The planets the the solar system as illustrated by a NASA computer illustration. Orbits and sizes space not displayed to scale.(Image credit: NASA)
While many human being can suggest to a picture of Jupiter or Saturn and call the a "planet," the an interpretation of this word is much much more subtle and has adjusted over time. Plenty of astronomers chose on a new an interpretation in 2006 after the discovery of several people at the fringes of the solar mechanism — a decision that stays controversial.

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The International huge Union characterized a earth as things that:

orbits the sunhas adequate mass to be round, or almost roundis no a satellite (moon) of another objecthas gotten rid of debris and little objects from the area approximately its orbit

The IAU likewise created a more recent classification, "dwarf planet," i beg your pardon is things that meets planetary criteria other than that it has actually not cleared debris from its orbital neighborhood. This definition meant the Pluto — thought about a world at the time — to be demoted and also reclassified as a dwarf planet.

But not all scientists agree v this classification, an especially after the new Horizons 2175forals.comcraft flew by Pluto in 2015. The 2175forals.comcraft revealed a complex world full of geological change. This consisted of mountains getting to as high as 11,000 feet (3,500 meters), a heart-shaped region nicknamed Tombaugh Regio that has methane ice and other substance, and also weird ice-ridged terrain that looks favor snakeskin, amongst many various other features because then, members that the brand-new Horizons team have made clinical presentations suggesting Pluto is without doubt a planet. The new scientific findings proceed to amaze the public, Alan Stern claimed in 2018.

"I think mine two greatest surprises were first, just exactly how utterly exceptional Pluto turned out to it is in — how numerous different type of functions were ~ above the surface and even in the atmosphere," Stern said in a NASA podcast interview. "There was something for everyone. And the 2nd amazing finding was how countless members of the general public really wanted to get involved in it and also just it is in a component of this exploration. We meant it would be a big response, however it was much bigger than we thought."

Planetary history

The term "planet" originally comes from the Greek word for "wanderer." Many old cultures observed these "moving stars," yet it wasn"t until the development of the telescope in the 1600s that astronomers to be able come look in ~ them in much more detail. Small telescopes revealed moons circling Jupiter — a big surprise to Galileo Galilei (the most likely discoverer) and his adversaries at the Catholic Church — and rings roughly Saturn and also an ice cap on Mars.

Telescopes likewise revealed the existence of objects not recognized to the ancients, since they room too much away and tiny to be spotted with the naked eye. Uranus was uncovered on march 13, 1781, by the prolific astronomer william Herschel. Ceres to be discovered between Mars and also Jupiter in 1801. That was originally classified as a planet, yet it was later on realized the Ceres to be the an initial of a course of objects eventually dubbed asteroids. Neptune was discovered in 1846.

Astronomers continued scouring the solar system"s external reaches searching for a big "Planet X" that was believed to be disturbing the orbits that Uranus and also Neptune. While these irregularities were later on discounted by additional observations, Clyde Tombaugh did spot a smaller object in 1930 past the orbit that Neptune. Called Pluto, the object (then called a planet) was reasonably small and also had a extremely eccentric orbit that sometimes even lugged it closer to the sun than Neptune is.

Discovery of an ext worlds

Nothing close come Pluto"s size was discovered in the solar device for more than two generations. That readjusted in the 2000s, as soon as Mike Brown — a young astronomer in ~ the California academy of technology — was looking for a defining research project and also decided top top searches because that objects in the outer solar system.

In rapid succession, Brown and his team discovered several large "trans-Neptunian objects," or icy bodies beyond Neptune"s orbit. While discovering icy objects that far away was not unanticipated — the an alleged Oort Cloud, the birthplace of comets, should have actually trillions of these things — it was the size that made various other astronomers pay attention.

Some that Brown"s remarkable discoveries had Quaoar; Sedna; Haumea; Erisand the moon, Dysnomia; and also Makemake. Every were uncovered in a fairly short period of time, in between 2001 and 2005. Eris (which was initially nicknamed "Xena" ~ a popular television show of the time) was large enough that part in the media were calling that the 10th planet.

Vote and also implications

With this collection of explorations confronting them, the International expensive Union spent two years evaluating the evidence and called a basic meeting in 2006 to poll on what the meaning of what a planet would be. The delegates present voted ~ above a meaning that excluded Pluto, Eris and also any various other objects the were close come the exact same size.

The newer designation "dwarf planet" is taken into consideration to be an item orbiting the sun that is round, or approximately round, however is smaller than Mercury. The object may additionally be in an area v several other objects orbiting v it, such as within the asteroid belt. The IAU has actually only accepted five objects as dwarf world so far: Ceres, Pluto, Eris, Makemake and Haumea. Yet there are countless other civilizations that can one day it is in classified as dwarf planets follow to your characteristics, such together Quaoar, Sedna, Orcus or Salacia. Much more observations are needed to for sure up countless suspected dwarf planets" size, because that example, i beg your pardon is difficult to accomplish since they are so much out in the solar system and so small.

There might be as plenty of as 200 dwarf planets in the solar system and also the Kuiper Belt, according to part astronomers. The IAU"s main list of dwarf plants mirrors a complex variety the worlds, with many of them including moons and distinctive surface ar compositions from each other.

Years after the vote, however, there space still researchers that describe Pluto together a planet. Because that example, NASA published a video in early 2014 from numerous speakers in ~ the Pluto science Conference in July 2013 that repeatedly referred to the civilization as a "planet". Also, human being such together NASA"s Alan Stern regularly current their debates for why Pluto need to still be considered a planet, citing difficulties with the IAU definition such as planets never totally clearing the zone about them.

The new Horizons mission come Pluto added much more fuel to the fire, together its complicated geological attributes had plenty of scientists saying that "planet" was the most befitting condition for the world. Data from new Horizons is still being analyzed, and also the jury is out on even if it is the an interpretation of "planet" will certainly be revisited. NASA"s Dawn mission also visited the dwarf planet Ceres beginning in 2015, revealing features such together a 4-mile-high (6.5-kilometer-high) mountain and also various bright clues on the surface.

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Searches for people are recurring in the external solar system, v the most prominent instance being the search for "Planet Nine." This is a theoretical planet that can be affecting the orbits the objects in the Kuiper Belt. If the exists, it would certainly be an ext of a "super-Earth", at four times the diameter of ours planet and also 10 times together massive.

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Elizabeth Howell is a contributing writer for who is one of the couple of Canadian journalists come report regularly on exploration. She is follow a Ph.D. Part-time in scientific researches (University of north Dakota) after completing an M.Sc. ( studies) in ~ the same institution. She likewise holds a bachelor the journalism level from Carleton University. Besides writing, Elizabeth teaches communications at the university and also community university level. To view her latest projects, monitor Elizabeth top top Twitter at