What Is a Planet?
The planets of the solar system as depicted by a NASA computer illustration. Orbits and sizes are not shown to scale.
Credit: NASA

While many people can point to a picture of Jupiter or Saturn and call it a "planet," the definition of this word is much more subtle and has changed over time. Many astronomers decided on a new definition in 2006 after the discovery of several worlds at the fringes of the solar system — a decision that remains controversial.

The International Astronomical Union defined a planet as an object that:

  • orbits the sun
  • has sufficient mass to be round, or nearly round
  • is not a satellite (moon) of another object
  • has removed debris and small objects from the area around its orbit

The IAU also created a new classification, "dwarf planet," which is an object that meets planetary criteria except that it has not cleared debris from its orbital neighborhood. This definition meant that Pluto — considered a planet at the time — was demoted and reclassified as a dwarf planet. But not all scientists agree with this classification.

[Related: New Horizons' Pluto Flyby: Latest News, Images and Video]

The term "planet" originally comes from the Greek word for "wanderer." Many ancient cultures observed these "moving stars," but it wasn't until the advent of the telescope in the 1600s that astronomers were able to look at them in more detail. Small telescopes revealed moons circling Jupiter — a big surprise to Galileo Galilei (the likely discoverer) and his opponents at the Catholic Church — as well as rings around Saturn and an ice cap on Mars.

Telescopes also revealed the existence of objects not known to the ancients, because they are too far away and small to be spotted with the naked eye. Uranus was found on March 13, 1781, by the prolific astronomer William Herschel. Ceres was discovered between Mars and Jupiter in 1801. It was originally classified as a planet, but it was later realized that Ceres was the first of a class of objects eventually called asteroids. Neptune was discovered in 1846. [Related: Solar System Planets: Order of the 8 (or 9) Planets]

Astronomers continued scouring the solar system's outer reaches in search of a large "Planet X" that was believed to be disturbing the orbits of Uranus and Neptune. While these irregularities were later discounted by further observations, Clyde Tombaugh did spot a smaller object in 1930 beyond the orbit of Neptune. Called Pluto, the object (then called a planet) was relatively small and had a highly eccentric orbit that sometimes even brought it closer to the sun than Neptune is.

Nothing close to Pluto's size was found in the solar system for more than two generations. That changed in the 2000s, when Mike Brown — a young astronomer at the California Institute of Technology — was in search of a defining research project and decided upon searches for objects in the outer solar system.

In quick 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 unexpected — the supposed Oort Cloud, the birthplace of comets, should have trillions of these things — it was the size that made other astronomers pay attention.

Some of Brown's notable discoveries included Quaoar; Sedna; Haumea; Eris and its moon, Dysnomia; and Makemake. All were found in a relatively short period of time, between 2001 and 2005. Eris (which was originally nicknamed "Xena" after a popular television show of the time) was large enough that some in the media were calling it the 10th planet.

With this series of discoveries confronting them, the International Astronomical Union spent two years examining the evidence and called a general meeting in 2006 to vote on what the definition of what a planet would be. The delegates present voted on a definition that excluded Pluto, Eris and any other objects that were close to the same size.

The new designation "dwarf planet" is considered to be an object orbiting the sun that is round, or approximately round, but is smaller than Mercury. The object may also be in an area with several other objects orbiting with it, such as within the asteroid belt. The IAU has only accepted five objects as dwarf planet so far: Ceres, Pluto, Eris, Makemake and Haumea. But there are many other candidate objects discovered by Brown and other astronomers in the years since.

Brown agreed with the demotion of "Xena" (which Eris was still called at the time) to dwarf planet, although he acknowledged other people felt emotional about the loss of Pluto. "I understood. Pluto was part of their mental landscape, the one they had constructed to organize their thinking about the solar system and their own place within it. Pluto seemed like the edge of existence. Ripping Pluto out of that landscape caused what felt to be an inconceivably empty hole," he wrote in his memoir, "How I Killed Pluto and Why It Had It Coming."

Years after the vote, however, there are still scientists that refer to Pluto as a planet. For example, NASA published a video in early 2014 from several speakers at the Pluto Science Conference in July 2013 who repeatedly referred to the world as a "planet". Also, people such as NASA's Alan Stern regularly present their arguments for why Pluto should still be considered a planet, citing problems with the IAU definition such as planets never fully clearing the zone around them.

A more recent discovery beyond Pluto's orbit — 2012 VP113, nicknamed "Biden," announced in early 2014 — shows that the zone between Pluto and the Oort Cloud could be populated with more objects like Sedna. The solar system is a more complex place than imagined even a couple of decades ago.

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