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Least massive earth like planet possible in the habitable zone?

  1. Aug 18, 2009 #1
    I mean the smallest, lowest mass, and/or lowest g terrestrial planet that can hold onto and support a favorable atmosphere for potential life, and is in the zone where water can exist as a liquid. I'm really asking: what's the smallest "Earth" possible?

    I heard that the upper limit for "Super-Earths" is 14 times the mass of the earth, and there is an extrasolar planet that is close to this mass. Someone calculated that it would have an upper limit of 3.5g at the surface, which would make life very squat if it even got past a bacterial stage. It made me wonder what the opposite conditions would be, and what the lower limit has been calculated at.

    I can't find any good information on the lower limit though (bad search queries?), so I'm asking here. Some requirements to considered to be an "Earth" raise these questions: What's the smallest a body can be and still hold onto a decent atmosphere (within the habitable zone, Titan has an advantage of extreme frigidity), and how long can it hold onto it for? How long can such a small planet stay warm and maintain a magnetic field? What are the lowest g conditions that life can start in, and would biological molecules have trouble forming in low percentages of earth's gravity?

    Could something, say, 10 times the mass of earth's moon (making it an 8th of earth) sustain favorable conditions for any significant length of time?
  2. jcsd
  3. Aug 19, 2009 #2


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    It must be massive enough to hold an atmosphere and warm enough to allow for chemical processes. Mars is probably close to the low limit on mass. It appears to have been marginably habitable for about a billion years.
  4. Aug 20, 2009 #3
    What do you mean by "Earth like"? By smallest "Earth", do you mean a planet with some kind of developed creatures walking around, or the smallest planet possible for exobiology? Enceladus is only 505 km in diameter and is certainly not in the Habitable Zone of the Sun, but it has subsurface water, thermodynamics and at least the possiblity of extremophile exobiology. A search for exobiology needn't be restricted to mini or super Earths.
    The HZ of a massive star is similarly misleading. If the Earth orbited in the HZ of Betelgeuse, then we would have liquid water, except perhaps for the 60+ years of winter we would experience in our larger orbit.
    Also, marine exobiology on a "Super Earth" with 3.5g surface gravity wouldn't necessarily be "squat". Fish-like creatures would be bouyant in water and therefore would not experience their mass at all.
    Just something to consider.
  5. Aug 20, 2009 #4
    My requirements are that it can maintain an atmosphere of similar components to the earth (I'm not sure what would be acceptable in terms of pressure), has a magnetic field and molten core, and has liquid water on the surface for 99% of the time. Also, that it can support large scale distribution and eco-system of life. Perhaps its still bacterial entirely, but its thriving on a large scale, and not hibernating under layers of rock.

    Also, doesn't Enceladus have subsurface water due to tidal heating? That's due to it being a moon in resonance with other moons, and it's water is underneath it's crust of ice?

    If you could wave a magic wand and have Enceladus replace the earth and the moon in their orbit around the sun, Enceladus would warm up to the point that it would be a very small water planet instead of an ice moon. However, how long could it hold onto an atmosphere and water? It doesn't have the gravity to stop the escape of large amounts of hydrogen. It couldn't maintain a magnetic core for very long either.

    The reply about Mars could be right. Mars is a lot smaller than the Earth (about 10%?), but a LOT bigger than Enceladus, and it has lost it's magnetic field, and then its favorable atmosphere and surface water, but I suppose it had a good enough run for life to sustain itself, but for how long? How long did water last on Mars? Billons of years or just millions? Maybe it is the lower limit for my conditions.
  6. Aug 20, 2009 #5
    If I read you correctly, you are only concerned with the smallest planets that meet your "Earth like" requirements and have exobiology, even though even smaller planetoids that are not Earth like, could meet the exobiology requirement too. You're correct about the water on Enceladus. However, my point was that watery Enceladus, right where it is, may have exobiology, whereas now barren Mars may not. Miles of frozen ice keep out harmful radiation from space just as well as an atmosphere and a magnetosphere. So you want to ignore the smallest planets that could support exobiology in your research and concetrate only on the smallest Earth like planets? In that case, I believe that Mars had surface water for something like a billion years.
  7. Aug 20, 2009 #6
    I am considering what you have said. I'm not disregarding it. The thing is, can Enceladus sustain life as a planet and not as a moon? Cause planets are what I'm looking for, not moons.

    If it was within its own orbit at the distance of Jupiter, it might have the thick ice to keep out radiation, but without Jupiter it wouldn't have the tidal heating that even allows it to have an ocean. It would be solid ice all the way through (well, when it formed it would be molten, but it would settle down without there being conditions for life), and as we've already covered, move it closer to the sun, and it's gravity/lack of magfield will not allow it to hold water for long.

    Is there anyway that it can generate enough internal heat to sustain this sub-surface ocean biology as a planet that far out, and on timescales that allow billions of years of biology (like earth)?

    EDIT: Could two planets the size of Enceladus orbit each other and create the effect desired?
  8. Aug 20, 2009 #7
    Or if an Enceladus type world had a radioactive core that heated it or had a tight orbit around a binary M class dwarf pair that provided tidal heating. Just thinking out loud.
  9. Aug 20, 2009 #8
    So, I guess the answer is that there are ways in which a planet that small can sustain oceanic life.
  10. Aug 21, 2009 #9
    Well, we won't find it if we don't know where to look, but let's say extremophile bacteria might have a chance there. BTW, just to clarify, Enceladus orbits Saturn, not Jupiter.
  11. Aug 21, 2009 #10
    Hi Researcher X

    The minimum mass for an independently habitable planet is usually thought to be ~0.2-0.4 Earth masses if made from similar material to the Earth. The lower 0.2 limit is based on sufficient gravity to retain atmosphere - especially against the much higher early solar wind and late heavy bombardment - while the 0.4 limit is for sufficient tectonic activity to maintain plate tectonics to the present day. But there are so many unknowns that we can't say for sure if Mars would've been habitable to the present day if it was closer to the Sun. I suspect it probably would still be, but that's just an educated guess. A major unknown is just how Mars' magnetic field was stopped dead. It still has a molten core, but the last major impact probably shut its magnetic field generation down for good. A magnetic field helps retain nitrogen against dissociative recombination.
  12. Sep 2, 2009 #11
    as venus nor mars has a magnetic field or much of one today
    is a large moon needed to keep up a magnetic field ??
  13. Sep 2, 2009 #12
    No one knows. Physically it doesn't seem necessary, but the coincidence is a bit suspicious. Venus seems to have insufficient spin to form an Earth-like field, and Mars seems to have suffered an impact that wiped its field out. Yet Mercury still has a substantial field, yet small size and slow rotation. Why? It's an area of active research.
  14. Sep 2, 2009 #13
    could mercury be so close to the sun
    that it replaces the large moon effect ???
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