Pluto is the largest known dwarf planet in the Solar System. It was previously considered the ninth and farthest planet from the Sun. In 2006, its status was downgraded. This strange world is located in the Kuiper Belt—a region beyond Neptune filled with hundreds of thousands of rocky, icy bodies, each over 100 kilometers in diameter, as well as 1 trillion or more comets. Pluto ceased to be a planet in 2006 when it was reclassified as a dwarf planet, sparking controversy and debate in the scientific community and among the general public.
American astronomer Percival Lowell first proposed Pluto’s existence in 1905, noticing strange deviations in the orbits of Neptune and Uranus. Lowell believed there must be another planet whose gravity was acting on these ice giants, causing irregularities in their orbits. In 1915, Lowell predicted the location of the mysterious planet, but died 15 years before its discovery. Pluto was discovered in 1930 by Clyde Tombaugh at the Lowell Observatory, based on predictions by Lowell and other astronomers.
The first close-up image of Pluto and one of its moons. Pluto is surrounded by sunlight reflected from Charon. NASA/Johns Hopkins APL/Southwest Research Institute/NOIRLab
Pluto was named by 11-year-old Venetia Burney of Oxford, England, who suggested to her grandfather that the new world be named after the Roman god of the underworld. Her grandfather later passed the name on to the Lowell Observatory. The name also honors Percival Lowell, whose initials are the first two letters of the word “Pluto.”
For a long time, Pluto was considered a unique object in the Kuiper Belt. But as astronomers learned more about the Kuiper Belt (and the asteroid belt between Mars and Jupiter), we learned that many Pluto-like objects exist. In some ways, Pluto is more similar to Pluto than Pluto is to the other planets. With the discovery of all these new objects, astronomers had to more precisely define what is meant by the word “planet” and determine what category Pluto belongs in.
A composite image of enhanced color images of Charon (upper left) and Pluto (lower right) taken by NASA’s New Horizons spacecraft during its flyby in 2015. NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute
The three rules that astronomers of the International Astronomical Union have formulated to define a planet are: an object must orbit the Sun; the object must be massive enough to be nearly spherical; and the object must clear its orbit of any objects comparable in mass to its own (that is, it must gravitationally dominate its orbit). Pluto meets the first two criteria, but not the third. Even one of its moons, Charon, is roughly half the size of Pluto. So, instead of being the runt of the planetary group, Pluto is now the “king” of the dwarf planet group!
Astronomers believe Pluto likely has a rocky core. Beyond this, but still deep within, there likely lies an ocean of water, covered by another layer of frozen water ice. The surface crust is composed of various ices, primarily nitrogen, with giant mountains of water ice and traces of methane and carbon monoxide ice.
Like the crust, Pluto’s atmosphere is composed of nitrogen, methane, and carbon monoxide. Pluto’s atmosphere also contains haze particles that scatter blue light. These particles originate high in Pluto’s atmosphere as ionized methane and nitrogen. Through interactions, the ions combine into more complex molecules and begin to assemble an outer shell of volatile ices. As the haze particles become more massive, they begin to penetrate the atmosphere, collecting more and more ice. This “snow” falls onto Pluto’s surface as a reddish-gray color.
Because Pluto is so far from Earth, little was known about the size and surface conditions of this dwarf planet until 2015, when NASA’s New Horizons spacecraft made a close flyby of Pluto. New Horizons revealed that Pluto’s diameter is 2,370 km, less than one-fifth the diameter of Earth and only about two-thirds the diameter of the Moon.
Observations of Pluto’s surface by the New Horizons spacecraft have revealed numerous surface features, including mountains up to 3,500 meters (11,000 feet) high, comparable to Earth’s Rocky Mountains. Although methane and nitrogen ice cover much of Pluto’s surface, these materials are not strong enough to support such enormous peaks, leading scientists to speculate that the mountains formed on bedrock made of water ice.
