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Comment by dr_zoidberg | Hacker News Reader

The asteroid belt doesn't have nearly enough mass to be/have been a planet[0]. Largest thing there is Ceres, followed by Vesta, of which only the first qualifies as a dwarf planet.

There was an hypothesis about it being the remnants of a destroyed planet[1], but that was mostly an idea to support the Titius-Bode law[2], which was disproved a few hundred years ago.

[0] Currently about 3x Ceres mass, though it may have about Earth mass early in its history. See https://en.wikipedia.org/wiki/Asteroid_belt#Formation and https://en.wikipedia.org/wiki/Asteroid_belt#Evolution

[1] https://en.wikipedia.org/wiki/Phaeton_(hypothetical_planet)#...

[2] https://en.wikipedia.org/wiki/Titius%E2%80%93Bode_law

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The qualifier "dwarf" doesn't mean that the celestial body is too small to be considered a planet, it means that it hasn't cleared the neighborhood around its orbit. (Blame the IAU if you find this counterintuitive). This means that your conclusion is wrong: the asteroid belt actually has enough mass to be a planet.

Other than not having enough mass, what would cause a celestial body to not clear its orbit? The only other factor that occurs to me is time, is there anything else? If not, does that mean Ceres may eventually become a planet?

> ... the Titius-Bode law[2], which was disproved a few hundred years ago

Er... the Wikipedia page you're citing at [2] has this to say:

"Results from simulations of planetary formation support the idea that a randomly chosen stable planetary system will likely satisfy a Titius–Bode law."

and

"96% of these exoplanet systems adhere to a generalized Titius–Bode relation to a similar or greater extent than the Solar System does."

"Disproved" in science often means "superseded by more accurate theory".

True, but the article doesn't mention a more accurate theory. It implies that the rule works better than we have theoretical reasons to expect. (But as I commented above, the article contradicts itself to some degree.)

The Wikipedia article is contradictory. As you quoted, it says that the law works well in simulation and in observation, and yet it's just a coincidence. Of course it's not exact, but it looks like a very good first approximation.

If the asteroid belt originally had the mass of Earth, then I'd say it's fair to count it as a planet in this context, assuming that the orbits of the planets were established fairly early and have stayed about the same since then.

Is it possible that this was a planet, but most of the mass was ejected out of the system?

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