Pluto, the former ninth planet in our solar system, has long captured the imagination of scientists and the public alike. Since its discovery in 1930, this small, icy world has played a significant role in our understanding of the outer reaches of our solar system. So buckle up and prepare to blast off to the edge of our solar system as we delve into the wonders of Pluto!
Northern hemisphere of Pluto in true color, taken by NASA’s New Horizons probe in 2015
Pluto’s existence was first proposed by the American astronomer Percival Lowell in 1905. His observations of the unusual deviations in the orbits of Neptune and Uranus led him to believe that another celestial body was exerting a gravitational pull on them, causing these discrepancies. In 1915, Lowell even predicted the location of this elusive planet. In 1930, Pluto was finally discovered by Clyde Tombaugh at the Lowell Observatory, based on predictions made by Lowell and other astronomers.
Despite the initial expectations that the unknown planet would be significantly larger and brighter than the object discovered by Tombaugh, Pluto was soon recognized as the anticipated ninth planet. Its name is derived from the Roman god of the underworld, Pluto (known as Hades in Greek mythology). The symbol ♇, which was invented for Pluto, represents the first two letters of its name as well as the initials of Percival Lowell.
Pluto is a dwarf planet in the Kuiper Belt, an area beyond Neptune that encompasses numerous small, frozen bodies. Initially, it was believed that Pluto was the sole object of its kind in the Kuiper Belt. However, as astronomers explored more about the Kuiper Belt and the asteroid belt between Mars and Jupiter, they realized that there are numerous objects like Pluto, and some share more similarities with Pluto than Pluto with other planets. Consequently, astronomers needed to clarify what they meant by the term “planet” and determine which category Pluto belonged to.
In 2006 the International Astronomical Union devised three criteria to define a planet. Firstly, the object must orbit around the sun. Secondly, the object must be massive enough to maintain a roughly spherical shape. Lastly, the object must have cleared its orbit of any bodies of comparable mass. Pluto fails to fulfill the third criterion. Even one of its moons, Charon, is roughly half the size of Pluto. Therefore, instead of being the smallest member of the planet group, Pluto is now the “ruler” of the dwarf planet classification and the largest known member of the Kuiper Belt!
Pluto and Charon
Pluto has a thin atmosphere consisting mainly of nitrogen, with traces of methane and carbon monoxide. At an average distance of 3.7 billion miles (5.9 billion kilometers), Pluto is 39 AU (astronomical units) away from the Sun, with 1 AU being the distance from the Sun to Earth.
If someone were standing on Pluto’s surface, they would see the Sun as an incredibly bright star in a dark sky. Sunlight takes about 5.5 hours to travel from the Sun to Pluto. Due to its distance from the Sun, Pluto receives only 1/1,600 of the amount of sunlight that reaches Earth, resulting in extremely low surface temperatures that can plummet to as low as -375°F to -400°F (-226°C to -240°C). As a result, gases such as nitrogen and carbon monoxide exist on Pluto’s surface as ice.
Panoramic view of Pluto’s icy mountains and flat ice plains, imaged by New Horizons 15 minutes after its closest approach to Pluto
Because of Pluto’s remote location and small size, even the most advanced telescopes on Earth and in Earth’s orbit could not resolve many details about its surface. It was not until the US spacecraft New Horizons flew by Pluto and its moon Charon in July 2015 that many crucial questions about the dwarf planet and its surroundings were answered.
Pluto has a diameter of approximately 1,430 miles (2,300 kilometers), making it smaller than Earth’s moon and approximately 1/6 of the size of Earth. To put it into perspective, if Earth was the size of a nickel, Pluto would be roughly the size of a popcorn kernel.
Pluto (bottom left) compared in size to the Earth and the Moon
It is likely that the dwarf planet possesses a rocky core enveloped by a layer of water ice. It has a unique reddish-brown color, possibly due to its surface containing organic compounds, such as tholins. These compounds are formed when ultraviolet light reacts with methane and nitrogen in Pluto’s atmosphere, producing complex organic molecules. Pluto’s terrain is distinguished by mountains, valleys, plains, and craters.
One day on Pluto is equivalent to approximately 153 hours, or 6.4 Earth days. One year on Pluto, or the time it takes to complete one orbit around the Sun, is equivalent to 248 Earth years. Pluto’s orbit is highly elliptical, with its closest point to the Sun being inside the orbit of Neptune and its farthest point being outside the Kuiper Belt.
Pluto’s orbit around the Sun. The sizes of the planets are not to scale, and Pluto is extra-large to show its spin axis. Source: explanet.info
Pluto has five known moons, Charon, Nix, Hydra, Kerberos, and Styx. These moons have varying shapes and sizes and are thought to have formed from debris left over after a giant impact with a protoplanet early in the solar system’s history.
And there you have it. The mysterious and captivating dwarf planet Pluto, the ruler of the Kuiper Belt. From its discovery in 1930 to its reclassification in 2006 and the groundbreaking flyby by the New Horizons spacecraft in 2015, Pluto has never failed to capture our curiosity and imagination. So let’s continue to explore and unravel the mysteries of our solar system, and who knows what other surprises might be waiting for us beyond the edge of our known universe. Keep looking up!
“I refuse to accept Pluto’s resignation as a planet.” —Amy Lee
“If you slid Pluto to where Earth is right now, heat from the sun would evaporate that ice, and it would grow a tail. Now that’s no kind of behavior for a planet.” —Neil deGrasse Tyson
“I’m sure the universe is full of intelligent life. It’s just been too intelligent to come here.” —Arthur C. Clarke
“Once you can accept the universe as matter expanding into nothing that is something, wearing stripes with plaid comes easy.” —Albert Einstein
Charles Q. Choi and Scott Dutfield. Pluto—Everything You Need to Know About the Dwarf Planet. Space.
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