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Before anybody opines with "How could Pluto have an atmosphere when it's less massive than the moon (which has no atmosphere)?", it's worth noting that the moon does in fact have an atmosphere.

Interestingly, NASA speculates that Pluto's surface pressure is around 3 µbar, whereas they state that the moon's surface pressure at night is dramatically less than that, at 3.0 × 10^-9 µbar. I'm curious to see what New Horizons' measurements will reveal.

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If we're talking outliers then Titan, the largest moon of Saturn has a surface atmospheric pressure 1.4x more than Earth's, while being 2.3% of Earth's weight

And then there's Venus' atmospheric pressure :-)

Curious, but how much of an effect would temperature have on a planet's ability to retain an atmosphere? A colder planet of equal size will presumably be capable of maintaining a thicker atmosphere, but I'm not sure if that'd be a significant factor in this case or not.

Proximity to the sun is a crucial attribute with respect to density of an atmosphere, both due to heat input (hot molecules escape easier) and solar wind density (energetic particles strip away molecules in collisions.)

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