This composite image from NASA’s New Horizons probe shows Pluto’s vast icy plain, called Sputnik Planitia. The box indicates the location of other detailed images of the glacier. NASA/JHUAPL/SwRI
Pluto’s surface is also covered in a rich layer of methane ice, but New Horizons scientists observed significant differences in how this ice reflects light from the dwarf planet’s surface. The dwarf planet also has icy ridges reminiscent of snakeskin; astronomers noted similarities to penitentes, or erosional features on highlands on Earth. The features on Pluto are much larger; their height is estimated at 500 meters (1,650 feet), while those on Earth are only a few meters.
Another distinctive feature of Pluto’s surface is a large, heart-shaped region, informally known as Tombaugh Regio (after Clyde Tombaugh; regio is the Latin word for region). The left side of this region (the ice cream cone-shaped region) is covered in carbon dioxide ice. Other variations in the composition of surface materials have been discovered within Pluto’s “heart.”
In the center and left of Tombaugh Regio lies a very flat region, informally known by the New Horizons mission team as “Sputnik Planitia,” after Earth’s first artificial satellite, Sputnik. This region of Pluto’s surface lacks impact craters, suggesting that this region is geologically very young—no more than 100 million years old. It is possible that this region is still being formed and reshaped by geological processes.
These icy plains also feature dark streaks several miles long, stretching in one direction. These lines are likely formed by strong winds blowing across the surface of the dwarf planet.
The setting sun illuminates the haze or mist near Pluto’s surface in this small section of this image taken by NASA’s New Horizons probe on July 14, 2015, when it was 18,000 kilometers (11,000 miles) from the dwarf planet. The image covers a landmass 185 kilometers (115 miles) wide. NASA/JHUAPL/SwRI
NASA’s Hubble Space Telescope has also found evidence that Pluto’s crust may contain complex organic molecules.
Pluto’s surface is one of the coldest places in the solar system. Temperatures there can drop to minus 375–400 degrees Fahrenheit (minus 226–240 degrees Celsius). Comparisons of images of Pluto taken by the Hubble Space Telescope with previous images have shown that the dwarf planet appears to have reddened over time, likely due to seasonal changes.
Pluto may have (or could have) a subsurface ocean, although evidence for this discovery is currently lacking. If a subsurface ocean existed, it could have significantly impacted Pluto’s history. For example, scientists discovered that the Sputnik Planitia zone altered Pluto’s orientation due to the amount of ice in the region, which was so heavy that it affected Pluto as a whole; New Horizons estimated the ice thickness at approximately 6 miles (10 km). A subsurface ocean is the best explanation for the evidence, the researchers added, although, considering less likely scenarios, a thicker layer of ice or movements in rocks could be responsible. If Pluto did have a liquid ocean and sufficient energy, some scientists believe life could have existed on Pluto.
Pluto’s highly elliptical orbit can take it more than 49 times farther from the Sun than Earth. Because the dwarf planet’s orbit is so eccentric—meaning far from circular—Pluto’s distance from the Sun can vary significantly. In fact, the dwarf planet gets 20 years closer to the Sun than Neptune every 248 Earth years, giving astronomers a rare opportunity to study this small, cold, and distant world. Pluto’s rotation is retrograde compared to other worlds in the solar system: it rotates in the opposite direction, from east to west.
NASA’s New Horizons spacecraft looked back at the Sun 15 minutes after its closest approach to Pluto on July 14, 2015, capturing this near-sunset view of the dwarf planet’s icy mountains and flat ice plains. The image was taken from a distance of 18,000 kilometers from Pluto; the frame is 1,250 kilometers wide. NASA/JHUAPL/SwRI
As a result of this orbit, after 20 years as the eighth planet (in order of distance from the Sun), in 1999 Pluto crossed the orbit of Neptune, becoming the farthest planet from the Sun (until it was demoted to dwarf planet status).
Some parameters of Pluto according to NASA:
Atmospheric composition: methane, nitrogen. Observations by the New Horizons spacecraft show that Pluto’s atmosphere extends up to 1,600 km above the surface of the dwarf planet.
Magnetic field: It is not yet known whether Pluto has a magnetic field, but the small size and slow rotation of the dwarf planet suggest that it has little or no such field.
Chemical composition: Pluto is likely composed of a mixture of 70 percent rock and 30 percent water ice.
