# What Makes a Dwarf Planet > On 27 October 2026 astronomers using the new Vera C. Rubin Observatory announced 2026 ED138, an icy body perhaps 1,000 kilometres across, about 46 times farther from the Sun than Earth: if confirmed, the sixth recognised dwarf planet and among the largest and brightest bodies known beyond Neptune. This film explains how a moving dot is found among billions, why its size is still uncertain, the three tests the International Astronomical Union set for a planet in 2006 and the vote that made Pluto a dwarf planet, why big bodies pull themselves round, the five recognised dwarf planets from Ceres to Eris, and the crowded, cold Kuiper belt where more are probably waiting. Interactive film: https://www.planetpast.com/stories/what-makes-a-dwarf-planet/ Publisher: PlanetPast (https://www.planetpast.com/) · d => T('readPublished', l, { d: dateIn(d, l) }) 2026-10-28 · Space, Physics ## Prologue ### Round, orbiting, and not alone *Space · Physics* A new world, and an old argument. A faint dot crept across thirty pictures of the sky. On the twenty-seventh of October 2026, astronomers announced what it is: an icy world beyond Pluto, about forty-six times as far from the Sun as Earth is. It is called 2026 ED138. It may be about a thousand kilometres across, among the largest and brightest bodies known out there, and it is the first possible dwarf planet found by the new Vera C. Rubin Observatory. If it is confirmed, it would join Ceres, Pluto, Eris, Haumea and Makemake as the sixth recognised dwarf planet. So what exactly is a dwarf planet? - **≈ 46 AU**: from the Sun; 1 AU = Earth–Sun distance - **600–1,500 km**: estimated diameter - **30**: Rubin sightings, March to July 2026 ![A mountaintop observatory in Chile at dusk. The find was announced at a planetary-science meeting in Spokane, Washington. AI-generated illustration.](https://assets.planetpast.com/a/42/42d4b907ba429436.jpg) - How a moving dot is found - The 2006 rules, and Pluto - Why big bodies are round - The five, and the Kuiper belt Prologue #### A new candidate SLAC and University of Washington (27 Oct 2026): 2026 ED138 is a trans-Neptunian object about 46 astronomical units (AU) from the Sun, against Pluto’s average of 40 AU; its size is estimated at 600 to 1,500 kilometres, “roughly 1,000 kilometers across” according to Mario Jurić; SLAC calls it among the largest and brightest bodies in this region, and GBH among the seven largest objects identified in the outer solar system. Its orbit is tilted almost 25 degrees. It was announced at the Division for Planetary Sciences meeting in Spokane, Washington, as the Rubin Observatory’s first dwarf planet candidate. One astronomical unit (AU) is the average distance from Earth to the Sun. 2026 ED138 is a candidate: its size and shape are not yet confirmed, and the IAU has not classified it. The solar-system map, the sky field and the bodies in the scenes are teaching models: distances are to scale only where stated, the bodies are enlarged, and colours are schematic. Sources: [SLAC National Accelerator Laboratory (27 Oct 2026): NSF–DOE Vera C. Rubin Observatory reveals its first dwarf planet candidate](https://www6.slac.stanford.edu/news/2026-10-27-nsf-doe-vera-c-rubin-observatory-reveals-its-first-dwarf-planet-candidate-our-solar); [University of Washington (27 Oct 2026): Rubin Observatory reveals its first dwarf planet candidate in our solar system](https://www.washington.edu/news/2026/10/27/rubin-observatory-new-dwarf-planet-candidate/); [U.S. National Science Foundation (27 Oct 2026): New dwarf planet candidate discovered by NSF–DOE Rubin Observatory](https://www.nsf.gov/news/new-dwarf-planet-candidate-discovered-nsf-doe-rubin); [GBH News (27 Oct 2026): Potential new dwarf planet found by Harvard astrophysicists](https://www.wgbh.org/news/local/2026-10-27/potential-new-dwarf-planet-found-by-harvard-astrophysicists) ## Chapter I · A moving dot ### A moving dot *Chapter I* Stars stay put. Worlds in the solar system move. The Rubin Observatory sits on Cerro Pachón, a mountain in Chile. Its 8.4-metre mirror feeds a 3,200-megapixel camera, the largest digital camera ever built, and since June 2026 it has been photographing the whole southern sky again and again. Stars stay in place from night to night. Objects in the solar system