Pluto is a small, icy world that circles the Sun. It used to be called the ninth planet. Now scientists call it a dwarf planet. A dwarf planet is round and goes around the Sun, but it is much smaller than a big planet. Pluto is the most famous dwarf planet of all.
Why Pluto is tricky to understand
A long time ago, Pluto was on the list of planets. In 2006, scientists made a new rule about what a planet is. A real planet has to sweep its path clear of other objects. Pluto shares its space with lots of other icy worlds. So scientists moved it to a new group called dwarf planets.
That does not mean Pluto went away. It is still out there, still circling the Sun. It just has a new name for its group.
Pluto is also very far away. It is so far that it is hard to picture. From Pluto, the Sun would look like just a very bright star, not the big yellow Sun we see. That is why Pluto stays so cold and dark.
Key facts about Pluto
Pluto is a dwarf planet. It is round and goes around the Sun, but it is small.
Pluto is smaller than the Moon. It is only about 1,477 miles across. That is even smaller than Earth’s own Moon.
Pluto is super cold. The temperature drops to about -375 to -400 degrees Fahrenheit. That is far colder than anywhere on Earth.
Pluto is far, far away. It is about 39 times farther from the Sun than Earth is.
A year on Pluto is very long. It takes about 248 Earth years for Pluto to go around the Sun once.
A day on Pluto is long too. Pluto spins around once every 6.4 Earth days.
Pluto has a big moon named Charon. Charon is about half as wide as Pluto, which is huge for a moon.
Pluto has five moons in all. They are named Charon, Nix, Hydra, Kerberos, and Styx.
Pluto has a heart on it. There is a big bright patch shaped like a heart on its surface.
Pluto was found in 1930. A young scientist named Clyde Tombaugh spotted it from Arizona.
Common myths about Pluto
Myth: Pluto stopped being a planet because it is tiny. Being small was not the real reason. Pluto became a dwarf planet because it shares its path with many other icy worlds. A planet is supposed to keep its path clear, and Pluto does not.
Myth: Pluto was named after the cartoon dog. It was the other way around. Pluto the dwarf planet was named first, in 1930. It is named after a Roman god of the dark underworld. An 11-year-old girl named Venetia Burney thought of the name.
Myth: Pluto is close to Earth. Pluto is one of the farthest worlds from the Sun. It is billions of miles away, out past all the big planets. You cannot even see it without a powerful telescope.
Myth: You could land and walk on Pluto without a spacesuit. Pluto is far too cold, and it has almost no air to breathe. A person could only visit with a spaceship and a spacesuit.
Myth: Pluto’s heart shape was painted by astronauts. No person has ever been to Pluto. The heart is a natural patch of bright ice on the surface. Scientists named it after Clyde Tombaugh, who found Pluto.
Frequently asked questions about Pluto
Is Pluto a planet?
Pluto used to be called the ninth planet. In 2006, scientists changed the rules and called it a dwarf planet instead. It is still a real, round world that circles the Sun. It just belongs to a different group now.
Why is Pluto so cold?
Pluto is very far from the Sun. It gets only a little bit of sunlight, so it stays freezing all the time. The ground there is covered in ice, and the temperature drops to hundreds of degrees below zero.
How many moons does Pluto have?
Pluto has five moons. The biggest one is Charon, which is about half as wide as Pluto itself. The other four are smaller and are named Nix, Hydra, Kerberos, and Styx.
Has anyone been to Pluto?
No people have been to Pluto. But a robot spaceship called New Horizons flew past it in 2015. It took the first close-up pictures of Pluto ever. Before that, Pluto looked like just a tiny dot in the sky.
Why does Pluto have a heart on it?
Pluto has a big bright patch shaped like a heart. Part of it is a giant plain of frozen nitrogen ice. The heart is natural, not painted. Scientists named it after the man who discovered Pluto.
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Pluto is a dwarf planet, which is a small, round world that orbits the Sun. It is the most famous member of the Kuiper Belt, a wide ring of icy worlds beyond the planet Neptune. Pluto is smaller than Earth’s Moon, and it is one of the coldest places we know of. For most of the 1900s it was called the ninth planet, but scientists reclassified it as a dwarf planet in 2006.
Why Pluto is surprising
The biggest surprise about Pluto is why it stopped being called a planet. Many people think it was because Pluto is small. That is not quite right. In 2006, astronomers wrote a new definition of the word planet. A planet has to do three things: orbit the Sun, be round because of its own gravity, and clear other objects out of its orbit. Pluto does the first two. It fails the third, because it shares the Kuiper Belt with many other icy worlds instead of having its path to itself.
