If there was ever an object in the solar system that had astronomers confused, it was Pluto. At first their consensus was that it was a planet; then they decided it was not. The New Horizons spacecraft was supposed to answer questions and solve mysteries about this mysterious object, but when it flew past Pluto at about 31,000 mph, it uncovered more mysterious things. Pluto has a nice round, planet-like shape, but it also has a surface with huge areas covered in comet-like ice. It confused astronomers to see large swathes of Pluto’s surface looking blemish-free. Pluto looked youthful instead of fitting neatly into the extremely old-age thought pattern of many astronomers today. Data gathered from the New Horizons spacecraft flyby indicated that there was a heavier mass beneath the surface ice. It was at the right density to indicate a liquid water ocean below the surface. It’s almost as if Pluto was a warm body that has started to freeze on the surface just like a pond in the wintertime.
Despite being icy, Pluto looks rather spicy! This picture was taken by the New Horizons spacecraft and overlaid with an infrared photograph showing exaggerated colors to highlight a variety of different compositions of ice.
One exotic world isn’t enough in Pluto’s neighborhood; it’s a small object but still holds on to five of its own moons. Spicing things up a little more are two of the moon’s names, Styks and Niks. They look like big rocky boulders. The biggest moon, Charon, orbits Pluto once every six Earth days—about the same amount of time it takes Pluto to make one rotation. This makes Charon appear in the same area of Pluto’s sky at all times.
Pluto sounds like a far-out planet, and it literally is! It’s forty times farther away from the Sun than Earth. If you were to stand on Pluto and look back toward the Sun, the Sun would look like a point of light only about the size of Jupiter and would make Pluto about as bright as a full moon night. Since it’s so far away from the Sun, Pluto is about -400°F (-240°C). It takes 247.9 years to orbit the Sun once. Even though it’s far away, Pluto’s orbit is elliptical enough that for about 8% of its orbit, it’s closer to the Sun than Uranus.
Pluto is so small that it would cover only about half of the width of the United States. This causes an extremely low gravity too. A 100-pound (45 kg)person would only weigh 7 pounds (3 kg) on Pluto.
Pluto’s small size was part of the reason for the confusion on whether or not Pluto should be considered a planet. Astronomers have already found four other objects in the solar system that are about the same size as Pluto. Should those objects be called planets too?
The International Astronomical Union decided that an object needed to meet three criteria before it could be considered a planet. First, it had to orbit the Sun. Second, it needed to have enough mass, creating enough gravity, to pull itself together into a sphere. Third, it needed to have enough gravity to clean out its orbit of all significant competing objects. Pluto didn’t clean out its orbit, so its planetary status was wiped out. In the process, Pluto acquired a new official name—a lowly number, the minor-planet designation 134340. Even the lowly things of God’s creation are captivating and mysterious—and sometimes they seem a little more confusing than necessary!