Internal structure: The dwarf planet likely has a rocky core surrounded by a mantle of water ice, and more exotic ices such as methane, carbon monoxide, and nitrogen ice cover the surface.
Average distance from the Sun: 3,670,050,000 miles (5,906,380,000 km) – 39,482 times the distance from Earth.
Perihelion (closest distance to the Sun): 2,755,773,000 miles (4,434,987,000 km) – 30.151 times that of Earth
Aphelion (greatest distance from the Sun): 4,538,698,000 miles (7,304,326,000 km) – 48.023 times that of Earth
Pluto has five moons: Charon, Styx, Nix, Kerberos, and Hydra, with Charon being closest to Pluto and Hydra being farthest.
As Pluto approaches the Sun, its surface ices melt, temporarily forming a tenuous atmosphere composed primarily of nitrogen with some methane. Pluto’s low gravity, just over one-twentieth that of Earth, causes this atmosphere to extend much higher than Earth’s. It is believed that as Pluto moves further from the Sun, most of its atmosphere freezes and virtually disappears. However, even when Pluto has an atmosphere, it appears to have strong winds. The atmosphere is also subject to variations in brightness, which could be explained by gravity waves or air currents flowing over mountains.
A close-up of a photo taken by NASA’s New Horizons probe on July 14, 2015, when it was just 18,000 kilometers from Pluto. The 380-kilometer-wide image, taken near sunset, shows rugged, icy mountains up to 3,500 meters high and broad, flat plains. NASA/JHUAPL/SwRI
Although Pluto’s atmosphere is now too thin for liquids to move, they may have flowed across the surface in ancient times. The New Horizons spacecraft photographed a frozen lake in Tombaugh Regio, near which ancient canals likely existed. In ancient times, the planet’s atmosphere may have been approximately 40 times thicker than that of Mars.
In 2016, scientists announced they may have detected clouds in Pluto’s atmosphere using data obtained by the New Horizons spacecraft. The researchers observed seven bright objects near the terminator (the boundary between day and night), where clouds typically form. All of these objects are located at low altitudes and are approximately the same size, indicating they are distinct objects. The composition of these clouds, if they are indeed clouds, likely consists of acetylene, ethane, and hydrogen cyanide.
In 1978, astronomers discovered a very large moon around Pluto, almost half the size of the dwarf planet itself. This moon was named Charon, after the mythological demon who transported souls to the underworld in Greek mythology.
This illustration shows Pluto and its largest moon, Charon. NASA/Robert Lea
Because Charon and Pluto are so similar in size, their orbits differ from those of most planets and their moons. Both Pluto and Charon orbit a point in space located between them, similar to the orbits of binary star systems. For this reason, scientists call Pluto and Charon a binary dwarf planet, a binary planet, or a binary system.
Pluto and Charon are only 19,640 km apart, less than the flight distance between London and Sydney. Charon orbits Pluto once every 6.4 Earth days, while Pluto orbits once every 6.4 Earth days—a Plutonian day. This is because Charon orbits the same spot on Pluto’s surface and always displays the same side of Pluto—a phenomenon known as tidal locking.
While Pluto has a reddish tint, Charon appears more gray. In its early years, the moon may have harbored an ocean beneath its surface, although this likely no longer exists. Compared to most planets and moons in the Solar System, the Pluto-Charon system is tilted sideways relative to the Sun.
Observations of Charon by the New Horizons spacecraft revealed canyons on the moon’s surface. The deepest of these canyons descends 6 miles (9.7 km). A long strip of rocks and depressions extends 600 miles (970 km) across the middle of the moon. A patch of the moon’s surface near one pole is covered with a much darker material than the rest of the planet. Similar to regions of Pluto, much of Charon’s surface is free of craters, indicating that the surface is relatively young and geologically active. Scientists have seen evidence of landslides on its surface, the first such features discovered in the Kuiper Belt. Our Moon may also have had its own version of plate tectonics, which causes geological changes on Earth.
