Pluto and the other dwarf planets
The majority of you were likely taught in your high school science class that Pluto was the ninth and last planet out from the sun. That was true, until 2006, when the International Astronomical Union (IAU) decided to change the definition of what constituted a planet.
According to the new definition, a planet is a body that orbits the sun without being a moon of another object; is large enough that its own gravity has rounded it into a sphere (but not so large that it undergoes fusion reactions, like a star); and has cleared its neighbourhood of most other bodies.
Pluto meets the first two criteria but fails the third and, as such, was demoted to dwarf planet. This, according to their calculations, is due primarily to the fact that Pluto is located on the edge of the Kuiper Belt (a vast belt of trillions of small, icy objects beyond Neptune), and has not cleared this neighbourhood of most of these objects. I won’t delve here into all the pros and cons of the IAU’s decision (it would take pages and pages of text), but shall, instead, just mention that the IAU’s demotion of Pluto is still an active subject of discussion in astronomical circles; Pluto may well be reinstated to planethood in the near future.
In the meantime, a few facts about Pluto. Much as Neptune was discovered as a result of the investigation of deviations in the orbit of Uranus, so, too, was the discovery of Pluto based on calculations rather than direct observation. In 1905, American astronomer, Percival Lowell, noticed deviations in the orbit of Neptune, suggesting the presence of another, as-of-yet undiscovered body beyond the eighth planet. Though he predicted this body's probable location in 1915, he, unfortunately, died before finding it. It was, in fact, discovered (using Lowell's calculations) by American astronomer Clyde Tombaugh in 1930. It was named Pluto (as suggested by 11-year old British schoolgirl, Venetia Burney) after the Roman god of the underworld. Incidentally, the first two letters, “P” and “l”, honour Percival Lowell.
Pluto orbits at approximately 5.1 billion kilometres (39.9 AUs) from the sun; has an equatorial diameter of 2,302 kilometres; has an orbital period of about 248 Earth years; and has five moons. Not much was known about Pluto until NASA’s New Horizon mission flew by the planet in July 2015. Pluto has a highly eccentric orbit, which brings it inside (for about 20 years) the orbit of Neptune every 230 years.
Pluto’s largest moon, Charon (the boatman who ferried the dead souls across the Styx River into the underworld in Greek mythology), was discovered in 1978. It is about half the size of Pluto; the two are often referred to as a “binary (double) dwarf planet system.”
The other Plutonian moons were discovered more recently — Nix (originally “Nyx” — Greek goddess of night, and mother to Charon, but later changed to “Nix” — Egyptian goddess of night and darkness) and Hydra (a nine-headed serpent said to guard the entrance to the underworld) were discovered in 2005 from a Hubble Space Telescope (HST) photo of Pluto; Kerberos (the three-headed dog of Greek mythology which guarded the entrance to the underworld ) discovered in 2011 from a HST photo; and Styx (a Greek deity, and the river separating the living world from the underworld) was discovered in 2012.
There are currently four other dwarf planets classified as such, though there are a number of other objects being considered for designation. Ceres (for the Roman goddess of the harvest), the largest object in the main asteroid belt between Mars and Jupiter, was discovered in 1801 by Italian astronomer Giuseppe Piazzi; it has no known moons. Haumea (for the Hawaiian goddess of childbirth) was discovered in 2004; it has two moons. Eris (after the Greek goddess of strife and discord) was found in 2005; it has one moon. Makemake (after the Polynesian creator of humanity), found in 2015, has one moon (Dysnomia, the second largest moon of a dwarf planet after Pluto's moon, Charon). It was the discovery of Eris, which is only slightly larger then Pluto, and often called Pluto's twin, that resulted in Pluto's demotion from planet to dwarf planet.
Next week — the Kuiper Belt, the asteroid belt and the Oort Cloud
Until next time, clear skies.