There might have once been a Mars-size planet out by Neptune, but where'd it go?

It is thought that a rocky orb with a mass close to that of Mars used to hang out with Neptune in the outer solar system, but if it existed, what happened?

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United Kingdom ,British ,Nancy Volk ,Brett Gladman ,University Of British Columbia ,University Of Arizona ,British Columbia ,Annual Review ,Solar System ,Protoplanet ,Space ,