How to grow a gas giant
Watching gas planets during their youth is key to understanding how they’re born
How clouds of dust transform into fully fledged gas giants is one of the key questions for astronomers researching young exoplanets.
“There are two competing theories,” says Jaehan Bae of the University of Florida. “The first is the ‘bottom-up’ process, where you initially form a small, rocky core that becomes more massive. It starts to collect gas from the disc and forms a gas giant like Jupiter. Then there’s the ‘top-down’ theory, where the disc becomes unstable due to its own gravity and collapses to form a planet. We don’t know which is right, even for Jupiter.”
Both of these ideas have problems, however. In the ‘bottom-up’ model, regions of the disc far from the star are too cold for planets like Neptune to form within the 10 million-year window before the disc disappears. On the other hand, the ‘top-down’ model forms planets that are too massive to remain in stable orbits. Planets that are many times the mass of Jupiter fall into their stars far too readily. To delve deeper into the mystery, astronomers observe young planets while they’re forming. Planets formed via the ‘top-down’ process are expected to be warmer than their ‘bottom-up’ counterparts, so mapping out the mass and temperature of forming planets could yield vital clues. Meanwhile, tracking how planets are distributed in the disc at different points in their lives will reveal more about their patterns of migration.
“It’s not a yes/no question,” Bae says. “Both models can form giant planets, but they may work in different regions of the disc, or under different conditions.”