How Many Dwarf Planets Are in the Solar System?
The Solar System holds many minor worlds that could be considered planets
Until recent times, most people thought there were nine planets in the Solar System, but as of 2006, Pluto, the smallest of the planets and farthest from the Sun, was bumped from the group, becoming a so-called dwarf planet. But many people, experts and others, think Pluto should still be a planet, and perhaps it always will be one in the minds of many.
At any rate, let’s launch an exploratory mission to see what brought about this change in the number of planets and see if there’s still hope for Pluto - and perhaps other worlds - to be included in the list of planets.
Please keep reading!
History of Dwarf Planets
In antiquity, there were five planets, but eventually the Earth, Uranus and Neptune were added to this illustrious assemblage. Many smaller worlds such as Ceres, nearly 600 miles in diameter, were eventually found, and these were considered planets for awhile, but this designation didn’t last long, mainly because astronomers didn’t consider them large enough; instead, they were considered asteroids.
The number of planets increased to nine when Pluto was discovered in 1930. The other objects in the Solar System were satellites (commonly known as moons), and there were also thousands of asteroids and comets.
But this changed in 2006, when the International Astronomical Union (IAU), decided that Pluto no longer fits the criteria of a planet. So, there are eight planets once again, and Pluto is now a lesser world – a dwarf or minor planet. At present, the IAU has identified five dwarf planets.
According to the IAU, a celestial object can only be a planet if it qualifies on three essential points:
1. The object must have the shape of a planet, that is, mostly spherical. This happens when a body has enough mass to pull everything – dirt, water, ice and dust - into a round shape, a state known as hydrostatic equilibrium. This distinction pertains to the size and mass of the object, of course, but the IAU hasn’t specified an upper or lower limit to the size of a dwarf planet.
2. The object must be in direct orbit around the Sun, not in orbit around another celestial body, such as the Moon orbits the Earth, or the satellite Titan orbits the planet Saturn.
3. The object must have cleared virtually all debris – asteroids or comets, in particular – from its orbital path around the Sun. Therefore, the object would be in a state of orbital dominance.
The IAU says Pluto fails point number three, because its mass is only 0.07 times the mass of other objects in its orbit and therefore it does not have orbital dominance. In comparison, the Earth’s mass is a million times the mass of other objects in its orbit.
Five Dwarf Planets
It took a long time to find Pluto, what used to be the ninth planet. Since the discovery of Neptune in 1846, astronomers speculated that a still-undiscovered planet lay beyond the orbit of Neptune. Some scientists referred to this hypothetical body as Planet X. Finally in 1930, astronomer Clyde Tombaugh discovered Pluto by comparing two sets of photos taken over a period of months. (Pluto had actually been photographed many times before by other people, but not identified as another planet.)
At first, scientists thought Pluto was as large or larger than the planet Mercury, or about 3,000 miles in diameter, but Pluto is considerably smaller, about 2,368 kilometers in diameter or 1,471 miles. Pluto also has a tenuous atmosphere, mostly methane and nitrogen, which is only gaseous during the planet’s summer; otherwise, it’s frozen solid.
Back in 1930, Pluto had no known satellite, but now it appears to have at least five, including, Charon, a sizable rock at 750 miles in diameter, about half the size of Pluto. Interestingly, Pluto and Charon are only about 12,000 miles apart and comprise a binary dwarf planet system. Pluto’s other four satellites – Nix, Hydra, Kerberos and Styx - are much smaller than Charon.
These days, Pluto is considered one of many thousands of objects in the Kuiper belt, a realm of asteroids and comets beyond the orbit of the planet Neptune, as opposed to the asteroid belt, which is much smaller and closer to the Sun.
Also of interest, Pluto’s highly elliptical orbit sometimes carries it closer to the Sun than Neptune, a situation which has prompted many scientists to call it nothing more than a captured asteroid, rather than a planet, all of which have mostly circular orbits.
New Horizons Discoveries at Pluto
According to press releases by NASA, available on the website, www.NASA.gov, the New Horizons spacecraft has made numerous discoveries in the Pluto-Charon dwarf planet system. The following is a list of these mind-boggling astronomical revelations:
1. Close-up images of the equatorial region of Pluto reveal mountains as high as 11,000 feet. Formed only about 100 million years ago, the mountains show that Pluto is geologically active. Generally, small planets or moons can be heated by gravitational interactions with large planets such as Jupiter and Saturn, but Pluto has no such planet nearby that could help it build mountain ranges. So, how these mountains were formed remains a mystery.