What pushed scientists to make this new rule was a discovery in 2005. They found a faraway world called Eris that is even heavier than Pluto. If Pluto counted as a planet, then Eris would too, and so might dozens of other objects. So astronomers chose a stricter definition. That left eight planets, with Pluto and Eris grouped as dwarf planets.
Pluto also has a strange orbit. Instead of a near-circle, its path is a stretched oval. For about 20 years out of every long trip, Pluto comes closer to the Sun than Neptune does. You might think they could crash, but they never do. Pluto’s orbit is tilted, and the two worlds are timed so they are never near each other at the crossing point.
Key facts about Pluto
Pluto is a dwarf planet in the Kuiper Belt. The Kuiper Belt is a ring of icy worlds beyond Neptune.
Pluto is smaller than Earth’s Moon. It is about 1,477 miles (2,377 km) across, about two-thirds the Moon’s width.
Pluto is very far from the Sun. It orbits about 39 times farther out than Earth does.
A year on Pluto lasts about 248 Earth years. It has such a long path that it has not finished one trip since it was discovered in 1930.
A day on Pluto lasts about 6.4 Earth days. Pluto spins slowly compared with Earth.
Pluto is freezing cold. Its surface drops to about -375 to -400 °F (-226 to -240 °C).
Pluto has a thin atmosphere. It is made mostly of nitrogen, with a little methane and carbon monoxide.
Pluto has a heart-shaped region. Part of it is a huge glacier of frozen nitrogen called Sputnik Planitia.
Pluto has five moons. The biggest is Charon, which is about half Pluto’s width.
A spacecraft has visited Pluto. NASA’s New Horizons flew past it in 2015.
Common myths about Pluto
Myth: Pluto became a dwarf planet because it is too small. Size was not the deciding reason. The new rule is about whether a world has cleared its orbit of other objects. Pluto shares the Kuiper Belt with many icy worlds, so it does not pass that test.
Myth: Pluto and Neptune will crash one day. Their orbits seem to cross, but the two worlds never come close. Pluto’s orbit is tilted, and the timing keeps them apart. They have safely missed each other for billions of years.
Myth: Pluto is a plain, boring ball of ice. When New Horizons flew past, it found a busy, varied world. Pluto has mountains of frozen water, a giant heart-shaped ice plain, and even possible ice volcanoes. Some parts of the surface have almost no craters, which means they are young and still changing.
Myth: Charon is a tiny moon. Charon is huge compared with Pluto, about half its width. Pluto and Charon are so close in size that they each keep the same side pointed at the other as they turn. Some scientists call them a double system.
Myth: Pluto was named after the cartoon dog. The dwarf planet was named first, in 1930. An 11-year-old girl named Venetia Burney suggested the name, after the Roman god of the underworld. The cartoon dog came later that year.
Frequently asked questions about Pluto
Why is Pluto a dwarf planet and not a planet?
In 2006, scientists agreed that a planet must orbit the Sun, be round, and clear other objects out of its orbit. Pluto orbits the Sun and is round, but it shares its space with many other Kuiper Belt worlds. Because it has not cleared its orbit, it is called a dwarf planet.
What is the Kuiper Belt?
The Kuiper Belt is a wide ring of icy objects that circle the Sun beyond Neptune. Pluto is one of the largest objects in it. The belt holds many smaller frozen worlds, along with chunks of ice and rock.
What is the heart on Pluto made of?
Pluto’s heart-shaped region holds a giant plain of frozen nitrogen called Sputnik Planitia. It stretches more than 620 miles (1,000 km) across, about the size of two large U.S. states side by side. The ice slowly moves and churns, like a very slow glacier.
What is Charon?
Charon is Pluto’s largest moon, about half Pluto’s width. It is so big compared with Pluto that the two always show each other the same face as they turn. If you stood on Pluto, Charon would seem to hang in the same spot in the sky.
How did we get close-up pictures of Pluto?
NASA sent a spacecraft called New Horizons to study Pluto. It traveled for more than nine years and flew past Pluto on July 14, 2015. It took the first sharp pictures of Pluto and its moons, turning a tiny dot of light into a mapped world.
