Minor planet

A minor planet is a special kind of space object that orbits the Sun but is not big enough to be called a planet, nor is it a comet. These objects are found all around our Solar System, from close to Earth out to far-off areas beyond the planet Neptune. Some well-known minor planets include Pluto and Ceres.

Before 2006, all these objects were called minor planets, but the rules were changed to sort them better. Now, some are called dwarf planets, like Pluto, while others are called small Solar System bodies. Even with these new names, scientists still use the term “minor planet” to talk about these objects, especially when giving them numbers and names.

The first minor planet ever found was Ceres, spotted in 1801. At first, it was thought to be a planet, then later called an asteroid. As more of these objects were found, especially far from Jupiter, the names and rules became more complex. Today, over 1.5 million minor planets are known, though many have not yet been given official numbers or names.

Populations

Main article: List of minor-planet groups

Many minor planets have been found in our Solar System, and more are discovered each month. The Minor Planet Center keeps records of these objects.

There are several main groups of minor planets:

• Asteroids: These are mostly found close to the Sun. Some are near Earth’s path, while others are between Mars and Jupiter, called the Asteroid belt.
• Distant minor planets: These are found farther from the Sun, including objects like Centaurs between Jupiter and Neptune, Neptune trojans, and objects beyond Neptune such as those in the Kuiper belt and the Oort cloud.

Naming conventions

Main article: Astronomical naming conventions § Minor planets

All astronomical bodies in the Solar System need a special name. Minor planets have a three-step naming process. First, they get a temporary name when they are found, called a provisionally designated minor planet. After scientists watch the object long enough to know where it will go, it gets an official number and becomes a numbered minor planet. Finally, it may receive a full name chosen by the person who found it. But only a few minor planets have been given names. Most are either numbered or still have only a temporary name.

Provisional designation

Main article: Provisional designation in astronomy

When a minor planet is first found, it gets a provisional designation. For example, 2002 AT4 shows the year it was found (2002) and a code that tells which half-month and order it was in. Once scientists are sure of its path, it gets a number and may later get a name too (like 433 Eros). Minor planets with numbers but no names keep their temporary name, like (29075) 1950 DA. Because we find so many new minor planets now, most stay unnamed.

Numbering

Minor planets get an official number once their paths are confirmed. With so many being found now, these numbers have six digits. The change from five to six digits happened on October 19, 2005.

Naming

Main article: Name conflicts with minor planets

The first minor planets were named after figures from Greek and Roman mythology, but as those names ran out, people started using names of famous people, characters from stories, the discoverer's family, friends, and even TV characters.

Gender

The first minor planet with a non-mythological name was 20 Massalia, named after the Greek name for the city of Marseille. For a while, only female names were used. This changed when 334 Chicago was named.

Eccentric

As more minor planets were found, people began giving them more playful names. The first hints of this were with names inspired by pets. Unusual names are still suggested and accepted, like 4321 Zero, 6042 Cheshirecat, 9007 James Bond, 13579 Allodd, 24680 Alleven, and 26858 Misterrogers.

Discoverer's name

A rule says minor planets cannot be named after their discoverer(s). One way around this has been for astronomers to name their discoveries after each other.

Languages

Names have been changed to fit different languages from the start. 1 Ceres was named in different forms in various languages. In Greek, the name was changed to Δήμητρα (Demeter), the Greek version of the Roman goddess Ceres. In Chinese, names usually have one or two syllables for the character's meaning, followed by 神 'god(dess)' or 女 'woman' if just one syllable, plus 星 'star/planet'.

Physical properties of comets and minor planets

Commission 15 of the International Astronomical Union looks at the features of comets and minor planets.

Old records about these objects are kept in the PDS Asteroid/Dust Archive. This has common details about asteroids, like their sizes, weights, how fast they spin, and their surface temperatures. There is also a group called the European Asteroid Research Node that collects information about asteroids near Earth.

Environmental properties

Minor planets have special environments that help scientists study them and plan space missions. These environments include space around them, their surfaces, and their insides.

Without a protective atmosphere or strong magnetic field, minor planets are directly exposed to radiation from the Sun and space. This radiation includes energy from sunlight and particles from the solar wind.

The surface of minor planets can change in brightness as they rotate. This helps scientists learn about their shape, size, and surface materials. Their surfaces often have craters from impacts, and many are made of loose, rocky material. These surfaces can also show signs of movement and erosion.

Most minor planets do not generate their own magnetic fields because they are small and lack the internal structures that create such fields, unlike Earth.