© University of Arizona/JLP/NASAThe solar system's mightiest planet
Jupiter and Saturn are thought to have begun life as rocky worlds with the mass of at least a few Earths. Their gravity then pulled in gas from their birth nebula, giving them dense atmospheres.
In this picture, all gas giants should have cores of roughly the same size. Yet spacecraft-based gravity measurements suggest Jupiter's core weighs just two to 10 Earth masses, while Saturn's comes in at 15 to 30.
New simulations by Shu Lin Li of Peking University in China, and colleagues, may explain why. They calculated what would happen when a super-Earth of 10 times the mass of our planet slammed into a gas giant. The rocky body flattened like a pancake when it hit the gas giant's atmosphere, then barrelled into the giant's core about half an hour later. The energy of the collision could have vaporised much of the core.
These vaporised heavy elements would then have mixed with the hydrogen and helium of the gas giant's atmosphere, leaving only a fraction of the gas giant's former core behind. This could explain not only why Jupiter's core is so small, but also why its atmosphere is richer in heavy elements compared with the sun, whose composition is thought to mirror that of the nebula that gave birth to the solar system's planets.
The super-Earth might have grown into a gas giant itself one day if it had not collided with Jupiter, says study co-author Douglas Lin at the University of California, Santa Cruz. "It may very well have been on its way to becoming a gas giant, but lost the race and got gobbled up," he says.
Saturn has a similar overabundance of heavy elements in its atmosphere. The team argues that this could be due to impacts by rocky objects smaller than Earth that decelerated and broke up before they could reach Saturn's core. The team's simulations suggest that this would have left the core intact, or even added to its mass as fragments rained down onto it.
"It's an interesting explanation of why you might have a variety of core masses in giant planets," says William Hubbard of the University of Arizona in Tucson. "It's a very useful contribution."
David Stevenson of the California Institute of Technology in Pasadena says he is "favourably disposed" towards the idea to explain the core size difference. But he says it is not needed to explain Jupiter's heavy-element enrichment, which, like Saturn's, could be due to numerous impacts by much smaller rocky objects.
The new work adds to the evidence that the solar system's birth was a violent and chaotic business, with perhaps five of its eight extant planets having suffered impacts by other planet-sized objects, and the rest by objects not much smaller. Giant collisions are thought to have spawned Earth's moon, blasted away the outer layers of Mercury, reshaped Mars's northern hemisphere and knocked Uranus onto its side. Smash-ups may also have led Neptune to acquire a moon (New Scientist, 20 March 2010, p 14) and slowed Venus's spin rate.
The new work also bolsters the idea that chance collisions play a big role in determining planet properties. Planets that form in similar conditions, like Jupiter and Saturn, can end up with very different properties as a result of such crashes, says Hubbard.
Giant collisions could explain some of the diversity seen not only in our solar system, but in planetary systems around other stars, too. "I think this ought to be very common," Lin says.
Alien impacts
Giant collisions around other stars could account for some of the weirdest planets seen so far.
The measured size and mass of some exoplanets indicates that they have relatively puny atmospheres but extremely massive cores. The heart of recently discovered Corot-13b, for example, seems to possess the mass of at least 140 Earths.
In 2006, Masahiro Ikoma at the Tokyo Institute of Technology in Japan, and colleagues, suggested that a smash-up between gas giants could blast away much of the giants' gas while fusing their cores. This process may have created Corot-13b, as well as another core-heavy planet called HD 149026b.
Giant collisions may have also affected other exoplanet properties, such as their orbits: planets are thought to follow circular paths at birth, but some have very elongated orbits. "People are just beginning to look at this," says Douglas Lin at the University of California, Santa Cruz.
I say Scientific clap-trap this is a hypothesis and should in no way be taken as fact.