periods if they had to.

Gliese 667Cc is also thought to be hot, a few degrees hotter than Earth, which means it may well not have polar ice caps (or have them only seasonally, if it has seasons.) Organisms which live in hot climates are normally small, in order to maximise their surface-area-to-mass ratio and allow them to cool down easily. Given that Gliese 667Cc is larger than earth, its gravity may be stronger, which would also encourage smaller organisms. It would also encourage physical strength, possibly at the cost of quickness or endurance.

Tragically, infrared is a poor method of spotting stars. A species which evolved on Gliese 667Cc might never have developed astronomy, and as a result may never have thought to explore the galaxy.

Small sturdy blind heat-sensing underground dwellers with no desire to fly: that sounds like fantasy Dwarves to me, but that might just be me.

What else is in the system?

There may be as many as seven planets orbiting Gliese 667C. The other planet we know exists (Gliese 667Cb) is too close to the star to be habitable for humans, but Gliese 667Ce and Cf are within the Goldilocks zone. The downside of them is that they may not exist: they haven’t been confirmed by other studies and are thought to perhaps just be glitches in the data.

There may also be dwarf planets, asteroids, moons and other such things that we can’t see.

Further out, there are the twin yellow stars of Gliese 667A and B. From the surface of Gliese 667Cc they’d be little bright pinpricks – unless you had infrared vision, of course, in which case they’d be little dim pinpricks. Nonetheless, they might have the same symbolic meaning to inhabitants of Gliese 667Cc that Earth’s Moon has to us.


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