As Uranus is less massive than Neptune, would this be evidence that it was formed further away from the sun than Neptune, and later it swaps orbit with Neptune to move closer to sun?
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2 Answers
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The Nice model of solar system evolution proposes that both Uranus and Neptune migrated outward from their original orbits due to numerous encounters with planetesimals, stimulated by a temporary 2:1 resonance between Jupiter and Saturn. Simulations going back billions of years can put either Uranus or Neptune initially closer to the Sun. Desch 2007, assuming that the Nice model is correct, argues for a particular density profile of the protoplanetary disk and notes that a closer proto-Neptune fits that profile better. Some solar system scientists have problems with the Desch model, but Uranus's smaller mass is consistent with it.
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No. The difference in mass is less than 20%, and for astronomers that counts as "basically the same" (for the masses, check wikipedia).
Your question implies that more massive planets should form closer to their star. However, there is as yet no definitive theory on the formation of planetary systems - that might come with the next generation of telescopes, (E-ELT, TMT, GMT, JWST) as they will allow us both to study more than one planetary system and better observe the early stages of planet formation.
