Quote:
Originally Posted by The Lone Ranger
(In astronomy, hydrogen and helium are referred to as "gases"; all other elements and compounds are referred to as "dusts.")
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Just a minor quibble, but this isn't quite right. It's often joked that astronomers know only three chemical elements: hydrogen, helium and metals. Which makes a lot of sense for them, since the heavier elements make up only a tiny fraction (~0.1% by mass) of the matter in the Milky Way, and their only role in stellar spectra seems to be "create a lot of absorption lines". Of course, the people studying interstellar chemistry and molecular line emission tend to have a more nuanced view.
Dust in an astronomical context means mostly macromolecules (like polycyclic hydrocarbons) and solid state particles composed mostly of graphite and silicate. The sizes of these dust grains range from several nanometers to micrometers in interstellar conditions; in planet forming regions, the densities are high enough that they can efficiently coagulate to form larger particles, at least to a couple of millimeters.
Presumably dust grains grow beyond millimeter sizes, but at that point we can't see them anymore; as an analogy, it's hard to see a brick from a couple of miles away, but if you grind it to dust and scatter it in the air it's much easier to spot. From models, we can be pretty sure that the coagulation process continues at least to the meter scale. What happens after that is still a bit of a mystery, since meter sized dust grains aren't good at sticking to each other (again, try it with bricks), and they're not heavy enough that gravity can help them out. It's still an outstanding question how particles grow from meter sized objects to kilometer sized planetesimals.
Once planetesimals form the accretion process is very efficient in forming planets. At this point there are still two processes that stand in the way of a planetary system like the one we're familiar with. First, there is the gaseous disk that still surrounds the parent star. The friction caused by this disk, and the tidal effects it induces may cause a newly formed planet to spiral in towards the central star, or eject them from the system. Second, a solar system with more than one planet is notoriously chaotic, and we have no way of knowing whether it will be stable in the long run. Interaction among the planets may again eject some of them from the system or send them into the star.
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Apart from that, this was another excellent refresher course on planet formation, TLR. I'd just like to add this cartoon I made a couple of years ago, outlining the whole process from collapsing molecular cloud core via accretion disk with an acive outflow and passive disk to planetary system.