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Kepler Mission
marsophile
post Sep 15 2011, 08:18 PM
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I noticed at the press conference they flashed a graphic of a representation of the Alpha Centauri system. I suppose the implication is that this discovery increases the plausibility that the Alpha Centauri binary may have planets.

If it does, their orbital plane (and presumably that of the binary) must be nearly perpendicular to the line of sight; otherwise, we would have seen something in radial velocity measurements by now. Even so, shouldn't we have noticed timing variations in the orbit of the binaries caused by the tug of planets?
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Drkskywxlt
post Sep 16 2011, 01:54 AM
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QUOTE (marsophile @ Sep 15 2011, 03:18 PM) *
I noticed at the press conference they flashed a graphic of a representation of the Alpha Centauri system. I suppose the implication is that this discovery increases the plausibility that the Alpha Centauri binary may have planets.

If it does, their orbital plane (and presumably that of the binary) must be nearly perpendicular to the line of sight; otherwise, we would have seen something in radial velocity measurements by now. Even so, shouldn't we have noticed timing variations in the orbit of the binaries caused by the tug of planets?


There's been several studies that have shown that planets in sufficiently close orbits would be stable around Alpha Centauri A or B. Their average separation distance is something like 20 AU. I think orbits within a couple AU of each star are gravitationally stable. So I don't think Alpha Centauri is a true analog of this system.
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Reed
post Sep 16 2011, 04:49 AM
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QUOTE (Paolo @ Sep 15 2011, 11:46 AM) *
for those having access to Science, Kepler-16: A Transiting Circumbinary Planet

For those without, a preprint can be found at http://exoplanet.eu/star.php?st=Kepler-16%20%28Ab%29 edit: Also on arxiv now http://arxiv.org/abs/1109.3432

Related paper on arxiv http://arxiv.org/abs/1109.3198 "Spin-Orbit Alignment for the Circumbinary Planet Host Kepler-16A." which has some discussion of the age (the Science paper basically says "we don't know".)
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Mongo
post Sep 16 2011, 01:53 PM
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This one almost slipped past me, but if their adjustments are confirmed, this is HUGE.

Near-Infrared Spectroscopy of Low-Mass Kepler Planet-Candidate Host Stars: Effective Temperatures, Metallicities, Masses and Radii

Abstract: We report stellar parameters for low-mass planet-candidate host stars recently announced by the Kepler Mission. We obtained medium-resolution, K-band spectra of 84 low-mass Kepler Objects of Interest (KOIs). We identified one KOI as a giant; for the remaining dwarfs, we estimated effective temperatures by comparing measurements of K-band regions dominated by H2O opacity with predictions of synthetic spectra for low-mass stars. We measured overall metallicities ([M/H]) using the equivalent widths of Na I and Ca I absorption features and an empirical metallicity relation calibrated with nearby stars. With effective temperatures and metallicities, we estimate the masses and radii of the low-mass KOIs by interpolation onto evolutionary isochrones. The resultant stellar radii are roughly half of the values reported in the Kepler Input Catalogue and, by construction, correlate better with effective temperature. Our results significantly reduce the sizes of the corresponding planet-candidates, with many less than 1 Earth radius. Recalculating the equilibrium temperatures of the planet-candidates from the implied stellar luminosities and masses, and assuming Earth's albedo and re-radiation fraction, we find that six of the planet-candidates are terrestrial-sized with orbital semi-major axes that lie within the habitable zones of their low-mass host stars. The stellar parameters presented in this letter serve as a resource for further characterization of the planet-candidates.

******

Scaling the Earth’s equilibrium temperature of 255 K by the orbital semi-major axis, stellar Teff and stellar radius of the KOIs in this letter, we find that KOIs 463.01, 1422.02, 947.01, 812.03, 448.02 and 1361.01 all have equilibrium temperatures between 217 K and 261 K: the limits of the habitable zone as described in Kasting et al. (1993).

******

The six habitable zone candidates. Assuming that the planet densities are equal to that of the Earth, the surface gravity varies directly with the radius (Mass varies with radius cubed, but gravitational force varies with mass over distance (radius) squared, leaving the surface gravity directly proportional to the radius)

KOI 448.02 (M0-V Primary) -- Radius 1.85 Earth -- 240 K -- Year 43.62 days
KOI 463.01 (M3-V Primary) -- Radius 0.93 Earth -- 232 K -- Year 18.48 days
KOI 812.03 (M0-V Primary) -- Radius 1.16 Earth -- 228 K -- Year 46.19 days
KOI 947.01 (M1-V Primary) -- Radius 1.24 Earth -- 254 K -- Year 28.60 days
KOI 1361.01 (M0-V Primary) -- Radius 1.58 Earth -- 232 K -- Year 59.88 days
KOI 1422.02 (M2-V Primary) -- Radius 0.85 Earth -- 249 K -- Year 19.85 days

