Njorl said:
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
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
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