In 2005, scientists photographed Pluto with the Hubble Space Telescope in preparation for the New Horizons mission and discovered two other tiny moons of Pluto, now named Nix and Hydra. These moons are two and three times farther from Pluto than Charon. According to New Horizons measurements, Nix is estimated to be 42 kilometers long and 32 kilometers wide, while Hydra is 55 kilometers long and 113 kilometers wide. Hydra’s surface is likely covered predominantly by water ice.
In 2011, scientists using the Hubble Space Telescope discovered a fourth moon, Kerberos. This moon is shaped like two lobes: the larger lobe is approximately 8 km across, and the smaller is approximately 5 km across. On July 11, 2012, a fifth moon, Styx (estimated to be 10 km across), was discovered, further fueling the debate over Pluto’s planetary status.
This image of Pluto’s small moon Kerberos was created by combining four images taken by NASA’s New Horizons spacecraft on July 14, 2015, at a distance of 245,600 miles (396,100 kilometers) from Kerberos. NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute
The leading hypothesis for the formation of Pluto and Charon is that the nascent Pluto was a glancing blow from another Pluto-sized object. According to this hypothesis, most of the resulting material formed Pluto, and the rest separated and formed Charon. The other four moons could have formed in the same collision as Charon.
NASA’s New Horizons mission is the first probe to study Pluto, its moons, and other planets in the Kuiper Belt from close range. The probe was launched in January 2006 and successfully approached Pluto on July 14, 2015.
The New Horizons probe is carrying some of the ashes of Pluto’s discoverer, Clyde Tombaugh.
Limited knowledge of the Pluto system posed unprecedented risks for the New Horizons probe. Before the mission’s launch, scientists knew of only three moons around Pluto. The discovery of Kerberos and Styx during the spacecraft’s flight reinforced the idea that more moons, invisible from Earth, may orbit the dwarf planet. Collisions with these unseen moons or even small debris could have seriously damaged the spacecraft. However, the New Horizons team equipped the spacecraft with protective measures during its flight.
An artist’s impression of NASA’s New Horizons spacecraft. NASA/JHUAPL/SwRI
In October 2021, the New Horizons spacecraft made history by capturing the first close-up images of Pluto and its moons.
When NASA’s New Horizons spacecraft flew past Pluto in 2015, it revealed a stunning world of ice and haze shaped by various geological processes, hinting that an ocean may have played a role in the dwarf planet’s recent history. The abundance of scientific discoveries has spurred researchers to unravel some of the mysteries of this tiny world a decade after the spacecraft’s flyby.
“There are still many questions,” said Carly Howett, a planetary scientist at the University of Oxford and a member of the New Horizons team, at the “Progress in Understanding the Pluto Mission: 10 Years After Flyby” conference in Laurel, Maryland. With these questions in mind, Howett and her colleagues have designed the next mission in hopes of finally unraveling some of Pluto’s mysteries.
An infographic illustrating the properties of Pluto’s surface, mantle, and core. Future/Wikimedia Commons
Such a mission, aimed at exploring the outskirts of the solar system, would last for several decades. But approval is still a long way off. “This mission could last more than 50 years, posing engineering, operational, and data analysis challenges never before attempted,” Howett wrote in a 2021 study published in the Planetary Science Journal, detailing the mission concept.
For the return mission to the dwarf planet, Howett and her colleagues chose the name Persephone, after Pluto’s wife and the “queen of the underworld” in Greek mythology.
“Given that Pluto is named after the Roman god of the underworld, and that we wanted a feminine name that would reflect the diversity of our team, which has many women in leadership positions, this name seemed appropriate,” Howett wrote.
Persephone will be equipped with 11 instruments, all based on those used on previous missions but with some modifications. The main question it will attempt to answer is whether Pluto has a subsurface ocean.
The surface of Pluto as seen by NASA’s New Horizons spacecraft during its July 2015 flyby. NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute
If this question had been asked before the New Horizons flyby, most scientists would have said it was unlikely. While many icy worlds may initially have a water layer, it freezes over time. To remain liquid over the 4.5 billion years of the solar system’s existence, this ocean must remain warm.