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Pluto is a dwarf planet in the Kuiper Belt, the most famous of the small, icy worlds that orbit the Sun beyond Neptune. It is about 1,477 miles (2,377 km) across, smaller than Earth’s Moon, and orbits at an average distance of about 39 astronomical units, roughly 39 times Earth’s distance from the Sun. Discovered in 1930, Pluto was counted as the ninth planet for 76 years until the International Astronomical Union reclassified it as a dwarf planet in 2006. It has five known moons, a thin nitrogen atmosphere, and a surface marked by nitrogen-ice plains and mountains of frozen water.
What is often misunderstood about Pluto
The most common misconception about Pluto is why it lost its planet status. Pluto did not shrink, move, or break any rule about being too small in an absolute sense. In 2006 the International Astronomical Union adopted a three-part definition of a planet: it must orbit the Sun, it must have enough mass for its own gravity to pull it into a round shape, and it must have cleared the neighborhood around its orbit of other comparable bodies. Pluto satisfies the first two parts. It fails the third, because it shares the Kuiper Belt with many other icy objects rather than dominating its orbital zone.
The trigger for that debate was the discovery of Eris in 2005, a distant object more massive than Pluto. Astronomers faced a choice: admit Eris and likely many more bodies as planets, or define the word planet more precisely. They chose the stricter definition, which left eight planets and reclassified Pluto and Eris as dwarf planets. So Pluto’s demotion was about context, the company it keeps in the Kuiper Belt, not about a single number for its size.
Pluto is also not a pure ball of ice. Its mean density of about 1.85 grams per cubic centimeter is well above that of water ice, which means a large fraction of its interior is rock. Models picture a rocky core wrapped in a thick mantle of water ice, capped by a thin veneer of more volatile ices like nitrogen and methane. That layered structure helps explain both the bright surface ices New Horizons mapped and the heavier material that gives Pluto a high bulk density for a small, cold world.
Pluto’s orbit is also more unusual than a textbook planet’s. It is a stretched oval, carrying Pluto from about 30 astronomical units from the Sun at its closest to about 49 at its farthest. For roughly 20 years of each 248-year orbit, Pluto is actually closer to the Sun than Neptune. The two never collide, because Pluto’s orbit is tilted about 17 degrees out of the plane the major planets share, and because Pluto and Neptune are locked in a stable timing pattern that keeps them apart.
Key facts about Pluto
Diameter: about 1,477 miles (2,377 km), roughly two-thirds the width of Earth’s Moon and smaller than the planet Mercury.
Average distance from the Sun: about 39 astronomical units, with the distance ranging from about 30 to 49 astronomical units over one orbit.
Orbital period: about 248 Earth years to complete one trip around the Sun.
Rotation period: about 6.4 Earth days, so a single Pluto day lasts about 6.4 of ours.
Axial tilt: about 57 degrees, so Pluto spins nearly on its side, which gives it long seasons.
Surface temperature: about minus 375 to minus 400 °F (minus 226 to minus 240 °C), among the coldest in the solar system.
Atmosphere: thin and mostly nitrogen, with traces of methane and carbon monoxide, far less dense than Earth’s air.
Moons: five known. Charon is the largest, followed by Nix, Hydra, Kerberos, and Styx.
Charon: about 754 miles (1,212 km) across, roughly half Pluto’s diameter, the largest moon relative to its planet in the solar system.
Sunlight travel time: about 5.5 hours to reach Pluto, compared with about 8 minutes to reach Earth.
Discovery: found in 1930 by Clyde Tombaugh at Lowell Observatory in Flagstaff, Arizona.
Spacecraft visit: NASA’s New Horizons flew past on July 14, 2015, the first and only close study of Pluto so far.
Common myths about Pluto
Myth: Pluto was reclassified because it is too small. Size alone was not the rule. The International Astronomical Union’s definition turns on whether a body has cleared its orbital neighborhood. Pluto shares the Kuiper Belt with many comparable objects, so it fails that criterion. A larger body in the same crowded zone would face the same problem.
Myth: Pluto and Neptune will eventually collide because their orbits cross. Pluto’s path does carry it inside Neptune’s orbital distance for part of each orbit, but the two never come close. Pluto’s orbit is tilted about 17 degrees relative to Neptune’s, and the pair is locked in a stable timing pattern, so they are never near the crossing point at the same time.
Myth: Pluto is a frozen, featureless ball. New Horizons revealed a varied, active world. It has a heart-shaped region named Tombaugh Regio, a vast nitrogen-ice plain called Sputnik Planitia, mountains of water ice, and possible ice volcanoes. Parts of the surface have almost no craters, a sign they are geologically young.
