“M-Dwarf Planet Multiplicity”
A talk by Dr. Sarah Ballard
UW, October 7th, 2014
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It’s time again for ‘Astrobiology’ lectures , this one by Dr. Sarah
Ballard, who holds a ‘Carl Sagan Fellowship’ at UW. She spoke well,
rapidly and informatively, without notes. I regret not knowing her
better.
You may remember from last year’s lectures that our inventory of
‘exoplanets’ then topped 1500, with ‘several’ of them in the
Goldilocks zone where water may remain liquid and .. presumably ..
life may be possible. These interesting objects were detected by the
Kepler satellite observing dips in the light curves caused by the
planets crossing in front of their central stars. Most of them were
large ..Jupiter sized.. with close-in orbits around sun-like stars.
Alas, Kepler is now dead, but much data is still being analyzed.
Big surprise, to me anyway: the fashion has changed to searching
among smaller stars, ‘M-Dwarfs’ [*], for reasons that should have
been obvious. The dwarfs are dimmer, their disks are smaller, and
obscuring planetary transits easier to detect. Their ‘Goldilocks
Zones’ are closer in, where shorter orbiting periods permit detecting
more transits of smaller planets. It turns out that, owing to this
‘discovery bias’, the vast majority of revealed planets orbit stars
smaller and cooler than ours. The past year’s analysis of Kepler data
has revealed ‘many’ [**] planets orbiting M-Dwarfs, most of them
accompanied by at least one planet, with the median closer to six.
A direct quote from Dr. Ballard: “95% of M-Dwarfs are
accompanied by a habitable planet.” Wow!
This last bold statement stems from Dr. Ballard’s analysis using
Bayesian statistics upon Kepler observations. Yes. There are enough
of them to do statistics.
Of especial interest to me, spectral observations on a few of them
[only 2 or 3, so far] have separately revealed CH4, CO2, and H20 in
their atmospheres. Note that CH4 & CO2 are not photochemically stable
in the presence of each together. To find both on the same planet
would be strong evidence of dynamic sources, such as life.
Plans are advanced to launch “TESS” [Transiting Exoplanet Survey
Satellite] in 2017. Goody.
Cheers,
hh
[*] “M-Dwarf” stars are ‘main sequence’ [like the sun] between,
ruffly 0.1 and 0.4 solar masses, with surface temperatures less
than ~4000 degK. The sun’s surface temperature is ~6000 degK.
[**] Sorry, I don’t know the present count of stars with exoplanets.
Greater than 1500, at any rate.
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