Countless objects populate the solar system , completing orbits around the Sun in just a few days or thousands of years. Among them, the 8 major planets stand out, whose names and aspects you know very well. But there are also other equally interesting bodies that we pay less attention to. These are the dwarf planets . Not everyone in the scientific community adopts the same definition for these bodies, so the total number of dwarf planets is between 5 and about 120 , depending on who you ask, although it is true that the International Astronomical Union (IAU) and Most of those who are dedicated to research in astrophysics and astronomy leave the number at 5 bodies that are surely dwarf planets, with the possibility of adding up to 4 more planets in the coming years as we learn more about them.
But to begin with, what is a dwarf planet and what differentiates it from a regular planet? The definition of planet and dwarf planet given by the IAU in 2006 consists of three points for each category, two of which are shared. These are the condition that any object in the solar system, to be considered a planet (dwarf or not) must directly orbit the Sun and must have sufficient mass to have acquired hydrostatic equilibrium and, thus, a sufficiently spherical shape . This excludes all the large moons of the solar system, such as our own or the even larger Ganymede or Titan, which are larger than any dwarf planet and even larger than Mercury, but because they orbit around Jupiter and Saturn respectively they cannot be considered as planets in themselves.
The third point, where the official definitions of planet and dwarf planet differ, take opposite forms. A planet will be considered as such if it meets the two previous conditions and has also managed to clear its orbit of other smaller bodies that share it with it . That is, the mass corresponding to the planet will be the majority of the percentage of the total mass of all the objects with which it shares an orbit . Jupiter, for example, shares an orbit with, we believe, a million Trojan asteroids, but these do not account for more than a tiny percentage of the mass contained in that orbit. The dwarf planets , however, will be those bodies that meet the first two conditions but have not been able to clean their orbits sufficiently . In the case of Pluto , for example, the dwarf planet does not accumulate even 1% of the total mass of the objects with which it shares an orbit. Well, now that we know this, what are the dwarf planets in the solar system?
Pluto is the most famous of all the dwarf planets in the solar system, having been considered a planet until just 16 years ago. This is the largest dwarf planet in size, although not the most massive , as Eris has a slightly higher mass. Pluto has 5 moons , four of them are rocks less than 50 kilometers in size, while the fifth is Charon , which with a diameter of more than 1,200 kilometers could be considered a dwarf planet on its own if it did not orbit with Pluto. Charon is so large that it is not correct to say that it orbits around Pluto , since both actually orbit around the barycenter , located several thousand kilometers above Pluto’s surface. This is why Pluto is sometimes considered a double system .
This was the first dwarf planet discovered, in 1801 . In fact, we know of its existence even before Neptune. This is because, despite being considerably smaller than Pluto and even Charon , it lies much closer to Earth , in the asteroid belt between the orbits of Mars and Jupiter. Ceres is the only dwarf planet in the inner solar system. After its discovery and that of other asteroids several hundred kilometers in size located close to it, it was considered a planet, although it soon lost that consideration , until in 2006 it was reclassified as a dwarf planet. The existence of water volcanoes is known about it and it has been visited by NASA’s Dawn probe.
This is the most massive dwarf planet we know of and was responsible for questioning whether Pluto belonged to the category of a planet when it was discovered in 2005. Around it, a satellite has been observed that, although it is not as large as Charon itself would have a considerable size. This dwarf planet has a highly eccentric orbit that takes it within Pluto’s orbit at its closest point to the Sun and up to nearly 100 times the distance from Earth to the Sun at its furthest point. Eris is the largest object in the solar system that has not yet been visited by any probe . This will likely continue for a while as it is currently near the farthest point of its orbit, making missions there particularly difficult.
Haumea is the third largest trans-Neptunian object, with a mass one-third that of Pluto and a diameter at its longest part of more than 2,000 kilometers. Haumea is unique in its elongated appearance, likely due to its rapid rotation . This planet has two confirmed satellites around it, although what is truly remarkable about it is that it has a ring system around it. This ring, which would orbit about a thousand kilometers above the planet’s surface, was discovered in 2017, being the first ring system discovered for an object orbiting beyond Neptune. Haumea orbits somewhat further out than Pluto, taking about 283 years to complete each orbit.
Makemake is the smallest of the four confirmed dwarf planet bodies orbiting beyond Neptune, with a spherical shape and a diameter of just over 1,400 kilometers, although it is larger than Ceres. This planet has an orbit close to that of Haumea, but somewhat larger, taking just over 300 years to complete one revolution around the Sun. It is known to be a satellite and, given its density and low surface temperature, it is believed that it should be covered with a thick layer of methane, ethane and nitrogen in the form of ice .
Some new members could be added to this short list in the future, depending on what future more detailed investigations show. These members are Gonggong, Quaoar, Sedna, Orcus, and Salacia . All of them are smaller than Makemake and, except for Sedna, have at least one satellite orbiting around them. Sedna is a peculiar object because its orbit takes it every more than 11,000 years to move away from the Sun 900 times more than our planet . It is currently inside its orbit, which still places it twice as far from the Sun as Pluto.
Referencias:
NASA Science, Solar System Exploration, https://solarsystem.nasa.gov/planets/overview/
L. L. Christensen, 2006, The IAU draft definition of “planet” and “plutons”. International Astronomical Union