move. On the thirteenth of October, Matthew Holman ran Rubin’s data through a program that links billions of detections of moving dots into paths. One path stood out. Rubin had caught the object thirty times on twenty nights, and older images from another telescope carried its orbit back to 2016. Try blinking the nights yourself. - **8.4 m**: primary mirror - **3,200 MP**: camera, over 3,000 kg - **10 years**: of repeated sky survey ![A giant survey camera on a test stand. Rubin covers the visible sky about every three nights (GBH). AI-generated illustration.](https://assets.planetpast.com/a/92/9263f9d4ef48dcc9.jpg) - 2016: Earliest archival images (DECam) - 4 Mar–11 Jul 2026: 30 Rubin sightings on 20 nights - 13 Oct 2026: Holman’s code links the dots - 27 Oct 2026: Announced in Spokane Chapter I · A moving dot #### Finding a moving dot NSF: the Rubin Observatory on Cerro Pachón, Chile, has an 8.4-metre combined primary/tertiary mirror and a 3,200-megapixel camera weighing over 3,000 kg, the largest digital camera ever built, for a 10-year survey. SLAC and UW: the Legacy Survey of Space and Time began in June 2026. On the evening of 13 October 2026 Matthew Holman (Center for Astrophysics | Harvard & Smithsonian) ran Rubin data through search code built with Kevin Napier that links billions of detections of moving objects; Mario Jurić (University of Washington) then found more observations. Rubin saw the object 30 times on 20 nights between 4 March and 11 July 2026, and archival Dark Energy Camera (DECam) images found through the Asteroid Institute extend its orbit back to 2016. Try it: blink between nights, then link the dots. The star field and the object’s path are schematic. The solar-system map, the sky field and the bodies in the scenes are teaching models: distances are to scale only where stated, the bodies are enlarged, and colours are schematic. Sources: [NSF: Rubin Observatory installs the LSST Camera (3,200 megapixels; 8.4 m mirror)](https://www.nsf.gov/news/nsf-doe-vera-c-rubin-observatory-installs-lsst-camera); [SLAC National Accelerator Laboratory (27 Oct 2026): NSF–DOE Vera C. Rubin Observatory reveals its first dwarf planet candidate](https://www6.slac.stanford.edu/news/2026-10-27-nsf-doe-vera-c-rubin-observatory-reveals-its-first-dwarf-planet-candidate-our-solar); [University of Washington (27 Oct 2026): Rubin Observatory reveals its first dwarf planet candidate in our solar system](https://www.washington.edu/news/2026/10/27/rubin-observatory-new-dwarf-planet-candidate/); [GBH News (27 Oct 2026): Potential new dwarf planet found by Harvard astrophysicists](https://www.wgbh.org/news/local/2026-10-27/potential-new-dwarf-planet-found-by-harvard-astrophysicists) ### Why is the size so uncertain? From that far away, the object is just a point of light. Astronomers can measure how bright it is, but not how big. As Matthew Holman put it, it could be a very reflective object that is smaller, or a very dark object that is bigger. How much light a surface reflects is called its albedo. That is why the estimate runs from six hundred to fifteen hundred kilometres. Infrared observations, which measure the heat it gives off, are needed to pin it down. > “All we can see is how much light we’re getting from it.” (Matthew Holman, Harvard & Smithsonian, to GBH News) ![Fresh snow beside dark rock. A surface that reflects four times more light can be half the width and look equally bright. AI-generated illustration.](https://assets.planetpast.com/a/c1/c1db957c3adf41fc.jpg) - **600 km**: if very reflective - **1,500 km**: if very dark Chapter I · A moving dot #### Brightness is not size GBH News: Holman said “all we can see is how much light we’re getting from it”, so it could be “a very reflective object that’s smaller or a very dark object that’s bigger.” SLAC: the 600–1,500 km estimate comes from the object’s distance, its brightness and an assumed albedo; the analysis is preliminary, and infrared observations are needed to pin down its size and confirm whether it is a dwarf planet. The image caption’s “four times more light, half the width” is geometry: reflected light scales with area (the square of the width) times albedo. Try it: compare a small bright world with a large dark one. 2026 ED138 is a candidate: its size and shape are not yet