Posted on behalf of Ron Cowen
Planetary scientists don’t usually don catcher’s masks at the end of a professional talk, but Hal Levison of the Southwest Research Institute in Boulder, Colorado, wasn’t taking any chances. Acknowledging just how outrageous his new theory of planet formation is, Levison, who looks like an ex-hippie, joked that he wanted to be prepared in case his audience started throwing things. Levison presented the work on 6 October at a joint meeting of the European Planetary Science Conference and the American Astronomical Society’s Division for Planetary Sciences in Nantes, France.
Levison is a formidable force in this field, well known for his contribution to the so-called ‘Nice model’ of outer solar system formation (see Nature 435, 459-461; 26 May 2005) Thus, the audience of planetary scientists was playful but remarkably respectful, given that Levison, David Minton of Purdue University in Indiana and their colleagues are now suggesting that all the planets in the solar system began forming at roughly Earth’s distance from the Sun. As the researchers see it, each embryonic planet from Neptune to Mars would shoot outward in succession through the disk of gas and dust that surrounded the young Sun. During the journey, each would grab enough mass from the disk to reach its present planetary size in an incredibly fast one million years.
Neptune would be the oldest planet, since it shot through the disk first. That’s in stark contrast to the traditional planet-forming model, in which fledgling planets stay put and continue forming at about the same orbital radii where they first coalesced, accumulating material only from their immediate surroundings. In that conventional scenario, Neptune would rank among the youngest planets since it would have taken much longer for material in the outer reaches of the disk to collide and stick together.
The standard model has had several successes, so why are Levison and his colleagues messing with a perfectly good theory? Because it isn’t, they say. The theory can’t explain how the giant planets, especially Uranus and Neptune, can finish forming before all the gas in the disk dissipates — a roughly five million year window.
It’s widely accepted that all the planets started with rocky cores. In the case of the gaseous, outer planets, the rocky cores had to snare vast shrouds of gas. But in the outer part of the disk, where the standard theory says the outer planets formed, material orbits more slowly and it takes too long to form a core. By then the gas needed to build up the gas giants should already have departed. That apparent contradiction has left planetary scientists with an unsatisfying account of what happened.
In the new model — which Levison stresses is so far only a ‘fairy tale’ whose details have yet be worked out — he and his collaborators considered the complex dance that emerges between vast numbers of 50-kilometer-size solid bodies, or planetesimals, that form at around Earth’s distance from the Sun and a few bigger, moon-size bodies that also happen to coalesce every so often.They discovered that when certain conditions are met, interactions with the planetesimals will drive a lunar-size body outwards. It will then zip through the outer disk of virgin, undisturbed material, gathering up enough solids to grow a massive core and then collecting a hefty portion of available gas.
As long as there is enough time for the disk to settle back down before a new lunar-size body happened to emerge in the inner part of the disk, it, too will shoot out, rapidly accumulating a sizable solid core.
In this fairytale, Neptune ought to have the fattest core because it moved through the disk first and had the best chance at capturing solid material. There is tentative evidence that supports this, argues Minton, because Neptune puts out more heat than expected. If it formed first, with a larger core, it should have a greater abundance of aluminum-26, a radioactive element common in the very early solar system and a heat source.
Jupiter ought to have the smallest core because it’s the last gas giant to form, but because it spent more time in the disk, it grabs more gas and has a larger shroud.
Then there’s little Mars—as Levison and Minton see it, it’s the last planet to be pushed through the disk, which, by then, is severely depleted of material. There aren’t enough planetesimals to push the body very far and not enough material to make it very big. So it gets nudged just slightly beyond where it first grew and remains relatively small—solving the puzzle of why Mars is so much smaller than Earth and Venus.
And what about our own planet and its hot sister? The disk is essentially kaput by the time they come along, the researchers say, and these rocky planets grew by clumping together whatever remaining planetesimals were left in the inner region of the disk. (Mercury is yet another story, adds Minton.)
A determination of Neptune’s age would provide one clear test of the model, notes Minton. As to how one would go about determining the planet’s age, Minton jokes, with a wave of his hand, “I’ll leave that as an exercise to the readers.”
William McKinnon of Washington University in St. Louis seemed to sum up the reaction in the small, crowded lecture room: “It’s an interesting idea. We’ll have to see if it pans out.”
Levison acknowledges the theory is “radical”. If correct, he says, “it will turn planetary science on its head.” If it’s wrong, he adds, “maybe, they’ll have me turn in my planetary science badge and my decoder ring. So be it.”
Photo: Bill Bottke