Pluto
What is Pluto like? Why is Pluto no longer considered a planet?
Before the arrival of the New Horizons spacecraft, this Hubble Space telescope image of Pluto was our best image available.
This image of Pluto was taken by the New Horizons spacecraft during a flyby of Pluto in 2015. The robotic spacecraft was launched in 2006, taking some nine years to reach Pluto and its tidally-locked moon Charon, before heading out to explore more objects in the Kuiper belt.
The varied surface of Pluto was a somewhat surprising to astrophysicists. Pluto is very far from the Sun, so it is very cold. The average surface temperature is -380 degrees Fahrenheit (-218 degrees Celsius). Most people expected to find a heavily cratered object, with little resurfacing. The reason for the apparent resurfacing of Pluto is not yet well understood. It is probably due in part to gravitational coupling with its largest moon, Charon, and in part by internal radioactive decay.
The reddish color of Pluto is produced by hydrocarbons that fall to the surface after ultraviolet sunlight reacts with methane in the atmosphere. The light colored material is made of three kinds of ices—frozen nitrogen, methane and carbon monoxide. There may be a subsurface liquid ocean on Pluto, warmed by radioactive decay in the rocky interior.
Pluto has an axial tilt of about 120 degrees, much larger than that of Earth's 23.5 degrees. Pluto's thin atmosphere is also mostly nitrogen, methane and carbon monoxide. Pluto has five moons, with the smallest at about one kilometer in diameter.
Why is Pluto no longer considered a planet?
When the first asteroid, Ceres, was discovered, it was considered a new planet. It was soon realized that there were very many bodies in the asteroid belt, ranging down to very small objects, and that not all of them could be considered planets. Similarly, Pluto was the first of a large class of Kuiper belt objects to be discovered. In 2006, astrophysicists questioned whether or not Pluto should be considered a planet. It was realized that there was not an operational definition a planet.
The International Astronomical Union defined a planet to be:
"a celestial body that (a) has sufficient mass for its self-gravity to overcome rigid-body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (b) is in orbit around a star, and is neither a star nor a satellite of a planet"
By this definition, Pluto would have been considered to be a planet. But there was an added criterion: a planet must have "cleared the neighborhood around its orbit." Pluto did not fit this criterion.
Pluto's 248 year orbit is highly eccentric, compared to the orbits of the planets. Its semimajor axis is about 39.5 AU. For about twenty years of its total orbit, Pluto's position is closer to the Sun than Neptune. Pluto has not cleared other objects from its orbital path, and does not dominate its orbit. Pluto's orbit also has a large inclination angle of about seventeen degrees from the plane of the Solar system.
Charon
The largest of Pluto's five known moons is Charon, shown behind Pluto in this composite image. Charon is tidally locked to Pluto. The mass of Charon is about an eighth that of Pluto, so compared to moons orbiting other planets, Charon is relatively much more massive. Charon's diameter is about half of Pluto's diameter.
Charon shows evidence of considerable resurfacing, probably due to the tidal forces from Pluto. It is thought that the huge rift canyon near Charon's equator is evidence of major geological upheaval, almost splitting the moon in half. It is thought that a vast internal ocean of water that froze long ago, cracking the moon.