confirmed, and the IAU has not classified it. The solar-system map, the sky field and the bodies in the scenes are teaching models: distances are to scale only where stated, the bodies are enlarged, and colours are schematic. Sources: [GBH News (27 Oct 2026): Potential new dwarf planet found by Harvard astrophysicists](https://www.wgbh.org/news/local/2026-10-27/potential-new-dwarf-planet-found-by-harvard-astrophysicists); [SLAC National Accelerator Laboratory (27 Oct 2026): NSF–DOE Vera C. Rubin Observatory reveals its first dwarf planet candidate](https://www6.slac.stanford.edu/news/2026-10-27-nsf-doe-vera-c-rubin-observatory-reveals-its-first-dwarf-planet-candidate-our-solar) ## Chapter II · What is a planet? ### What is a planet? *Chapter II* A definition written in 2006. For most of history, nobody had written down what a planet is. Then in 2005 astronomers announced Eris, an icy body about the size of Pluto, and the question could no longer wait. On the twenty-fourth of August 2006, in Prague, the International Astronomical Union voted on three tests. A planet orbits the Sun. It has enough mass for its own gravity to pull it into a nearly round shape. And it has cleared the neighbourhood around its orbit. A dwarf planet passes the first two tests but not the third, and it is not a moon. Use the buttons to try each test. ![A pale icy world with a small moon. Eris has about 27% more mass than Pluto (NASA APOD). AI-generated illustration.](https://assets.planetpast.com/a/cd/cd726ccde8a89fbc.jpg) - 1. orbits the Sun - 2. is massive enough to be nearly round - 3. has cleared its orbital neighbourhood - **✓ ✓ ✗**: passes tests 1 and 2, fails 3 - **not a moon**: a satellite does not count Chapter II · What is a planet? #### The 2006 definition IAU Resolution 5A, adopted on 24 August 2006 at the 26th General Assembly in Prague: a planet (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a nearly round (hydrostatic equilibrium) shape, and (c) has cleared the neighbourhood around its orbit. A dwarf planet satisfies (a) and (b), has not cleared its neighbourhood, and is not a satellite (Wikipedia; NASA). NASA: the discovery of Eris (found on images taken in 2003, announced in July 2005, per Caltech) set off the debate behind the 2006 decision. Try it: each button highlights the bodies that pass one test. The map is schematic: planet sizes are enlarged, and the inner planets are crowded near the Sun. The solar-system map, the sky field and the bodies in the scenes are teaching models: distances are to scale only where stated, the bodies are enlarged, and colours are schematic. Sources: [Wikipedia: IAU definition of planet (Resolutions 5A and 6A, 24 August 2006; the vote)](https://en.wikipedia.org/wiki/IAU_definition_of_planet); [NASA Science: What is a planet?](https://science.nasa.gov/solar-system/planets/what-is-a-planet); [NASA Science: Dwarf planets (Pluto, Eris, Makemake; more than 100 may remain undiscovered)](https://science.nasa.gov/?p=73086); [Caltech (2006): Dwarf planet formerly known as Xena officially named Eris](https://www.pma.caltech.edu/news/dwarf-planet-formerly-known-xena-has-officially-been-named-eris-iau-announces-1187) ### Only 424 astronomers were in the hall for the vote, out of about nine thousand members of the union. The definition passed almost unanimously. A second resolution, naming Pluto a dwarf planet, passed 237 to 157. Pluto, a planet since its discovery in 1930, had a new label. Not everyone agreed. The planetary scientist Alan Stern objected that the third test is unclear: if Neptune had really cleared its zone, he said, Pluto would not be there. NASA notes another difficulty: roundness is hard to measure for faraway objects. ![A conference hall during a show of hands. The 26th General Assembly met in Prague in August 2006. AI-generated illustration.](https://assets.planetpast.com/a/80/80de4cc8845003d6.jpg) - For: 237 - Against: 157 - Abstained: 30 > “If Neptune had cleared its zone, Pluto wouldn’t be there.” (Alan Stern, planetary scientist) Chapter II · What is a planet? #### The vote Wikipedia (IAU definition of planet): only 424 astronomers were present for the vote out of roughly 9,000 IAU members; Resolution 5A passed almost unanimously, and Resolution 6A, making Pluto a dwarf planet and the prototype of a new category