I consider Pluto and Charon twins. They run around the sun, constantly together, and pretty much ignoring all their other siblings. Pluto was discovered in 1930, and was considered a planet until fairly recently, when it was demoted to dwarf planet.
Charon wasn’t discovered until 1978, and is usually considered Pluto’s closest and largest moon. However, there are scientists who (like me) think Pluto and Charon should be classified as a binary dwarf planet unit. For one thing, Charon’s diameter is slightly more than half of Pluto’s, which is very large for a moon. The relative sizes of a moon to its planet rarely approach that, from what we’ve been able to observe so far. Charon’s size is so large, compared to Pluto’s, that - strictly speaking - it doesn’t actually revolve around Pluto. Both of these twins revolve around a point somewhere between them. Kind of like 2 kids on a playground, holding each other’s hands and spinning around, laughing as they get dizzy and the world around them starts to look silly. And like those 2 kids, they don’t allow the 4 remaining ‘moons’ to join them. The tiny moons revolve around the pair, wishing they were part of the game.
Pluto’s diameter is 2,372 km[1,474 miles], making it the largest dwarf planet we know of. My quick research didn’t find Charon’s exact diameter, but it’s slightly more than 1,186 km[737 miles], which certainly makes it larger than Ceres (950 km)[590 miles].
While NASA’s probe thoroughly studied Pluto, it didn’t neglect Charon. Since it was there, why waste the opportunity? And aside from size, they do rather resemble each other.
Both Pluto and Charon are believed to have a rocky core surrounded by water ice, with other ices covering that. And at the temperatures experienced that far out, water ice is as hard as stone. I find that a little hard to fathom, but not impossible to accept.
Pluto and Charon are tidally locked, meaning each keeps the same side facing the other at all times. Rather like the 2 kids mentioned earlier. But they must have gotten so dizzy they fell over, because they travel around the sun on their sides - still revolving around each other. Maybe the twins got the idea of laying down from Uranus.
And like Uranus and Venus, Pluto rotates backwards, so that sun rises in the west and sets in the east. I found no mention of Charon doing that.
Both of the twins have some interesting features, like Pluto’s ‘heart’, which is a huge glacier made of Nitrogen ice, and Charon’s huge chasm that crosses its entire face. The ‘southern’ half of Charon is smoother and has less craters than the ‘northern’ half. The current thought is that when Charon’s internal water froze (and therefore increased in volume), the pressure forced some partially frozen water out as a type of lava. Remember Ceres’ cryovolcano?
For at least part of its year, Pluto has an atmosphere, or maybe it should be called ‘layers of haze’. And some of it is escaping into space, but not as much as scientists expected, and mostly methane, not the nitrogen they expected would be leaving. I thought I had heard that Charon also had some haze, possibly borrowed from Pluto, but I couldn’t find anything like that during my research, so I may have mis-heard or misunderstood what was said.
And now, I’m going to give a self-satisfied raspberry to those who decided Pluto was ‘just’ a dwarf planet. It (and Charon) were full of surprises and brain-twisting facts for the entire New Horizons team that studied the incoming data. Way to go, Twins!