Some moons constantly flex, gravitationally interacting with their host planet and neighboring moons to keep their oceans from freezing. Charon, Pluto’s largest moon, is nearly as massive as Pluto; they are often referred to as a “double planet” (though neither meets the criteria for a planet). Scientists believe Charon could potentially keep Pluto warm if its ocean contained enough non-water to lower its freezing point.
NASA’s New Horizons spacecraft has sent back a higher-quality version of the image of Pluto’s moon Charon, first taken on July 14, 2015. Image released September 10, 2015. NASA Applied Physics Laboratory/Johns Hopkins University/Southwest Research Institute
It wasn’t until New Horizons discovered Pluto’s surprisingly young, lightly cratered surface that most scientists began to consider the possibility of an ocean (though some had done so even before the spacecraft’s arrival). New Horizons studied Pluto in detail for only a few hours, though it continued to observe the dwarf planet for several months before and after its closest approach.
Persephone, by contrast, planned to orbit the tiny world for just three years, allowing for longer, close-up observations.
Persephone will study Pluto’s shape, searching for signs of a distinctive fossilized bulge—a sort of “beer belly” formed when gravity pulls on the planet’s mass. Bulges form more easily when the layers are liquid and can be frozen. New Horizons did not detect such a bulge, but Persephone will send a more sensitive instrument that will be able to conduct a more detailed study.
This image from NASA’s New Horizons spacecraft shows a 350-kilometer-wide (220-mile-wide) region of Pluto with vastly different surface reflectivities and geologic topography. Image released September 10, 2015. NASA Applied Physics Laboratory/Johns Hopkins University/Southwest Research Institute
Persephone will also attempt to determine the composition of Pluto and Charon using gravity and terrain measurements similar to those taken on Saturn’s moon Enceladus, and possibly calculate the thickness of their interior ice layers.
Pluto is suffering from a secular winter, and Persephone will arrive in the midst of it. Much of the dwarf planet will be shrouded in darkness, so the mission will need instruments capable of peering through the veil. Cameras will create a more detailed map of the entire planet’s surface in various wavelengths, including the half that was obscured during the New Horizons flyby. The cameras will look for hotspots, signs of ongoing activity and eruptions that could indicate a warm interior, as well as signs of surface changes since the 2015 observations. They will also provide a more detailed crater count on both Pluto and Charon, helping scientists better understand how active the Kuiper Belt has been over the years.
This perspective view of Pluto, based on the latest high-resolution images from NASA’s New Horizons spacecraft, creates the impression of looking down from about 1,800 kilometers above Pluto. Image released September 10, 2015. NASA Applied Physics Laboratory/Johns Hopkins University/Southwest Research Institute
Although the solar system is rich in icy worlds, both Pluto and Charon possess unusual surface features. In the Tartarus Dorsa region of Pluto, blades of methane ice cover the surface. Scientists speculate that these peaks formed through sublimation, when methane instantly transitioned from solid to gas, but this is not a definitive conclusion. Charon features a strange ice mountain embedded in a frozen moat—a unique feature among icy peaks. Both surfaces are covered in exotic ices, and understanding their properties in Pluto’s frigid temperatures will be a key component of the mission.
In 1988, astronomers discovered a tenuous atmosphere around Pluto, but its composition eluded them. The New Horizons probe not only answered these questions but also raised new ones. Interestingly, Pluto appears to be slowly shedding some of its atmosphere onto Charon, creating a distinctive red pole that could shift the hemispheres over time. One of the key goals of the Persephone probe will be to directly determine the atmospheric composition using mass spectrometry.
Persephone will also study the space around the tiny world. Pluto is so distant that light from the Sun takes just 5.5 hours to reach it. From Pluto, the Sun appears as a dot in the sky.
NASA’s New Horizons spacecraft captured this image of a 470-kilometer-wide (300-mile-wide) patch of rugged, uneven terrain at the edge of Sputnik Planitia (not its official name). Image released September 10, 2015. NASA Applied Physics Laboratory/Johns Hopkins University/Southwest Research Institute.
