Man, I am just a killjoy. So, let’s get the positive out of the way: it is way cool that 7 little – as in, not gas giant – planets were found around a ‘nearby’ in the sense of unimaginably and unreachably distant, star.
Almost got through a paragraph without getting snarky. Oh, well. Seriously, exoplanets are fun. If they ever actually find any sign of extraterrestrial life, that will be fun, too! But finding cool little planets isn’t the same as finding signs of extraterrestrial life. Oops, there I go again.
Let’s go with the NASA press release, to see Our Tax Dollars at Work: NASA Telescope Reveals Largest Batch of Earth-Size, Habitable-Zone Planets Around Single Star. Wow, the artist’s rendition, which seems to be required by law to accompany any NASA press release no matter how scanty the information, makes it look like what we have here are 7 very attractive and detailed – friendly, even – little earth-sized planets!
That looks like fun. Here, let me play:
But enough with the attempts at humor, at least until some other funny thought strikes me. The opening paragraphs state:
NASA’s Spitzer Space Telescope has revealed the first known system of seven Earth-size planets around a single star. Three of these planets are firmly located in the habitable zone, the area around the parent star where a rocky planet is most likely to have liquid water.
The discovery sets a new record for greatest number of habitable-zone planets found around a single star outside our solar system. All of these seven planets could have liquid water – key to life as we know it – under the right atmospheric conditions, but the chances are highest with the three in the habitable zone.
What NASA thinks the tax-paying public is most likely to be wowed by is: Alien Life! Therefore, it deploys the terms “habitable zone” (three time), “liquid water” (twice) and “life as we know it” in the first two paragraphs. The opening ends with a suggestion that there’s a chance – a pretty good chance, right? – that such conditions as would make a planet ‘habitable’ are right here on all 7 planets, but: “…the chances are highest with the three in the habitable zone.” So, one might suppose there’s a better than decent chance of life on those 3 planets in the habitable zone. Pretty exciting, eh?
One has to read all the way to paragraph 11 to discover that the star is an “ultra-cool dwarf”, which, while it sounds kind of cool, ultra cool, even, has some drawbacks: such stars are so cool for stars that their habitable zone is very, very close to them as opposed to stars like the sun. Planets must be very close, in other words, to potentially have the right temperature range for liquid water to exist on them.
Such close orbits present a problem: “The planets may also be tidally locked to their star.” At the very least (and some celestial mechanic out there please straighten me out on this if I’ve misunderstood) this means these planets orbiting close to such a star would be subject to tidal forces that strongly tend to slow down their rotation, sometimes, as is the case with our own moon, ‘tidally locking’ the smaller body so that it rotates exactly once per orbit. Sometimes, as with the roughly similar-sized Charon and Pluto, *both* bodies get tidally locked. Sometimes – and I don’t think this is very well understood (1) – the smaller body will fall into some sort of resonance period – 3 revolutions for every 2 orbits, as is the case with Mercury.
Full tidal locking would result in a planet with one relatively scorching side and one freezing side. If there were an atmosphere, it would tend to heat up and expand on the sunward side, and flow to the night side, where it would cool and maybe even freeze. If liquid water evaporated, it would suffer the same fate. I would imagine that, over time, like a few million years, the atmosphere would get thinner and thinner on the sunlit side until the ice on the dark side could evaporate into space – atmosphere and ice would be lost.
Be that as it may, a tidally locked planet seems very unlikely to be ‘habitable’ if we mean ‘life as we know it could live and develop there.’ (2) Like the economist with one foot on fire and one foot in a block of ice, on average things might be OK, but in practice they are not. The situation would be more complicated but not much better on any planets with resonance periods like Mercury – really slow rotational periods allow the sunward side to get hotter and the night side to get colder than a quicker rotation, which could result in the same situation as fully locker planets – it might just take longer. (3)
Enough of my pessimism. I can only think of one tidally locked planet in SciFi, a throw-away world in the third (I think) Foundation book, with stations on the thin twilight zone. I’m sure other have done it, too. It would much fun to make up a way, somehow, that an advanced civilization could develop on such a world….
But don’t hold your breath over TRAPPIST-1, even if that’s a pretty cool name.
- Meaning: I’ve given it a shot, but don’t understand it as well as I’d like. In a bit of astronomy/egomaniacal irony, the entire Universe revolves around ME! The Omphalos Wikipedia: “Mercury is tidally or gravitationally locked with the Sun in a 3:2 resonance, and rotates in a way that is unique in the Solar System. As seen relative to the fixed stars, it rotates on its axis exactly three times for every two revolutions it makes around the Sun.[a] As seen from the Sun, in a frame of reference that rotates with the orbital motion, it appears to rotate only once every two Mercurian years. An observer on Mercury would therefore see only one day every two years.”
- This is granting the as yet unevidenced principle that life will just ‘arise’ whenever conditions are ‘right’, given enough time. Let’s see example #2 of life – you know, extraterrestrial life – before we start generalizing principles, shall we?
- Some of the other articles I perused called ultra cool dwarf stars ‘overlooked’. I kind of doubt that – you’d focus on stars around the size of the sun, because that’s where planets can end up in the Goldilocks Zone without getting tidally locked – as we know from our own planet. Planets around much smaller stars will have that problem; much bigger stars tend to blow up well within the several billion years it is assumed to take for life to develop. So, if you’re looking for another earth, you’d look around stars that look like another sun.