Planets of Eternal Midnight

So planets are those lumpy things that orbit stars, right? Well, if all of the recent nonsense about definitions is to be taken seriously. That’s why there are ridiculous terms like “planetar” and “planemo” being thrown about.

As far as I’m concerned though, if it’s round and smaller than a star, it’s a planet. But planets get flung about quite wildly as they form. Statistically, a few must get thrown out of their parent star system altogether. You’d think they’d just end up lost in the voids of interstellar space, frozen, dead and alone… You’d think.

Dave Stevenson of the California Institute of Technology decided in 1998 that he wasn’t so sure. Submitting a paper to nature entitled “Life-sustaining planets in interstellar space?”. (If you don’t have a Nature subscription, you can download a copy of the original here — it was actually submitted on my birthday!)

With no stellar wind to ablate them all away, he hypothesised that these “rogue planets” could hold on to thick atmospheres of hydrogen gas. Due to the insulation provided by this hydrogen atmosphere, he even hypothesised that these planets could retain enough surface warmth to have liquid water. They could potentially remain hot for some time, courtesy of geothermal energy and radioactive isotopes decaying. In turn, should one of these planets retain a moon (and apparently, around 5% of ejected Earth-sized planets could), tidal heating could also be a contributory factor.

It sounds far fetched, but his theory looks pretty sound to me. Dare I say it, rogue planets have long been a mainstay of science fiction (by Arthur C Clarke amongst others), but it’s almost certain that there must be at least a few out there. By probability alone, I put it to you that the Milky Way is large enough that if it’s possible, it has probably happened. In turn, by basic celestial mechanics, conservation of momentum causes the smallest member of a three body system to be ejected readily. With the right trajectory, a gravitational slingshot could easily set such a planet on a course for interstellar space. Just like the Voyager probes.

It’s a lovely idea, if nothing else. As always, it’s enough to make you wonder what might be drifting silently out there waiting to be discovered…

By the way, that image was taken over the Atacama Desert. It’s by a guy called Serge Brunier as part of The World At Night. Go and check it out, some of those photos are truly awesome!

About Invader Xan

Molecular astrophysicist, usually found writing frenziedly, staring at the sky, or drinking mojitos.
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21 Responses to Planets of Eternal Midnight

  1. Given the etymology of “planet”, an interstellar one would actually more deserve the name. :-)

  2. invaderxan says:

    Re: Bee Sight
    Hmmm… Interesting, interesting. I wonder how many animals can discern it with the right preparation.
    The fact remains though — those shrimp are still probably a lot better at it than us!

  3. invaderxan says:

    Re: Vinge Classics
    Nice! Thanks for the recommendations!

  4. Anonymous says:

    Bee Sight
    Hi InvaderXan
    Humans can see polarised light too. Given the right preparation we can see the magnetic and electric axes as little crosses that are sharper or blurrier depending on the degree of polarization – see Nick Herbert’s book “Elemental Mind” for a discussion.

  5. Anonymous says:

    Vinge Classics
    Hi InvaderXan
    “The Peace War” and “Marooned in Real-Time” are two good ones about ‘survivors’ of the Singularity – Vinge almost single-handedly made it a reasonable topic of discussion at NASA for example. “A Fire in the Deep” and “A Deepness in the Sky” are good space-opera in a rather unique Galactography – the speed of computation varies in different parts of the Galaxy. In our part, “the Slowness”, it’s limited to lightspeed, but away from the Disk the speed increases rapidly, while closer to the Core it goes down.

  6. pax_athena says:

    Oh, thank you! Cool, I understood that (what is also a cool realisation) and I even think that it fits well in what I actually learn for my exams right now ;)

  7. invaderxan says:

    Hell yeah!!
    Actually I forgot — as well as the spectral bandwidth, mantis shrimp can also see polarised light. The little guys would make excellent astronomers… ;)

  8. invaderxan says:

    Actually, I can answer that to some extent… (Which is quite a cool realisation!)
    Any star’s environment depends on its wind speed and its mass loss rate. In the case of the Sun, solar wind alone shields us from extrasolar cosmic rays simply due to collisions. Within the Sun’s heliosphere, there are enough solar wind particles that there’s quite a high column density between Earth and interstellar space. We still get peppered by solar cosmic rays, but many from interstellar space are deflected or absorbed before they get to us.
    Interstellar UV makes no odds to us, as we get far more from the Sun. In AGB stars though, the high mass loss (coupled with the star’s lower UV output) means there are regions which are at least partially shielded from UV due to the number of molecules in the cloud. Molecules in the outer shell are ionised as they absorb the UV. It’s the same reason why dense clouds block UV photons above around 13eV due to hydrogen photolysis.
    I think that’s all correct, anyway… ;)
    Good fiction mixed with good science is always an excellent combination. I might have to find something by Vinge, just to have a look! :)

  9. ryttu3k says:

    …I want eyes like THAT!

