Dwarf Planets, Asteroids, Comets and Meteors: Complete Notes
Apart from the eight planets, the solar system contains a large family of smaller bodies - dwarf planets, asteroids, comets and meteoroids. Examinations ask this topic through the definition of a dwarf planet, the names and positions of the recognised dwarf planets, the location of the asteroid belt, the structure and parts of a comet, and the difference between a meteoroid, a meteor and a meteorite. The facts are direct and record-based, so a clear list of names, places and definitions is enough to score full marks.
What is a dwarf planet
According to the International Astronomical Union (IAU) definition of 2006, a dwarf planet is a celestial body that orbits the Sun, has enough mass for its own gravity to make it nearly round, has not cleared the neighbourhood around its orbit, and is not a satellite (moon). The same 2006 meeting reclassified Pluto from a planet to a dwarf planet, which is why the solar system now has eight planets. The IAU currently recognises five dwarf planets: Ceres, Pluto, Haumea, Makemake and Eris. Ceres is the only dwarf planet in the inner solar system and lies in the asteroid belt. Pluto, Haumea and Makemake lie in the Kuiper Belt beyond Neptune, while Eris lies in the scattered disc and is the most massive known dwarf planet. Pluto is the largest dwarf planet by volume and has a large moon called Charon.
The asteroid belt and asteroids
Asteroids are small rocky and metallic bodies left over from the formation of the solar system. The main asteroid belt lies between the orbits of Mars and Jupiter. Ceres, the largest object in the belt, contains about one-third of the belt's total mass. Other large asteroids include Vesta, Pallas and Juno. Asteroids that come close to Earth are called Near-Earth Asteroids (NEAs), and the Apollo group is an important example. Asteroids that share a planet's orbit are called Trojans, and the largest group is associated with Jupiter. Unlike comets, asteroids are mostly rocky, contain little ice, and usually stay within the inner solar system.
Comets and their parts
A comet is a small icy body that releases gas and dust when it comes near the Sun. A comet is often described as a "dirty snowball" because it is made of ice, dust and rocky material. Its main parts are the nucleus (the solid icy core), the coma (the glowing cloud of gas and dust around the nucleus) and the tail. The tail always points away from the Sun because the solar wind and radiation pressure push the gas and dust outward. Comets are divided into short-period comets (period less than 200 years, coming from the Kuiper Belt) and long-period comets (coming from the distant Oort Cloud). The most famous is Halley's Comet, which returns about every 76 years and was last seen in 1986, so it is expected again around 2061.
Meteoroids, meteors and meteorites
A meteoroid is a small rocky or metallic body travelling in space. When it enters the Earth's atmosphere and burns up, the glowing streak of light is called a meteor or a shooting star. If a part of it survives the journey and reaches the Earth's surface, it is called a meteorite. Most meteors burn up in the mesosphere, about 80 to 100 kilometres above the ground. A meteor shower happens when the Earth passes through the debris left behind by a comet, producing many meteors from the same direction; the Perseids, Leonids and Geminids are well-known showers. The Tunguska event of 1908 in Siberia and the Chelyabinsk meteor of 2013 in Russia are famous examples of large meteor events.
Difference between asteroids, comets and meteors
The three are often confused. Asteroids are rocky bodies, mostly found in the belt between Mars and Jupiter, and they orbit the Sun without developing a tail. Comets are icy bodies from the Kuiper Belt or Oort Cloud that grow a glowing coma and tail near the Sun. Meteoroids are fragments of asteroids or comets; when they burn in the atmosphere they become meteors, and if they land they are meteorites. In short, asteroid means a rocky object in space, meteor means the light seen in the sky, and meteorite means the piece that reaches the ground.
Quick revision
Remember that the IAU recognises five dwarf planets: Ceres, Pluto, Haumea, Makemake and Eris. Ceres is the only one in the inner solar system and the only one in the asteroid belt; Eris is the most massive; Pluto is the largest by volume and has the moon Charon. The asteroid belt lies between Mars and Jupiter, and Ceres is its largest member. A comet has a nucleus, coma and tail, and its tail points away from the Sun; Halley's Comet returns about every 76 years. Meteoroid, meteor and meteorite must be kept in order: the object in space, the streak of light in the sky, and the piece that reaches Earth.
How to prepare this topic
- Memorise the 2006 IAU definition of a dwarf planet with its four conditions, and learn the five recognised names with their positions.
- Fix the locations: asteroid belt between Mars and Jupiter, Kuiper Belt beyond Neptune, Oort Cloud far outside the solar system.
- Draw and label a comet with nucleus, coma and tail, and repeat that the tail points away from the Sun.
- Learn the meteoroid to meteor to meteorite sequence in order and note that most meteors burn up in the mesosphere.
- Keep the famous examples ready: Ceres, Vesta, Halley's Comet, Tunguska 1908 and Chelyabinsk 2013.
Common exam traps
- Ceres is a dwarf planet and also the largest asteroid; it is the only dwarf planet in the inner solar system.
- Eris is the most massive dwarf planet, while Pluto is the largest by volume; do not mix the two records.
- The comet tail points away from the Sun, not behind the direction of motion.
- A meteor is the streak of light in the atmosphere, and a meteorite is the fragment that lands on the ground.
- Halley's Comet is a short-period comet with a period of about 76 years; it is not a one-time visitor.
Frequently Asked Questions
What is a dwarf planet and how many are recognised?
A dwarf planet orbits the Sun, has enough mass for its own gravity to make it nearly round, has not cleared the neighbourhood around its orbit, and is not a moon. The International Astronomical Union recognises five dwarf planets: Ceres, Pluto, Haumea, Makemake and Eris. The 2006 definition reclassified Pluto from a planet to a dwarf planet.
Where is the asteroid belt and which is its largest object?
The main asteroid belt lies between the orbits of Mars and Jupiter. Ceres, which is also classified as a dwarf planet, is the largest object in the belt and contains about one-third of its total mass. Other large asteroids are Vesta, Pallas and Juno.
What are the parts of a comet and where does its tail point?
A comet has a nucleus, a coma and a tail. The nucleus is the solid icy core, the coma is the glowing cloud of gas and dust around it, and the tail is the long trail of gas and dust. The tail always points away from the Sun because of the solar wind and radiation pressure.
What is the difference between a meteoroid, a meteor and a meteorite?
A meteoroid is a small rocky or metallic body travelling in space. When it enters the atmosphere and burns up, the glowing streak is called a meteor or shooting star. If a piece survives and reaches the Earth's surface, it is called a meteorite. Most meteors burn up in the mesosphere.
What is Halley's Comet and where do short and long period comets come from?
Halley's Comet is the most famous short-period comet, returning about every 76 years; it was last seen in 1986 and is expected around 2061. Short-period comets come from the Kuiper Belt beyond Neptune, while long-period comets come from the distant Oort Cloud.