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Short Takes for the Weekend

In which the hapless author tries to clear out his growing backlog of material. This may have to become a regular feature, since the amount of new information coming in about the extrasolar planet hunt alone would be enough to keep Centauri Dreams busy all day, not to mention continuing work on propulsion concepts from solar and magnetic sails to antimatter and ongoing discoveries relating to dark matter and energy. Herewith, then, a few shorter items compressed only for reasons of space and time, so to speak.

On Transit Windows and Red Dwarfs

The planet around GL 581, an M-class red dwarf discovered last September, is unusually interesting because of its low mass, roughly 17 times that of Earth. This is probably a Neptune-class world with some possibility of being observable through transits — i.e., its orbit may cross its primary as seen from Earth, making it a candidate for the transitsearch.org collaboration. But the last transit window on March 28 was rendered useless by cloud cover in California, where transitsearch is based and where it has the most observers.

New windows open, however, on April 2 (Japan and Australia), April 8 (Europe, South Africa) and April 13 (North, South America). Let’s be clear on this: no apparent attempt has been made to check GL 581 for transits, and although the odds are small, efforts like transitsearch.org could pay off in a big way. From Greg Laughlin’s systemic site:

If the planet transits, we would know that it truly has a Neptune mass. The depth of the transit would give us the planetary size, which, coupled with the mass, would yield the density. The density would tell us what the planet is made of. If it is primarily water, like Uranus or Neptune, then we expect a radius of ~0.3 Jupiter radii. If the planet is made of rock and metal, however, like the terrestrial planets in our solar system, then the radius will be smaller, more in the neighborhood of ~0.22 Jupiter radii. A water-rich composition would tell us that the planet formed further away from the star, and then migrated inward to its steamy current location. This information, in turn, would give us valuable insight into the conditions that held sway in the disks surrounding low mass stars, and would help guide our hypotheses regarding the presence of habitable worlds orbiting the lowest mass stars.

Laughlin and colleagues have come to believe that M-class red dwarfs are commonly orbited by terrestrial-mass planets. More on the case for this hypothesis was provided in a presentation by astronomer Ryan Montgomery at the Astrobiology Science Conference in Washington in late March; the transitsearch team advocates an immediate targeting of the nearest low-mass stars in hopes of snagging a transit. Keep an eye on systemic for more — anyone seriously interested in exoplanetary research needs to be checking this site regularly.

Centauri Dreams‘ take: it’s fascinating to see the speed with which red dwarfs have moved into serious consideration for terrestrial planets. And if such are indeed common around these stars, our estimate of the number of terrestrial worlds must go up significantly. Red dwarfs comprise (depending on whose figures you use) roughly 70 percent of all stars in the Milky Way. What exotic worlds these would be, tidally locked to their stars, but capable according to some current thinking of nourishing sustainable biospheres. Their long lifetimes also make red dwarfs intriguing, offering life extraordinary evolutionary opportunites provided it can cope with persistent flare activity.

The Migrations of Gas Giants

Gas giant planets are a problem to current theories of planetary formation. Interactions between the protoplanetary disc and the massive planetary core should cause such worlds to migrate into the inner parts of a forming solar system in short order. So swift is this migration — about 100,000 years — that gas should not have time to accrete on the fledgling planet. In fact, the young gas giant-to-be ought to merge with its central star before planets ever form, making its presence in numerous exoplanetary systems (not to mention our own) problematic.

British astronomers Paul Cresswell and Richard Nelson tackle this issue in an article to be published in Astronomy & Astrophysics, showing that even when the gravitational interactions with a swarm of other protoplanets are taken into consideration, the migration inward cannot be avoided in their simulations. What may solve the problem is a generational approach; only the last generation of planets that forms as the disk dissipates is the one to survive. Another possibility: altering our view of the physical properties of the protoplanetary disk, which may be more turbulent than assumed by conventional models.

Also note: the migration of gas giants is studied in relation to the planetary system around HD128311 in a paper called “On the evolution of the resonant planetary system HD128311,” by Zsolt Zandor and Willy Kley, available at the arXiv site.

New Horizons’ Splendid Report

The latest from the New Horizons mission to Pluto/Charon and the Kuiper belt is that six of the seven instruments onboard have now completed tests demonstrating their viability post-launch. Computers boot up properly, thermal control systems are functional, commands are received and processed, and it is clear that the entire payload package survived the launch in good order. Spring and summer will see a variety of payload performance tests and calibration activities for the various instruments, with payload commissioning complete by early fall. The next major instrument checkpoint: the Jupiter flyby in February of 2007.

Good News on Astrobiology Funding

Current plans now call for NASA to restore about $30 million of the funding it had cut from its astrobiology research program. Although not making up for the original $160 million cut over five years, the restored dollars do provide some hope for funding proposals in this vital work. All told, the agency had cut science funding by $3.1 billion to cover cost overruns on the manned spaceflight program, in what Centauri Dreams views as a classic case of short-term thinking, one that emphasizes highly visible missions with human crews despite their lack of scientific payoff at the expense of the long-range goals that should motivate both manned and robotic space ventures. And yes, I know that the process is political and that all budgets are a matter of give and take; I also know that the money flowing back in is less than hoped for, but in today’s fiscal climate, even $30 million restored is good news.

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  • ljk November 13, 2008, 0:08

    On the evolution of mean motion resonances through stochastic forcing: Fast and slow libration modes and the origin of HD128311

    Authors: Hanno Rein, John C. B. Papaloizou

    (Submitted on 12 Nov 2008)

    Abstract: We clarify the response of extrasolar planetary systems in a 2:1 mean motion commensurability, with masses ranging from the super Jovian range to the terrestrial range, to stochastic forcing that could result from protoplanetary disk turbulence. The behaviour of the different libration modes for a wide range of system parameters and stochastic forcing magnitudes is investigated. The growth of libration amplitudes is parameterized as a function of the relevant parameters. The results are applied to provide an explanation of the configuration of the HD128311 system.

    We first develop an analytic model without making the assumption that the eccentricities are small. We also perform numerical N-body simulations with additional stochastic forcing terms to represent the effects of putative disk turbulence. We isolate two distinct libration modes for the resonant angles. These react to stochastic forcing in a different way and become coupled when the libration amplitudes are large. Systems are quickly destabilized by large magnitudes of stochastic forcing but some stability is imparted should systems undergo a net orbital migration. The slow mode, which mostly corresponds to motion of the angle between the apsidal lines of the two planets, is converted to circulation more readily than the fast mode which is associated with oscillations of the semi-major axes. This mode is also vulnerable to the attainment of small eccentricities which causes oscillations between periods of libration and circulation.

    Stochastic forcing due to disk turbulence may have played a role in shaping the configurations of observed systems in mean motion resonance. It naturally provides a mechanism for accounting for the HD128311 system for which the fast mode librates and the slow mode is apparently near the borderline between libration and circulation.

    Comments: 14 pages, 7 figures

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0811.1813v1 [astro-ph]

    Submission history

    From: Hanno Rein [view email]

    [v1] Wed, 12 Nov 2008 16:01:00 GMT (581kb,D)

    http://arxiv.org/abs/0811.1813