Charon won’t be the only moon targeted by the mission. Persephone’s launch timing will determine how many small moons it will be able to observe, but it will likely be able to observe four other moons.
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The New Horizons spacecraft has detected bands of water ice on Nix, Hydra, and Kerberos, as well as traces of ammonia on Nix and Hydra. Persephone will obtain more detailed spectra of these three moons, as well as Styx, and attempt to determine how much of their surface, along with Charon’s, is littered with debris left over from the collision that likely formed them. While the best observations of the small moons will likely be obtained from the closely orbiting Styx, and the worst from Pluto’s outermost moon, Hydra, a flyby of either Kerberos or Hydra may be possible, depending on when the mission launches and reaches Pluto.
Persephone will remain on Pluto for just over three Earth years. During this time, the spacecraft will be able to orbit the binary system and eventually escape.
This mosaic, composed of high-resolution images of Pluto acquired by NASA’s New Horizons spacecraft between September 5 and 7, 2015, shows the icy plain known as Sputnik Planum, the bright region at the center. NASA Applied Physics Laboratory/Johns Hopkins University/Southwest Research Institute
A one-year mission extension will allow the spacecraft to visit another Kuiper Belt object, similar to how New Horizons visited Arrokoth after its flyby of Pluto. The mission extension will depend on its launch and arrival times, but it could yield valuable information about the remnants left behind from the formation of the Solar System. This will have significant scientific benefits, as the remoteness of the Kuiper Belt objects makes them difficult to study from Earth.
Returning to Pluto would not be a haphazard undertaking. While New Horizons’ initial journey took less than ten years, changes in the planetary alignments would mean a subsequent mission would take just over 27.5 years and require five Next-Generation Radioisotope Thermoelectric Generators (NGRTGs), nuclear batteries that use the decay of radioactive materials to power the spacecraft. A long mission lifespan would require several NGRTGs to maintain heat in the frigid temperatures of space. This is a challenging task at a time when plutonium for spaceflight is still in short supply. NASA’s current goal is to produce 1.5 kilograms of plutonium per year; current RITTGs use 4.8 kilograms.
This would require significant investment in plutonium for outer planet exploration. It would also require financial investment. According to Howett, the estimated cost of the Persephone mission is $3 billion.
A backlit oblique view of a New Horizons image shows ice (likely frozen nitrogen) flowing from Pluto’s mountains through valleys (circled by red arrows) onto plains known as Sputnik Planitia; the “flow front” is indicated by blue arrows in this image and spans a region 390 miles (630 kilometers) wide. NASA Applied Physics Laboratory/Johns Hopkins University/Southwest Research Institute
Designed for a half-century mission, the spacecraft is equipped with several backup systems. However, Stern noted that despite the long lifespan, this is not unusual. NASA’s Hubble Space Telescope is still operational after 35 years, as are the Voyager 1 and 2 spacecraft, launched 48 years ago.
Another challenge for the Persephone mission relates to the people working on it. Given the potential mission duration of 50 years, researchers estimate that Persephone will be a project involving three generations of scientists, succeeding each other throughout their careers. Information and training will need to be passed down from generation to generation. According to Stern, the delay in recruiting participants was one of the most challenging aspects of planning the New Horizons mission.
This image from New Horizons shows ice (likely frozen nitrogen) flowing from Pluto’s mountains through valleys (circled by red arrows) onto plains known as Sputnik Planitia; the “flow front” is indicated by blue arrows in this image and spans an area 390 miles (630 kilometers) wide. NASA/JHUAPL/SwRI
Opportunities to launch the spacecraft will be available annually from 2029 to 2032. After that, Jupiter’s orbit will preclude further launch opportunities for a full decade.
Persephone was part of NASA’s Planetary Mission Concept Study, which funds a series of projects to inform decisions about potential future missions through a ten-year study.
But don’t count on Persephone flying anytime soon: the power requirement alone could prevent its official inclusion. Howett’s mission proposal was one of several solicited for NASA’s 10-year study on future mission priorities and prospects.
However, the space agency continues to work on improving the RTGs and their supporting technologies. Howett hopes that continued technological advancements will improve the mission’s chances.
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