What is a dwarf planet?

A dwarf planet is a world that orbits the Sun and is nearly round, but has not cleared its orbit of other debris.^[1] The term dates from 2006, when the International Astronomical Union, the body that names objects in the sky, wrote a definition of a planet and reclassified Pluto.^[1] Five bodies carry the label today, and the same definition is what makes a newly found object beyond Neptune a candidate rather than a confirmed dwarf planet.

What the definition says

The IAU's definition asks three things of a planet: it orbits its star, it is mostly round, and it is big enough for its gravity to have cleared away other objects of similar size near its orbit.^[1] A dwarf planet passes the first two tests and fails the third.^[1] Pluto fails it because other bodies cross its orbit; it was reclassified as a dwarf planet in 2006 after being counted as the ninth planet since its discovery in 1930.^[2] The discovery of Eris, a body of about the same size, helped trigger the debate that led to that reclassification.^[1]

The five the IAU recognises

In order of distance from the Sun they are Ceres, Pluto, Haumea, Makemake and Eris.^[1]

• Ceres is the only dwarf planet in the inner solar system: it orbits in the asteroid belt at an average distance of 2.8 astronomical units from the Sun, was the first member of the belt to be discovered, by Giuseppe Piazzi in 1801, and was called an asteroid for many years before being classified as a dwarf planet in 2006.^[3] Its radius is 476 km, about one thirteenth of Earth's.^[3]
• Pluto is about 2,377 km across and orbits the Sun once every 248 years on an oval path that takes it from 30 to 49.3 astronomical units away; it has five known moons.^[2]
• Haumea is about 1,740 km across, one seventh the width of Earth, and one of the fastest-rotating large objects in the solar system; it has two known moons.^[4][1]
• Makemake is about 1,430 km across and the second-brightest object in the Kuiper belt as seen from Earth, after Pluto; it has one provisional moon.^[5][1]
• Eris is about 2,400 km across, close to Pluto's size, and orbits about 68 astronomical units from the Sun on average, once every 557 years; it has one moon, Dysnomia.^[6] It is the largest known member of the scattered disk, the far part of the Kuiper belt.^[7]

Pluto, Haumea, Makemake and Eris are all Kuiper belt objects, and all four have moons.^[7]

Why the list grows so slowly

The IAU says there may be many more dwarf planets, perhaps more than a hundred, waiting to be discovered.^[1] The reason so few are confirmed is that the test is roundness, and roundness cannot be seen from Earth for a body billions of kilometers away. What a telescope measures is brightness, and size has to be inferred from it.

The inference needs one more number: how reflective the surface is, its albedo. NASA's size estimator for small bodies gives the relation: for a given absolute magnitude, a surface that reflects 5 per cent of sunlight must be far larger than one that reflects 30 per cent to look equally bright, and an error in the assumed albedo can produce a significantly wrong diameter.^[8] That is why a new discovery arrives with a range rather than a size. The Rubin Observatory's candidate 2026 ED138, with an absolute magnitude of 3.19, could be anywhere from 612 to 1,530 km across for albedos between 0.25 and 0.04.^[9] Pinning the size down takes observations in the infrared, which measure the heat the body gives off rather than the light it reflects.^[10]

The candidates

A handful of objects beyond Neptune are bright enough that most astronomers treat them as dwarf planets in all but name. JPL's catalogue lists only 67 trans-Neptunian objects with an absolute magnitude below 4.5, and the brightest after the four recognised ones are Sedna at 1.50, Gonggong at 1.82, Orcus at 2.13 and Quaoar at 2.41.^[11] 2026 ED138, at 2.85 in the same catalogue, sits among them.^[11] Each will have its own page here, with the measured sizes where a spacecraft, a stellar occultation or an infrared telescope has provided one.

How new ones are found

The outer solar system is surveyed by wide-field telescopes that photograph the same sky night after night and look for points of light that move. The Rubin Observatory in Chile began its ten-year survey in June 2026; in April, before the survey formally started, its team had already submitted observations of over 11,000 new asteroids to the Minor Planet Center, including roughly 380 new trans-Neptunian objects.^[10] Its first dwarf-planet candidate was found on 13 October 2026 when a search code compared billions of detections and matched 30 observations of one object taken on 20 nights between 4 March and 11 July.^[10] It had escaped earlier surveys because it lies far south, near the crowded plane of the Milky Way.^[10] The discovery paper estimates that about three more objects as bright as it remain undiscovered away from the Galactic plane, and probably more within it.^[9]

Questions people ask

Is Pluto a planet?

Not under the IAU's definition. Pluto orbits the Sun and is round, but other objects cross its orbit, so it fails the third test and has been a dwarf planet since 2006.^[2] It remains the largest object in the Kuiper belt, although Eris, which is similar in size, has slightly more mass.^[7]

How many dwarf planets are there?

The IAU recognises five: Ceres, Pluto, Haumea, Makemake and Eris.^[1] It also says there may be more than a hundred more to be found, and several known objects are widely regarded as likely dwarf planets that have not been formally classified.^[1]

Is 2026 ED138 a dwarf planet?

Not yet. 2026 ED138 is a dwarf-planet candidate: its brightness and distance imply a body between 600 and 1,500 km across, and icy bodies that large are expected to be round, but infrared observations are needed to confirm its size and nature.^[10] If confirmed, it would join the five the IAU recognises.^[10]

Why is Ceres both an asteroid and a dwarf planet?

Ceres was the first object found in the asteroid belt and was called an asteroid for many years; it is so much bigger and so different from its rocky neighbours that scientists classified it as a dwarf planet in 2006.^[3] It is the only dwarf planet in the inner solar system.^[1]