What do we know about Pluto?
Mostly as a result of occultations of background stars, and of so-called 'mutual events' in which Pluto and its large satellite Charon occult and eclipse each other, we know quite a lot. The key results are that the radius is 1150 km (with some uncertainty caused by effects in Pluto's weak atmosphere) and the density is 2030+/-60 kg/m^3. The mass is a miniscule 0.002 Earth masses. The density (about twice that of ice) shows that Pluto has a significant rock component. Depending on what kind of rock that might be, the rock/(ice + rock) ratio is of order 0.6, which means that 2/3rds of the mass of Pluto is in ice. It is very likely that Pluto is differentiated, with a rock sludge core and a more pure ice mantle. Some models suggest a liquid water ocean at some depths, for some times.
The discovery of the numerous bodies in the Kuiper Belt raises the question of the nature and significance of Pluto. This was evident immediately upon the discovery of 1992 QB1 and every KBO discovered since then has emphasized the same point: Pluto looks less and less special with every new KBO we find. Most recently, several large KBOs, including one as large as Pluto itself, have shown clearly that Pluto is not unique. As described on the Plutino Page, the orbit of Pluto is not different from the orbits of a large number of Plutinos already discovered. If you doubt this, take a look at this plan of the solar system and see if Pluto stands out from the other trans-Neptunian bodies (hint: it doesn't). Other distingushing characteristics such as the size and Pluto's satellite have slipped away with the flood of new discoveries. Pluto's size conveys several secondary benefits, notably the ability to retain a thin atmosphere from which surface frosts are deposited. For this reason, Pluto's albedo is very high (about 60% compared to the 4% assumed to prevail elsewhere in the Kuiper belt). But this difference really is secondary and, in any case, other large KBOs (notably 2003 UB313) have high albedos that are best explained by the deposition of surface frosts
So, until recently, one had two choices. One could either regard Pluto as the smallest, most peculiar planet moving on the most eccentric and most inclined orbit of any of the planets or one could accept that Pluto is the largest known, but otherwise completely typical, Kuiper Belt Object. From the point of view of trying to understand the origin and significance of Pluto it has always made sense to take the second option. Pluto's eccentricity and inclination were pumped up along with the eccentricities and inclinations of the ~few x 1000 other Plutinos (diameters > 100 km), probably driven by the radial migration of Neptune. The processes that shaped the orbits of the KBOs are the same ones that gave Pluto its prominent dynamical characteristics.
In August 2006, the International Astronomical Union, a group concerned with naming and pronouncing on astronomical matters, decided the issue formally through a new definition of what defines a planet. Pluto is not a real planet, they said, it's a dwarf planet. Then, in 2008, they said it's a Plutoid. So be it.
Some people see this as a demotion for Pluto. I think that it can reasonably be portrayed as a promotion. Our perception of Pluto has been transformed from a singularly freakish and unexplained anomaly of the outer solar system to the leader of a rich and interesting family of trans-Neptunian bodies whose study will tell us a great deal about the origin of the solar system.
So, we have discovered -1 planets and +1 Kuiper Belt. It seems like a fair trade to me. (See also Dan Green's page).
These new scientific developments have overshadowed plans to send a spacecraft to Pluto. In its new role, Pluto is interesting primarily as an end member of the known KBOs, and we stand to gain as much or more by studying the smaller (less evolved) KBOs as by studying Pluto alone. This new purpose has been accepted only slowly by some Plutophiles.
An interview with Pluto.
A postcard to Pluto.
Last Updated February 2010