There is another candidate (with a calculated radius that is a close match to that of Earth) that has a calculated equilibrium temperature just above the upper limit given above, which I think should be considered as well:

KOI 494.01 (M1-V Primary) -- Radius 1.05 Earth -- 268 K -- Year 25.70 days
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ngunn
post Sep 16 2011, 10:21 PM
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Thanks Reed for those links. I don't fully trust the integration of the orbital motion over 2 million years, and in any case that's a very short time in cosmic terms.

Mongo - thanks also for your reporting.
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Explorer1
post Sep 16 2011, 10:44 PM
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Mind, these are still only 'candidates', and unconfirmed so not time to party just yet (but worth keeping and eye on!)
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dilo
post Sep 17 2011, 05:10 AM
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so many M-class possible planets! rolleyes.gif


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Drkskywxlt
post Sep 17 2011, 11:47 AM
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QUOTE (Explorer1 @ Sep 16 2011, 05:44 PM) *
Mind, these are still only 'candidates', and unconfirmed so not time to party just yet (but worth keeping and eye on!)


Yes, but the Kepler team has done a variety of studies to show that their false positive rate is very small. So 95-99% of their "candidates" are planets.
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Greg Hullender
post Sep 17 2011, 10:57 PM
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QUOTE (ngunn @ Sep 15 2011, 12:05 PM) *
Does the full paper discuss the stability of this arrangement?

It's in the "Supporting Online Information," and it references these two papers:

An Emperical Condition for Stability of Hierarchical Triple Systems

Long-Term Stability of Planets in Binary Systems

The SOI says that in addition to the criteria given in these two papers, their own long-term integration suggests the orbit is stable.

--Greg
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nprev
post Sep 18 2011, 08:50 AM
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It's an exciting discovery, but frankly I'm not surprised.

One thing we seem to find with almost repetitive regularity is that Nature always finds a way to do the unexpected.

That's probably entirely due to the fact that we are limited in our imaginative capability...but, that's okay.

That's precisely why we explore. wink.gif


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A few will take this knowledge and use this power of a dream realized as a force for change, an impetus for further discovery to make less ancient dreams real.
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ngunn
post Sep 18 2011, 10:09 AM
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QUOTE (Greg Hullender @ Sep 17 2011, 11:57 PM) *
these two papers:


Thanks Greg. So if I understand correctly the newly discovered planet is close to the inner limit for stability - a hint perhaps that it is the innermost member of a dynamically 'full' family.
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Greg Hullender
post Sep 18 2011, 07:30 PM
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QUOTE (ngunn @ Sep 18 2011, 03:09 AM) *
So if I understand correctly the newly discovered planet is close to the inner limit for stability

That's what I get. The report puts the planet at 0.70 AU from the barycenter. Using the formula from Holman and Wiegert, the limit for stability around the pair is at about 0.65 AU, so we probably shouldn't expect anything closer, unless it just orbited one of the two stars. In that case, though, the outer stability limits are 0.068 and 0.032 AU (respectively), so they'd be quite toasty.

By contrast, the limit around Alpha Centauri AB is 87 AU, so nothing closer than that is likely to orbit the pair. On the other hand, the individual limits for A and B are 2.8 and 2.5 AU, respectively, so planets in the habitable zone of either star should be quite stable; there just likely wouldn't be any outer planets.

--Greg
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kap
post Sep 19 2011, 10:45 PM
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QUOTE (Greg Hullender @ Sep 18 2011, 12:30 PM) *
By contrast, the limit around Alpha Centauri AB is 87 AU, so nothing closer than that is likely to orbit the pair. On the other hand, the individual limits for A and B are 2.8 and 2.5 AU, respectively, so planets in the habitable zone of either star should be quite stable; there just likely wouldn't be any outer planets.


So if planets were actually orbiting in the habitable zone, they'd likely have little protection from bombardment?

-kap
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Holder of the Tw...
post Sep 19 2011, 10:55 PM
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If you're talking about the lack of gas giants, who needs them when each star has the other star? Sounds like everything out to 87 au will be cleared out.
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Greg Hullender
post Sep 19 2011, 11:50 PM
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Yep. No stable orbits from about 2.5 AU out to 87 AU. Any debris would be swept into one or the other star or ejected from the system entirely. This occurs very quickly--millions of years at most.

--Greg
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