of trans-Neptunian objects, passed 237 to 157 with 30 abstentions. Alan Stern criticised the definition: “If Neptune had cleared its zone, Pluto wouldn’t be there.” NASA (What is a planet?): some scientists found the terms unclear, and roundness is hard to measure for distant objects. NASA: Clyde Tombaugh discovered Pluto in 1930. The 3D bars show the three vote counts to scale. The solar-system map, the sky field and the bodies in the scenes are teaching models: distances are to scale only where stated, the bodies are enlarged, and colours are schematic. Sources: [Wikipedia: IAU definition of planet (Resolutions 5A and 6A, 24 August 2006; the vote)](https://en.wikipedia.org/wiki/IAU_definition_of_planet); [NASA Science: What is a planet?](https://science.nasa.gov/solar-system/planets/what-is-a-planet); [NASA Science: Dwarf planets (Pluto, Eris, Makemake; more than 100 may remain undiscovered)](https://science.nasa.gov/?p=73086) ## Chapter III · Round and crowded ### Round and crowded *Chapter III* Big enough to be round, too small to clear its path. Why should size make a world round? Every bit of a body pulls on every other bit. A small rock is too weak to change its own shape, so it stays lumpy. Make it bigger, and its gravity grows until it overcomes the strength of the rock and ice. Material slumps toward the centre, and the body settles into a ball. Astronomers call this hydrostatic equilibrium. For icy bodies the line is roughly four hundred kilometres across, according to the astronomer Jonti Horner; other estimates put it nearer a thousand. Saturn’s moon Mimas, 396 kilometres wide, shows the line: moons follow the same physics, even though they do not count as dwarf planets. - Gravity weak - Material strength wins - Shape stays lumpy ![A small potato-shaped asteroid in front of a large round icy world. Fast spin can also flatten a body into an oval. AI-generated illustration.](https://assets.planetpast.com/a/e4/e425216b5a7b4d9c.jpg) - Horner: ≥ 400 km - Brown: 200–400 km - Grundy et al.: fully solid, a stricter test: 900–1,000 km - **396 km**: about the smallest body that may be round by its own gravity Chapter III · Round and crowded #### Why big things are round Jonti Horner (University of Southern Queensland), The Conversation: gravity pulls material toward the centre; once it beats the strength of the material, the body settles into hydrostatic equilibrium, and fast spin flattens it into an oblate shape. An icy body needs to be “at least 400 kilometres” across; Saturn’s moon Mimas, at 396 km, is the smallest that may qualify, and rocky bodies need to be larger still. Wikipedia (List of possible dwarf planets): Mike Brown suggests about 200–400 km for icy bodies, Grundy and colleagues about 900–1,000 km for fully solid bodies; the IAU’s 2006 questions and answers said bodies over 800 km are normally round. Try it: change the size of the icy body and watch it settle. The transition in the scene is schematic. The solar-system map, the sky field and the bodies in the scenes are teaching models: distances are to scale only where stated, the bodies are enlarged, and colours are schematic. Sources: [Jonti Horner (University of Southern Queensland), The Conversation: Why are stars, planets and moons round?](https://australiascience.tv/education/why-are-stars-planets-and-moons-round/); [Wikipedia: List of possible dwarf planets (Gonggong, Quaoar, Orcus; roundness thresholds)](https://en.wikipedia.org/wiki/List_of_possible_dwarf_planets) ### Clearing the neighbourhood is the test Pluto fails. The eight planets dominate their orbits: their gravity has swept up, flung away or captured most of what was nearby. Pluto does not. Its stretched, tilted orbit even crosses inside Neptune’s, and it shares its region with a swarm of other icy bodies in the Kuiper belt, a ring of leftovers beyond Neptune. The two never collide: their orbits are timed so they never meet. The same is true of every known dwarf planet. Each is round, but none is alone on its path. - Planets dominate their orbital zones - Dwarf planets share theirs ![Icy bodies scattered across the dark outer solar system. NASA’s 2001 estimate: about 70,000 Kuiper belt objects wider than 100 km. AI-generated illustration.](https://assets.planetpast.com/a/b2/b23ac8f42ba50fa6.jpg) - **39.4 AU**: