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Pluto, the ninth-largest body orbiting the Sun, has been a matter of contention in recent
years. Originally considered a planet following its discovery in 1930, Pluto was regarded
as the ninth planet in the solar system for more than 75 years. In 2006, following the
discovery of several other small planet-like objects similar to Pluto, astronomers were
forced to decide what makes a planet a planet, instead of something else. According to the
rules agreed upon by the International Astronomical Union, Pluto was not really a planet, and
on September 13, 2006, it was reclassified, along with the other similar small bodies,
as a 'dwarf planet.'
Pluto is small - with a diameter of only two-thirds that of Earth's moon - and is made of rocky
ice. Astronomers believe that it is about two-thirds rock and one-third water ice. Even
though Pluto is small, this much ice represents more than three times the amount of water
in all of Earth's oceans combined. Pluto is the largest dwarf planet in the solar system,
but not the most massive: that title belongs to Eris. Unlike the gas giants of the outer
solar system, Pluto has no rings, but it does have a thin atmosphere made mostly of nitrogen,
with small amounts of methane and carbon monoxide.
Pluto is 3.7 billion miles or 5.9 billion kilometers away from the sun, with an orbit
that takes 248 years to complete. Its orbit is highly elliptical. Because of this, Pluto
crosses inside of Neptune's orbit for 20 years of each 248-year orbit. Between 1979 and 1999,
Pluto was actually closer to the Sun than Neptune, something that won't happen again
for more than 200 years. Because Pluto is so far from the Sun, it is a cold world. Temperatures
there are about -390 F or -230 Celsius. Pluto is also very dark: the amount of sunlight
that reaches its surface is so little that even on its brightest day, the sky would be
in twilight.
A day on Pluto lasts for about six and one-third Earth days. Like Uranus, Pluto rotates on
its side, something which results in extreme seasonal changes with part of its surface
in constant darkness and part of it in constant daylight for decades.
Pluto has five known moons, with the latest moon discovered as recently as 2012. Its largest
and closest moon, Charon, is so large compared to Pluto that they actually orbit a point
between each other, causing some astronomers to call Pluto and Charon binary, or double,
dwarf planets.
Like Neptune, Pluto was initially discovered through mathematical predictions rather than
by observation. After Neptune was discovered and studied, astronomers realized that there
must be another planetary body out at the edge of the solar system, and in 1906 Percival
Lowell began an all-out search for a ninth planet, called at the time 'Planet X.' Lowell
died before the planet could be found, and the search was interrupted for more than ten years.
In 1929, young astronomer Clyde Tombaugh arrived at Lowell's observatory and was assigned to
search through photographs of the night sky to find anything that was shifting position.
On February 18, 1930, after almost a year of searching, Tombaugh found the elusive planet.
The new planet was named Pluto after the Greek god of the underworld, a name suggested by
an eleven year old girl from England, Venetia Burney.
Beginning in 1992, many other objects were found to be orbiting in the same area as Pluto.
Currently more than 1,000 objects have been discovered there, and scientists believe there
may be as many as 100,000. This collection of objects at the edge of the solar system
is called the Kuiper Belt. It is similar to the asteroid belt between Mars and Jupiter,
but it is 20 times wider and perhaps as much as 200 times more massive. As well, the Kuiper
Belt is though to consist mainly of icy material, whereas the asteroid belt is composed mostly
of rock and metal.
As more and more objects in the space surrounding Pluto were discovered, astronomers began to
question whether Pluto could truly be called a planet. When Eris was discovered in 2005,
it was briefly hailed as a tenth planet, but it sparked a debate in the astronomical community
about what makes a planet a planet.
On August 24, 2006, the International Astronomical Union declared that there are three conditions
that must be met for an object in the Solar System to be a planet. One, it must orbit
around the Sun. Two, the object must be massive enough for its own gravity to pull it into
a spherical shape. And three, it must have cleared the neighborhood around its orbit.
Because of all the objects it shared its orbit with, Pluto failed to meet the third condition,
and along with Eris and other similar bodies, was reclassified as a dwarf planet. There
are currently five accepted dwarf planets in the solar system, with hundreds more possible.
Only one mission has traveled to Pluto, the New Horizons space probe. It was launched
on January 19, 2006, while Pluto was still considered the ninth planet. The three billion
mile journey took eight years, and it arrived in Pluto's system in July of 2015. New Horizons
learned a great deal about Pluto's atmosphere and geography, as well as collecting information
on Charon and Pluto's smaller moons. The mission uncovered many surprises about the dwarf planet,
including strange formations that some are calling 'ice volcanoes' on its surface.
Dark, cold, and tiny, Pluto is still a source of fascinating new discoveries for astronomers.
I hope you enjoyed learning about Pluto, the king of the Kuiper Belt. Goodbye till next time!