Dwarf planet

A dwarf planet is a specific type of celestial body located within our Solar System. These objects orbit the Sun directly rather than orbiting another planet. To be classified as a dwarf planet, an object must possess enough mass to be gravitationally rounded. This means its own gravity pulls it into a sphere. However, a dwarf planet lacks orbital dominance. It has not cleared its neighborhood of other objects, such as planetesimals, along its orbital path. This distinction separates them from the eight classical planets, which exert much stronger gravitational control over their surroundings.

The physical nature of a dwarf planet is defined by the relationship between mass and gravity. When an object reaches a certain mass, its internal pressure becomes high enough. This causes the mantle to become plastic, which is a soft, flowing state. Under this weight, gravity pulls the material into a round shape. This process allows for planetary geology, meaning the surface can change over time. Even though they lack orbital dominance, many dwarf planets are geologically active. Spacecraft missions have confirmed this activity on worlds like Pluto and Ceres.

Astronomers generally agree on the nine largest candidates for dwarf planet status. These objects are listed in decreasing order of diameter: Ceres, Pluto, Eris, Haumea, Makemake, Gonggong, Quaoar, Orcus, and Salacia. There is significant uncertainty regarding the tenth candidate, Sedna, which is considered a borderline case. Understanding these worlds often requires studying their moons. Seven of these ten candidates have at least one known moon. Measuring a moon's orbit allows scientists to calculate the dwarf planet's mass. From there, they can estimate the object's density to build geophysical models.

The history of planetary classification has seen many shifts. In 1801, the discovery of Ceres began a period where many small bodies were called planets. By 1851, the number of known planets had reached 23. Astronomers eventually began using the term "asteroid" to distinguish smaller bodies from major planets. Pluto was discovered in 1930 and was considered a major planet for nearly 50 years. However, the 1978 discovery of Pluto's moon, Charon, allowed scientists to measure its mass accurately. They found Pluto was only about one-twentieth the mass of Mercury. This revealed that Pluto was much smaller than previously estimated.

In the 1990s, new discoveries in the Kuiper belt changed the debate. Astronomers found many objects sharing orbital characteristics with Pluto. These objects are sometimes called plutinos. This led to a realization that either these new bodies were planets, or Pluto needed to be reclassified. In January 2005, the discovery of Eris, which was thought to be larger than Pluto, intensified the discussion. This prompted the International Astronomical Union (IAU) to hold a General Assembly in August 2006. The goal was to establish a formal definition for planets and other bodies.

The 2006 IAU decision created the dwarf planet category. Two astronomers, Julio Ángel Fernández and Gonzalo Tancredi, proposed an intermediate category. They suggested that objects should be round but must also clear their orbits to be planets. This proposal led to the removal of Pluto, Ceres, and Eris from the list of major planets. The decision was not without controversy. Some scientists, like Mike Brown, agreed with the change. Others, such as Alan Stern, rejected the definition. Stern argued that dwarf planets should be considered a subtype of planet rather than something separate.

Today, dwarf planets provide vital clues about the structure of the Solar System. Missions like Dawn, which visited Ceres, and New Horizons, which visited Pluto, have provided incredible data. These missions showed that even small, distant worlds can be complex and active. Some scientists use different terms to describe these bodies, such as planetoids or quasi-planets. In the trans-Neptunian region, the term "plutoid" is used for dwarf planets located beyond Neptune. By studying the mass, density, and orbits of these objects, astronomers continue to refine our understanding of how planetary systems form and evolve.