It used to be the farthest planet from the Sun in our Solar System, until it was declassified.<\/p>\n
It’s an icy relic in the Kuiper Belt on the edge of our Solar SYystem, and has only been visited once, by NASA’s New Horizons spacecraft.<\/p>\n
Perhaps unsurprisingly, planetary scientists are still teasing out the details when it comes to Pluto, but we have learned a lot since it was first discovered.<\/p>\n
Here are some of the best and most interesting facts about Pluto.<\/p>\n
![\"Saturn](\"https:\/\/www.newsbeep.com\/ca\/wp-content\/uploads\/2026\/06\/04.STScI-01H41MRAT34FZJRVA1BNSSYTEK.jpg\")
Saturn and its rings, as seen by the James Webb Space Telescope, 30 June 2023. Credit: NASA, ESA, CSA, STScI, Matt Tiscareno (SETI Institute), Matt Hedman (University of Idaho), Maryame El Moutamid (Cornell University), Mark Showalter (SETI Institute), Leigh Fletcher (University of Leicester), Heidi Hammel (AURA)<\/p>\n
Pluto used to be a planet<\/p>\n
More on all of this below, but in a nutshell\u2026 astronomers were looking for a so-called Planet X, discovered Pluto and thought they\u2019d found it.<\/p>\n
But as the years went by and technology improved, they realised Pluto was a lot smaller than they\u2019d previously assumed \u2013 around 500 times smaller, in fact \u2013 and just one of thousands of similar objects. And so Pluto was officially downgraded<\/a> by the International Astronomy Union in 2006.<\/p>\n It can be described in a number of ways<\/p>\n If Pluto isn\u2019t a planet, what is it? Well, it\u2019s a dwarf planet, for starters \u2013 but there are a few of those. It\u2019s also classed as a trans-Neptunian object, or TNO, because it lies beyond the orbit of Neptune (unlike the giant asteroid Ceres<\/a>, which is also a dwarf planet but doesn\u2019t).<\/p>\n More specifically, it\u2019s a Kuiper Belt<\/a> object \u2013 all TNOs are either Kuiper belt objects (KBOs) or scattered disc objects. It can further be categorised as two other types of object: it\u2019s both a plutoid and a plutino! But that\u2019s enough names for now, so we\u2019ll come back to those.<\/p>\n Today we know of over 5,500 TNOs, and it\u2019s believed there are tens of thousands more waiting to be discovered. But most of them are tiny: Pluto, with a mean radius of 1,188km (738mi), is the largest.<\/p>\n It is, however, only the second most massive: Eris, with a mean radius of 1,163km (723mi), weighs in at 0.0027 Earth masses, compared to Pluto\u2019s 0.00218.<\/p>\n It\u2019s smaller than the Moon<\/p>\n Pluto\u2019s volume is 0.00651 times that of Earth, making it just under one-third the volume of Earth\u2019s Moon, with its 0.02 Earth volumes.<\/p>\n However, as Pluto contains a lot of ice while the Moon is made of rock, the dwarf planet is considerably less massive, weighing in at 1.3 x 1022 kg, or just over one-sixth of the Moon\u2019s 7.346 x 1022kg.<\/p>\n It\u2019s smaller than quite a few moons<\/p>\n No fewer than seven of the Solar System\u2019s natural satellites can beat out Pluto in the size stakes: take a bow Ganymede, Titan, Callisto, Io, the Moon, Europa and Triton.<\/p>\n Pluto\u2019s orbit is eccentric<\/a>, which means it’s not a perfect circle, and therefore Pluto’s distance from the Sun varies as it orbits.<\/p>\n But on average, it\u2019s about 39 times further from the Sun than Earth is \u2013 it takes sunlight 5.5 hours to reach Pluto, whereas light reaches Earth from the Sun in just over eight minutes<\/a>.<\/p>\n Its orbit is both eccentric and inclined<\/p>\n The planets\u2019 orbits are roughly circular but Pluto\u2019s is elliptical, which means the Sun-Pluto distance can range from 29.568 AU (where 1 AU = the distance from Earth to the Sun) at perihelion to 49.305 AU at aphelion.<\/p>\n Pluto also spins more or less on its side: its axis is tilted by 60\u00b0 relative to its orbital path (or 120\u00b0 the other way, depending how you look at it).<\/p>\n It has extreme seasons<\/p>\n Pluto\u2019s steeply inclined orbit means it experiences a high degree of seasonal variation, with its solstices ushering in long periods of either continuous darkness or continuous daylight for around half of its surface.