Minor planet

Euler-Diagram bodies in the Solar System

Minor planet count

Minor planets are celestial bodies in the Solar System that orbit the Sun but are not planets or comets. The term is often used interchangeably with asteroids, although it also includes other small Solar System bodies such as dwarf planets and small Solar System objects. The distinction between a minor planet and a planet is not only their size but also their lack of certain characteristics that define a planet, such as clearing their orbit of other debris.

Classification

Minor planets are classified into various groups based on their orbits, composition, and size. The main types include the asteroid belt asteroids, Trojan asteroids, Near-Earth Objects (NEOs), and Centaurs. The asteroid belt, located between the orbits of Mars and Jupiter, contains the majority of known minor planets. Trojans are bodies that share an orbit with a larger planet but are located at the Lagrangian points, L4 and L5, leading and trailing the planet in its orbit. NEOs are asteroids and comets with orbits that bring them close to Earth's orbit. Centaurs are icy bodies located between Jupiter and Neptune that have characteristics of both asteroids and comets.

Discovery and Observation

The discovery of minor planets has accelerated over the past few decades thanks to advancements in telescope technology and dedicated astronomical surveys. The first minor planet, Ceres, was discovered in 1801 by Giuseppe Piazzi. Since then, the number of known minor planets has grown to over half a million, with many more expected to be discovered. Observations are conducted using both ground-based telescopes and space-based telescopes, which can detect minor planets by their movement against the background stars or by their thermal emission.

Physical Characteristics

Minor planets vary widely in size, composition, and surface features. They can range from a few meters to hundreds of kilometers in diameter. Compositionally, they can be made of rock, metal, ice, or a combination thereof. Surface features, such as craters, grooves, and ridges, are common, and some minor planets have been found to have moons of their own.

Importance

The study of minor planets provides valuable insights into the early Solar System. Their compositions and distributions can reveal information about the conditions and processes that were present during the formation of the Solar System. Additionally, understanding the orbits and physical characteristics of NEOs is crucial for assessing the potential threat they pose to Earth and for developing possible deflection strategies.

Exploration

Several space missions have been launched to study minor planets up close. Notable missions include NASA's Dawn mission, which explored Vesta and Ceres, and the JAXA's Hayabusa2 mission, which returned samples from the asteroid Ryugu. These missions have provided invaluable data on the surface composition, geology, and history of these minor planets.