It says a planet must do three things:. Discussion—and debate—will continue as our view of the cosmos continues to expand. Science is a dynamic process of questioning, hypothesizing, discovering, and changing previous ideas based on what is learned. Scientific ideas are developed through reasoning and tested against observations.
Scientists assess and question each other's work in a critical process called peer review. Our understanding about the universe and our place in it has changed over time. New information can cause us to rethink what we know and reevaluate how we classify objects in order to better understand them.
New ideas and perspectives can come from questioning a theory or seeing where a classification breaks down. Defining the term planet is important, because such definitions reflect our understanding of the origins, architecture, and evolution of our solar system. Over historical time, objects categorized as planets have changed. Earth was not considered a planet, but rather was thought to be the central object around which all the other celestial objects orbited.
The first known model that placed the Sun at the center of the known universe with the Earth revolving around it was presented by Aristarchus of Samos in the third century BCE, but it was not generally accepted.
It wasn't until the 16th century that the idea was revived by Nicolaus Copernicus. By the 17th century, astronomers aided by the invention of the telescope realized that the Sun was the celestial object around which all the planets—including Earth—orbit, and that the moon is not a planet, but a satellite moon of Earth.
Uranus was added as a planet in and Neptune was discovered in Ceres was discovered between Mars and Jupiter in and originally classified as a planet. But as many more objects were subsequently found in the same region, it was realized that Ceres was the first of a class of similar objects that were eventually termed asteroids star-like or minor planets.
Pluto, discovered in , was identified as the ninth planet. But Pluto is much smaller than Mercury and is even smaller than some of the planetary moons. Charon, its huge satellite, is nearly half the size of Pluto and shares Pluto's orbit. Though Pluto kept its planetary status through the s, things began to change in the s with some new discoveries.
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 , revealing features such as a 4-mile-high 6.
Searches for worlds are ongoing in the outer solar system, with the most prominent example being the search for "Planet Nine. If it exists, it would be more of a "super-Earth", at four times the diameter of our planet and 10 times as massive.
Join our Space Forums to keep talking space on the latest missions, night sky and more! And if you have a news tip, correction or comment, let us know at: community space. Elizabeth Howell is a contributing writer for Space. She is the author or co-author of several books on space exploration. Elizabeth holds a Ph. She also holds a bachelor of journalism degree from Carleton University in Canada, where she began her space-writing career in Besides writing, Elizabeth teaches communications at the university and community college level, and for government training schools.
This orbit-clearing criterion is what the IAU decided to use in its definition of a planet. It's a criterion based on dynamics and not geophysics. This difference explains why Pluto is not classified as a planet. Unlike any of the planets, Pluto is embedded in a vast swarm of bodies similar to itself.
Pluto is therefore analogous to the asteroid Ceres in the asteroid belt between Mars and Jupiter. Pluto has many friends orbiting nearby, which is not the case for any of the planets.
The planets accumulate, eject, or otherwise control all the mass in their immediate proximity. Pluto and Ceres are not able to do that; therefore they belong to a class that is really quite distinct from the eight planets. Some who argued for maintaining Pluto as a planet proposed the following arguments: Pluto has a tiny atmosphere.
Several moons of the Jovian planets have an atmosphere, so the presence of an atmosphere is not a distinctive feature of planets. Pluto has satellites. Many small asteroids and trans-Neptunian objects have satellites, so the presence of a satellite is not a distinctive feature of planets. Pluto is round. Many moons are round and some asteroids are round, so roundness is not a distinctive feature of planets. Pluto is no longer a planet!
That's ok. Science is all about recognizing that earlier ideas may have been wrong. For a long time biologists thought that all microbes causing diseases in humans were bacteria. At some point scientists realized that there was another class of microbe more properly described as viruses, and they had to change their ideas about which infective agent was what. We are all better off now as the new classification has clarified meaning and has allowed researchers and health professionals to communicate with each other and the public.
Astronomers had to revise their classification in light of our improved understanding of the solar system. Pluto is now recognized as a large member of the trans-Neptunian population. Did the decision cause a cultural revolution? It caused a storm in a teapot. Pluto was considered a planet for only 75 years, and questions about its planetary status had been raised for more than 10 years. Compare that to the thousands of years during which schoolchildren were taught that planets revolved around the Earth.
When scientists demonstrated that planets revolve around the Sun, people had to make very serious adjustments to their ways of thinking. But people adjusted. And they also adjusted quite well to a solar system with eight planets.
Some people resisted. People are resistant to change, and this resistance is sometimes obvious in discussions involving Pluto. Most non-scientists understand the arguments quite well and have no problems with the current classification of planetary bodies. What about the mnemonic? New mnemonics are easy to make. Try it, it's fun! A very small number of astronomers have been opposing the classification quite vocally.
Some of them have said that they prefer a classification based on geophysics and not dynamics. However the language that they are using "Save Pluto" reveals an emotional attachment to Pluto. According to the geophysics proposal, the large asteroid Ceres would also be counted as a planet, but the people who became so upset about Pluto in had done absolutely nothing to try to "Save Ceres" prior to Ceres was considered a planet in the first half of the nineteenth century, and was demoted from its planetary status when new discoveries showed that it had many friends orbiting nearby - just like Pluto.
This inconsistency in behavior suggests but does not prove that the objection to Pluto's demotion has more to do with attachment to Pluto than it has to do with geophysics.
How did the IAU arrive at its decision? On Aug 24, , the assembly of IAU members voted overwhelmingly in favor of a resolution that defines three distinct classes of objects in the solar system: planets, dwarf planets, and small solar-system bodies.
Merriam-Webster's Words of the Week - Oct. Homophones, Homographs, and Homonyms. Time Traveler. Love words? Need even more definitions? Ask the Editors 'Everyday' vs.
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