average distance from the Sun - **5**: known moons Chapter III · Round and crowded #### Never alone on its path IAU definition (Wikipedia; NASA): a dwarf planet has not cleared the neighbourhood around its orbit. Alan Stern’s objection notes that Pluto’s orbit crosses Neptune’s zone. NASA (2001): David Jewitt estimated about 70,000 Kuiper belt objects larger than 100 km between 30 and 50 AU. NASA: Pluto’s average distance from the Sun is 39.44 AU and it has five known moons. Astronomy magazine: Pluto and Neptune are locked in a 3:2 orbital resonance (three Neptune orbits for every two of Pluto’s) and never come within about 16 AU of each other. The orbits in the scene are drawn to scale in distance; Pluto’s orbit is drawn with an approximate tilt and stretch. Bodies are enlarged. The solar-system map, the sky field and the bodies in the scenes are teaching models: distances are to scale only where stated, the bodies are enlarged, and colours are schematic. Sources: [Wikipedia: IAU definition of planet (Resolutions 5A and 6A, 24 August 2006; the vote)](https://en.wikipedia.org/wiki/IAU_definition_of_planet); [NASA Science (2001): the Kuiper belt and 1992 QB1](https://science.nasa.gov/?p=66068); [NASA Science: Pluto facts (distance, moons, Charon)](https://science.nasa.gov/?p=464928); [Astronomy magazine (Feb 2023): Will Pluto and Neptune ever collide? (3:2 resonance; never within about 16 AU)](https://www.astronomy.com/science/ask-astro-will-pluto-and-neptune-ever-collide/) ## Chapter IV · The five ### The five *Chapter IV* One near, four far. The first dwarf planet was found long before the word existed. On the first of January 1801, Giuseppe Piazzi, in Palermo, spotted Ceres, the first object found in the asteroid belt between Mars and Jupiter. Ceres is about 940 kilometres across and holds roughly forty percent of the asteroid belt’s mass. In 2015 NASA’s Dawn spacecraft went into orbit around it, the first spacecraft to orbit a dwarf planet. - **939 km**: mean diameter, measured by Dawn - **2.77 AU**: from the Sun - **4.6 years**: to orbit once ![A grey cratered dwarf planet with bright spots. Ceres is the only dwarf planet in the inner solar system (NASA). AI-generated illustration.](https://assets.planetpast.com/a/fb/fb25119d28bdca36.jpg) - 6 Mar 2015: Captured into orbit - 1 Nov 2018: Mission ends - Counted as a dwarf planet since 2006 - Not a plutoid: it lies inside Neptune’s orbit Chapter IV · The five #### Ceres NASA: Giuseppe Piazzi discovered Ceres on 1 January 1801 in Palermo; it was the first object discovered in the asteroid belt and is the only dwarf planet in the inner solar system. Wikipedia: its mean diameter is 939.4 km (measured by Dawn); it orbits at 2.77 AU in 4.6 years and holds about 40% of the asteroid belt’s mass. Dawn was captured into orbit on 6 March 2015 and its mission ended on 1 November 2018, the first spacecraft to orbit a dwarf planet. In 2008 the IAU named dwarf planets beyond Neptune “plutoids”; Ceres, inside Neptune’s orbit, is not one (Britannica). The solar-system map, the sky field and the bodies in the scenes are teaching models: distances are to scale only where stated, the bodies are enlarged, and colours are schematic. Sources: [NASA Science: Ceres](https://science.nasa.gov/ceres); [Wikipedia: Ceres (dwarf planet) — diameter, orbit, Dawn mission](https://en.wikipedia.org/wiki/Ceres_(dwarf_planet)); [Encyclopaedia Britannica: Dwarf planet](https://britannica.com/science/dwarf-planet) ### Pluto was the first of these worlds found beyond Neptune. When NASA’s New Horizons flew past on the fourteenth of July 2015, it measured Pluto at 2,370 kilometres across, the largest known object beyond Neptune. Pluto has five moons. The largest, Charon, is 1,208 kilometres wide, more than half Pluto’s size, with about twelve percent of its mass. The two seem to dance around a point between them. - **2,370 km**: diameter, New Horizons - **1930**: discovered by Clyde Tombaugh - **14 Jul 2015**: New Horizons flyby ![A pale icy world with a large bright plain. New Horizons settled a decades-long debate about Pluto’s size. AI-generated illustration.](https://assets.planetpast.com/a/5b/5b5c0b93a35a7659.jpg) - **1,208 km**: diameter - **1978**: discovered - **≈ 12%**: of Pluto’s mass - Five known moons - Charon is by far the largest