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Pluto and Charon seem to have surprisingly young surfaces, considering that resurfacing events were expected to be rare for them.

Could the explanation be that they actually are young? Formed later than the Solar system at some other star and then ejected and captured? Or that they are ancient but have extra-Solar origin and have little cratering because they have spent billions of years as a vagabond planet system in empty interstellar space? Sedna is believed to be captured.

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The surface of Pluto is not just young, but very young, and also differentiated. The 'heart', Sputnik Planum, may be currently active, with flowing glaciers, and nitrogen snow. This area may have be resurfaced during Pluto's perihelion. Your theory cannot account for the very young age of the surface.

Next capture of a rogue planet is not easy. It would need Pluto to interact with the outer planets, to slow it down to enter solar orbit (otherwise a rogue planet would just pass through the solar system and come out the other side) A captured rogue would be unlikely to have the relatively low eccentricty of pluto.

Finally, while we don't know how common rogue planets are, they are probably not common, and given the massive distances between stars, the chances of any rogue just happening to have passed anywhere near the solar system in the last billion years in the right direction to be captured to Pluto's orbit is very low.

Conclusion. Pluto formed around the sun. A capture theory does not explain the observed young surface of Pluto, and modelling suggests a capture would be very unusual.

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  • $\begingroup$ Pluto was at perihelion in 1989. Is Sputnik Planum only 26 years old? :-D I agree that capture seems unlikely, unless there are raining Pluto's out there. And wherever it was formed, it probably was in a proto-planetary disk with heavy cratering anyway. But recent resurfacing is also a mystery. A closer look at some other Kuiper Belt objects might be helpful, but unfortunately there's only one of each planet. I bet the New Horizons data already holds a good explanation to be understood and published in a few years. $\endgroup$
    – LocalFluff
    Oct 5, 2015 at 18:12
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    $\begingroup$ Rogue planets are estimated to be upwards of exceedingly numerous. The ratio rogue planets:stars has estimates ranging from ~2 to ~100,000. A very large range, so quite consistent with your statement that we don't know how common they are, but by sheer numbers they appear to be no more rare than stars themselves. Encountering them may still be quite rare, though. Space is extremely vast, after all. $\endgroup$
    – zibadawa timmy
    Oct 6, 2015 at 10:15
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(Edit, I think my original conclusion here was wrong, having read up about it).

While it seems probable that Pluto and Charon were formed by collision, I gather it's unlikely that this collision was so recent as to explain their lack of craters and young surface. The Collision, by most articles I've read, happened when the solar system was young, not in the last 100 million years.

Now, it's possible that none of this is certain, but I retract that part of my original post.

But, the creation of Charon by giant collision remains likely See here, and here. That would suggest formation within our solar-system, not from outside.

Pluto/Charon are also among a fairly common orbital region called Trans-Neptunian objects.

According to Wikipedia they suggest the collision that formed Charon happened 4.5 billion years ago.

Sedna (love the story, by the way), has a very elongated and distant orbit which seems more likely with capture of an object. I agree with everything James Kilfinger said. Rogue planet/Rogue object capture may happen from time to time, but the Rogue would need to get quite close to the sun for the sun to have sufficient gravitation to achieve the capture. Highly elongated orbits would seem likely for solar-system orbital capture. Pluto is only slightly elongated and not a good candidate. More massive suns should have significantly greater capture numbers, but that's kinda obvious I suppose.

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