Reality Is Always Stranger Than Fiction
I’ve talked before about the perils of common sense. This is especially true in astronomy. This is being highlighted by the ongoing discoveries in exoplanets. You see, in years past, people assumed other solar systems must be like ours. Terrestrial planets close in, gas giants further out, and if you were looking for ‘inhabitable’ planets, then red dwarfs were absolutely out. Only it’s emerging that this nice and tidy, familiar picture has the common sense problem – put simply, it’s just wrong.
This has big implications for scientists and fiction writers alike. If you’re considering using an M dwarf as a setting, for instance, then it’s worth having a think about which bits of our ‘common sense’ might be horrifically misleading there. And this is all quite topical at the moment.
Red dwarfs have been in the news again – in particular, Gliese 581. It seems that Gliese 581 does indeed have a 3-Earth-mass planet sat in the star’s biozone. (Just to expand on my previous comments, I’ve read the paper now, and I believe them.) Now, there’s a small health warning here – this detection is a radial velocity measurement, not a transit measurement. What this means is that the planet has a minimum mass estimate, but not a radius to go with it. Depending on the inclination of the planet’s orbit, the actual mass may be rather different from the measured mass (I can expand on this if anyone would like, but the discussion will be a bit technical).
Normally, this would be a big issue. However, apparently, for the Gliese 581 system, computer modelling of the orbits suggests that they pretty much have to be in a certain arrangement for the system to be stable. The planets are tightly packed, close enough together that you can’t just ignore their gravitational interactions. All of these are bigger and heavier than Earth, and all seven of them are packed into a region only just bigger than the orbit of Venus. (It’s possible that they actually look like small discs, not points, in each others’ skies. These planets are *close*.)
Incidentally, 20 years ago, no-one would have expected this. If anyone had described a system architecture like this, the result would have been amusement or incredulity. Except, like the title says, reality is always stranger than fiction. (Hot Jupiters, pulsar planets, densely-packed terrestrials … if anything, it’s beginning to look like our solar system might be the oddity, not the other way round.)
The point is, because of the required architecture for the Gl 581 system, this actually constrains the possible inclinations quite nicely. And it largely gets rid of the usual m sin i issue. For once, we can be relatively happy with the numbers for the Gliese planets.
So, we appear to have a biozone planet. It’s not an exact Earth clone – it’s heavier by a factor of three and it’s presumably cooler too. The ‘equilibrium temperature’ (temperature ignoring the effects of atmosphere) is something like -50 C. Assuming a terrestrial atmosphere, the planet as a whole goes up to about -20 C. But you’ll see the word ‘assuming’ lurking in there.
This is where I start getting twitchy.
You see, the planet is ‘terrestrial’ in the sense that it’s very likely rocky in composition. Whatever else it’s like, however, it certainly won’t be a duplicate of Earth. Surface conditions will be quite different. First off, it orbits close to an M dwarf. This means it is likely to be tidally locked – the star’s tides will have spun the planet’s rotation down, until one day lasts the same time as one orbit. One face permanently looks at the sun, the other looks away. (And the tidal bulge remains radially-aligned with the star at all times, so it can’t exert any friction against the rest of the planet, so no further spin-down occurs.)
So the daylight hemisphere is continually sunny. Presumably this means it will be a bit warmer than -20!
But there will be more differences.
I’m not specifically describing Gl 581g here, but think of a roughly Earth-mass planet around an M dwarf. The weather will be different. There won’t be much in the way of hurricanes (slower rotation, so not much Coriolis effect). Also there won’t really be any trade windws, for the same reason – not much rotation, so not much deflection of Hadley cells. (Odd implication … if we lived on a planet around an M dwarf, Columbus would never have got to the Americas. He’d end up becalmed somewhere in the Atlantic. Cross-continental travel would presumably have to wait until the invention of the propellor.)
Also, although there are no hurricanes, some aspects of the weather may be much more extreme.
‘Instability’ occurs when air is warmer than its surroundings. At this point, it expands and will rise. Once unstable air starts moving updwards, it will keep going until something stops it. In our atmosphere, the ozone layer creates an effective ‘lid’ on the instability. It does this because ozone absorbs ultraviolet – and heats up in the process. Thus, after cooling with altitude for many miles, the air suddenly starts to get warmer again. This point is the bottom of the stratosphere.
But, M dwarfs don’t emit much light in the UV. In fact, they emit hardly any at all.
Presumably, then, a terrestrial planet around an M dwarf won’t have an ozone layer. (Ozone needs UV to form in the first place.) Also, no ozone, no UV absorption so no inversion layer. A cloud on such a planet presumably carries on rising until it hits the point where the mechanical pressure of the air can no longer support its weight.
Presumably at this point, many, many miles up in the sky, it drops like a rock.
So you don’t get a day-night cycle. You don’t get hurricanes, trade winds or sunburn. But you do get huge, torrential downpours every now and then, when a cloud decides to fall out of the sky. Probably also there are accompanying vertical gusts of wind, plunging down from what should be (but isn’t) a stratosphere. Probably also thunder storms, hail and lightning. And that’s just on the day face – the night side is likely to be a completely different kettle of fish.
The point I’m making is that, for any purely-speculative human colony*, such an environment would be a very strange place to live. Lots of things that we’ve been conditioned – evolved, even – to expect would simply be false. ‘Common sense’ would be even more of a train-wreck then it is here. Reality would indeed be stranger than any fiction.
*This ignores, of course, the rather significant logistical log-jam that is getting to another star in the first place.