Chapter IV · The five #### Pluto NASA: Clyde Tombaugh discovered Pluto in 1930; it was reclassified in 2006, and New Horizons flew through the system on 14 July 2015, measuring Pluto at 2,370 km in diameter and Charon at 1,208 km; Pluto is the largest known object beyond Neptune. NASA facts: Pluto has five moons; Charon, discovered in 1978, has about 12% of Pluto’s mass. Because Charon is so massive, Pluto and Charon both circle a point in the space between them; the scene shows this schematically, not to scale in distance. The solar-system map, the sky field and the bodies in the scenes are teaching models: distances are to scale only where stated, the bodies are enlarged, and colours are schematic. Sources: [NASA (2015): How big is Pluto? New Horizons settles decades-long debate](https://www.nasa.gov/directorates/how-big-is-pluto-new-horizons-settles-decades-long-debate); [NASA Science: Pluto facts (distance, moons, Charon)](https://science.nasa.gov/?p=464928); [NASA Science: Dwarf planets (Pluto, Eris, Makemake; more than 100 may remain undiscovered)](https://science.nasa.gov/?p=73086) ### Eris, found by Mike Brown’s team, is about 2,400 kilometres across, nearly Pluto’s size. Pluto is slightly wider, but by timing Eris’s moon Dysnomia, which circles it every sixteen days, astronomers worked out that Eris has about twenty-seven percent more mass. Haumea spins once about every four hours, so fast that it is stretched into the shape of a rugby ball. In 2017 it became the first Kuiper belt object known to have a ring. Makemake, about 1,430 kilometres wide, is the second-brightest object in the Kuiper belt after Pluto. Where would the new candidate fit? Use the buttons to compare both ends of its estimate. - **+27%**: more mass than Pluto, from Dysnomia’s 16-day orbit - **≈ 10 billion km**: from the Sun ![An elongated icy world with a thin ring. Haumea has two moons, Hiʻiaka and Namaka, both found in 2005. AI-generated illustration.](https://assets.planetpast.com/a/4d/4d286d04c64d4f5d.jpg) - Pluto: 2,370 - Eris: 2,326 - Haumea (equator): ≈ 1,740 - Makemake: ≈ 1,430 - Ceres: 939 - 2026 ED138: 600–1,500? - **≈ 1,430 km**: diameter - **2005**: found by Mike Brown’s team - **MK 2**: its one known moon Chapter IV · The five #### Eris, Haumea, Makemake NASA: Eris is about 2,400 km across, nearly Pluto’s size, about 10 billion km from the Sun; NASA gives 5 January 2005 as its discovery date, by Mike Brown, Chad Trujillo and David Rabinowitz, using Palomar images from 2003, and Caltech says it was formally announced on 29 July 2005; its moon is Dysnomia. NASA APOD (2007): Dysnomia’s orbit shows Eris has about 27% more mass than Pluto. NASA: Haumea is about 1,740 km across at the equator, 43 AU from the Sun, spins about once every 4 hours into a football shape, has two moons found in 2005, and was found in 2017 to have a ring, the first known for a Kuiper belt object; two teams dispute its discovery. NASA: Makemake is about 1,430 km across, found in March 2005 by Mike Brown’s team, with one moon, MK 2; it is the second-brightest Kuiper belt object after Pluto. Britannica gives Eris as 2,326 km across; figures differ slightly by source and method. Try it: show the candidate at 600 km and at 1,500 km beside the five. 2026 ED138 is a candidate: its size and shape are not yet confirmed, and the IAU has not classified it. The solar-system map, the sky field and the bodies in the scenes are teaching models: distances are to scale only where stated, the bodies are enlarged, and colours are schematic. Sources: [NASA Science: Dwarf planets (Pluto, Eris, Makemake; more than 100 may remain undiscovered)](https://science.nasa.gov/?p=73086); [NASA Astronomy Picture of the Day (2007): Eris is more massive than Pluto](https://science.nasa.gov/?p=1241294); [Caltech (2006): Dwarf planet formerly known as Xena officially named Eris](https://www.pma.caltech.edu/news/dwarf-planet-formerly-known-xena-has-officially-been-named-eris-iau-announces-1187); [NASA Science: Haumea](https://science.nasa.gov/dwarf-planets/haumea/); [Encyclopaedia Britannica: Dwarf planet](https://britannica.com/science/dwarf-planet) ## Chapter V · The Kuiper belt ### The Kuiper belt *Chapter V* A cold, crowded ring of leftovers. Pluto turned out to be the first known member of a vast ring of icy bodies