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Understanding the structure of the solar system isn’t inherently difficult. Essentially, the solar system comprises the Sun and eight other planets in order of proximity to the Sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Dwarf planets are…
According to the International Astronomical Union (IAU), the official organization responsible for naming celestial objects and phenomena, an object in the sky is considered a planet only if it meets three specific criteria: 1) it must orbit the Sun, 2) it must be large enough and have sufficient gravity to assume a nearly round shape, and 3) it must “clear the neighborhood” of similar objects from its orbit.
Dwarf planets satisfy the first two criteria but fail the third one: They share their orbital space with similar objects, thus lacking the unique dominance of a planet like Earth or Mars.
Why is Pluto considered a dwarf planet nowadays?
Pluto has always been an enigmatic planet. It is smaller than all other planets in the solar system. Its diameter is merely 2,377 kilometers, and its volume is only 1/500th that of Earth. Pluto orbits the Sun on an unusually inclined axis, forming a highly flattened orbit. Since 1992, astronomers have discovered other objects orbiting the Sun, even farther away than Neptune. This implies that Pluto isn’t the only peculiar planet.
In 2003, when astronomer Michael Brown at the California Institute of Technology (Caltech) discovered Eris, an object nearly the size of Pluto located on the outskirts of the solar system, he remarked that it was apparent that there were not just 9 planets in the solar system, but potentially many more.
In 2006, an IAU committee introduced the term “dwarf planet” and applied it to both Pluto and Eris. The frigid, distant region where Eris and Pluto reside is called the “Kuiper Belt,” and numerous small objects orbit these two planets, referred to simply as “Kuiper Belt objects.”
Do minor planets and small planets resemble dwarf planets?
No, minor planets and small planets are distinct from dwarf planets. The term “minor planet” is outdated and used to refer to any small object in the solar system that isn’t a comet or moon. “Minor planet” is generally used to describe rocky objects that orbit the Sun, either in the same orbit as Earth or in smaller orbits, mainly within the asteroid belt between Mars and Jupiter.
Most minor planets are too small to gravitationally force themselves into a spherical shape, thus not meeting the criteria for being labeled as dwarf planets. An exception is the minor planet Ceres, with a diameter of 946.294 kilometers, the largest known minor planet. Many scientists consider Ceres a “proto-planet” or the only remaining object from the material that formed the original eight major planets around 4.5 billion years ago.
How many dwarf planets are there?
Officially, there are five recognized dwarf planets: Pluto, Eris, Ceres, and two Kuiper Belt objects, Haumea and Makemake. However, there are more objects awaiting classification, and there’s a high likelihood of discovering more beyond our current knowledge.
Astronomers continue to discover objects in the Kuiper Belt and beyond that are potential dwarf planets, but measuring the size of these extremely distant objects to determine if they qualify as dwarf planets is challenging and time-consuming.
Furthermore, IAU seems hesitant to classify new dwarf planets since reclassifying Pluto as a dwarf planet in 2006. While waiting, astronomer Brown has provisionally categorized five candidates and 661 highly probable candidates for dwarf planets.
Additionally, Brown and other astronomers speculate that there might be a much larger object lurking at the edge of the solar system. However, if this hypothetical object, referred to as “Planet Nine,” exists, it wouldn’t be a dwarf planet but rather a much larger planet, potentially five to seven times the size of Earth.
Could Pluto be reinstated as a regular planet?
This remains uncertain. Despite the hopes of enthusiasts and fervent scientists, there’s no indication from the IAU that they’re considering revisiting this issue. Instead, the IAU is busy carefully redefining the term “planet.” In the 2018 IAU meeting, NASA astronomer Eric Mamajek proposed an expanded definition to apply not only to objects in our solar system but also to objects orbiting other stars.
A major challenge in considering Pluto as a regular planet again is dealing with the countless objects similar to planets existing in space. Many scientists find it amusing that if this were defined, there could be hundreds of planets in the solar system.
Nonetheless, this doesn’t diminish Pluto’s significance as a dwarf planet. Pluto is an intriguing, small world, regardless of the terminology used to describe it.