<\/p>\n Solar System years<\/a> \u2013 the length of time it takes a given object to orbit the Sun \u2013 get longer the further out you get.<\/p>\n Mercury\u2019s year is just 88 days long, while Neptune\u2019s lasts 60,190 days, or nearly 165 Earth years. But even that pales in comparison to Pluto, whose year lasts 248 Earth years. 247.94, if you want to get all precise about it.<\/p>\n Its days are longer than Earth’s<\/p>\n Pluto rotates on its axis once every 6.387 Earth days. It\u2019s hardly an outlier in this regard, though: the length of a \u2018day\u2019 on any given Solar System body can range from less than 10 hours (in the case of Jupiter) to 243 Earth days (if you happen to be standing on Venus).<\/p>\n It is never a naked-eye object<\/p>\n Nearby rocky planets like Mars, Venus and Mercury can quite often be spotted with the naked eye. But in the case of Pluto, which is rated at mag. +13.65-15.1, you\u2019re going to need a large (at least 8-inch) telescope and very dark skies.<\/p>\n Even then, it will be case of capturing images over successive nights and comparing them: Pluto\u2019s quite small, and a long way away!<\/p>\n In 1929, a 23-year-old American astronomer called Clyde W Tombaugh<\/a> took on a new job at the Lowell Observatory in Arizona. His job was to capture images of a particular region of space on consecutive nights, then use a device called a \u2018blink comparator\u2019 to examine them in rapid succession, as described above.<\/p>\n In February 1930 he spotted an apparent anomaly in images captured on 21, 23 and 29 January, and on 13 March his discovery of Pluto was announced.<\/p>\n That particular region of space had been chosen because it was where Percival Lowell<\/a>, founder of the observatory, had predicted there should be a putative \u2018Planet X\u2019, based on perturbations detected in the orbit of Uranus. But that\u2019s something else we\u2019re going to need to come back to\u2026<\/p>\n It was named by an 11-year-old British girl<\/p>\n Well, sort of! Once Pluto was discovered, over 1,000 different names for the new \u2018planet\u2019 were suggested: by astronomers, by the press and by the general public. Minerva, Pluto and Cronus were the most popular, and obviously in the end Pluto was chosen \u2013 partly because the name began with Lowell\u2019s initials, PL.<\/p>\n Of the 150-plus letters that the Lowell Observatory had received suggesting that name, the earliest date was on a telegram sent by astronomer Herbert Hall Turner. But Turner was merely passing on a proposal from his friend Falconer Madan, librarian at Oxford University\u2019s Bodleian Library, based on a suggestion made by Madan\u2019s 11-year-old granddaughter Venetia Burney.<\/p>\n Pluto was the Greek god of the underworld<\/p>\n He was originally known as Hades, but over time that name came to be used more commonly to mean the underworld itself. In Ancient Rome, the god of the underworld was known variously as Dis Pater, as Orcus or, again, as Pluto \u2013 though the precise relationships and equivalences between these deities can vary depending on which classical scholar you speak to.<\/p>\n The five natural satellites of Pluto are Charon, Styx, Nix, Kerberos and Hydra. All five are believed to consist of debris left over from giant impacts early in Pluto\u2019s life.<\/p>\n Charon, the largest, is around half the size of Pluto itself. It is regular in shape (ie, spherical), and Pluto and Charon are mutually tidally locked, meaning they always present the same face to each other. Some astronomers regard Pluto-Charon as a binary system.\u00a0<\/p>\n The smaller moons, all with diameters below 160km (100 miles), are irregular in shape and are not tidally locked.<\/p>\n Its moons also have underworld-derived names<\/p>\n The Styx was the river that separated the worlds of the living and the dead, Charon was the boatman who carried the recently deceased across it, Nix was the goddess of darkness (and Charon\u2019s mother), and Kerberos and the Hydra were the three-headed dog and nine-headed serpent (respectively) that guarded the gates of the underworld.