Abraham Lincoln is said to have once asked his cabinet: "If you call a horse's tail a leg, how many legs would a horse have?" After his advisors muttered, "five," he gave the answer: four, calling a tail a leg doesn't make it one!
Members of the public got very excited when Pluto was "demoted" by the International Astronomical Union from being called a planet to a dwarf planet in 2006. I ask students each semester what they would try to discover if they had the Hubble Space Telescope at their control. Sometimes students say they would try to discover whether Pluto really is a planet or not.
Scientists aren't hung up so much on what we call things. It's what things are that fascinates us!
And we're about to learn a lot more about what that fascinating ball of ice and rock Pluto is, whether we call it a planet or not! NASA's New Horizons spacecraft is making an historic flyby. Because it takes so long to plan a space mission and then travel three billion miles to the outer solar system, we may not see new pictures of Pluto in our lifetimes. New Horizons was chosen as a NASA mission in 2001 and then launched in 2005.
Pluto is the only "planet" discovered by an American, by Clyde Tombaugh in 1930 (some of his ashes are aboard the spacecraft flying by his discovery today!) Most of the time Pluto is the furthest "planet" from the Sun, although its non-circular orbit periodically brings it closer than Neptune (the two will never collide). At an average of 39 times further from the Sun than Earth, Pluto is a cold, dark place. Its irregular orbit means patterns of heating and cooling could lead to evaporation and re-freezing. Scientists are now figuring out how dark and light spots on Pluto's surface (shaped a bit like a whale and a heart) might be related to those conditions.
In 1978 Pluto's moon Charon was discovered. (Now we count five moons for Pluto.) Charon is a pretty sizeable moon for so small a "planet." From watching Pluto and Charon pull on each other, we are able to measure Pluto's mass. This has told us that it has a low density, meaning that unlike Earth it is more ice than rock.
New Horizons has multiple instruments aboard to study the Pluto system scientifically. It sees not just visible light, but can also "see" in ultraviolet (forming spectra to learn what Pluto's thin atmosphere is made of), and can detect particles and dust. The small spacecraft doesn't have enough energy to slow down and stay in orbit around Pluto. Instead it will continue on through the "Kuiper Belt" of countless comet-like objects, of which Pluto is probably the largest. New Horizons has nailed down the exact size of Pluto, so we know it is somewhat larger (but less massive) than the more distant Eris, discovered in 2005.
Whether New Horizons can add to its mission by flying by another Kuiper Belt object or not, its ultimate fate is to escape the solar system's gravity entirely, joining the two Pioneer spacecraft and two Voyager spacecraft already flying out to interstellar space, never to return.