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On July 21 at 10 am, Seti Talks will be discussing the topic of "Could Rogue Planets Harbor Life". Supposedly there are 50 billion such planets in the Milky Way. I would think that the answer was obvious: no. So what is there to discuss?

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    $\begingroup$ I wouldn't rule out life anywhere in the universe. $\endgroup$
    – PeterH
    Jul 15, 2021 at 14:22
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    $\begingroup$ Life doesn't have to take a form in any way similar to what we've previously been exposed to in this world. When we look at things like the need for water, carbon-based chemistry, etc., these types of natural characteristics are simply based on past experience, not proven laws of the universe. $\endgroup$ Jul 15, 2021 at 20:05
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    $\begingroup$ Somewhere on a rogue planet - "Could star-bound planets harbor life? Seems unlikely, all that radiation." $\endgroup$ Jul 15, 2021 at 20:51
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    $\begingroup$ Life, uh, finds a way. $\endgroup$
    – AnoE
    Jul 16, 2021 at 9:07
  • $\begingroup$ @Panzercrisis: It's hard to imagine any form of life that doesn't require an external energy source though. $\endgroup$ Jul 16, 2021 at 23:05

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There's also the possibility that a rogue giant planet may have a moon with a subsurface ocean of liquid water due to tidal heating in an orbit close enough to its parent planet. E.g. if Jupiter was a rogue planet its moon Europa could still harbor life because of tidal heating.

Around five percent of Earth-sized ejected planets with Moon-sized natural satellites would retain their satellites after ejection. A large satellite would be a source of significant geological tidal heating26.

26. Debes, John H.; Steinn Sigurðsson (20 October 2007). "The Survival Rate of Ejected Terrestrial Planets with Moons". The Astrophysical Journal Letters. 668 (2): L167–L170. arXiv:0709.0945

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    $\begingroup$ @Pureferret Why would you think rogue planets can't have satellites? $\endgroup$
    – John
    Jul 15, 2021 at 16:25
  • $\begingroup$ I thought if a planet was knocked out of orbit, that the moon would be almost certainly knocked away? Is that wrong? $\endgroup$ Jul 15, 2021 at 18:26
  • $\begingroup$ @Pureferret From Wikipedia "Around five percent of Earth-sized ejected planets with Moon-sized natural satellites would retain their satellites after ejection. A large satellite would be a source of significant geological tidal heating.[26]" $\endgroup$
    – Aubreal
    Jul 15, 2021 at 18:36
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    $\begingroup$ A rogue planet could also pick up moons after being ejected. $\endgroup$ Jul 15, 2021 at 20:57
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    $\begingroup$ @Pureferret An ejected planet can retain its moon, see my answer here: astronomy.stackexchange.com/questions/41832/… $\endgroup$
    – Connor Garcia
    Jul 15, 2021 at 22:20
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In short, if a rogue planet has enough internal heat and retains enough of that internal heat by a thick atmosphere or within a miles-deep ice crust, it could be warm enough for liquid water and thus possibly for Earth-like lifeforms.

here is a link to an article on the subject:

https://futurism.com/life-could-exist-on-rogue-planets-that-dont-have-stars

And an advanced civlization capable of building artifical habitats on lifeless worlds or in outer space could settle a rogue planet, providing their own energy source from nuclear fusion.

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The discussion will focus on

  • the term rogue planet is an oxymoron, as it's missing the principle defining feature of a planet.

  • a Jupiter-sized body without a sun will be warmer than you would have thought. And they could be even larger, up to being brown dwarfs.

  • moons of such a body can be heated by tidal forces, without regard to whether there's a sun or not.

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