SOLAR SYSTEM FAMILY-FACTS ABOUT THE FOUR DWARF PLANETS OF THE ASTEROID BELT
The asteroid belt consists of all types of irregular rocks from tiny bits and pieces of loose debris to considerably larger chunks. Among them, the largest asteroids viz., Ceres, Pallas, Hygiea and Vesta deserve special mention. These asteroids have also been designated as dwarf or minor planets that account for nearly 40% of the belt's mass of which Ceres alone occupies nearly 25%. This article throws more light upon their individual characteristics and certain interesting features. However, the story behind their discovery has already been mentioned in the following article-Facts About The Asteroid Belt.
The Largest Object, Ceres: Since the day of its discovery in 1801, up to recent times there has been a widespread disagreement among the scientific community about its classification as an asteroid or a planet. As per the earlier definitions provided by the International Astronomical Union(IAU) a planet must be a celestial body with sufficient mass for its self-gravity to overcome rigid body forces to assume a spherical shape, and it has to orbit a star. But the modern definition provided by the IAU adds another criteria that a planet must have cleared the neighbourhood around its orbit. Based upon earlier definitions, Ceres was designated as a planet. But quite recently it has been reclassified as a large asteroid or a dwarf planet because of the fact that it shares its orbit along with countless other smaller objects.
Ceres is located within the asteroid belt, at a distance of 2.7 AU from the Sun(1 AU or Astronomical Unit is the distance between the Sun and Earth i.e., 150 million km) and completes its orbit in about 4.6 Earth years. This celestial body, with a mean diameter of 950 km and a mass of 9.38✕10²⁰ kg is massive enough for gravity to give it a nearly spherical shape, and is the largest object within the main asteroid belt. Interestingly, Ceres is a protoplanet that escaped further development because of Jupiter's strong gravitational influence. Thus it got settled as a dwarf planet and is a perfect relic of the processes that took place in the early days of our solar system. Different theoretical models based upon observational data suggest that Ceres might hold more water than Earth in the form of water ice, subsurface oceans and pools of brine. This dwarf planet is also thought to possess a differentiated interior and is quite similar to the inner planets i.e., Mercury to Mars. Ceres has got a heavily cratered surface, but those have smoothened out because of past geological processes.
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| Ceres/Image Credits: Justin Cowart, CC BY 2.0 , via Wikimedia Commons |
The Second Largest Object, Vesta: Vesta, discovered on March 29, 1807 by Heinrich Olbers, and named after the Roman goddess of family or hearth, is the second largest object of the asteroid belt. This celestial body has been classified as a dwarf planet because of its oblate spheroid shape and is supposedly a surviving protoplanet just like Ceres. Vesta is also the brightest asteroid in the night sky and might be visible to the naked eye under extremely clear skies. It is good to note that this dwarf planet has hurled a distinct group of meteorites towards Earth, called the HEDs or the Howardite-Eucrite-Diogenite Group. And an analysis of those rocks have revealed much about Vesta's history of formation.
Vesta is located at a distance of 2.5AU from the Sun, and completes its orbit once in every 3.6 Earth years. It has a mean diameter of 530 km, a rotational period of 5.34 hours and has a surface temperature ranging from 85-225K(or -188℃ to -48℃). Surface features include two prominent impact craters viz., the Rheasilvia(width of 500 km) and Veneneia(width of 400 km) along with some other minor craters, troughs and a thin layer of regolith.
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| Vesta/Image Credits: NASA / JPL / MPS / DLR / IDA / Björn Jónsson, Public domain, via Wikimedia Commons |
The Third Largest Object, Pallas: Discovered by the astronomer Heinrich Olbers on March 28, 1802, Pallas is the third largest object of the asteroid belt. It is located at a distance of 2.7 AU from the Sun and has an orbital period of 4.6 Earth years. Pallas has a mean diameter of 512 km and like others might be a surviving protoplanet. This asteroid has not yet been explored by a space-craft and observations made with the help of ground based telescopes reveals a heavily cratered surface.
The Fourth Largest Object, Hygiea: Hygiea, discovered on April 12, 1849, by the astronomer Annibale de Gasparis, might be the fourth largest object within the asteroid belt. Having a mean diameter of 434 km and a comparatively spherical shape, this asteroid is waiting for the official designation of the smallest dwarf planet in our solar system. At present, the smallest dwarf planet is Ceres. Hygiea is located at a distance of 3.14 AU from the Sun and has an orbital period of 5.5 Earth years.
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| The four large asteroids/Image Credits: Wikimedia Commons |
Explorations: There has been only one mission to the asteroid belt, out of the dozen missions proposed by various countries since 1990. The ''Dawn'' mission developed by the American Space Agency(NASA) and launched on September 27, 2007 became the first mission dedicated to the study of Ceres and Vesta. The primary objective of the Dawn spacecraft was to gather data to answer questions about the evolution of our solar system and the formation of the eight major planets. The Dawn spacecraft while visiting Ceres and Vesta became the first manmade object to explore a dwarf planet, and also the first to orbit an object of the main asteroid belt. Dawn gathered immense data on their geology, composition, volcanic features, presence of atmosphere, magnetic field, gravity field to help scientists answer some of the profound questions regarding the formation of our solar system and its subsequent evolution. The mission was however concluded on 2018 when the craft used up its fuel and was placed in a stable orbit around Ceres.
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