Why are gas giants round

New model explains frequency of super-earths in narrow orbits

Gas giant

Leicester, United Kingdom - Rocky planets like our Earth may emerge from large gas planets that are formed far away from their star. This model, now presented by a Russian researcher, turns the previous ideas of astronomers about the formation of planets on their head. In the classic model, rocky planetary cores initially form, which then capture extensive gas envelopes - in the new model, the formation is exactly the other way around. The alternative scenario, which will soon be presented in the journal "Monthly Notices of the Royal Astronomical Society", provides an explanation for the large number of planets with several times the mass of the earth, so-called super-earths, which orbit their stars in narrow orbits.

Planets form in rotating clouds of gas and dust around young stars. In the classic core accretion model, dust grains clump together in the cloud and gradually form larger and larger objects through collisions and mergers. When the objects have exceeded a critical size, they attract gas from the environment with their gravity. This creates gas planets with rocky cores like Jupiter and Saturn. Close to the stars, the strong radiation sweeps the gases out so quickly that the nuclei remain here as Earth-like planets without extensive gas envelopes.

But the core accretion model is difficult to explain the large number of super-earths orbiting their stars in relatively narrow orbits. Sergei Nayakshin from the University of Leicester therefore sees a different process at work. In his model, gas compressions first form far out in the rotating cloud. When these gas protoplanets become massive and dense enough, they begin to collect dust. The dust falls into the centers of the gas clumps and forms rocky cores there.

The interaction with the surrounding gas disk then causes the young giant planets to migrate inward. If they get too close to their central star, the star's gravity tears the outer gas envelope away from the planet - rocky planets remain behind. The analysis by Nayakshin shows that this process typically takes place at a distance of 0.03 to 0.2 Earth orbit radii - and it is in this zone that astronomers have tracked down many super-earths.