Wednesday, October 19, 2005

Itsy bitsy little planets

Now, this is interesting...

Short versions: M star GJ876 already known to have planets. Berkeley group is now formally claiming a third planet of 7.5 Earth masses (inclination is known from modeling of planet-planet interactions, assuming all 3 planets are co-planar), in 2 day orbit!


So much for the "M stars don't have planets", well, except for this one we know about...

Much to think about here.


Blogger Adam Solomon said...

Oy! Wonder how I missed that one. Thanks for pointing that out ;) I'm curious as to how rare that is or is not...why exactly are M dwarfs not supposed to have planets? I can understand how they may not have so many habitable planets, but...

9:43 PM  
Blogger Steinn said...

We'll soon know how rare, since M dwarfs are now enthusiastically added to large telescope surveys for planet by Very Large Telescopes.
My personal suspicion is: not very rare

Theoretically, if you think disks are minimal mass and M_disk scales with M_star, then M dwarfs should have few or no planets, and fewer giant planets still. Further, the habitable zone
is in the tidal lock regime, so who cares.

In reality, disk masses are all over the place, and since formation time scales are long for low mass stars, the M dwarfs may have substantially massive protoplanetary disks that persist longer in a quiescent slow accretion phase, permitting more prolonged and more efficient planet formation.
Further, there are subtle issue with planet-planet interactions in disks which argue the lower mass stars might be prone to forming relatively high mass planets (ie M_planet/M_star is not constant as M_star changes for some average planet). Or not.

Fact is that there were no planet around M stars except for that one which had been carefully studied. Microlensing surveys put some limits on preponderance of high mass planets at moderate separation around bulge and halo M dwarfs, but not very stringent.

The M star habitable zone is interesting again: one reason is that atmospheric circulation may make a planet surprisingly habitable, even in 1:1 tidal lock; second reason is that there are tidal locks that are not 1:1; and third reason is that there may be habitable massive moons of uninhabitable planets.

So, interesting. Lots more data soon, possibly as soon as next week.

10:44 AM  

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