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Binary Planets

  1. Jan 16, 2004 #1
    Would it be possible for binary planets, or even a planet and it's moon, to simultaneously sustain life? I know the odds of a planet having life on it is very low, but for a planet that does (earth) would it's sister planet or moon be able to sustain life as well? Maybe if the bodies are the same and simply broke up, then they would both contain the same conditions (soil, weather, etc.), right? I don't know, I am just curious.

    Also, how do bodies get into a binary state?
     
    Last edited: Jan 16, 2004
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  3. Jan 16, 2004 #2

    LURCH

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    Interesting question, especially in ligth of the fact that the Earth/Moon system could be seen as a binary planetary system. The model of our planet's formation that is currently most popular is that two protoplanets in their ealry formation colided. When the two coers "stuck" to each other, the lighter debris was blown off to form a ring which eventually clumped to form the Moon.

    So if there had been more debris, the two resulting bodies would be the same mass (appr.). Or, if they had jsut missed colliding, there would have been two planets formed in orbit around a common center of gravity, which in turn would orbit the Sun. In either case, tidal forces on each planet would be much greater.

    Or, if the two were far enough appart to keep the tidal forces as we see them now, then the distance from the Sun would very more than it does. I don't know if the amount of veriation would be enough to make the formation of life significantly more difficult.

    Of course, both of these problems are obviated if the plantes' orbit around one another is 90o from their orbit around the star. Also, if they orbit a common C of G with their poles pointed at each other (the way Uranus orbits the Sun), tidal effects would not be a factor.
     
  4. Jan 16, 2004 #3

    Njorl

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    It might not be as big of a coincidence as you think. If one planet has life, than the other is at a viable distance from the star. Simple life developed before there was much atmosphere. Meteor collisions could kick viable debris into orbit, to be captured by the other planet. Not only could you have life on both, it would not be suprising to have very similar biochemistries on the two planets.

    Njorl
     
  5. Jan 19, 2004 #4

    Phobos

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    1 in 9? (based on the available data :wink:)

    It depends. But I suspect that a third (temporary) body is involved (e.g., the Earth-impactor that resulted in the creation of the moon) because 2-body capture doesn't really work (the approaching object would get a gravitational slingshot rather than being captured unless there was a third object with enough gravity to change the velocity/trajectory). Although I suppose binary star systems could form straight from the original nebula. Hmm...don't know...guess I'll have to look into it.
     
  6. Jan 19, 2004 #5

    Nereid

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    Creation of binary solar system objects?

    In the case of the Earth/Moon, it was (most likely) a collision between the proto-Earth and a Mars-sized object, with the Moon condensing out of the debris left in orbit around the Earth.

    In the case of Pluto-Charon (some misguided folk continue to think of Pluto as a planet, rather than the largest EKB object), we don't know, but could be collision (like Earth-Moon) or capture (as Phobos says). IIRC, 9 binary EKB objects have been found to date. IMHO, the statistics aren't yet good enough (not enough binaries, too few EKBs examined in enough detail) to test among competing formation hypotheses, but in another 5 to 10 years ...

    A number of asteroids have been found to have satellites, rather too many for current models. However, these aren't binaries.

    Then there's binary stars ... later. :smile:
     
  7. Jan 20, 2004 #6
    "Not only could you have life on both, it would not be suprising to have very similar biochemistries on the two planets." - Njorl

    So when the planets are revolving each other, they would have 2 points. The first would be it's position closest to the sun in it's binary revolution (during that particular time in it's orbit around the sun), and the second would be it's farthest distance from the sun. During this change in distances, would the temperature change so dramatically that the life would die and simply not be able to even live?
     
  8. Jan 20, 2004 #7

    Janus

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    No. There is a limit as to how far the planets can orbit from each other before the tidal effect from the Sun will pull them apart into independent orbits. For instance, in the case of Earth sized bodies at Earth distance form the Sun, this distance is about twice the present distance that the Moon orbits.

    This distance is small compared to the distance the Earth already moves in and out from the Sun due to the eccentricity of its orbit, and we feel no real difference in the temp of the Earth due to this already existing change in distance.
     
  9. Jan 20, 2004 #8

    russ_watters

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    Re: Re: Binary Planets

    How about 1 in 1 (based on known planets with liquid water)?

    We reallllly need to send a probe with a drill to Europa.
     
  10. Jan 21, 2004 #9

    Phobos

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    Re: Re: Re: Binary Planets

    You got my vote!
     
  11. Jan 21, 2004 #10
    So, are you saying that if the moon was actually a second 'earth', it would need to be twice as far away from us in order to maintain the binary state?
     
  12. Jan 21, 2004 #11

    Janus

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    No, it would be the upper limit as to how far apart they could be.
     
  13. Jun 6, 2004 #12

    Jenab

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    On the contrary, it's still improbable. It is possible, though, that there could exist a binary planet with both components habitable.

    There are two strategies for getting the right orbital conditions.

    1. You can either put the two components close enough together to tide-lock within a short time, with a mutual orbit (month) short enough to provide an acceptable diurnal (day/night) cycle. But beware of the Roche limit.

    2. You can separate the two components far enough apart that they won't tide-lock to each other before they can become habitable. But beware of the tidal disruption of their mutual orbit by the gravity of their sun.

    Dmax = 719,000 km (Ma+Mb)^(1/3) Mstar^1.742

    Where Dmax is the maximum allowed separation for the binary planet, Ma and Mb are the masses of the components of the binary planet in Earth masses, and Mstar is the mass of the star in solar masses. Note: the exponent 1.742 on Mstar is approximately correct for 0.8 < Mstar < 2.5.

    For both cases, there is an upper bound on the permitted mass for the sun, 1.5 solar masses, resulting from the requirement that the star remain on the main sequence for at least 3 billion years.

    For Case 2, there is a lower bound, resulting from the requirement that the two components be immune from the tidal disruption of their mutual orbit by their sun, while at the same time remaining freely rotating (i.e., not tide locked) with respect to each other. The former requirement demands that the separation of the components be not greater than some distance. The latter requirement demands that the separation of the components be not less than some other distance.

    There's such a conflict-in-requirements for single planets, too, though a more relaxed one. To be habitable, a single planet must be in the liquid water zone (the ecosphere), and must orbit far enough from the star so that the star's gravity doesn't induce a low-ratio tide lock with the planet's rotation (as is the case with Mercury and Venus). The minimum acceptable mass for the sun of a single habitable planet is about 0.8 solar masses.

    But, getting back to binary habitable planets...

    Assuming that both of these planets are like the Earth thermally, the lower bound to their sun's mass is 1.3 solar masses, or thereabout.

    If we may grant that habitable planets might have a stronger greenhouse effect than Earth presently does, so that such a planet, if placed in our own solar system, would be habitable if it were in Mars' orbit, the lower bound to the acceptable star masses can be reduced to about 1.1.

    So whereas a single habitable planet can be found orbiting stars from 0.8 to 1.5 solar masses (K0 to F0/A9), binary planets (with both components habitable) are restricted to the upper portion of this range, to stars having masses from ~1.3 to 1.5 solar masses (F3 to F0/A9).

    Jerry Abbott
     
    Last edited: Jun 17, 2004
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