Myth: Charon is a small moon like our own. Charon is enormous relative to Pluto, about half its diameter. The balance point the two orbit lies in open space above Pluto’s surface rather than inside Pluto, which is why some astronomers describe the pair as a double system.
Myth: Pluto was named after the Disney dog. The name came first. An 11-year-old girl in England, Venetia Burney, suggested Pluto, the Roman god of the underworld, in 1930. The Disney character debuted later that year and is generally thought to have followed the newly named world, not the reverse.
Frequently asked questions about Pluto
Why is Pluto not a planet anymore?
In 2006 the International Astronomical Union defined a planet as a body that orbits the Sun, is round due to its own gravity, and has cleared the neighborhood around its orbit. Pluto meets the first two requirements but not the third, because it shares the Kuiper Belt with many other objects. It was therefore reclassified as a dwarf planet, a category it shares with Eris, Ceres, Makemake, and Haumea.
How big is Pluto compared with Earth’s Moon?
Pluto is about 1,477 miles (2,377 km) across, roughly two-thirds the diameter of Earth’s Moon, which is about 2,159 miles (3,474 km) wide. Pluto is also smaller than the planet Mercury. Its modest size was part of why its planetary status came into question once similar Kuiper Belt objects were found.
What is the heart on Pluto?
The bright, heart-shaped region is named Tombaugh Regio, after Pluto’s discoverer. Its left lobe is Sputnik Planitia, a plain of nitrogen ice more than 620 miles (1,000 km) across that behaves like a slow glacier. The ice slowly churns in convection cells, which erases craters and keeps the surface looking young.
How many moons does Pluto have, and what is Charon?
Pluto has five known moons: Charon, Nix, Hydra, Kerberos, and Styx. Charon, discovered in 1978, is by far the largest, about half Pluto’s diameter. Pluto and Charon are mutually tidally locked, each keeping the same face toward the other, and they orbit a shared balance point in space above Pluto’s surface.
Has any spacecraft visited Pluto?
Yes. NASA’s New Horizons flew through the Pluto system on July 14, 2015, passing within about 7,800 miles (12,500 km) of the surface after a journey of more than nine years. It returned the first detailed images of Pluto and Charon, transforming Pluto from a blurry point of light into a mapped world. It did not orbit or land; it was a single high-speed flyby.
Why is Pluto so cold?
Pluto orbits about 39 times farther from the Sun than Earth, so it receives very little solar warmth. Surface temperatures fall to about minus 375 to minus 400 °F (minus 226 to minus 240 °C). At that cold, nitrogen, methane, and carbon monoxide freeze into ices on the ground, and the thin atmosphere partly freezes out as Pluto moves farther from the Sun.
Source notes
Pluto’s diameter, distance, orbital period, rotation period, axial tilt, surface temperature, atmosphere, and moon list come from NASA’s Pluto facts page and the NASA in-depth Pluto page. The reclassification and the three-part planet definition are explained by the Library of Congress. Charon’s size, the mutual tidal locking, and the shared balance point are from the Johns Hopkins APL Pluto system page. The New Horizons flyby date and close-approach distance are from NASA’s New Horizons mission page.
Each of this topic’s quiz questions cites a source for the specific fact tested. You can play at any level: Rookie, Curious, Sharp, or Expert.
Pluto is a dwarf planet and the largest known body in the Kuiper Belt by volume, a trans-Neptunian region of icy remnants left from the formation of the solar system. It has an equatorial diameter of about 1,477 miles (2,377 km), smaller than Earth’s Moon and the planet Mercury, and a strongly eccentric, inclined orbit averaging about 39 astronomical units from the Sun. Discovered by Clyde Tombaugh in 1930, Pluto was the ninth planet for 76 years until the International Astronomical Union’s 2006 resolution reclassified it as a dwarf planet. It possesses a thin, mostly nitrogen atmosphere in vapor-pressure equilibrium with its surface ices, five known moons, and a geologically active surface that New Horizons revealed in 2015.
Why Pluto’s classification and dynamics are non-intuitive
Pluto’s reclassification is widely misremembered as a verdict on its size. The actual basis is dynamical. The 2006 IAU definition sets three tests for a planet: it must orbit the Sun, it must be in hydrostatic equilibrium, meaning its self-gravity has pulled it into a nearly round shape, and it must have cleared the neighborhood around its orbit. Pluto satisfies the first two. It fails the third because it has not dynamically dominated its orbital zone; it is one of many comparable bodies in the Kuiper Belt rather than the gravitationally dominant object on its path. Metrics proposed to quantify orbit-clearing, such as the ratio of a body’s mass to the residual mass in its orbital zone, place Pluto orders of magnitude below the eight planets, separating the two populations cleanly.
