If you are old enough, you might remember a time when scientists thought Pluto was a planet just like Mars or Jupiter. But then they discovered objects orbiting beyond Pluto, some quite large. Today, Pluto is known as a dwarf planet. Pluto is found in the icy outer edges of our Solar System in what is called the Kuiper Belt. While Pluto is too small to be considered a planet, it is the largest object in the Kuiper Belt.
Who Decides What a Planet is?
Scientists once agreed that nine planets orbited the Sun, from Mercury to Pluto. But then they discovered object orbiting beyond Pluto, some quite large. Which ones were planets? Scientists no longer agreed. The International Astronomical Union (IAU), comprised of scientists from nations around the world, debated the issue. In 2006 they voted on a new definition for a planet.
A “planet” is a celestial body that:
(a) is in orbit around the Sun,
(b) has enough mass for its gravity to make the object have a (nearly) round shape, and
(c) has cleared other large objects from the region it crosses during its orbit. (Its gravity caused other orbiting objects to impact, or crash into, its surface or be ejected from our solar system).
Unfortunately, Pluto does not meet requirement (c).
The IAU Prague General Assembly also approved a new term, dwarf planet.
A “dwarf planet” is a celestial body that:
(a) is in orbit around the Sun,
(b) has enough mass for its gravity to make the object have a (nearly) round shape,
(c) has not cleared other large objects from the region it crosses during its orbit. (Its gravity is not great enough to cause other orbiting objects to impact, or crash into, its surface or be ejected from our solar system).
(d) is not a satellite of another object.
Pluto meets all of these requirements. It is now the best example of a dwarf planet.
Characteristics of Pluto
The atmosphere of Pluto comes and goes depending on how close Pluto is to the Sun. When it is closer to the Sun and its surface temperature is warmer, methane on the surface turns to gas and forms a temporary atmosphere. However, when Pluto moves further away from the Sun and Pluto becomes cooler, the methane freezes again and the atmosphere disappears. New Horizons captured this image of Pluto. Backlit by the Sun, the dwarf planet is dark, while its atmosphere resembles a bright halo.
Before New Horizons arrived at Pluto, scientists carefully looked for orbiting debris that could pose a risk to the spacecraft. They discovered four tiny moons.
While studying Pluto with a telescope in 1978, astronomer James Christy noticed that Pluto’s shape seemed to change every 6.4 days. That change in shape turned out to be a large moon circling Pluto. The moon Charon is so close to Pluto that it makes a single orbit in that short time. Pluto’s large moon Charon has a surprising dark red north pole, caused by trapped hydrocarbons. Canyons are visible near the equator.
Like our own Moon and Earth, Charon always shows the same face to Pluto. Unlike our Moon, it always faces one side of Pluto and never rises or sets in the sky.
Pluto's Discovery
In the early 1900s, astronomer Percival Lowell began to search for a planet beyond Neptune. He thought (incorrectly) that a ninth planet was needed to account for unexplained motions in the orbits of Uranus and Neptune. Clyde Tombaugh, a young astronomer at the Lowell Observatory in Arizona, later took up the search. In 1930 at age 24, he discovered the object he later named Pluto.
Exploration of Pluto
New Horizons was the first spacecraft to explore Pluto and its moons. Its flyby visit in 2015 transformed our vision of Pluto from mysterious to amazing. New Horizons revealed that Pluto and moon Charon have dramatic landscapes and altered surfaces that are surprisingly young. Scientists had expected to find an old and heavily cratered surface.