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A New Take on Planetary Migration

It’s shaping up to be a good week for exoplanet findings, with yesterday’s intriguing work on ‘planemos’ and their disks and now, also presented at the AAS Calgary meeting, word of new findings on planetary migration. This is a significant issue, because so many of the exoplanets we know about are huge ‘hot Jupiters’ in tight orbits around their star. The effects such planets would have on smaller worlds in the habitable zone could be devastating if the gas giants migrated through that region early in the system’s life.

And migration is assumed to be what happens. The assumption is that such planets form a long way from their stars, as much as 20 AU out, and move to their present positions as the planet interacts tidally with the surrounding gas disk. But migration is tricky business, implying that most planets would fall into their stars within a million years. Preserving a solar system with gas giants and low-mass terrestrial worlds becomes challenging business (and recall that it wasn’t so long ago that ‘hot Jupiters’ were considered more or less an impossibility, a reminder of the nascent state of our migration theories).

Perhaps ‘dead zones’ can save the day. They were the topic of a presentation by Ralph Pudritz (McMaster University) at Calgary yesterday, reporting on a theoretical case that could give us more leeway in the formation of planetary systems. Extending out to about 13 AU, a dead zone is a region of low viscosity gas that can slow planetary migration. Moving inward toward its star, a gas giant opens a gap in the circumstellar disk; its migration speed then becomes locked to the inward drift of the gas. After entering the dead zone, the planet opens a much wider gap, and its migration is substantially slowed by the gas within the zone.

In contrast, low mass planets do not open gaps in the disk as they migrate inward, but their migration can be reversed if they encounter a steep gradient in gas density, as would be found at the edge of the dead zone. Lighter worlds that formed within the dead zone in the first place can open gaps in the zone and have their inward migration slowed.

From the paper on this work, which has been submitted to The Astrophysical Journal, one of several interesting conclusions: “Jovian or super Jovian planets are likely to be formed beyond a dead zone. Inside dead zones, a gap opens for smaller mass planets – ice giants or even terrestrial planets.” What follows is interesting indeed: most massive planets in other solar systems remain undiscovered, but will be found in orbits at 5 AU and greater from their parent star. That’s close enough to Solar System parameters to fuel Centauri Dreams‘ continuing interest in this work. The paper is Matsumura and Pudritz, “Dead Zones and Extrasolar Planetary Properties,” now available here.

Comments on this entry are closed.

  • ljk May 23, 2007, 14:43

    Massive planet migration: Theoretical predictions and comparison with observations

    Authors: Philip J. Armitage

    (Submitted on 21 May 2007)

    Abstract: We quantify the utility of large radial velocity surveys for constraining theoretical models of Type II migration and protoplanetary disk physics. We describe a theoretical model for the expected radial distribution of extrasolar planets that combines an analytic description of migration with an empirically calibrated disk model. The disk model includes viscous evolution and mass loss via photoevaporation. Comparing the predicted distribution to a uniformly selected subsample of planets from the Lick / Keck / AAT planet search programs, we find that a simple model in which planets form in the outer disk at a uniform rate, migrate inward according to a standard Type II prescription, and become stranded when the gas disk is dispersed, is consistent with the radial distribution of planets for orbital radii 0.1 AU

  • ljk July 19, 2007, 11:58

    The effect of type I migration on the formation of terrestrial planets in hot-Jupiter systems

    Authors: Martyn J. Fogg, Richard P. Nelson

    (Submitted on 18 Jul 2007)

    Abstract: Context: Our previous models of a giant planet migrating through an inner protoplanet/planetesimal disk find that the giant shepherds a portion of the material it encounters into interior orbits, whilst scattering the rest into external orbits. Scattering tends to dominate, leaving behind abundant material that can accrete into terrestrial planets.

    Aims: We add to the possible realism of our model by simulating type I migration forces which cause an inward drift, and strong eccentricity and inclination damping of protoplanetary bodies. This extra dissipation might be expected to enhance shepherding at the expense of scattering, possibly modifying our previous conclusions.

    Methods: We employ an N-body code that is linked to a viscous gas disk algorithm capable of simulating: gas accretion onto the central star; gap formation in the vicinity of the giant planet; type II migration of the giant planet; type I migration of protoplanets; and the effect of gas drag on planetesimals. We use the code to re-run three scenarios from a previous work where type I migration was not included.

    Results: The additional dissipation introduced by type I migration enhances the inward shepherding of material but does not severely reduce scattering. We find that greater than 50% of the solids disk material still survives the migration in scattered exterior orbits: most of it well placed to complete terrestrial planet formation at less than 3 AU. The shepherded portion of the disk accretes into hot-Earths, which survive in interior orbits for the duration of our simulations.

    Conclusions: Water-rich terrestrial planets can form in the habitable zones of hot-Jupiter systems and hot-Earths and hot-Neptunes may also be present. These systems should be targets of future planet search missions.

    Comments: Accepted by A&A. 15 pages, 14 figures. Higher resolution pdf available at this http URL

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0707.2674v1 [astro-ph]

    Submission history

    From: Martyn Fogg [view email]

    [v1] Wed, 18 Jul 2007 08:44:10 GMT (908kb)


  • ljk April 21, 2008, 14:59

    Dead Zone Accretion Flows in Protostellar Disks

    Authors: N. J. Turner, T. Sano

    (Submitted on 17 Apr 2008)

    Abstract: Planets form inside protostellar disks in a dead zone where the electrical resistivity of the gas is too high for magnetic forces to drive turbulence. We show that much of the dead zone nevertheless is active and flows toward the star while smooth, large-scale magnetic fields transfer the orbital angular momentum radially outward. Stellar X-ray and radionuclide ionization sustain a weak coupling of the dead zone gas to the magnetic fields, despite the rapid recombination of free charges on dust grains. Net radial magnetic fields are generated in the magneto-rotational turbulence in the electrically conducting top and bottom surface layers of the disk, and reach the midplane by Ohmic diffusion. A toroidal component to the fields is produced near the midplane by the orbital shear. The process is similar to the magnetization of the Solar tachocline. The result is a laminar, magnetically-driven accretion flow in the region where the planets form.

    Comments: 12 pages, 4 figures

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0804.2916v1 [astro-ph]

    Submission history

    From: N. J. Turner [view email]

    [v1] Thu, 17 Apr 2008 22:32:07 GMT (377kb)