<\/p>\n It\u2019s mostly made of rock and ice<\/p>\n Pluto is thought to have a rocky core surrounded by a mantle of water ice, with the crust and regolith consisting of ices of water, nitrogen, methane and CO2. Many scientists believe it may also have a subsurface ocean of liquid water, though this remains unproven.<\/p>\n Pluto\u2019s atmosphere is about 99% nitrogen. The other one per cent is mostly made up of methane (around 0.25%) and carbon monoxide (roughly 0.05%), with traces of acetylene, ethylene, hydrogen cyanide and other compounds.<\/p>\n Its atmosphere varies in size<\/p>\n Like other trans-Neptunian objects, Pluto shares some of its characteristics with comets<\/a>.<\/p>\n One of these is that its atmosphere varies in size\/volume depending on how close Pluto is to the Sun at any given moment: it expands as Pluto nears our star, and contracts as it gets further away. Which is one reason it took us a while to figure out Pluto\u2019s actual physical size\u2026<\/p>\n Once we found it, Pluto’s mass kept shrinking!<\/p>\n The \u2018Planet X\u2019 that Lowell was looking for was estimated to have seven times Earth\u2019s mass, based on his calculations regarding Uranus\u2019s orbit, but when Tombaugh found Pluto, he estimated it to have only around the same mass as Earth. But a lot of that was due to albedo effects \u2013 Pluto shines brightly because of all the methane ice on its surface, which makes it look bigger than it is.<\/p>\n By 1948, estimates of Pluto\u2019s mass had been reduced to around 10 per cent that of Earth, by 1976 it was calculated at less than 1 per cent of Earth\u2019s and by 2006, when Pluto was finally downgraded, it was known to have only a little over one-fifth of 1 per cent of Earth\u2019s mass.<\/p>\n When Tombaugh discovered Pluto, he believed it to be the seven-Earth-masses \u2018Planet X\u2019 that Lowell said should be there. But given that it has turned out to be so much smaller, it clearly wasn\u2019t. In fact, it turns out we never \u201cneeded\u201d a Planet X in the first place!<\/p>\n In 1992, astronomer Myles Standish recalculated the mass of Neptune, based on data sent back by Voyager 2. He calculated that Neptune\u2019s mass had been overestimated by around 0.5% \u2013 and on the basis of Neptune\u2019s revised mass, the irregularities perceived in the orbit of Uranus ceased to be irregularities at all.<\/p>\n The fact that Pluto was where Lowell had said Planet X should be was simply a coincidence, and nothing more.<\/p>\n Its surface is uneven<\/p>\n Pluto has mountains that can reach up to 3km (1.86mi) in height. By way of contrast, Mount Everest is roughly 8.8km (5.46mi) tall, while the UK\u2019s largest mountain, Ben Nevis, stands at just 1.35km (0.83mi). It also has impact craters, deep valleys that can stretch to over 600km (350mi) in length, and large, smooth plains made up almost entirely of nitrogen ice.<\/p>\n Key surface features include the Heart, the Whale and the Brass Knuckles<\/p>\n The Heart, or Tombaugh Regio, is a large, light-coloured plain in the shape of a love heart. The Whale, or Belton Regio, lies along the Plutonian equator and is a darker region some 2,990km (1,860mi) long, while between these two landmarks lie the Brass Knuckles \u2013 a series of smaller, irregularly-shaped dark \u2018blobs\u2019, each around 480km (300mi) in diameter.<\/p>\n Being so far from the Sun, poor little Pluto doesn\u2019t get a huge amount of sunlight \u2013 around 1\/900th as much as Earth receives \u2013 and the average surface temperature is a very, very chilly -232\u00b0C (-387\u00b0F).<\/p>\n Plutonium is named after it<\/p>\n When US chemist Glenn T. Seaborg discovered a new radioactive element in 1941, he promptly named it in Pluto\u2019s honour. In doing so, he was following convention. Uranium (discovered in 1789) had previously been named after the planet Uranus (discovered in 1781), while neptunium (discovered in 1940) was named after Neptune (discovered in 1846).