Why is Pluto no longer classed as a planet?

Pluto was reclassified as a dwarf planet in 2006 by the International Astronomical Union (IAU). The IAU had adopted a formal definition of a planet that includes a requirement to have cleared the neighbourhood around its orbit. Pluto orbits the Sun and is nearly round, but it shares its region with many other objects, so it fails that third test. [1] [2]

IAU Resolution B5 sorts bodies orbiting the Sun into three categories. A planet orbits the Sun, has enough mass for self-gravity to pull it into a nearly round shape (hydrostatic equilibrium), and has cleared the neighbourhood around its orbit. A dwarf planet meets the first two conditions, has not cleared its neighbourhood, and is not a satellite. Everything else orbiting the Sun, apart from satellites, is a Small Solar System Body. [3] [4]

Clearing the neighbourhood means being gravitationally dominant, with no bodies of comparable size nearby other than its own satellites or those under its influence. Pluto shares its space with Kuiper Belt objects such as the plutinos, and thousands of known objects orbit in that region. [2] [5]

Pluto was discovered in 1930 and counted as the ninth planet. It is much smaller than Mercury, with a diameter of about 2,300 kilometres, and smaller than some planetary moons. Its orbit is highly inclined and elliptical, and it crosses Neptune's orbit during its 247-year journey around the Sun. Its large moon Charon is nearly half its size. [6] [3]

In the early 1990s astronomers began finding many icy bodies in the Kuiper Belt, beyond Neptune. It was then proposed that Pluto is better thought of as the biggest Kuiper Belt object. In 2005 a team announced a Pluto-sized object, later named Eris, which prompted the IAU to define the word planet. Pluto, Eris, Ceres, Haumea and Makemake are the dwarf planets the IAU recognises. [3]

NASA notes that astronomers and planetary scientists did not unanimously accept the IAU definitions. Some felt the scheme was designed to limit the number of planets, and others found it incomplete or its terms unclear. [3]

One alternative is to call any natural object massive enough to be roughly spherical a planet. Critics of that idea say it does not specify how round is round enough, and the shapes of distant objects are hard to measure. Another view holds that an object's location or composition matters, and that orbital dynamics, such as whether it sweeps away or holds its neighbours, should not be the deciding factor. Some astronomers also argue that context is important for understanding how the solar system formed and evolved. The search results do not say how many specialists hold each view, but the IAU definition remains the official one. [3]