2. Pluto’s largest satellite, Charon - about half the size of Pluto at 750 miles in diameter - has an apparent lack of craters, because this is no dead world like the Moon or the planet Mercury. Marked by cliffs and troughs, Charon, like Pluto, is geologically active. In fact, Charon has a canyon four to six miles deep, dwarfing any comparable rift on Earth.
3. Pluto, like many other moons in the Solar System, has ice volcanoes, some of which miles in elevation, and at least two of which show deep calderas at the summits.
4. Although Pluto has geologically active areas, one of which informally known as Sputnik Planum, it also has many craters, some of which billions of years old. It appears Pluto is a mixed bag of both young and old landforms. Counting such craters, more than a thousand of them, gives scientists a better estimate of the size of the Kuiper Belt, from which Pluto formed some four billion years ago.
5. Pluto's four other major satellites – Nix, Hydra, Kerberos and Styx – are not tidally locked to Pluto, that is, keeping one face toward the planet, as Earth’s moon does. Instead, Pluto’s tiny moons spin very quickly, and all at different rates. This may happen because Charon’s sizable gravitational field keeps the moons from slowing down to a synchronous orbit around Pluto.
Ceres was discovered by Giuseppe Piazzi in 1801. At 590 miles in diameter, Ceres is the largest asteroid in the asteroid belt and contains roughly a third of the entire mass of the asteroid belt. It has a very thin atmosphere and is covered with ice. Also, similar to Pluto, Ceres may have a large ocean of water under a layer of ice and rock. In fact, Ceres has so much ice, it may have once been an enormous comet.
Discovered in 2004, Haumea is about one third the mass of Pluto and has an ellipsoidal shape caused by its rapid rotation. This odd shape is unique among dwarf planets. Haumea, named for the Hawaiian goddess of childbirth, has two tiny satellites. These bodies comprise a so-called collisional family, which was formed by a collision with another object billions of years ago.
Makemake was discovered in 2005 and is roughly two-thirds the size of Pluto. Since Makemake has no known satellites, its size can only be estimated at about 1,400 kilometers in diameter. Also, Makemake is the second brightest object in the Kuiper belt after Pluto.
Believe it or not, Eris may be the largest dwarf planet, perhaps some 27 per cent more massive than Pluto. But the size of each dwarf planet is not known with certainly, so only time will tell which one is the largest. Anyway, Eris has one known satellite, Dysnomia. Interestingly, Eris is three times farther from the Sun than Pluto. This makes Eris and Dysnomia the most distant natural objects in the Solar System, excluding some comets.
Other Possible Dwarf Planets
There could be many more dwarf planets in the Solar System – perhaps hundreds or even thousands of them! At least six more are currently under consideration by the IAU. All discovered in the early 2000s, the names of these celestial bodies are: Orcus, 2002 MS4, Salacia, Quaoar, 2007 OR10 and Sedna.
Discovered in October 2016 and known as 2014 UZ224, this dwarf planet candidate is 330 miles in diameter, making it smaller than Pluto’s moon, Charon. It is approximately 8.5 billion miles from the Sun and takes 1,100 years to complete one orbit around the Sun.
Relative sizes of dwarf planets
Of course, the rules regarding inclusion in the group of dwarf planets may change, so its ranks could swell dramatically in the coming years and decades, increasing the number well beyond what it is today. Some scientists think giant satellites such as Ganymede, Titan and Triton should be considered dwarf planets, and perhaps they will be one day. Just ask the IAU and see what its members say about it.
Then again, Pluto may one day regain its “planethood” and become a planet once more. Regarding this issue and dwarf planets in general, in his book, The Pluto Files: The Rise and Fall of America’s Favorite Planet, published in 2009, astrophysicist Neil DeGrasse Tyson wrote:
“At a July 2008 meeting in Oslo, Norway, the Executive Committee of the IAU approved a proposal from the IAU Committee on Small Body Nomenclature for the name plutoid to classify all dwarf planets that one might find orbiting beyond Neptune. As of this printing, two dwarf planets orbit there – Pluto and Eris – with many more surely to come. This obviously makes Pluto a plutoid.”
But Tyson doesn’t like the word plutoid, because it sounds like hemorrhoid. Nevertheless, he thinks Pluto should be considered what it is – a kind of planet.
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© 2014 Kelley
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