From an article in Scientific American on Pluto’s moons:

Pluto’s biggest moon, Charon, weighs in at a hefty 11% of the mass of Pluto itself. It may have formed during a cosmic collision early in the Solar System’s history, billions of years ago. Debris from that smash may have coalesced into the much smaller moons Styx, Nix, Kerberos and Hydra.

Pluto’s no longer officially a planet. Fine. Don’t care.

One of the questions about extra-terran biospheres (and regenerating atmospheres) is the importance of meaningful tidal stresses. Those pretty much require moons which are an appreciable percentage of the primary in mass. And as far as I know, we don’t know how likely that is.

But let’s look at the small planets (Mercury, Venus, Earth, Mars), and the previously-a-planet Pluto, and the would’ve-been-a-planet Eris, which is roughly the same size as Pluto. There are more probable dwarf planets, but they’re all substantially smaller than either – Makemake and Haumea have less than two-thirds the diameters of Eris and Pluto, which is a lot less mass, both all things equal and as measured so far. (About 70% less.)

Two of these six planet- and near-planets have moons big enough for meaningful tides: Earth and Pluto. And both of them have moons thought to have been formed by early collision in solar system formation.

A third is a pretty healthy percentage. Now, I don’t know how critical this is thought to be at this point, because I’m not real current in my solar-system-formation and xenobiology theory. But it used to be thought that tides helped prevent carbon dioxide buildup (see: Venus) and a quick check online seems to imply that’s still the case.

If all these ideas about atmospheric regeneration and the like haven’t been discredited, this is a pretty interesting result. From the data we have, large moons around small planets (and, yes, dwarf planets) are not unlikely things.

And that’s pretty neat.