<\/p>\n The case of a certain Disney cartoon dog is less clear-cut! Pluto (the dog) first appeared in a Mickey Mouse cartoon in 1930, certainly. But no one seems to know for sure if Walt Disney was actually aware of Tombaugh\u2019s (then very recent) discovery, or whether it was pure coincidence.<\/p>\n Only one spacecraft has ever visited it<\/p>\n The only spacecraft to visit Pluto so far has been NASA\u2019s New Horizons mission, which launched in 2006 and flew by Pluto in 2015, making its closest approach on 14 July. During its six months observing Pluto, New Horizons imaged its northern hemisphere in its entirety, and the southern hemisphere down to about 30\u00b0 south.<\/p>\n The Hubble Space Telescope had previously managed to image around 85 per cent of the surface, down to around 70\u00b0 south. But Pluto\u2019s southern polar regions have only ever been captured (in very low resolution) from Earth.<\/p>\n As well as being a dwarf planet, a trans-Neptunian object and a KBO, Pluto is also both a plutoid and a plutino.<\/p>\n![\"A](\"https:\/\/www.newsbeep.com\/ca\/wp-content\/uploads\/2026\/06\/Kuiper-Belt-a5d1d25.jpg\")
A computer model of the Kuiper Belt. Credit: NASA
\nPluto is the largest trans-Neptunian object<\/p>\n![\"Image](\"https:\/\/www.newsbeep.com\/ca\/wp-content\/uploads\/2026\/06\/crop_p_color2_enhanced_release_small-5832a13-e1579511756305.png\")
The first images of Pluto sent back by the New Horizons spacecraft revealed the dwarf planet’s icy heart. Credit: NASA\/JPL – NASA\/Johns Hopkins University Applied Physics Laboratory\/Southwest Research Institute – https:\/\/solarsystem.nasa.gov\/resources\/855\/color-pluto\/
\nPluto is a long way from the Sun<\/p>\n![\"Pluto\u2019s](\"https:\/\/www.newsbeep.com\/ca\/wp-content\/uploads\/2026\/06\/pluto-blue-haze.jpg\")
Pluto\u2019s blue haze layer captured by the New Horizons Ralph\/Multispectral Visible Imaging Camera (MVIC). Credit: NASA\/JHUAPL\/SwRI
\nPluto has long years<\/p>\n![\"Clyde](\"https:\/\/www.newsbeep.com\/ca\/wp-content\/uploads\/2026\/06\/Clyde_Tombaugh_Pluto-f64d02b-e1648112263558.png\")
Clyde Tombaugh pictured at a Zeiss Blink Comparator, a machine used in the discovery of Pluto. Credit: Lowell Observatory Archives
\nWe didn\u2019t know Pluto existed until 1930<\/p>\n![\"Hubble](\"https:\/\/www.newsbeep.com\/ca\/wp-content\/uploads\/2026\/06\/Hubble-Pluto-Styx-Hydra-Nix-430e1f3.jpg\")
Hubble Space Telescope image showing some of Pluto’s moons. Credit: NASA, ESA, H. Weaver (JHU\/APL), A. Stern (SwRI), and the HST Pluto Companion Search Team
\nPluto has five known moons<\/p>\n![\"A](\"https:\/\/www.newsbeep.com\/ca\/wp-content\/uploads\/2026\/06\/sky_165_web_3_charon_moon_of_pluto-63a86a7.jpg\")
A New Horizons image of Charon, the largest of Pluto’s five moons. Credit: NASA\/JHUAPL\/SwRI
\nPluto’s atmosphere is mostly nitrogen<\/p>\n![\"A](\"https:\/\/www.newsbeep.com\/ca\/wp-content\/uploads\/2026\/06\/Pluto-Mountains-Plains-1557eee-e1607438292396.jpg\")
A view of icy mountains and ice plains on Pluto, captured by New Horizons just 15 minutes after its closest approach to Pluto on 14 July 2015 as the spacecraft looked back towards the Sun. Credit: NASA\/Johns Hopkins University Applied Physics Laboratory\/Southwest Research Institute
\nPluto isn\u2019t Lowell\u2019s \u2018Planet X\u2019<\/p>\n![\"\"](\"https:\/\/www.newsbeep.com\/ca\/wp-content\/uploads\/2026\/06\/pluto-brass-knuckles.jpg\")
Annotated image showing the ‘Brass Knuckles’ on Pluto. Credit: NASA\/JHUAPL – Mika McKinnon
\nPluto is extremely cold<\/p>\n![\"Topographic](\"https:\/\/www.newsbeep.com\/ca\/wp-content\/uploads\/2026\/06\/Pluto-Wright-Mons-Piccard-Mons.jpg\")
Topographic map showing two of Pluto’s mountains, Wright Mons (left) and Piccard Mons (right). False colour shows changes in elevation, with blue indicating lower terrain and brown showing higher elevation; green terrains are at intermediate heights. Credit: NASA\/Johns Hopkins University Applied Physics Laboratory\/Southwest Research Institute
\nPluto lends its name to two classes of object<\/p>\n