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I know that tidal forces are pushing Jupiter farther from the Sun, but I couldn't find exactly the yearly amount. In a few billion years would this effect (and subsequent decrease in gravity pull) allow the formation of a new planet from the asteroid belt or would Jupiter take the asteroids with it farther away?

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The total mass of the asteroid belt is just about 4% of the mass of our Moon. Even if the asteroids don't collide with other planets in the meanwhile, the mass is too low to form a planet in the sense of the 2006 IAU definition.

Even if Ceres would accrete all asteroids in the asteroid belt, its radius wouldn't grow to the double of its present radius, hence stay smaller than Mercury. But Ceres is already large enough to fit the present IAU definition of a planet, with the exception, that it hasn't cleared its neighbourhood.

With a Stern-Levison parameter of $8.32 \cdot 10^{−4},$ Ceres is too small to fulfill the third IAU criterion for a planet, and too small to clear its neighbourhood.

Even a dwarf planet of the total mass of the asteroid belt couldn't fulfill the criterion on a pure Stern-Levison parameter basis, hence wouldn't be able to keep its neighbourhood clear. Its Stern-Levison parameter would be about $7.5\cdot 10^{-3}$. That's well below 1, the estimated value necessary to fulfill the third IAU criterion, but well above the Stern-Levison parameter of $2.95 × 10^{−3}$ for Pluto.

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Sometime after 4.5 billion years, the sun will change into a red giant star and expand to about 1 AU. Some people think it will be beyond Earth's orbit. (In any case, Earth's orbit would have changed and likely to have struck the surface of the expanded sun.) The asteroids would experience a stronger solar wind and their orbits would become more elliptical, so Jupiter's influence would be significant.

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so there is no time for the formation of another planet from the asteroid belt? – symbiotech Apr 29 '14 at 22:52
As the Sun loses mass Earth's orbit will expand, making it less likely to be engulfed by the expanding Sun. As for the asteroids' orbits becoming more elliptical, can you support that claim? I would think that an asteroid with a nearly circular orbit would have its orbit expand more or less uniformly. I also wonder how much effect the solar wind would have on the asteroids' orbits, particularly the more massive ones. – Keith Thompson May 1 '14 at 1:11

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