bystander wrote:Astronomers Explain Formation of Seven Exoplanets around TRAPPIST-1
Netherlands Research School for Astronomy (NOVA) | 2017 Jun 08
Astronomers from the University of Amsterdam (the Netherlands) explain with a model how seven earth-sized planets could have been formed in the planetary system Trappist-1. The crux is on the line where ice changes in water.
Now, the Amsterdam researchers come up with a model where pebbles migrate instead of complete planets. The model begins with pebbles that are floating from outside regions to the star. Such pebbles consist largely of ice. When the pebbles arrive near the so-called ice line, the point where it is warm enough for liquid water, they get an additional portion of water vapor to process. As a result, they clot together into a protoplanet. Then the protoplanet moves a little closer to the star. On its way it sweeps up more pebbles like a vacuum cleaner, until it reaches the size of the Earth. The planet then moves in a little further and makes room for the formation of the next planet.
The crux, according to the researchers, is in the clotting of pebbles near the ice line. By crossing the ice line, pebbles lose their water ice. But that water is re-used by the following load of pebbles that is drifting from the outer regions of the dust disk. At Trappist-1, this process repeated until seven planets were formed. ...
[i]Formation of TRAPPIST-1 and Other Compact Systems - Chris Ormel, Beibei Liu, Djoeke Schoonenberg
This is a very interesting model. It reminds me somewhat of how hail stones grow in thunderstorms here on Earth. In the case of hail, gravity is pulling ice pellets down while strong up-drafts blow the pellets repeatedly up into freezing temp regions of the cloud. In the case of young red dwarf stars, there might be considerable back and forth migration of the ice line due to all the flaring of the star.
Presumably, such a process could be at work around any young star, no matter its mass, as all young stars are very active at first. I also wonder if other chemicals or elements besides H2O might play similar roles at their respective ice lines.
I've always wondered how pebbles with negligible gravitational attraction manage to stick together and grow into a protoplanet massive enough to start gravitational growth. This model provides a very logical explanation. Nice