Space questions, answered
Is Pluto a planet?
No — since 2006, Pluto has been classified as a dwarf planet, not a planet. It orbits the Sun and is round, but it hasn't 'cleared' its orbit of other objects.
In 2006 the International Astronomical Union set three tests for a planet: it must orbit the Sun, be round under its own gravity, and have cleared its orbital neighbourhood of other bodies. Pluto passes the first two but fails the third — it shares the Kuiper Belt with countless other icy objects, including some, like Eris, that are close to its size.
So Pluto became the best known of the 'dwarf planets'. It's still a fascinating, geologically active world — New Horizons revealed nitrogen glaciers and water-ice mountains there in 2015 — it just isn't a full planet by the modern definition.
Fly Pluto's nitrogen-ice heart →How many planets are there? →
People also ask
When did Pluto stop being a planet?
In August 2006, when the IAU adopted its formal definition of a planet.
How many dwarf planets are there?
At least five are recognised — Pluto, Eris, Haumea, Makemake, and Ceres — with more likely to be confirmed.
Sources: IAU — 2006 planet definition