---
title: "When did the number of planets last change?"
description: "Pluto was reclassified as a dwarf planet, leaving eight, 24 August 2006. What happened, why it has not happened since, and whether it plausibly could again."
url: https://atoneplace.net/the-last-time/planet-count/
site: "At One Place"
---

# When did the number of planets last change?

**When did the number of planets last change?: 24 August 2006**

Pluto was reclassified as a dwarf planet, leaving eight

The International Astronomical Union adopted a definition requiring a planet to have cleared its orbital neighbourhood, which Pluto has not — it shares its region with a great many other Kuiper Belt objects. The vote was prompted by the discovery of Eris, which is more massive than Pluto: either Eris was a tenth planet or Pluto was not a ninth. Only about four per cent of the IAU membership was present for the vote, which is part of why the decision remains contested.

- **What:** Pluto was reclassified as a dwarf planet, leaving eight
- **When:** 24 August 2006
- **Category:** Science and measurement
- **Could this change?:** Not in any foreseeable way. The event is closed.

## Science and measurement

| Question | When |
| --- | --- |
| When were the SI base units last redefined? | 20 May 2019 |
| When was the last leap second added? | 31 December 2016 |
| When was an element last added to the periodic table? | 28 November 2016 |
| When was the metre last redefined? | 1983 |

> This page gives a date and never a "years ago". These pages are built as static files, so a computed interval would freeze at build time and quietly become wrong — which on a reference site is worse than making the reader do the subtraction.

## Common questions

### When did the number of planets last change?

24 August 2006 — Pluto was reclassified as a dwarf planet, leaving eight.

### What happened?

The International Astronomical Union adopted a definition requiring a planet to have cleared its orbital neighbourhood, which Pluto has not — it shares its region with a great many other Kuiper Belt objects. The vote was prompted by the discovery of Eris, which is more massive than Pluto: either Eris was a tenth planet or Pluto was not a ninth. Only about four per cent of the IAU membership was present for the vote, which is part of why the decision remains contested.

## Related pages

- [When were the SI base units last redefined?](https://atoneplace.net/the-last-time/si-base-units/)
- [When was the last leap second added?](https://atoneplace.net/the-last-time/leap-second/)
- [When was an element last added to the periodic table?](https://atoneplace.net/the-last-time/new-element/)
- [When was the metre last redefined?](https://atoneplace.net/the-last-time/metre-redefined/)
- [When was the last Boeing 747 delivered?](https://atoneplace.net/the-last-time/boeing-747/)
- [When did Microsoft Office last change its default font?](https://atoneplace.net/the-last-time/office-default-font/)
- [When did the London Underground last gain a line?](https://atoneplace.net/the-last-time/london-underground-line/)
- [When were the SI base units last redefined?](https://atoneplace.net/the-last-time/si-base-units/)
- [When was the last leap second added?](https://atoneplace.net/the-last-time/leap-second/)
- [When was an element last added to the periodic table?](https://atoneplace.net/the-last-time/new-element/)
- [Everything, most recent first](https://atoneplace.net/the-last-time/)

## Sources

- NASA — National Aeronautics and Space Administration (https://www.nasa.gov/) [Public domain (US Government)]