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.