Why Is Pluto Not a Planet?

What did the IAU decide in 2006?

The International Astronomical Union (IAU) voted on a formal definition of "planet" at the closing ceremony of its 2006 General Assembly in Prague, and published the result on 24 August 2006.^[1][2] Under the definition, a planet is a body that orbits the Sun, has enough mass for its own gravity to pull it into a nearly round shape, and has cleared the neighbourhood around its orbit.^[1] The IAU said this leaves the Solar System with eight planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.^[1]

A second resolution states that Pluto is a "dwarf planet" and the prototype of a new category of trans-Neptunian objects, meaning objects beyond Neptune.^[1][3] The planet definition passed by a large majority without a count, and the Pluto resolution passed with 237 votes in favour, 157 against and 17 abstentions.^[1] One NASA page dates the vote Aug 26, 2006, but the IAU's own release is dated 24 August.^[4][1]

Which of the three tests does Pluto fail?

Pluto fails the third test.^[1] Resolution 5A defines a dwarf planet as a nearly round body orbiting the Sun that has not cleared the neighbourhood around its orbit and is not a satellite, and Resolution 6A places Pluto in that group.^[1] NASA's short explanation is that other objects might cross Pluto's path around the Sun.^[3] Pluto belongs to the Kuiper Belt, a disc-like zone beyond the orbit of Neptune that holds thousands of small icy worlds.^[3]

The IAU's plainer wording of the third test is that a planet must have a major effect on how stable nearby orbits are.^[2] Jupiter still qualifies even though asteroids lead and trail it in its orbit, because the IAU says they are held around two stable points by Jupiter's gravity.^[2]

Pluto is also small: it is much smaller than Mercury and about two-thirds the diameter of Earth's Moon.^[5][3]

• Pluto2,376 km
• Earth's Moon3,475 km
• Mercury4,879 km
• Earth12,756 km

Source: NASA Goddard Space Flight Center (NSSDCA) [7]

NASA pages give slightly different diameters for Pluto: 2,376 km in its planetary fact sheet, about 2,377 km on its Pluto page and about 2,370 km on its New Horizons page.^[7][3][6]

How did the Kuiper Belt and Eris lead to the vote?

Clyde Tombaugh found Pluto in 1930 at the Lowell Observatory, and it was long counted as the ninth planet.^[2][3] Astronomers suspected that a belt of objects lay beyond Neptune, and in 1992 David Jewitt and Jane Luu of the University of Hawaii confirmed it.^[2] As thousands of icy bodies turned up there, it was proposed that Pluto was better seen as the biggest Kuiper Belt object than as a planet.^[5]

Then came an object first labelled 2003 UB313 and later named Eris.^[4] Mike Brown, Chad Trujillo and David Rabinowitz discovered it on Jan. 5, 2005, in data taken on Oct. 21, 2003, during a Palomar Observatory survey.^[4] Eris is about the same size as Pluto and almost 30% more massive.^[4][2] The find was announced in 2005 as a tenth planet, and NASA says it helped trigger the debate that led to the IAU's 2006 decision.^[5][4]

There was a precedent: Ceres was called a planet when it was discovered in the asteroid belt in 1801, then lost that status as many other bodies were found in the same region.^[2]

What is a dwarf planet?

In the IAU's plain-language version, a dwarf planet orbits the Sun and is nearly round but has not cleared its orbit of debris.^[2] The IAU treats planets and dwarf planets as two separate classes of objects.^[1]

The first three dwarf planets were Ceres, Pluto and Eris, which was then still known as 2003 UB313.^[1][4] The IAU's Pluto page lists five known dwarf planets, Pluto, Ceres, Eris, Makemake and Haumea, and NASA names the same five as the dwarf planets the IAU recognizes.^[2][5] The IAU says dozens more, perhaps more than a hundred, may be waiting to be discovered.^[2] Objects that orbit the Sun but are too small to meet either definition are called small Solar System bodies.^[1][2]

What did New Horizons find at Pluto in 2015?

NASA's New Horizons was launched on Jan. 19, 2006, and became the first spacecraft to explore Pluto up close when it flew past on July 14, 2015.^[6] The IAU says its images established that Pluto's diameter is larger than that of Eris.^[2]

The images revealed broad plains and mountain ranges several kilometres high.^[2] NASA describes Pluto's tallest mountains as blocks of water ice 2 to 3 km (6,500 to 9,800 feet) high.^[3] A heart-shaped nitrogen glacier named Sputnik Planitia is about 1,000 km (600 miles) wide.^[6] Some surfaces have no visible craters, a sign that they formed or changed in the recent past.^[2] NASA says scientists were surprised by how much current activity the surface shows, and that there are hints of an internal water-ice ocean.^[6]

The IAU says its resolution remains in place, and that the New Horizons results show dwarf planets can be just as interesting to science as planets.^[2]

Do all scientists accept the definition?

NASA notes that the IAU's definitions never had the full agreement of astronomers and planetary scientists: to some the scheme seemed designed to limit the number of planets, and to others its terms were unclear.^[5]

A study published online in the journal Icarus in 2018, led by Philip Metzger of the University of Central Florida, reviewed scientific literature from the past 200 years and found only one publication, from 1802, that used orbit-clearing to classify planets.^[8] Metzger argues that, read literally, no planet clears its orbit, and recommends calling a body a planet if it is large enough for its gravity to make it spherical.^[8] His co-authors included Alan Stern, the principal investigator of the New Horizons mission.^[8][3]

NASA records an objection to that simpler idea: it does not say how round an object must be, and the shapes of some distant objects are hard to measure accurately.^[5]

The IAU's own page on Pluto quotes a former IAU president who described such decisions as conventions that no national or international law enforces.^[2]