The term "dwarf planet" has gained unheard-of popularity over the past couple of years. As part of a three-way categorization of objects orbiting the Sun, this term was adopted in 2006 due to the discovery of objects beyond the orbit of Neptune, comparable in size to Pluto. Since then, it has been used to describe many objects in the solar system, upsetting the old classification system, which had nine planets.
Also, this term has generated confusion and controversy, in particular, associated with its application to bodies like Pluto. Nevertheless, the International Astronomical Union (IAU) recognizes five bodies within our solar system as dwarf planets, six more will be identified in the coming years and about 200 such bodies may be within the Kuiper belt.
Definition
According to the definition adopted by the IAU in 2006, a dwarf planet is “a celestial body orbiting a star that is massive enough to be rounded off by its own gravity, but does not clear the nearest region of planetesimals, and is not a satellite. In addition, it must have sufficient mass to overcome the compressive strength and achieve hydrostatic equilibrium."
In essence, the term refers to any planetary mass object that is neither a planet nor a natural satellite that meets two basic criteria. First, it must be in direct orbit of the Sun and not be the moon around another body. Secondly, it must be massive enough to take on a spherical shape under the influence of its own gravity. And, unlike a planet, it doesn't have to clean up the surroundings around its orbit.
Size and weight
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For a body to round, it must be massive enough for gravity to become the dominant force affecting the shape of the body. The internal pressure generated by this mass will lead to the surface becoming plastic, smoothing high rises and filling depressions. Small bodies less than a kilometer in diameter do not do this (like asteroids), they are controlled by forces outside their own gravitational forces, which tend to maintain irregular shapes.
The largest known trans-Neptunian objects (TNO)
Meanwhile, bodies several kilometers across - when gravity is significant, but not dominant - take the shape of a spheroid or "potato". The larger the body, the higher its internal pressure until it becomes sufficient to overcome the internal compressive force and achieve hydrostatic equilibrium. At this point, the body becomes as round as it can possibly be, given its rotation and tidal effects. This is the definition of the limit of a dwarf planet.
However, rotation can also affect the shape of the dwarf planet. If the body doesn't rotate, it will be a sphere. The faster it spins, the more elongated or versatile it will become. An extreme example of this is Haumea, which is almost twice as long on the main axis as at the poles. The tidal forces also cause the rotation of the body to gradually become tidally blocked, and the body remains on one side of the companion. An extreme example of such a system is Pluto - Charon, both bodies are tidally locked between themselves.
The IAU does not determine the upper and lower limits for the size and mass of dwarf planets. Although the lower limit is determined by the achievement of an equilibrium hydrostatic shape, the size or mass at which this object reaches that shape depends on its composition and thermal history.
For example, bodies made of hard silicates (like rocky asteroids) must reach hydrostatic equilibrium with a diameter of about 600 kilometers and a mass of 3.4 x 10 ^ 20 kg. For a less rigid body made of water ice, this limit will be closer to 320 km and 10 ^ 19 kg. As a result, there is currently no specific standard for defining a dwarf planet based on its size or mass, but instead it is usually defined based on its shape.
Orbital position
In addition to hydrostatic equilibrium, many astronomers have insisted on drawing a line between planets and dwarf planets on the basis of their inability to "clear the vicinity of their orbit." In short, planets can remove smaller bodies near their orbits by collision, capture, or gravitational disturbance, whereas dwarf planets do not have the necessary mass to achieve this.
To calculate the likelihood that a planet will clear its orbit, planetary scientists Alan Stern and Harold Levinson presented a parameter that they denote by the letter "lambda".
This parameter expresses the probability of a collision depending on the given deviation of the object's orbit. The value of this parameter in the Stern model is proportional to the square of the mass and inversely proportional to the time and can be used to estimate the body's potential to clear the vicinity of its orbit.
Astronomers like Stephen Soter, a New York University scientist and a fellow at the American Museum of Natural History, suggest using this parameter to draw a line between planets and dwarf planets. Soter also proposed a parameter he calls the planetary discriminant - denoted by the letter mu - that is calculated by dividing the mass of a body by the total mass of other objects in the same orbit.
Recognized and possible dwarf planets
There are currently five dwarf planets: Pluto, Eris, Makemake, Haumea, and Ceres. Ceres and Pluto alone have been observed enough to be indisputably in this category. The IAU has ruled that unnamed trans-Neptunian objects (TNOs) with an absolute magnitude brighter than +1 (and mathematically limited to a minimum diameter of 838 km) should be classified as dwarf planets.
Potential candidates currently under consideration include Orc, 2002 MS4, Salazia, Kwavar, 2007 OR10 and Sedna. All of these objects are located in the Kuiper belt; with the exception of Sedna, which is considered separately - a separate class of dynamic TNOs in the outer solar system.
It is possible that there are 40 more objects in the solar system that can rightly be designated dwarf planets. It is estimated that up to 200 dwarf planets can be found in the Kuiper belt after studying it, and beyond this belt, their number could exceed 10,000.
Disagreements
Immediately after the decision of the IAU regarding the definition of the planet, a number of scientists expressed their disagreement. Mike Brown (leader of the Caltech group that discovered Eris) agrees to reduce the number of planets to eight. However, a number of astronomers like Alan Stern have criticized the definition of the IAU.
Stern argues that, like Pluto, Earth, Mars, Jupiter, and Neptune also do not completely clear their orbital zones. The Earth revolves around the Sun with 10,000 near-Earth asteroids, which Stern estimates are at odds with clearing the Earth's orbit. Jupiter, meanwhile, is accompanied by 100,000 Trojan asteroids on its orbital path.
In 2011, Stern referred to Pluto as a planet and considered other dwarf planets like Ceres and Eris, as well as large moons, to be complementary planets. However, other astronomers argue that although the large planets do not clear their orbits, they have complete control over the orbits of other bodies within their orbital zone.
Another controversial application of the new definition of planets concerns planets outside the solar system. Methods for detecting extrasolar objects do not directly determine whether an object clears the orbit, only indirectly. As a result, in 2001, the IAU approved separate “working” definitions for extrasolar planets, including the dubious criterion: “The minimum mass / size required for an extrasolar object to be considered a planet must match the parameters accepted for the solar system.”
Although not all members of the IAU were in favor of adopting this definition of planets and dwarf planets, NASA recently announced that it will use new guidelines set by the IAU. Nevertheless, the debate over the 2006 decision has not yet stopped, and we can well expect further developments on this front when more "dwarf planets" are discovered and identified.
It is fairly easy to define a dwarf planet by IAU standards, but fitting the solar system into a three-tier classification system will become more difficult as our understanding of the universe expands.
Ilya Khel