  10. invaderxan says:

    There are some that have even more than four! The mantis shrimp has hyperspectral vision. It apparently has up to 18 different channels, from ultraviolet down to infrared! It’s a pretty amazing little animal, actually…

  11. ryttu3k says:

    Oooh, good point! Seeing in the UV spectrum would be pretty damn interesting – there are animals on Earth that can do that, right? And then there’s tetrachromacy (I think?), where you have three cones for visible light and one for UV light… damn, that’d be cool.
    I think I read something about radio astronomy on the moon on the BABlog XD It’d definitely be cool, cutting out all the radio noise…

  12. pax_athena says:

    Oh, interesting… I also wonder how the solar wind helps to shield the earth from the cosmic radiation and which affect that would have.
    The seminar is on Wednesdays, so let’s see whether somebody knows anything about it…
    Vinge is not one of my most favourite authors, though I’ve read just two books by him so far, but I highly enjoyed “Rainbows End”, which is about a possible Internet-dominated future, with rather a lot of science next to a very imaginative fiction part!

  13. invaderxan says:

    Re: Just tell those rogues
    Heh… I wouldn’t worry. Between them, Jupiter and Saturn will take care of that! ;)

  14. invaderxan says:

    Well, if I might speculate flagrantly for a while… ;)
    They may not see the stars as we see them. Aside from a dense hydrogen atmosphere, having no parent star would mean they’d have no reason to evolve the ability to see “optical” light. Life on such a world might learn to see in the infrared being emitted by the planet, or the ultraviolet coming from all of those stars If the latter, they’d probably learn to measure rotational speed using those stars, just like sailors used to do on Earth.
    It is a lovely picture, isn’t it? The World At Night is full of beautiful photography!
    Astronomy from Mars? Now there’s an interesting idea. To my knowledge, it hasn’t been done yet. Though I suppose, if you’ve travelled all the way to Mars, it’s far more interesting to look down.
    There’s a lot of talk about using The Moon for astronomy — but there’s a whole post I’ve been meaning on writing about that! ;)

  15. invaderxan says:

    Oh, they’re doing another one? I saw some of the entries from last year. :)
    I don’t know… I might!

  16. beepbeep says:

    Just tell those rogues
    Not to smash into us…we have enough problems ;)

  17. ryttu3k says:

    That is extremely cool XD And a good setting for sci fi! Hmm, growing up on a planet with no concept of a sun of one’s own… wonder how they’d measure rotational speed? The sun is a pretty good point of reference…
    And gorgeous picture! At first glance I thought it was Mars, but the Atacama is close enough XD (If my novel ever becomes a movie, I’m totally filming the Mars scenes there. Hell, they even have perchlorate!)
    And actually, that just reminded me. Do you know if any of the Rovers or Landers on Mars do astronomy from there? I imagine with Mars’s thin atmosphere, the night sky would be dazzling!

  18. Off topic, but are you entering the “dance your phd” competition?

  19. invaderxan says:

    Unless I’m missing something, he hasn’t taken account of the interstellar radiation field (chemistry in the outer shells of stars can be powered by starlight!)… I’m not sure how this would affect planets though.
    If you do pester people in seminars and get some interesting answers, please do share them. I’d be fascinated to know more! :)
    Oh, and Vernor Vinge? I don’t think I’ve read anything by him. Can you recommend any good titles? :)

  20. pax_athena says:

    That sounds interesting – I think I’ll very much need to read the paper! Especially since I think that I’ve just recently read a story in a setting alike, though by Vernor Vinge, not by Clark… I also makes me wonder what happens in clusters, where the gravitational pull of the neighbouring suns is much bigger that say near the sun. Hm, perhaps I’ll bug some people in our “general seminar” about it, I can imagine that someone has calculated something alike.

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