Pluto is big news right now; no wonder since this year is the 30th anniversary of its discovery. But what do we really know about this enigmatic ninth "planet?" (quotes used advisedly, more on this later.)
Not much. We know that it is an average of forty Astronomical Units from the Sun; that is to say it orbits forty times farther from the Sun than does the Earth. At this distance, its surface temperature must be a balmy -380 degrees Fahrenheit, cold enough to freeze almost all gasses. We know that it reflects the sun's light ,displaying the feeble brightness of a 14th magnitude star–about 1600 times fainter than the faintest star that can be seen with the naked eye. We have some guesses about its mass… which is how the body was found in the first place. That remarkable story is worth review.
After the discovery of the 8th planet Neptune by measuring the wiggle it caused gravitationally on 7th planet Uranus' orbit, there was the fervent hope that finding further, unexplained wiggles in those outer planets' orbits would betray a 9th planet. Famed astronomer and Mars enthusiast, Percival Lowell, spent the last years of his life trying to find it. As it turns out, he did spot Pluto and even snapped pictures of it, but he took it for a star at the time, so slow is the planet's movement at the far end of the Solar System (similarly heartbreaking stories abound regarding early sightings of Uranus and Neptune.) In fact, the existence of Pluto was not confirmed until Clyde Tombaugh definitively found it, right around my 11th birthday, on February 18, 1930.
But is Pluto the planet Lowell was looking for? "Planet X?" There were doubts as soon as Tombaugh made his announcement. For instance, per Lowell's calculations, for Pluto to have the effect it did on the orbits of the outer planets, it would have to have a mass seven times that of Earth (if, indeed, the effect is genuine–we haven't even mapped Neptune through a complete orbit yet, so the non-Neptune-caused Uranus wiggle is our only source of data). Yet Pluto is so tiny, optically, that for it to have a mass that high, it would need to be a fragment of a dead, collapsed star. In fact, early on, that's just what was opined by some–that Pluto was a piece of an old White Dwarf.
Well, soberer heads did the math and determined that, based on its size (computed from its brightness at its distance) and its confirmed effect on Uranus, Pluto couldn't have a mass of more than that of the Earth, and probably somewhere between .5 and 1 Earth masses, depending on who you ask.
So, Pluto is not Planet X, which may still be floating out there. One astronomer suggests that there is a big planet nearly twice as far from the Sun as Pluto perturbing Neptune's orbit.
Then the next question is: Is Pluto even a planet at all? This is probably a good question to settle before everyone gets so comfortable with the idea that there are nine planets in the Solar System that they become stubbornly resistant to any change in that perception.
As early as 1936, a Raymond Littleton proposed that because of Pluto's unexpected tiny-ness and its strange orbit (it is tilted nearly 17 degrees to the plane in which all the other planets orbit, and the orbit is much less circular than that of the other planets–almost halfway to a comet's orbit), it is actually probably some rogue moon of Neptune that somehow got separated from the giant planet, perhaps via some primordial impact when the Solar System was formed. Further evidence in favor of this hypothesis is the fact that Neptune's moon, Triton, orbits backwards, and at a weird angle. Gerard Kuiper recently endorsed this origin story for Pluto.
My views tend to be more "Uniformitarian" than "Catastrophic," so I hold to the hypothesis of Dr. Frederick Leonard of UCLA. In August 1930, just six months after Pluto's discovery, he suggested that Pluto might not be unique: "Is it not likely that in Pluto there has come to light the first of a series of ultra-Neptunian bodies, the remaining members of which still await discovery but which are destined to still be detected?" Compare this to Ceres, the body discovered in 1801–it was once thought to be a planet, but it turned out to be the first of a new class of worlds, the asteroids.
Is there a slew of Pluto-like objects in the outer solar system? Only time, and more observation, will tell.
