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 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. Some minor planets even sometimes release gas and can also be called comets.

Populations

Main article: List of minor-planet groups

Many minor planets have been found in our Solar System, and thousands more are discovered each month. The Minor Planet Center records over 213 million observations of these objects.

There are several main groups of minor planets:

• Asteroids: These are mostly found in the inner Solar System. Some are near Earth’s orbit, while others are between Mars and Jupiter, known as the Asteroid belt.
• Distant minor planets: These are found farther out in the Solar System, 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 follow 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). The official way to write the number uses parentheses, but many people leave them out. 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. The first one to stay unnamed for a long time was (3360) 1981 VA, now called 3360 Syrinx. In May 2021, (4596) 1981 QB became the lowest-numbered unnamed minor planet.

Some minor planets can also be seen as both comets and asteroids, like 4015 Wilson–Harrington, which is also called 107P/Wilson–Harrington.

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, when the highest number jumped from 99947 to 118161.

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. The first with a completely non-Classical name was 45 Eugenia, named after Empress Eugénie de Montijo, the wife of Napoleon III. For a while, only female names were used; Alexander von Humboldt was the first man, but his name was changed to a female form, 54 Alexandra. This continued until 334 Chicago was named, after which female names were still common for years.

Eccentric

As more minor planets were found, people began giving them more playful names. The first hints of this were 482 Petrina and 483 Seppina, named after the discoverer's pet dogs. This became more noticeable in 1971 with 2309 Mr. Spock, named after the discoverer's cat. Even though rules now discourage using pet names, 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), unlike comets. One way around this has been for astronomers to name their discoveries after each other. Rare exceptions are 1927 Suvanto and 96747 Crespodasilva. 1927 Suvanto was named after its discoverer, Rafael Suvanto, after he passed away four years after the discovery during the Finnish winter war of 1939–40. 96747 Crespodasilva was named after its discoverer, Lucy d'Escoffier Crespo da Silva, because she passed away shortly after the discovery at age 22.

Languages

Names have been changed to fit different languages from the start. 1 Ceres, called Cerere in Italian, was named in that form. German, French, Arabic, and Hindi use forms close to English, while Russian uses Tserera, similar to Italian. In Greek, the name was changed to Δήμητρα (Demeter), the Greek version of the Roman goddess Ceres. In Chinese, names are not the Chinese form of the deity but 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', so most names have three Chinese characters. For example, Ceres is 穀神星 'grain goddess planet', and Pallas is 智神星 'wisdom goddess planet'.^{[citation needed]}

Physical properties of comets and minor planets

Commission 15 of the International Astronomical Union studies the physical features of comets and minor planets.

Old records about these objects are kept in the PDS Asteroid/Dust Archive. This includes common details about asteroids, such as their sizes, weights, how fast they spin, and their surface temperatures. There is also a special group called the European Asteroid Research Node that collects information about asteroids close to 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. They affect how we understand these objects and design tools to explore them.

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, particles from the solar wind, and cosmic rays from outside our Solar System.

The surface of minor planets can change in brightness as they rotate, helping 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. Some have layers of soil that change over time due to space conditions. Larger rocks on these surfaces give clues about their history and composition.

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. However, some minor planets may have magnetic fields from past interactions or from the solar wind, though these fields can change over time.