Astronomers Radically Reimagine the Making of the Planets

Pebble accretion is now a favored theory for how gas giant cores are made, and many astronomers argue it may be taking place in those ALMA images, allowing giant planets to form in the first few million years after a star is born. Research papers appear nearly every week about the early stages of planet growth, with astronomers arguing about the precise condensation points in the solar nebula; whether planetesimals start out with rings that fall onto the planets; when the streaming instability kicks in; and when pebble accretion does, and where. Most seem to have planets we lack, however: hot Jupiters as well as a class of midsize worlds that are bigger than Earth but smaller than Neptune, uncreatively nicknamed "Super-Earths" or "Sub-Neptunes." No star systems have been found that resemble ours, with its four little rocky planets near the sun and four gas giants orbiting far away. In the Nice model, the giant planets changed their orbits wildly at that time, which sent an asteroid deluge toward the inner planets. "Unless there is something to arrest that process, we would end up with giant planets mostly close to their host stars," said Jonathan Lunine, an astronomer at Cornell University. "Is inward migration really a necessary outcome of the growth of an isolated giant planet? What are the combinations of multiple giant planets that could arrest that migration? It's a great problem." In a paper published in April in Nature, he, Beibei Liu of Zhejiang University in China and Sean Raymond of the University of Bordeaux in France argued that gas flow dynamics may have caused the giant planets to migrate only a few million years after they formed - 100 times earlier than in the original Nice model and probably before Earth itself arose.