How Pluto Was Demoted From Ninth Planet to Dwarf Status

By: Contributors  | 
Four images from NASA's New Horizons' Long Range Reconnaissance Imager (LORRI) were combined with color data to create this global view of Pluto. NASA/Johns Hopkins

Despite its reclassification in 2006 from the ninth planet to a dwarf planet, Pluto remains one of the most fascinating celestial bodies in our solar system. With its unique composition, climate and orbital characteristics, this distant world offers us remarkable insights into the diversity and complexity of the universe.


The Discovery and Naming of Pluto

American astronomer Percival Lowell first theorized the existence of Pluto in 1905. Observing odd discrepancies in the orbits of Neptune and Uranus, Lowell predicted that these irregularities were the result of another planet's gravitational pull. He even managed to predict its location. It took until 1930 for Clyde Tombaugh at the Lowell Observatory to confirm the existence of Pluto.

Interestingly, the name "Pluto" came from an 11-year-old English girl, Venetia Burney. The name Pluto, the Roman god of the underworld, also honored Lowell, as his initials are the first two letters of Pluto.


Pluto's classification as a planet was uncontroversial until 2006.

The Downgrading of Pluto: From Planet to Dwarf Planet

Over the years, as more data has been collected, the understanding of our solar system has evolved. The discovery of the Kuiper belt, a ring of small celestial bodies beyond Neptune, raised new questions about Pluto’s status as a planet. The Kuiper belt was first proposed by Julio Angel Fernandez Alves and was later confirmed when astronomers discovered its first official member, an object known as Albion. Later, in 2005, astronomer Mike Brown discovered an even larger object in the Kuiper belt, dwarf planet Eris, which was nearly identical in size to Pluto.

The discovery of Eris and other similar-sized objects in the Kuiper belt led to a renewed debate over what exactly constituted a planet and whether Pluto and solar system objects like it might constitute a new category or class of object. This was a particularly urgent question since Eris, given its size, might technically qualify as the tenth planet in our solar system. If Eris was a planet, then what about Albion and the multitude of other Kuiper belt objects? Are they planets or dwarf planets? It became increasingly apparent that the traditional understanding of a planet needed to be revisited.


Dwarf planet Pluto is shown here, lit by the sun.
Dotted Yeti/Shutterstock

This led to the historic vote carried out by the International Astronomical Union in 2006. It was during this meeting that astronomers decided on a new definition for a planet. According to the new criteria, a celestial body needed to fulfill three conditions to be classified as a planet:

  1. It had to orbit a star.
  2. It had to be massive enough to assume a round shape due to its own gravity.
  3. It had to have cleared its neighborhood, implying that it needed to be the gravitationally dominant object in its orbit.

Under the new definition, Pluto could not be considered a planet. While it was in orbit around the sun and was massive enough to hold itself together into a round shape, it had not cleared its neighborhood of other objects, as evidenced by its location within the cluttered Kuiper belt.


A Glimpse Into Pluto's Interior and Atmosphere

Composed of rock and various types of ice, dwarf planet Pluto is home to an extraordinary variety of geological features. Deep beneath its icy surface, it is believed that Pluto houses a rocky core surrounded by a possible ocean of water, frozen over by a layer of water ice. The surface crust, composed mostly of nitrogen ice, also contains mountains of water ice along with traces of methane and carbon monoxide ices.

Pluto's atmosphere, while thin, is mainly composed of nitrogen, methane and carbon monoxide. The existence of haze particles within its atmosphere scatters blue light and gives Pluto's atmosphere a distinct blue tint. The freezing temperatures on Pluto, ranging from minus 375 to minus 400 degrees Fahrenheit, cause the majority of this atmospheric layer to freeze and precipitate as a reddish-gray "snow."


Unveiling the Surface Features of Pluto

Pluto's surface was largely a mystery until the historic flyby of NASA's New Horizons spacecraft in 2015. This mission revealed a diverse terrain with mountains as tall as 11,000 feet (3.35 kilometers) and an intriguing snake-skin-like texture caused by eroded ice ridges.

One of the most iconic features of Pluto's surface is the heart-shaped region known as Tombaugh Regio. Within this region lies a smooth area, "Sputnik Planum," which has a noticeably lower number of impact craters, indicating its relatively young age. The existence of dark streaks across these icy plains suggests the possibility of winds sweeping across Pluto's surface.


An Eccentric Orbit and Unique Moons

Pluto's orbit is unique in its significant eccentricity, allowing it to cross Neptune's orbit and come closer to the sun for approximately 20 years in its 248-Earth-years-long orbit. Pluto's five known moons, Charon, Styx, Nix, Kerberos and Hydra, also contribute to its intrigue. Charon, being the largest, shares a unique binary relationship with Pluto due to their similar sizes. This makes them orbit a point in space between them, rather than Charon orbiting Pluto directly.

One of the fascinating aspects of the Pluto-Charon system is its additional four moons, for a total of five. The small moons, Nix, Hydra, Kerberos and Styx, were discovered by the Hubble Space Telescope during preparations for the New Horizons mission. They have added layers of complexity to our understanding of the Pluto system.


For instance, Nix and Hydra, discovered in 2005, are around two to three times farther away from Pluto than Charon. Nix is estimated to be about 26 miles (42 kilometers) long and 20 miles (32 kilometers) wide, while Hydra is estimated at 34 miles (55 kilometers) long and 25 miles (40 kilometers) wide. Observations suggest Hydra's surface is likely coated primarily in water ice.

The moon Kerberos, discovered in 2011, has a double-lobed shape, with the larger lobe being about 5 miles (8 kilometers) across and the smaller one approximately 3 miles (5 kilometers) across. In 2012, the smallest moon, Styx, was discovered, with an estimated width of 6 miles (10 kilometers). The discovery of these moons added fuel to the debate surrounding Pluto's planetary status.

The leading hypothesis for the formation of Pluto, Charon and the four smaller moons is that a nascent Pluto was struck by another Pluto-sized object. The impact caused the formation of Pluto and the spinning off of most of the remaining matter to form Charon. The other four moons may have formed from debris left over from the same collision that created Charon.

Pluto with Charon and Polaris
This artist's rendering shows how Pluto, on the right, might look from a position high above its equator. To the left and beyond Pluto is Charon, its satellite moon. The bright star between Pluto and Charon is Polaris, the north star, which hangs directly over Earth's north pole.
Stocktrek Images/Getty Images


Pluto's Potential for Life

While the prospect may seem far-fetched given the extreme conditions on Pluto, the existence of a subsurface ocean and complex organic molecules on the dwarf planet's surface has led some scientists to speculate about the potential for life. If Pluto does or did have a liquid ocean and sufficient energy, it could theoretically harbor life.

This idea is based on findings from New Horizons, which suggested that the ice-heavy zone of Sputnik Planitia had redirected Pluto's orientation. The ice in this area is estimated to be about 6 miles (10 kilometers) thick. A subsurface ocean is the most plausible explanation for this, although other less likely scenarios such as a thicker ice layer or movements in the rock may also explain the movement. However, while intriguing, the evidence for a subsurface ocean — and thus the potential for life — on Pluto is still inconclusive.


This article was updated in conjunction with AI technology, then fact-checked and edited by a HowStuffWorks editor.