Of all of the planets in our solar system that have been the topic of controversy and debate, Pluto is at the top of the list. Generations grew up thinking that Pluto was the ninth planet and then in 2006, they demoted little Pluto to become a dwarf planet.

At one time, Pluto wasn’t just thought to be our ninth planet but was also thought to be the largest item in the Kuiper Belt which is located in the areas beyond Neptune’s orbit.

It was so far out that even the most powerful telescopes of the time could only show Pluto as a fuzzy grey image.

Earth, the Moon, and Pluto

The Kuiper Belt is home to hundreds of thousands of icy, rocky objects, many bigger than 62 mi/100 km across and over 1 trillion or more comets.

The demotion of Pluto outraged many in the scientific community as well as public opinion.

There was a proposal of a new definition of “planethood” in 2017 that included scientists and members of the New Horizon spacecraft mission to include “round objects in space smaller than stars” as part of the classification criteria.

This would change the number of planets in our solar system from the current eight to around one hundred.

Pluto Statistics:

Discovered By: Clyde W. Tombaugh

Date Discovered: February 18, 1930

Diameter: 2,372 km

Mass: 1.31 × 10^22 kg (0.17 Moons)

Orbit Distance: 5,874,000,000 km (39.26 AU)

Orbital Period: 248 years

Surface Temperature: -229 degrees C

Moons: 5

Pluto is smaller than our moon and is a mysterious world that has valleys, plains, craters, and possibly glaciers. It has five moons and the biggest moon is Charon, which is almost half the size of Pluto.

Charon and Pluto are listed as a “double planet” because they orbit so close to each other. Charon is listed as the largest satellite relative to its parent planet in our solar system.

It is thanks to the New Horizon spacecraft that we have learned so much about Pluto and its complex geology.


In 1905, Percival Lowell, an American astronomer first discovered Pluto when he found strange deviations in the orbits of both Uranus and Neptune. These alterations in their orbits suggested that there was another object’s gravity that was tugging on them.

Lowell predicted that there was a mysterious planet in 1915 but he passed away before he could ever find it. In 1930, Clyde Tombaugh at the Lowell Observatory officially discovered Pluto based on the information from Lowell and other astronomers.

Tombaugh was actually looking for the mysterious “Planet X” and instead discovered Pluto. It was revealed later that Planet X didn’t exist.

Discovery photographs of Pluto

Pluto was named by an 11-year old from Oxford, England named Venetia Burney. She had suggested to her grandfather that the new discovery be named after the mythological Roman god of the underworld.

Her grandfather gave the suggestion to the Lowell Observatory where they acknowledged the new name, which coincidentally has the first two letters of Percival Lowell’s name.

In preparation for the New Horizons space mission, scientists used the Hubble Space Telescope to photograph Pluto in 2005. In the process, they discovered two other small moons orbiting around Pluto, now named Nix and Hydra.

These two moons are 2-3 times farther away from Pluto than the moon Charon. The New Horizons spacecraft measured Nix at 26 mi/42 km long and 22 mi/36 km wide, and Hydra at 34 mi/55 km long and 22 mi/36 km wide.

The data suggested that the surface of Hydra is probably covered in water ice.In 2011, the Hubble Space Telescope discovered a fourth moon orbiting around Pluto, now named Kerberos.

It’s estimate that it’s 8-21 mi/13-34 km in diameter. In 2012, the fifth moon of Styx was discovered and it is estimated to be 6 mi/10 km wide. All four of the new moons have highly chaotic orbits.

The discovery of the four additional moons acted as ammunition for those that wanted Pluto’s status to be a planet.


Pluto is part of the Kuiper Belt which are objects beyond Neptune’s orbit and are believed to have been formed 4.5 billion years ago as our solar system formed.

The objects in this disc-like zone are rocky and icy and are referred to as Kuiper Belt objects, plutoids, or transneptunian objects.

The most popular theory about the formation of the moon Charon and Pluto is that Pluto was hit by another object that was around its size.

Most of the material gathered to form the Pluto that we see today and the rest was spun off and then came together to become Charon.

