Thread by @DrPhiltill

Fun fact: “Protoplanet” literally translates to “first planet” or “primitive planet”. They are the first set of planets that formed in the solar system. Vesta, the 2nd largest asteroid, is believed to be a surviving protoplanet, but it is no longer round due to collisions. /1

2/ In the early solar nebula before there were any planets, dust grains joined together and grew slowly into planetesimals. It takes maybe a million years to get planetesimals about 1 km in diameter.

3/ After reaching about 1 km in size, their gravity is big enough to pull in the dust grains faster, so runaway growth begins. After about 10 million years, the planetesimals (icy ones in the Kuiper Belt) may reach, say, 300 km diameter. Then a different growth phase begins...

4/ At this stage only a few planetesimals have experienced runaway growth, while the vast majority are a swarm of small bodies. The planetesimals are big enough to start giving gravitational kicks to each other. But as each one gets slingshotted faster, whipping it away, then...

5/...the gravitational drag of the swarm slows it back down again. This interplay of large kicks and gravitational drag both separates & recircularizing the orbits, causing the large planetesimals to separate themselves away from each other. Now they are called “Oligarchs.”

6/ This is the phase when they are big enough to be round and are considered Protoplanets, or “first planets”. As oligarchs they grow, but their growth rate has slowed because they tend to clear out the debris around themselves by flinging it away rather than just pulling it in.

7/ In the literature that describes this phase, scientists call these round bodies “oligarchs”, or “protoplanets”, or often simply “planets”. (It doesn’t matter that no orbit clearing has occurred, or that they are unable to clear orbits.) Example: arxiv.org/pdf/0807.1134.…

8/ The fate of these small, primitive planets, which are still swimming in a sea of planetesimals, depends on their location: terrestrial zone, asteroid zone, giant planet zone, or outer zone. (And it may be different for stellar systems with different architectures than ours.)

9/ In the inner zone they grow and merge to form terrestrial planets. Some giant collisions occur (like the one that formed the Moon, or the one that knocked much of the crust off Mercury) but this eventually settles down and the survivors clear their orbits to a large degree.

10/ Skipping the asteroid zone for a moment, the next farther zone is far enough from the star that lots of volatiles can be gathered by the gravity of the growing protoplanets. Those protoplanets are destined to become giants. They are the solid “cores” of the Jovian planets.

11/ Once giants begin forming, their huge gravity stirs the protoplanets and planetesimals that are just sunward of their location. This stirring prevents this planetesimals from coalescing like they do in the terrestrial zone. Instead, they endlessly bust apart & form again.

12/ This is the zone we call the Main Asteroid Belt. Only about two or three of the original protoplanets in that region survived: Ceres, Vesta, and possibly Pallas. Apparently the rest busted apart and remixed with the surviving planetesimals. (They, too, bust apart into dust.)

13/ This busted-up material in that zone re-clumps, forming “rubble pile” asteroids, which my colleague Dan Britt calls “sandbars in space.” The busted material also forms regolith coating all the asteroids.

14/ So far we have covered three zones, and in each zone the protoplanets had a different fate. What about the fourth zone? In our solar system we call it the Edgeworth-Kuiper Belt (or just Kuiper Belt). In other solar system architectures the locations of belts could differ,

15/ depending on how the giant planets moved around, which depends on lots of random things, like whether passing stars disrupted everything, etc. As a result, the term “Kuiper Belt” May not apply to other systems. But for our stellar system, we can call it the KB. So,...

16/...what happened to all the “first planets” in the KB? Well, they continued to grow like in the terrestrial zone. But there is so much space out there, they can coexist happily with the leftover planetesimals. So again, it was different in each zone. Also,...

17/...There wasn’t nearly as much gas that far from the sun so they couldn’t grow into giant planets. Also, while they are tugged by the giants, there is so much space out there that this tugging doesn’t make them collide & bust apart like the protoplanets in the asteroid zone.

18/ So in each zone, the “first planets” had a different destiny. Four zones, four types of planets.

1. Rocky planets
2. A few surviving protoplanets
3. Giant planets
4. Small icy planets (like Pluto)

Each type is the “end state” of the planet formation process in its own zone.

19/ I was thinking about planet formation tonight because of NASA Administrator @JimBridenstine saying he believes Pluto is a planet. E.g., express.co.uk/news/science/1…

20/ In 2006 the IAU voted that the relatively unimportant process of orbit-clearing is what matters: merely sweeping the workshop floor after planet formation is done. It doesn’t matter if a protoplanet did EVERY growth phase becoming an oligarch with complex active geology🤦‍♂️

21/21 they say only what happens AFTER planet formation is what matters. Did it sweep up the floor after it’s oligarchic growth was done, or didn’t it? However, the scientific literature shows many scientists call these bodies “planets” regardless whether the floor got swept.