The edge of the solar system

Pluto – the outermost planet (or not)

Estimated reading time: 5 minutes

Pluto’s fascinating history began with its discovery by American astronomer Clyde Tombaugh in 1930. Tombaugh was conducting research at the Lowell Observatory in Arizona when he identified a small object beyond the orbit of Neptune that seemed to be moving against the background of stars. Through further observations, he determined that this object was a previously undiscovered planet, which he called Pluto, like the Roman god of the underworld.

The first hints

In the early 1900s, astronomers noticed that the outer planets Uranus and Neptune were not following their predicted orbits, suggesting that the gravitational pull of an unknown planet beyond Neptune was affecting their movement. In 1906, Percival Lowell, a wealthy American businessman and amateur astronomer, began searching for this hypothetical “Planet X” at his observatory in Flagstaff, Arizona. Lowell died in 1916 without finding Planet X, but his search continued after his death.

Despite its initial classification as a planet, Pluto’s status as a planetary body has been the subject of much debate over the years. In 2006, the International Astronomical Union (IAU) redefined the criteria for a planet, leading to Pluto’s reclassification as a dwarf planet. This decision was controversial, as many people objected to demoting Pluto from planetary status.

The process that led to Pluto’s reclassification as a dwarf planet was long and complex. Before the IAU’s decision, there had been a growing recognition among astronomers that Pluto did not fit neatly into the traditional definition of a planet. Its small size and irregular orbit made it, unlike the other eight planets in the solar system. Additionally, the discovery of other small bodies in the outer solar system that was similar in size to Pluto further complicated the issue of what should be considered a planet.

In the end, the IAU defined a planet as a celestial object that orbits the sun, is spherical, and has cleared its orbit of other debris. By this definition, Pluto did not qualify as a planet, as its orbit intersects with Neptune’s, and it shares its region of the solar system with other similar-sized objects. As a result, it was reclassified as a dwarf planet, along with several other objects in the outer solar system.

Whether or not it is fair that Pluto is no longer considered a planet is a matter of debate. Some people feel that Pluto’s reclassification was arbitrary and deserves to be recognized as a planet due to its historical significance and unique properties. Others argue that the new definition of a planet is more scientifically rigorous and that Pluto’s classification as a dwarf planet is based on sound scientific principles.

Dwarf or not

Despite its controversial status, Pluto remains a fascinating object of study for astronomers. We now know that Pluto has five known moons, including Charon, Nix, Hydra, Kerberos, and Styx. Charon is fascinating, as it is nearly half the size of Pluto and has a unique geology that suggests it may have had a violent history. Pluto is a small, icy world covered in nitrogen ice with a thin atmosphere of methane and nitrogen gas.

In recent years, NASA’s New Horizons spacecraft has provided us with an unprecedented look at Pluto and its moons, sending back detailed images and measurements that have allowed us to learn more about this distant world. The data collected by New Horizons has helped us better understand the formation and evolution of the outer solar system and the potential for finding other objects like Pluto in our galaxy.

Pluto-like dwarf planets that orbit our sun are called “Plutoids.” This term was coined by the IAU in 2008, shortly after Pluto was reclassified as a dwarf planet. Plutoids are celestial bodies that share specific characteristics with Pluto, such as being small, round, orbiting the sun beyond Neptune. Currently, there are only two known Plutoids – Pluto and Eris.

Farther from the sun, the Oort cloud is a hypothetical space region far beyond Neptune’s orbit and the Kuiper Belt. It is thought to be a spherical cloud of frozen objects that orbit the sun at distances ranging from 2,000 to 200,000 astronomical units (AU). The Oort cloud is named after the Dutch astronomer Jan Oort, who proposed its existence in 1950.

The objects in the Oort cloud are acknowledged as remnants from the early solar system, and they may include both comets and small icy bodies. Due to their great distance from the sun, the objects in the Oort cloud are difficult to observe directly, and their existence is inferred mainly from the study of long-period comets, which are believed to originate from the Oort cloud. The Oort cloud is thought to harbor trillions of objects and is believed to be the source of most of the comets that enter the inner solar system.

A region of space closer to the sun than the Oort cloud is the Kuiper Belt is a. It is thought to be a vast, flat disc-shaped region made up of icy objects, such as dwarf planets, comets, and other small bodies. It is named after Gerard Kuiper, the Dutch-American astronomer who first predicted its existence in 1951. The Kuiper Belt is believed to contain many objects that have remained relatively unchanged since the solar system’s formation, making it a valuable source of information about the early history of our planetary system. It is also the source of many short-period comets that orbit our sun in less than 200 years.

Both these regions are beyond the heliosphere and, thus, considered part of interstellar space. In other words: outside the solar system.

Pluto matters!

Since 2006, the IAU has enforced a stricter definition of a planet, which includes three criteria: the entity must be in orbit around the sun, be large enough to have a spherical shape, and have cleared its orbit of other debris. Based on this definition, there are eight planets in the solar system: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. However, many other objects in the solar system could also be considered planets by some definitions, such as Pluto and other dwarf planets in the Kuiper Belt. Ultimately, the number of planets in the solar system depends on how one defines a planet.

Overall, while Pluto’s status as a planet may be controversial, there is no denying that it remains a fascinating and essential object of study for astronomers and scientists alike. As we continue to explore the outer reaches of our solar system and beyond, there is much more to discover about this intriguing world and its many mysteries.

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"Pluto – the outermost planet (or not)" in Space Expert, 2024
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