beyond Neptune. According to NASA, Gerard Kuiper proposed such a belt in 1951. The second was not found until 1992, when David Jewitt and Jane Luu, after searching since 1987, spotted a small world forty-four times as far from the Sun as Earth is. Some bodies go much farther. Sedna, found in 2003, swings out to about 937 times Earth’s distance from the Sun and takes around 11,400 years to orbit once. - 1930: Pluto - 1951: Kuiper proposes a belt - 1992: 1992 QB1: Jewitt and Luu - 2003: Sedna - 2005: Eris, Makemake ![A dome on a dark mountain under the Milky Way. Jewitt and Luu’s 1992 find was 44 AU from the Sun. AI-generated illustration.](https://assets.planetpast.com/a/f7/f7f2aed098ce115e.jpg) - **76 AU**: closest to the Sun - **937 AU**: farthest - **≈ 11,400 yr**: per orbit Chapter V · The Kuiper belt #### The Kuiper belt NASA (2001): Dave Jewitt and Jane Luu found 1992 QB1 in 1992, 44 AU from the Sun, after searching since 1987; Gerard Kuiper proposed such a belt in 1951, and Jewitt estimated about 70,000 objects larger than 100 km between 30 and 50 AU. Wikipedia: Sedna was found on 14 November 2003 by Brown, Trujillo and Rabinowitz; its orbit runs from 76.19 AU to 937 AU and takes about 11,400 years; it is about 906 km across from thermal data, or 1,025 ± 135 km from an occultation. Who first proposed the belt is debated by historians; this film follows NASA’s account. The 2001 NASA count is dated. The belt in the scene is schematic; distances are to scale. The solar-system map, the sky field and the bodies in the scenes are teaching models: distances are to scale only where stated, the bodies are enlarged, and colours are schematic. Sources: [NASA Science (2001): the Kuiper belt and 1992 QB1](https://science.nasa.gov/?p=66068); [Wikipedia: Sedna (orbit and size)](https://en.wikipedia.org/wiki/Sedna_(dwarf_planet)) ### Other large candidates are already known, among them Gonggong, Quaoar and Orcus. NASA thinks more than a hundred dwarf planets may remain to be found. So why was 2026 ED138 missed? Its orbit is tilted by almost twenty-five degrees, out of the flat plane most bodies share, a sign that Neptune’s gravity once pushed it, and it was hiding against a crowded field of stars near the plane of the Milky Way. As Kevin Napier put it, distant dwarf planets are faint, so more may still be undiscovered. Rubin will keep watching the southern sky for ten years. - Gonggong: ≈ 1,230 - Quaoar: ≈ 1,098 - Sedna: ≈ 906–1,025 - Orcus: ≈ 910 ![A dense star field along the Milky Way. In April 2026 Rubin had already reported about 380 new trans-Neptunian objects. AI-generated illustration.](https://assets.planetpast.com/a/34/344e7cb93b2b4a2a.jpg) - **> 100**: dwarf planets may remain undiscovered - **10 years**: of Rubin’s survey ahead Chapter V · The Kuiper belt #### Where the candidate fits SLAC: 2026 ED138’s orbit is tilted almost 25 degrees; it is a “hot-classical” trans-Neptunian object, which suggests Neptune’s gravity pushed it out of its original orbit; it hid in a crowded star field near the plane of the Milky Way, likely why earlier telescopes missed it. GBH: Kevin Napier said distant dwarf planets are faint, so more may remain undiscovered. UW and SLAC: in April 2026 Rubin reported more than 11,000 new asteroids, including about 380 new trans-Neptunian objects. NASA: there may be more than 100 undiscovered dwarf planets. Wikipedia (possible dwarf planets): Gonggong 1,230 ± 50 km, Quaoar 1,098 ± 2 km, Orcus about 910 km. The candidate’s orbit in the scene is drawn as a circle at 46 AU tilted 25°; its real shape and period have not been published. 2026 ED138 is a candidate: its size and shape are not yet confirmed, and the IAU has not classified it. The solar-system map, the sky field and the bodies in the scenes are teaching models: distances are to scale only where stated, the bodies are enlarged, and colours are schematic. Sources: [SLAC National Accelerator Laboratory (27 Oct 2026): NSF–DOE Vera C. Rubin Observatory reveals its first dwarf planet candidate](https://www6.slac.stanford.edu/news/2026-10-27-nsf-doe-vera-c-rubin-observatory-reveals-its-first-dwarf-planet-candidate-our-solar); [GBH News (27 Oct 2026): Potential new dwarf planet found by Harvard astrophysicists](https://www.wgbh.org/news/local/2026-10-27/potential-new-dwarf-planet-found-by-harvard-astrophysicists); [University