So I was watching some TV, and I heard Dr. Plait mention that the planets Uranus and Neptune are too big to be located so far out in our solar system. Now, I heard his explanation on the show as to how they may have ended up where they are, but I want to also leave this open for some education.

Why are these two planets considered too big for their respective orbits, and how did they get to their current size and locations?


In the disc of material expected to be left over after the Sun forms, there is not expected to be enough material for planets of the mass of Uranus and Neptune to form on reasonably short timescales at their current locations. One possibility is that they accreted mass at a smaller semi-major axis, somewhere in the vicinity of Jupiter and Saturn, and wound up being scattered outwards to their current locations.

From The Formation of Uranus and Neptune among Jupiter and Saturn over on The Astronomical Journal:

"Instead of forming in the trans-Saturnian region, Uranus and Neptune underwent most of their growth among proto-Jupiter and proto-Saturn, were scattered outward when Jupiter acquired its massive gas envelope, and subsequently evolved toward their present orbits."


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