Pluto has a radius of 715 mi/1,151 km. and is around one sixth the width of our Earth. To give you a comparison, if Earth was the size of a nickel, Pluto would be the size of a kernel of popcorn.

Pluto is incredibly far away. Astronomers use the measurement of “AU” (astronomical unit) to list distances for objects in space.

One AU is the distance from the Earth to the sun. Pluto is 39 AU’s from the sun and it takes sunlight 5.5 hours to travel to Pluto.

Pluto has five moons: Charon, Styx, Nix, Kerberos, and Hydra. Charon is the closest to Pluto and the largest of all of the moons, and Hydra is the farthest away. The discovery of Charon in 1978 also showed astronomers that this moon was almost half the size of Pluto.

It was named Charon after the mythological character that took souls to the underworld on a ferry. The close proximity of Pluto and Charon make them a “binary planet” which is unlike any other planet in our solar system.

They orbit very close to each other, both being tidally locked so that only one side faces the other one. Charon and Pluto are only 12,200 mi/19,640 km apart from each other, which is the distance between Sydney, Australia and London, England.

Charon is a gray-tone in color and Pluto has a reddish tint. It’s thought that early in its history, Charon might have had a subsurface ocean, although there is no evidence of one today and it more than likely couldn’t support one. Both Pluto and Charon are tipped on their side in relation to the sun.

The New Horizon spacecraft observations have shown that there are canyons on Charon’s surface, some plunging as deep as 6 mi/9.7 km.

It also has a large area of troughs and cliffs that stretch for 600 mi/970 km around the center section of the moon. One area of Charon has darker material that is similar to some of Pluto’s regions

Surface and Structure:

Until the 2015 New Horizons spacecraft visited Pluto, scientists new very little about it. Even some of the images taken by high-powered telescopes showed a grey fuzzy blob.

New Horizons revealed that Pluto has a diameter of 1,473 mi/2,370 km and it’s only around 2/3 the size of our moon.

New Horizons also showed that Pluto’s surface has mountains as tall as 11,000 ft/3,500 m which are close to the size of Earth’s Rocky Mountains.

Methane and nitrogen ice cover a majority of Pluto’s surface, neither of these materials have the strength to support such high mountainous peaks. Scientists believe that the mountains were formed on a water ice bedrock.


Pluto has an area that close to the Tombaugh Regio that is smooth compared to other areas of Pluto and is unofficially called “Sputnik Planum” after the first article satellite “Sputnik.”

This area doesn’t have any craters due to impacts and is believed to be relatively young and new at around 100 million years old. Scientists believe that there is some geological process that has resurfaced this region so that none of the older impact craters can be seen.

Pluto also has dark streaks that are a few miles long that are icy plains and they are all lined up in the same direction. This characteristic may be due to high winds that blow across the surface.

The NASA Hubble Space Telescope has discovered that the crust of Pluto could possibly contain some complex organic molecules.

The surface of Pluto is one of the coldest that exist on any planet in the solar system. It has temperatures of -375 degrees F/-225 degrees C.

Scientists have been comparing older images with newer ones and it seems that Pluto is getting oddly more reddish in color. Scientists believe this may be due to seasonal changes.

Researchers aren’t sure if Pluto ever had a subsurface ocean. It’s thought that if it did have one sometime in its past that it would have had a big effect on its history.

One example is the amount of ice that has been found in the area of Sputnik Planitia which is so heavy that it’s affected Pluto overall.

In this region, the ice is 6 mi/10 km thick and the only way scientists can explain such a large quantity of ice is with a subsurface ocean.

Pluto is made up of around 70% rock and 30% water ice with a rocky core that’s surrounded by a water ice mantle.

It is also believed that it has additional ices such as carbon monoxide, methane, and nitrogen at the surface. Scientists used information from the New Horizons spacecraft to detect the additional ices based on the way that the light is reflected from them.

Pluto has an ice ridge terrain that has an appearance of snakeskin. It’s similar to areas on Earth that are eroded mountainous terrain, but those on Pluto are a lot larger.

