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Today I will talk about a recent work on planetesimal formation by eugene and I.

Here I show a size ladder of the solids in protoplanetary disks which

I call it the solid's stairway to heaven. As every solid particle has their ultimate goal which is to become protoplanets. The planetesimals are these km size solid objects, and they are believed to be the building blocks for protoplanets. . Once you are at this km size, the gravity would promote further growth to protoplanets very radpidly. For particles size smaller than cm, particle-particle sticking would enable grains to stick to one another. .The real difficulty here is to climb up all this stairs from cm to km, as both sticking and gravity fall short in this intermedia size regime. One possible way to get around this is by direct gravitational collapse from those small size dust particles, which skips the particle growth from cm to km.

There are several reasons why this approach is attracting: 1) this path skips the meter size boulders, therefore, avoid the meter size barrier problem: as we know, meter size boulders undergoes a very rapid orbital decay, within a hundred years it could drift from 1 AU to the central star. 2) this approach happens very quickly, the timescale is much shorter than the particle growth through sticking. This is an old idea, its origin can go back to Safronov and Goldreich's work in the 60s and 70s.