How planets form is not an issue that will be settled any time soon, but two models have emerged that continue to energize research. We saw yesterday in a review of Alan Boss’ new paper that gravitational instability is one way to create a gas giant. But I spent most of yesterday’s post talking about UV radiation and its effects on the atmospheres of planets around M stars, a key part of Boss’ explanation of so-called ‘super-Earths’ in these environments.

So let’s back up and talk about gravitational instability itself. As early as 1997, the astrophysicist had proposed that planet-sized clumps could form relatively quickly due to instabilities in the disk of dust and gas surrounding a young star. Boss believed these clumps could be massive enough to form a gas envelope, but the model was hard to use in any predictive sense and demanded more intensive computer simulations than were then available.

Later work by Thomas Quinn, however, bears Boss out. Quinn (University of Washington) set up simulations of a protoplanetary disk, or proplyd, using powerful new software developed by his team and 30,000 processor hours on LeMieux, the Pittsburgh Supercomputing Center’s terascale system. The result? Well, let Quinn tell it: “We used a new model of planet formation that couldn’t adequately be tested without this kind of computing power, and we found that these giant planets can form in hundreds of years, rather than the millions that the standard model predicts.”

Planetary formation model

Image: Simulations by Quinn and colleagues shows how a protoplanetary disk surrounding a young star begins, in a relatively short time, to fragment and form gas giant planets with stable orbits. Credit: Pittsburgh Supercomputing Center.

Got that? Hundreds of years. Which is quite a change from the older core-accretion model. The latter is a much lengthier process that relies on clumps of solids forming within the young protoplanetary disk; these merge gradually into larger and larger pieces due to the force of gravity. In a million years or so, you get a planet, and some of these objects will grow, over perhaps ten million years, into Jupiter-class gas giants. The catch has always been that a gaseous planetary disk like this doesn’t seem to last long enough to allow the model to function over these time scales. And that’s one reason we have a battle between differing models of planetary formation.

Greg Laughlin (UC-Santa Cruz) sees serious problems with Boss’ model, though he agrees that the process can be involved in some giant planet formation, and he pays particular attention to the Gl 876 system we talked about yesterday. “Another important point to stress is that Alan’s simulations certainly aren’t in error in the sense of being computationally wrong,” says Laughlin. “It’s just that I don’t agree with the generic validity of the initial conditions.” Read all of Laughlin’s comments on gravitational instability here.

The key paper by Boss is probably “Giant Planet Formation by Gravitational Instability,” Science 276, 1836-39 (2002). And here’s a quick take from Scientific American on the same subject. Quinn’s work is found in Mayer, Quinn, Wadsley et al., “Formation of Giant Planets by Fragmentation of Protoplanetary Disks,” Science 298, 1756-59 (2002).