Another feature that is unusual on Pluto is the heart-shaped area that is unofficially called the Tombaugh Regio. It’s made of carbon monoxide ice and some have called it the “heart” of Pluto.

The surface of Pluto includes valleys, craters, plains, and mountains, with mountains made up of large blocks of ice. Some of the mountains are coated with frozen gases such as methane.

The most noticeable plains on Pluto seem to be made up of frozen nitrogen that have no crater activity.

Atmosphere and Magnetosphere:

During Pluto’s orbit it gets closer to the sun and it’s during this time that the surface ices thaw and create a temporary atmosphere that’s made up of mainly nitrogen and a little methane.

Pluto has such low gravity that the atmosphere extends out higher than that on Earth. However, when Pluto orbits away from the sun a majority of this atmosphere is believed to freeze and disappear.

During the time that Pluto does have an atmosphere there seem to be strong winds, and the atmosphere itself has variations in brightness which might be explained by air flowing over mountains or gravity waves.

The atmosphere is too thin to allow liquids to freely flow but scientists think that at one time in it’s past, Pluto might have had liquids. The New Horizon spacecraft showed images of a frozen lake that seemed to have channels near it that were probably ancient.

It may have had an atmosphere in its ancient past that was forty times thicker than the Martian atmosphere.

Using the New Horizons data, scientists announced in 2016 that they found clouds in the atmosphere on Pluto.

The researchers revealed seven bright areas around the boundary between darkness and daylight, which is the typical area for the formation of clouds. The clouds were all about the same size and in lower altitude. They believe that they are made up of acetylene, ethane, and hydrogen cyanide.

Scientists don’t know if Pluto has a magnetic field, however, due to the dwarf planet’s small size and slower rotation, they don’t think that it has one or if it does, it is minor.

Rotation and Orbit:

Pluto’s orbit is very elliptical, taking it over 49 times as far away from the sun as the Earth’s orbit. The orbit is considered to be eccentric or non-circular and also has variations.

Pluto does get closer to the sun than Neptune every 20 years out of its 248 Earth-years-long orbit cycle. During this time, astronomers have a rare opportunity to study the dwarf planet. During the years of 1979-1999 Pluto experienced one of these orbits.

One day on Pluto takes about 153 hours. Pluto has an axis of rotation that is tilted 57 degrees with respect to the plane of orbit around the sun and it therefore spins on its side.

Its orbit is also like that of Uranus and Venus in that it is “retrograde,” spinning from east to west.

Could Life Exist?

Pluto’s surface is too cold and it’s believed that life couldn’t exist there. Liquid water is one of the requirements for life and on Pluto, water is turned into rock-like ice.

There are some that theorize that Pluto could have an ocean deep below its surface, if that was the case, additional study would have to be done as a possibility for life.

Interesting Information:

  • For many years, people thought that Pluto was named after the cartoon character that has the same name. This may be why so many became endeared with the little dwarf planet.
  • For Pluto to complete a full orbit of the sun it takes 246.04 Earth days.
  • Pluto is small, in fact it’s smaller in size than most of the moons in the solar system.
  • Sunlight on Pluto looks like moonlight on Earth.
  • Objects in the solar system that orbit around the sun beyond the orbit of Neptune and that have a rounded shape are called “plutoid.”
  • The New Horizons spacecraft carried some of the ashes of Clyde Tombaugh, the astronomer that discovered Pluto.


The 2006 New Horizons spacecraft mission is the first to have studied Pluto, its moons, and some other objects in the Kuiper Belt. It made the closest approach to Pluto in 2006 and then a second close approach in 2015.

The 2015 flyby included details of Pluto’s moon Charon, and researchers have created a high-quality global map of the moon

Facts about Pluto for Kids:

  • Pluto was named by an 11-year old girl from England.
  • Pluto is considered to be the largest dwarf planet discovered so far.
  • There was at one time quite a bit of controversy as to whether Eris or Pluto was the largest dwarf planet.
  • Pluto is thought to be made up of around one third water which is in the form of ice.
  • Pluto sometimes has an atmosphere and then sometimes not.