The proximate cause of the debate was the 2005 discovery of Eris, a scattered-disk object then estimated to be more massive than Pluto. Admitting Eris as a tenth planet, with the prospect of many more discoveries to follow, forced a choice between an ever-growing planet count and a stricter definition. The IAU chose the latter and created the dwarf planet category, which now includes Pluto, Eris, Ceres, Makemake, and Haumea. Trans-Neptunian dwarf planets in Pluto’s dynamical class are sometimes called plutoids.
Pluto’s orbit is itself atypical. Its eccentricity carries it from a perihelion near 30 astronomical units to an aphelion near 49, and for roughly 20 years of each 248-year period it lies inside Neptune’s semi-major axis. Despite the apparent orbit crossing, the two never approach closely, for two reasons. First, Pluto’s orbital inclination is about 17 degrees, well out of the planetary plane, so its perihelion passage occurs far above Neptune’s path. Second, Pluto occupies a stable 3:2 mean-motion resonance with Neptune: it completes two orbits for every three of Neptune’s, a libration that prevents the two from ever being near the mutual node simultaneously. The minimum Pluto-Neptune separation stays on the order of tens of astronomical units.
Key facts about Pluto
Bulk properties. Equatorial diameter about 1,477 miles (2,377 km), roughly two-thirds the lunar diameter, with a mean density near 1.85 grams per cubic centimeter, implying a substantial rock fraction within an icy body.
Orbit. Semi-major axis about 39 astronomical units, eccentricity high enough to range from about 30 to 49 astronomical units, inclination about 17 degrees, orbital period about 248 Earth years.
Resonance. A stable 3:2 mean-motion resonance with Neptune, two Pluto orbits per three Neptune orbits, which protects against close encounters despite the orbit crossing.
Rotation and obliquity. Sidereal rotation about 6.4 Earth days, axial tilt about 57 degrees, producing extreme seasonal insolation patterns over the long year.
Thermal state. Surface temperatures about minus 375 to minus 400 °F (minus 226 to minus 240 °C), cold enough to freeze nitrogen, methane, and carbon monoxide as surface ices.
Atmosphere. Thin and predominantly molecular nitrogen with minor methane and carbon monoxide, in vapor-pressure equilibrium with the surface ice. Surface pressure is on the order of one hundred-thousandth of Earth’s, and it varies as Pluto’s heliocentric distance changes.
Charon. Diameter about 754 miles (1,212 km), roughly half Pluto’s, the largest satellite relative to its primary in the solar system. Pluto and Charon are mutually tidally locked.
Barycenter. The Pluto-Charon center of mass lies in free space above Pluto’s surface, a configuration that motivates describing the system as a binary or double dwarf planet.
Moons. Five known: Charon (1978) plus the small satellites Nix and Hydra (2005), Kerberos (2011), and Styx (2012), the latter four found with the Hubble Space Telescope.
Surface geology. Sputnik Planitia, a nitrogen-ice plain within a basin, shows convective polygons, glacial flow, and a near-absence of craters; candidate cryovolcanoes Wright Mons and Piccard Mons appear elsewhere.
Exploration. NASA’s New Horizons performed the first reconnaissance on July 14, 2015, passing about 7,800 miles (12,500 km) from Pluto’s surface.
Common misconceptions at expert level
Misconception: Pluto’s demotion was a size threshold. No absolute diameter or mass cutoff appears in the IAU definition. The decisive criterion is dynamical dominance of the orbital zone. A body of Pluto’s size embedded in a swept-clean orbit would not face the same problem; conversely, a larger object sharing the Kuiper Belt would still fail. The definition partitions by orbital context, not by a single dimension.
Misconception: The 3:2 resonance is a coincidence of similar periods. The resonance is an actively maintained dynamical state, most likely established as Neptune migrated outward early in solar system history and captured Pluto along with many other resonant bodies, the plutinos. The libration of the resonant argument keeps Pluto’s perihelion longitude away from Neptune. Without the resonance and the inclination, the orbit crossing would not be safe.
Misconception: Charon is a conventional moon. With a mass ratio near 1 to 8 relative to Pluto and a barycenter outside Pluto’s body, the Pluto-Charon pair behaves dynamically more like a binary than a planet-satellite system. The leading formation hypothesis is a giant impact analogous to the one proposed for the Earth-Moon system, followed by tidal evolution to the present doubly synchronous, circular configuration.
