Categorization of Celestial Objects

Why is Pluto no longer a planet?

It happened on 24 August 2006: instead of the nine planets it had up to that time, our Solar System suddenly had only eight – the planet Pluto was no longer a planet. What happened?
This picture shows the eight classical planets and the three dwarf planets in the Solar System. Since the resolution at the 26th plenary meeting of the International Astronomical Union (IAU), Pluto is now 'only' a dwarf planet.

This picture shows the eight classical planets and the three dwarf planets in the Solar System. Since the resolution at the 26th plenary meeting of the International Astronomical Union (IAU), Pluto is now 'only' a dwarf planet.

In August 2006, astronomers from all over the world gathered at the 26th General Assembly of the International Astronomical Union (IAU) in Prague. Among other things, they reorganized our planetary system and agreed on the scientific definition of a planet. A reorganization had become necessary as an increasing number of heavenly bodies were being discovered beyond Pluto's orbit that were about the same size as Pluto. If these bodies were also granted the status of being planets, this would lead to a real flood of planets in the long term. Under the chair of the well-known female astronomer Jocelyn Bell, the astronomers thus agreed on three criteria that a heavenly body must fulfill in order to be a planet. First, the body must orbit the Sun or a star and must not be a star itself. Second, it must have sufficient mass that is has become spherical due to its own gravity. (See also: Why aren’t all heavenly bodies perfectly spherical?). Thirdly, since its formation, it must have cleared the area around its orbit of small bodies.

Three classes: classical planets, dwarf and planetoids

Pluto does not satisfy the third criterion - although it fulfills the first two, it was named a 'dwarf planet' together with Ceres (see also the astronomic question from week 31) and Eris (which orbits the Sun outside Neptune's orbit). The updated Solar System now has three categories of planet: the eight classical planets – Mercury to Neptune, a slowly growing number of dwarf planets and the irregularly formed planetoids or Small Solar System Bodies (SSSBs).

There are already new mnemonics for remembering the order of the eight planets (moving away from the Sun), including "My Very Elegant Mother Just Served Us Nachos" - Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune.

Source: German Aerospace Center
» print article
Search
Astronomy Software

Redshift Android

Redshift for Android

The award winning Astronomy Software Redshift for Android. » more

Redshift Pro

Redshift Pro - Astronomy for iOS

The most advanced Redshift app » more

Redshift Astronomy

Redshift - Astronomy for iOS

The award winning Astronomy Software Redshift for iPhone, iPod touch and iPad. » more

Redshift Discover Astronomy deutsch

Redshift Compact - Discover Astronomy for iOS

The beginners version of the leading astronomy App Redshift » more

Redshift 8 Premium

Redshift 8 Premium - Download Edition (Multilingua Edition)

Explore the universe from your PC with the award-winning and professional planetarium software - Languages: German, English, French
 » more

Redshift 8 Premium DL deutsch/engl 2

Redshift 8 Premium - Update from older versions

Update from Redshift 7 or older to the current version of the professional planetarium software - Languages: German, English, French
 » more

Redshift 8 Compact

Redshift 8 Compact - Download Edition

The professional planetarium software for beginners » more