of Washington (27 Oct 2026): Rubin Observatory reveals its first dwarf planet candidate in our solar system](https://www.washington.edu/news/2026/10/27/rubin-observatory-new-dwarf-planet-candidate/); [NASA Science: Dwarf planets (Pluto, Eris, Makemake; more than 100 may remain undiscovered)](https://science.nasa.gov/?p=73086); [Wikipedia: List of possible dwarf planets (Gonggong, Quaoar, Orcus; roundness thresholds)](https://en.wikipedia.org/wiki/List_of_possible_dwarf_planets) ## Epilogue ### Round enough to pull itself into a ball, too small to rule its orbit, and never alone. That is a dwarf planet. Whether 2026 ED138 joins the five will be settled by infrared measurements still to come. Meanwhile, every few nights, the largest camera ever built sweeps the southern sky again, looking for the next slow dot. - 1801: Ceres - 1930: Pluto - 1992: First Kuiper belt object after Pluto - 2005: Eris - 2006: IAU defines “dwarf planet” - 2026: Rubin’s first candidate ![The survey camera. Rubin’s survey is planned to run for ten years. AI-generated illustration.](https://assets.planetpast.com/a/92/9263f9d4ef48dcc9.jpg) - Stills and pictures are AI-generated - Scenes are schematic; bodies are enlarged - 2026 ED138 is an unconfirmed candidate Epilogue · Sources #### Sources Every fact in this film comes from the sources below. The chapter stills and pictures are AI-generated illustrations; they are not photographs of the objects. The solar-system map, sky field and bodies in the scenes are schematic teaching models. 2026 ED138 is a candidate announced on 27 October 2026; its size is an estimate, and it has not been classified by the IAU. Sources: [SLAC National Accelerator Laboratory (27 Oct 2026): NSF–DOE Vera C. Rubin Observatory reveals its first dwarf planet candidate](https://www6.slac.stanford.edu/news/2026-10-27-nsf-doe-vera-c-rubin-observatory-reveals-its-first-dwarf-planet-candidate-our-solar); [University of Washington (27 Oct 2026): Rubin Observatory reveals its first dwarf planet candidate in our solar system](https://www.washington.edu/news/2026/10/27/rubin-observatory-new-dwarf-planet-candidate/); [U.S. National Science Foundation (27 Oct 2026): New dwarf planet candidate discovered by NSF–DOE Rubin Observatory](https://www.nsf.gov/news/new-dwarf-planet-candidate-discovered-nsf-doe-rubin); [GBH News (27 Oct 2026): Potential new dwarf planet found by Harvard astrophysicists](https://www.wgbh.org/news/local/2026-10-27/potential-new-dwarf-planet-found-by-harvard-astrophysicists); [NSF: Rubin Observatory installs the LSST Camera (3,200 megapixels; 8.4 m mirror)](https://www.nsf.gov/news/nsf-doe-vera-c-rubin-observatory-installs-lsst-camera); [Wikipedia: IAU definition of planet (Resolutions 5A and 6A, 24 August 2006; the vote)](https://en.wikipedia.org/wiki/IAU_definition_of_planet); [NASA Science: What is a planet?](https://science.nasa.gov/solar-system/planets/what-is-a-planet); [Encyclopaedia Britannica: Dwarf planet](https://britannica.com/science/dwarf-planet); [NASA Science: Dwarf planets (Pluto, Eris, Makemake; more than 100 may remain undiscovered)](https://science.nasa.gov/?p=73086); [NASA Science: Ceres](https://science.nasa.gov/ceres); [Wikipedia: Ceres (dwarf planet) — diameter, orbit, Dawn mission](https://en.wikipedia.org/wiki/Ceres_(dwarf_planet)); [NASA (2015): How big is Pluto? New Horizons settles decades-long debate](https://www.nasa.gov/directorates/how-big-is-pluto-new-horizons-settles-decades-long-debate); [NASA Science: Pluto facts (distance, moons, Charon)](https://science.nasa.gov/?p=464928); [NASA Astronomy Picture of the Day (2007): Eris is more massive than Pluto](https://science.nasa.gov/?p=1241294); [NASA Science: Haumea](https://science.nasa.gov/dwarf-planets/haumea/); [NASA Science (2001): the Kuiper belt and 1992 QB1](https://science.nasa.gov/?p=66068); [Wikipedia: Sedna (orbit and size)](https://en.wikipedia.org/wiki/Sedna_(dwarf_planet)); [Wikipedia: List of possible dwarf planets (Gonggong, Quaoar, Orcus; roundness thresholds)](https://en.wikipedia.org/wiki/List_of_possible_dwarf_planets); [Jonti Horner (University of Southern Queensland), The Conversation: Why are stars, planets and moons round?](https://australiascience.tv/education/why-are-stars-planets-and-moons-round/)