Misconception: Pluto’s surface is geologically dead. New Horizons found extensive evidence of recent activity. Sputnik Planitia is nearly crater-free, implying a surface age of only millions of years, and its polygonal cells are interpreted as solid-state convection in nitrogen ice. Glacial flow, sublimation-driven landforms, and the candidate cryovolcanoes Wright Mons and Piccard Mons point to ongoing or geologically recent resurfacing on a body once expected to be inert.
Misconception: The case for a subsurface ocean is settled. The location of Sputnik Planitia near the Pluto-Charon tidal axis has been interpreted as evidence of true polar wander driven by a positive mass anomaly, which is most easily reconciled with a liquid-water layer beneath the ice shell. This is a leading hypothesis, supported by reorientation modeling, but it remains debated. Alternative models, including an oceanless impactor-remnant interpretation of the basin, have been proposed, so the ocean is a strong inference rather than a confirmed detection.
Frequently asked questions about Pluto
What exactly disqualifies Pluto under the IAU definition?
Only the third clause. Pluto orbits the Sun and is in hydrostatic equilibrium, so it clears the first two requirements. It fails to have cleared the neighborhood around its orbit, because it is one member of a populous Kuiper Belt rather than the dominant body on its path. Quantitative orbit-clearing parameters place the eight planets far above Pluto and the other dwarf planets, which is the dynamical justification for the split.
Why can Pluto cross inside Neptune’s orbit without a collision risk?
Two protections operate together. The high orbital inclination, about 17 degrees, lifts Pluto’s perihelion well above the plane near Neptune’s path. The 3:2 mean-motion resonance constrains the orbital phase so Pluto is never at the crossing region when Neptune is nearby. The resonant argument librates rather than circulates, which fixes the geometry of closest approach and keeps the minimum separation large.
How large is Charon, and why is the pair called a double system?
Charon’s diameter is about 754 miles (1,212 km), roughly half Pluto’s, giving a Pluto-to-Charon mass ratio near 8 to 1, far smaller than for any planet-moon pair. The barycenter lies in space above Pluto’s surface, so both bodies orbit a point outside either one. That configuration, together with the doubly synchronous rotation, is why many planetary scientists treat Pluto-Charon as a binary.
What does Sputnik Planitia tell us about Pluto’s interior and activity?
Sputnik Planitia is a basin filled with nitrogen ice that displays convective polygons and glacial flow and lacks impact craters, indicating a very young, mobile surface. Its position close to the axis pointing toward Charon suggests reorientation of Pluto’s figure, true polar wander, driven by a local positive mass anomaly. The most discussed explanation invokes a dense subsurface liquid-water ocean beneath the ice shell, though oceanless alternatives have been proposed and the question is not closed.
How does Pluto’s atmosphere behave over its orbit?
The atmosphere is dominated by molecular nitrogen sublimating from surface ice and is in vapor-pressure equilibrium with that ice. As Pluto moves from perihelion toward aphelion and cools, the equilibrium vapor pressure falls and atmospheric gas should partly condense back onto the surface, so the atmosphere thins and thickens over the long orbit. New Horizons also detected layered haze and a colder upper atmosphere than pre-flyby models had predicted.
What did New Horizons accomplish, and what were its limits?
New Horizons executed a single high-speed flyby on July 14, 2015, after a journey of more than nine years, passing about 7,800 miles (12,500 km) from Pluto. It mapped Pluto and Charon, characterized the atmosphere and surface composition, and imaged the small moons. Because it was a flyby rather than an orbiter, coverage of the far hemisphere was limited and the encounter was brief. The spacecraft then continued into the Kuiper Belt, flying past the object Arrokoth in 2019.
Source notes
Pluto’s diameter, density, orbital elements, rotation, obliquity, thermal state, atmosphere, and moon inventory are drawn from NASA’s Pluto facts page and the NASA in-depth Pluto page. The IAU three-part definition and the reasoning behind reclassification are summarized by the Library of Congress, with broader context on Eris and the dwarf planet category in the Pluto overview. Charon’s diameter and the mutual tidal locking are from the NASA Charon page. Sputnik Planitia’s convection, glacial flow, and crater scarcity are described in the Sputnik Planitia entry, and the subsurface-ocean inference from true polar wander is from the peer-reviewed analysis in Nature.
Each of this topic’s quiz questions cites a source for the specific fact tested. You can play at any level: Rookie, Curious, Sharp, or Expert.
