Inhabitable planets. Aaah, that phrase.
It’s a phrase that can hide a world of sins. You come across it a lot in science fiction, and it’s turning into one of my pet peeves. Briefly, the ‘consensus’ definition appears to be a planet where an adult human being wouldn’t need a spacesuit. Call this the spacesuit test, if you will!
Presumably, this planet doesn’t make the grade…
But there’s a problem there. That can cover a huge range of environments. Sure enough, you see this in SF, from really well-thought-through and believable worlds all the way to single-biome stereotype-pools.
But it’s worse than that. Even if a planet is technically inhabitable, that doesn’t mean it will be Earthlike. I got to thinking about this in depth a few years ago, during my brief (and wholly-disastrous) experience of GM’ing an RPG campaign. Although the campaign was a trainwreck (I take full responsibility for being useless) and failed after just four sessions, it did get me to thinking about just what exactly you would experience when setting foot on an alien planet. When you have to describe things to people, you really need to think about them first, and it does help if you can create a strong visual image. Let’s take an example.
A terrestrial-mass planet orbiting a red dwarf.
It’s a dwarf and it’s red(ish). They do exactly what they say on the tin…
Red dwarfs are the most common type of star (around 80%, by numbers) and it appears many of them have planets. So they aren’t absurd to pick on. However, unlike our Sun, red dwarfs are very faint (a very bright red dwarf might just scrape up to eight percent the luminosity of our Sun). There isn’t a single one that you can see with the naked eye here on Earth; no-one had any idea they even existed until the invention of the telescope.
So, any planet that could pass the spacesuit test is going to have to orbit close to the star. If it’s as far out as the Earth, it will be a frozen ball of ice and the weather forecast may involve it raining nitrogen – not humanly-convenient, to say the least! In principle, you can snuggle planets up close enough to such stars – we know of red dwarfs with close-in Jovian planets, and even a few terrestrial-mass-range companions.
However, there is a problem. It’s called tidal locking. The star’s tides will, over millions of years, spin the planet down until it completes one rotation in the time it takes one orbit, thus always presenting the same face to the star. One side will be daylight, forever, and the other night, forever. (And this is quite unavoidable for a planet so close in.) It used to be thought that this was a planet-killer; such extreme temperature variations would prevent any kind of life. However, current research suggests that atmospheres can ameliorate the extremes. Even a thin atmosphere should be enough to keep the dayside hot-but-liveable and the nightside cold-but-liveable. Wind – isn’t it great?
Let’s just quickly take stock. We have a planet orbiting a small, reddish sun. Its ‘year’ lasts several terrestrial days to maybe a month at the most. On one side of the planet, the sun never sets. On the other, it never rises. So far, so weird!
On the surface
Red dwarf, strange shoreline.
The planet would be tide-locked, so the sun would never move in the sky*. Plants would presumably grow toward the sun, but unlike Earthly sunflowers, they wouldn’t turn during the day. Also, the vegetation would be rather Goth. It would need to make the most of the weak light, and be particularly good at soaking up the near-infrared. Plants would probably be black rather then green.
The sky would be odd. Our sky is blue due to Rayleigh scattering; short-wavelength photons such as blue light scatter more than longer-wavelength ones. But a red dwarf is brightest in the infrared; they don’t supply much ‘blue’ flux at all**. So without much light to scatter, even during the day the sky will be dark. It will be brightest near the horizon, where you’re looking through the thickest cross-section of air (so more air to scatter the few blue photons). Presumably you would get a sort of turquoise band, that fades rapidly to navy-blue or even black. There’ll probably be another bluer region near the star itself, too. But overhead, inkiness rules. If you shade your eyes from the sun, you could probably see stars, even by daylight.
Also, the sun itself will be odd. It will seem much bigger than ours does, even though M dwarfs are all physically-smaller. A habitable planet around an M dwarf will have to be close. (A terrestrial world around Proxima Centauri, for instance, would have a 7-Earth-day orbit.) Even though the star is smaller than our Sun, the planet will be so much closer, and that will result in a huge, fat-looking sun.
But that sun won’t dazzle you. I mentioned that most of M dwarf flux is in the infrared – you would probably be able to stare right at the star, even by daylight. It would be bright, yes, but it won’t blind you. What you would notice, though, would be the heat. It would be something like standing next to a baker’s oven with the door open. Human skin can register infrared, and there is a lot of IR coming off this thing.
The point I’m trying to make is, this planet might be inhabitable, but it wouldn’t be Earthlike.
*Or at least, not much. Libration and obliquity would complicate this picture, and also any eccentricity in the orbit. However, you wouldn’t expect much of either obliquity or eccentricity this deep in a star’s gravity well, so although I’m wrong, I’m not too wrong here.
**B-R > 1.0, if I recall correctly. Or to put it another, non-logarithmic way, the ‘blue’ band (B) has less than half the amount of flux as the ‘red’ band (R).