Small bodies of the Solar System are object of a wide range of sizes that orbit around the Sun. They are comets, meteors and asteroids. They are neither planets, nor dwarf planets, nor satellites of planets that have their own classification. The International Astronomical Union first defined the term Small Solar System Bodies in 2006. The classification does not include smaller-sized space debris such as cosmic dust, meteoroids and solar wind.
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The Oort Cloud is the most distant zone of comets that orbit the Sun. It lies at about 50,000 astronomical units (AU), or 7.5 billion kilometres – roughly one light year – away from the Sun. This is a quarter of the distance of the Sun from the nearest similar star, Alpha Centauri. Astronomers believe that long-period comets, whose orbital period around the Sun ranges from 200 years to many thousands or even millions of years, originate here. This also could be the source of Halley-type comets with orbital periods of between 20 and 200 years.
The Kuiper Belt is a region around the Sun that extends from the orbit of Neptune at 30 AU (450 million kilometres) to the start of the Oort Cloud at 50 AU. This region is believed to hold the remnants of the materials that created the Solar System. It is where most short-period comets originate.
Comets form like ‘dirty snowballs’ in the Solar System from dust, rock fragments and ice from water, methane, carbon dioxide and ammonia. These bodies travel around the Sun in elliptical orbits. They are usually of small mass and shaped like potatoes or peanuts. Their low gravitational pull does not allow the body to become spherical. Dust and gas ions around the surface of a comet nucleus form a coma, or a type of atmosphere. As the comet travels along its orbit, solar radiation pushes further dust and ionised gas behind it, creating a tail. The coma and tail are the visible parts of a comet as it passes by Earth.
Asteroids are mostly rocky debris left over from the formation of the Solar System. These occupy a belt, the Asteroid Belt, between the orbits of Mars and Jupiter. Some of the asteroids may have originated from larger bodies. The largest asteroid, Ceres, has a diameter of 950 kilometres. Some astronomers believe that the gravitational pull of Jupiter obstructed the coalescence of asteroids in this belt into larger objects. Others think that the asteroids are the remains of a planet.