Planets with radii between Earth and Neptune and small radii are the most common in planetary systems. These planets are challenging to explain with classical models of planets. Do planets form instead in-situ in an inside-out manner?
Earth’s composition seemingly does not fit into planet formation theory. Ripping apart its building blocks by collisions during accretion might sound violent, but can be a way to go.
How to form clumps in the intermediate ranges of massive protoplanetary disks? Could these later be planets?
A model that needs fewest parameters to explain a scenario is favourable. The fact that mm-size dust grains (chondrules) are present in the entire solar system brings rise to the question, whether all bigger solid objects are a collection of chondrules.
Last year, an image was released that took our breath away. Exquisite rings carved in a disk of material around a nearby star. Now, astronomers want to know if forming planets are responsible, and why the image might look different from the cartoon in your textbook.
Planets are ubiquitous in the Milky Way. Therefore, building them must be straightforward, right? Not at all!