Pluto

We explain everything about the dwarf planet Pluto, its characteristics and its satellites. Also, why is it no longer considered a planet.

What is Pluto?

Pluto is the best known of the trans-Neptunian objects in the solar system, that is, those that are beyond the orbit of Neptune. Reclassified by the International Astronomical Union as a dwarf planet In 2006, Pluto has a very eccentric orbit that takes 248 Earth years to complete one revolution around the Sun, so It is an icy and distant star whose surface there was no further evidence until 2015 when the New Horizons space probe revealed some images of its appearance.

The name Pluto pays tribute to Greco-Roman mythology, specifically to the god of the same name (“Hades” for the Greeks), who was the brother of Zeus and Poseidon, and reigned alongside Proserpina (“Persephone” for the Greeks) in the world. of the dead, the underworld. This name was given in 1930, shortly after its discovery, due to the weakness of its brightness and the spatial darkness that it inhabits, and in part also because it contained the initials of the responsible for its discovery, the American Percival Lowell (1855-1916), founder of the Lowell Observatory in Arizona.

Before this happened, Pluto was referred to as “planet X” by those who were determined to find it. Since the mid-19th century there have been suspicions of its existence, due to the disturbances it causes in the orbit of Uranus, thanks to the calculations of Urbain Le Verrier (1811-1877). But it was not discovered until almost a century later, when the young astronomer Clyde William Tombaugh (1906-1997) was assigned to search for it.

For decades, Pluto was considered the ninth and most distant planet in the solar system, ignoring the oddities of its constitution, size and orbit. However, in 2015, a name was created for similar astronomical objects: “dwarf planets”, and in turn a category within this classification, for objects similar to Pluto: “plutoids”.

See also: Astronomy

Characteristics of Pluto

Among the general characteristics of Pluto the following stand out:

• It has a mass of 1.30 x 10²² kg (0.2% of Earth's mass) a radius less than half that of Mercury and is located, on average, about 39.5 astronomical units (5.9 billion kilometers) from the Sun, so it receives just 1/1600 of the sunlight that the Earth receives.
• Its orbit around the Sun is atypical: much more elongated and eccentric and takes 248 Earth years. Likewise, its rotation movement takes 6.3873 Earth days and occurs in the opposite direction to the rest of the planets (retrograde direction) and at an inclination of 120° on the plane of its orbit.
• Its surface is covered in ice, due to its very low temperatures of around -233 °C This ice is composed mainly of nitrogen, methane and carbon monoxide, and hides a diverse topography underneath, with mountains several kilometers high.
• Little is known about its internal structure, but it is thought that it may have important cryovolcanic activity, that is, geysers of cold gases, as well as a rock core covered in water ice which would occupy 70% of its size.
• Its diameter is 2370 km: so small that it would fit in the territory of the United States and there would be several states left over.
• It has a tremendously weak atmosphere formed by the same gases that make up its ice layer, and whose sublimation and deposition cycle produces notable alterations in the brightness and appearance of this dwarf planet.
• It is orbited by its largest satellite, Charon and by four more satellites: Nix, Hydra, Cerberus and Styx, smaller than the first.
• It is the largest member of the Kuiper Belt a stable region of trans-Neptunian space (between 30 and 50 AU from the Sun), along with other similar stars, such as Eris, Sedna, Makemake or Haumea.

Pluto Satellites

Pluto It has a particular relationship with the largest of its satellites, called Charon, with which it actually forms a binary system. Charon is 1,208 kilometers in diameter and 11.5% the mass of Pluto, making it the largest satellite in the solar system in terms of size compared to its respective planet.

On the other hand, they orbit Pluto four more satellites, much smaller in size:

• Hydra (115 km diameter)
• Nix (93 km diameter)
• Cerberus (29 km diameter)
• Styx (10 km diameter).

These four satellites were discovered in 2005, thanks to the Hubble space telescope. Charon, on the other hand, has been known since 1978.

Why is Pluto not a planet?

The recategorization of Pluto as a dwarf planet, that is, a category different from that of a conventional planet, It is largely due to the discovery in 1992 of numerous celestial bodies orbiting beyond Neptune endowed with physical and orbital characteristics very similar to those of Pluto. This led scientists to wonder if Pluto was not rather part of this heterogeneous community of small space bodies, all located in the Kuiper Belt.

The debate in the scientific community was immediate, despite the fact that there were precedents for this change: Ceres, Pallas, Juno and Vesta, belonging to the asteroid belt after Mars, were once also considered as possible planets and later recategorized as asteroids.

Thus, as more and more trans-Neptunian objects similar to Pluto were discovered (such as Quaoar, 1070 km in diameter, almost half of Pluto; or Sedna, 1000 km in diameter), the scientific discussion intensified, as the idea gained arguments. that Pluto did not belong to the ordinary planets of the solar system.

In 2005, Eris, a trans-Neptunian object 2,326 km in diameter, slightly larger than the size of Pluto, was discovered and the astronomical community was forced to choose between considering the new discovery as a tenth planet, or rather removing Pluto from the planetary list and creating a new category for these small planets, which was “dwarf planets.” . In 2006, the International Astronomical Union issued its verdict: Pluto was no longer a planet and entered the list of “dwarf planets”.

From then on, for a celestial body to be considered a planet, it must meet the following three requirements:

• It must be in orbit around the Sun (Pluto does this).
• It must have enough mass so that its own gravity has given it a rounded shape, that is, its gravity has led it to hydrostatic equilibrium (Pluto does this).
• It must have orbital dominance, that is, with sufficient gravity to clear its orbit of other similar celestial objects, attracting them towards itself until it collides with them and absorbs them, or converts them into satellites that orbit around it. (Pluto does not do this, as it has only 0.07 times the total mass of its neighbors and therefore insufficient gravity to take over its orbit).

Those stars that meet only two of these requirements are considered “dwarf planets,” as are many other trans-Neptunian objects.

Continue with: Jupiter

References

• “Discovery of Pluto” on Wikipedia.
• “Pluto, the planet of discord” in National Geographic.
• “All about Pluto” at NASA.
• “16 years ago Pluto stopped being a planet. Nine years later it became a world” in Forbes Magazine.
• “Pluto (dwarf planet)” in The Encyclopaedia Britannica.