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How Does a Planet Grow?

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© NASA/ROGER RESSMEYER/CORBIS
The drama over Pluto's planethood is certainly something that pits researchers against one another, but it obscures an actual scientific mystery: how some of those undeniably planetlike planets formed. Researchers agree that the terrestrial planets--Mercury through Mars--are the product of increasingly large chunks of rock smashing together over millions of years. How gas giants like Jupiter and Saturn arose in our solar system and around other stars is a more nebulous question. Recent discoveries of planets orbiting low-mass stars provide fodder for either of two competing theories of gas giant formation, depending on who you ask.

The standard scenario for the birth of gas giants posits a continuation of the rocks-crashing-together process, also known as core accretion. In this view, when the growing core reaches about ten Earth-masses it starts slowly pulling in material from the wispy penumbra of gas enshrouding a young star. Eventually gas accumulation kicks into high gear, and then somehow stops, perhaps as the gas is depleted. The model is eminently plausible, argues Geoff Bryden, an astronomer at the Jet Propulsion Laboratory in Pasadena, Calif., because we know it already occurs for the rocky planets.

One puzzling detail is why Jupiter seems to have a relatively light core of no more than 10 Earth-masses, given that the core accretion model suggests a likely value of 20 to 30 Earth-masses. A decade ago, stimulated by this discrepancy and the discoveries of the first extrasolar planets, astronomer Alan Boss of the Carnegie Institution resurrected an alternative idea. The spinning disk of material around a star is barely stable, held in equilibrium by gravity and the outward pressure of its own warmth. If small variations arose in the density of the disk, ripples would form and sweep up mass until the gaseous disk clotted into spiral arms, which would later condense into balls of gas. The denser dust would finally accumulate in the middle to form the core. This process, called gravitational instability, would be faster than core accretion, Boss argued, taking a mere few hundred thousand or million years. "It seemed pretty crazy and oddball at the time," he recalls.

But he continues to accrete evidence for it. Boss points out that to explain the mass of Jupiter's core, researchers have to suppose that part of its initial core melted away, or the finishing chunks of rock got stuck in the atmosphere. "It's an amazing thing--claiming new physics because otherwise they have a problem," he says. Boss and his colleagues have hypothesized that solar ultraviolet (UV) radiation could account for the stunted core, if gravitational instability is at play. The UV would essentially blow dust away from the stellar disk out to about the orbit of Saturn, meaning that gas giants closer to the sun would have less dust as raw material.

Boss has recently proposed a similar effect to explain the discovery of two gas giants and two so-called super-Earths, or big rocky planets, each orbiting a small red dwarf star. Near the super-Earths were more powerful stars that put out a lot of UV radiation, so the two planets could have formed by disk instability and had their gaseous coverings stripped off by the UV. The planets without nearby massive stars would remain gas giants. Explained in terms of the core accretion model, the super-Earths formed too slowly to begin sucking in gas, whereas the gas giants got lucky, having grown a little faster. The statistics are too few to tell yet which model stacks up better, Bryden contends. "It would be nice if there was a much clearer result."

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