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 newer 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, particularly after the New Horizons spacecraft flew by Pluto in 2015. The spacecraft revealed a complex world full of geological change. This included mountains reaching as high as 11,000 feet (3,500 meters), a heart-shaped region nicknamed Tombaugh Regio that contains methane ice and other substance, and weird ice-ridged terrain that looks like snakeskin, among many other features Since then, members of the New Horizons team have made scientific presentations arguing Pluto is indeed a planet. The new scientific findings continue to amaze the public, Alan Stern said in 2018.

"I think my two biggest surprises were first, just how utterly amazing Pluto turned out to be — how many different kinds of features were on the surface and even in the atmosphere," Stern said in a NASA podcast interview. "There was something for everyone. And the second amazing finding was how many members of the public really wanted to participate in it and just be a part of this exploration. We expected it would be a big response, but it was much bigger than we thought."

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; Erisand 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 newer 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 worlds that could one day be classified as dwarf planets according to their characteristics, such as Quaoar, Sedna, Orcus or Salacia. More observations are needed to firm up many suspected dwarf planets' size, for example, which is hard to achieve since they are so far out in the solar system and so small.

There could be as many as 200 dwarf planets in the solar system and the Kuiper Belt, according to some astronomers. The IAU's official list of dwarf plants shows a complex variety of worlds, with most of them including moons and distinctive surface compositions from each other. 

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

The New Horizons mission to Pluto added more fuel to the fire, as its complex geological features had many scientists arguing that "planet" was the most befitting status for the world. Data from New Horizons is still being analyzed, and the jury is out on whether the definition of "planet" will be revisited. NASA's Dawn mission also visited the dwarf planet Ceres starting in 2015, revealing features such as a 4-mile-high (6.5-kilometer-high) mountain and various bright spots on its surface.

Searches for worlds are ongoing in the outer solar system, with the most prominent example being the search for "Planet Nine." This is a theoretical planet that might be influencing the orbits of objects in the Kuiper Belt. If it exists, it would be more of a "super-Earth", at four times the diameter of our planet and 10 times as massive.

  • Space.com
  • Dwarf Planet
  • Full-fledged Planet
  • I'm not sure - we need to wait and see.