Explainer: What is a planet?

The ancient Greeks first coined the name “planet.” The term means “wandering star,” explains David Weintraub. He’s an astronomer at Vanderbilt University in Nashville, Tenn. Aristotle, a Greek philosopher who lived more than 2,000 years ago, identified seven “planets” in the sky. These are the objects that today we call the sun, moon, Mercury, Venus, Mars, Jupiter and Saturn. This view of planets would hold for the next 1,500 years, Weintraub notes.

“The seven planets according to the Greeks were the seven planets at the time of the Copernicus,” he says. “And those seven included the sun and the moon.”

Nicolaus Copernicus was a Polish astronomer. In the early 1500s, he suggested that the sun, and not the Earth, was at the center of what we today call the solar system. By doing that, he removed the sun from the list of planets. Then, in 1610,  Galileo Galilei pointed a telescope at the sky. In doing so, this Italian mathematician saw not only Jupiter but also four of its moons.

Later in that century, astronomers Christiann Huygens and Jean-Dominique Cassini spotted five additional objects orbiting Saturn. We now know them as moons. But at the end of the 1600s, astronomers agreed to call them planets. That brought the total number of apparent planets to 16.

Between then and the early 1900s, the number of planets fluctuated. From that high of 16, it later fell to six. That’s when the objects circling planets were reclassified as moons. With the 1781 discovery of Uranus, the planet count bumped up to seven. Neptune was discovered in 1846. Later, it jumped to 13 as telescopes unveiled several objects orbiting the sun from a distance between Mars and Jupiter. Today we call these objects asteroids. And now we know even asteroids can have moons. Finally, in 1930 little Pluto was spotted orbiting the sun from a cold, distant outpost.

Clearly, scientists have been naming, re-naming and categorizing parts of the solar system ever since people began following the paths of objects in the night sky, thousands of years ago. In 2006, the International Astronomical Union defined Pluto in a way that kicked it out of the planet tribe.

But wait…the definition of planet may not be settled.

“The word has changed meanings many times, for many different reasons,” noted Lisa Grossman in a 2021 Science News review of the science. “So there’s no reason,” she says, “why it couldn’t be changed once more.” Indeed, she cited scientists who are now arguing that Pluto should be given back its planet status. And some scientists suspect yet another planet may be orbiting the sun well beyond Pluto.

Nor are planets found only in our solar system. Astronomers have been logging stars throughout our galaxy that also appear to host their own planets. To differentiate these from planets in our solar system, those around other stars are now referred to as exoplanets. As of March 2022, the count of known exoplanets had already topped 5,000.

Note: This story has been periodically updated to account for emerging developments in planetary science and discovery.

Aristotle: An ancient Greek philosopher who lived during the 300s B.C. He studied many scientific topics, including biology, chemistry, physics and zoology. But science was far from his only interest. He also probed ethics, logic, government and politics — the underpinnings of what would become European culture.

asteroid: A rocky object in orbit around the sun. Most asteroids orbit in a region that falls between the orbits of Mars and Jupiter. Astronomers refer to this region as the asteroid belt.

astronomer: A scientist who works in the field of research that deals with celestial objects, space and the physical universe.

exoplanet: Short for extrasolar planet, it’s a planet that orbits a star outside our solar system.

galaxy: A group of stars — and usually invisible, mysterious dark matter — all held together by gravity. Giant galaxies, such as the Milky Way, often have more than 100 billion stars. The dimmest galaxies may have just a few thousand. Some galaxies also have gas and dust from which they make new stars.

host: (in biology and medicine) The organism (or environment) in which some other thing resides. Humans may be a temporary host for food-poisoning germs or other infective agents. (v.) The act of providing a home or environment for something.

Jupiter: (in astronomy) The solar system’s largest planet, it has the shortest day length (9 hours, 55 minutes). A gas giant, its low density indicates that this planet is composed mostly of the light elements hydrogen and helium. This planet also releases more heat than it receives from the sun as gravity compresses its mass (and slowly shrinks the planet).

Mars: The fourth planet from the sun, just one planet out from Earth. Like Earth, it has seasons and moisture. But its diameter is only about half as big as Earth’s.

mercury: Sometimes called quicksilver, mercury is an element with the atomic number 80. At room temperature, this silvery metal is a liquid. Mercury is also very toxic. Sometimes called quicksilver, mercury is an element with the atomic number 80. At room temperature, this silvery metal is a liquid. Mercury is also very toxic. (in astronomy and here the term is capitalized) The smallest in our solar system and the one whose orbit is closest to our sun. Named after a Roman god (Mercurius), one year on this planet lasts 88 Earth days, which is shorter than one of its own days: Each of those lasts 175.97 times as long as a day on Earth. (in meteorology) A term sometimes used to refer to the temperature. It comes from the fact that old thermometers used to use how high mercury rose within a tube as a gauge for temperature.

moon: The natural satellite of any planet.

philosopher: Researchers (often in university settings) who ponder fundamental truths about relationships between things, including people and the world. The term also is used to describe truth seekers in the ancient world, ones who sought to find meaning and logic out of observing the workings of society and of the natural world, including the universe.

planet: A large celestial object that orbits a star but unlike a star does not generate any visible light.

Pluto: A distant world that is located in the Kuiper Belt, just beyond Neptune. Known as a dwarf planet, Pluto is the ninth largest object orbiting our sun.

Saturn: The sixth planet out from the sun in our solar system. One of the two gas giants, this planet takes 10.6 hours to rotate (completing a day) and 29.5 Earth years to complete one orbit of the sun. It has at least 82 moons. But what most distinguishes this planet is the broad and flat plane of bright rings that orbit it.

solar system: The eight major planets and their moons in orbit around our sun, together with smaller bodies in the form of dwarf planets, asteroids, meteoroids and comets.

star: The basic building block from which galaxies are made. Stars develop when gravity compacts clouds of gas. When they become hot enough, stars will emit light and sometimes other forms of electromagnetic radiation. The sun is our closest star.

sun: The star at the center of Earth’s solar system. It is about 27,000 light-years from the center of the Milky Way galaxy. Also a term for any sunlike star.

telescope: Usually a light-collecting instrument that makes distant objects appear nearer through the use of lenses or a combination of curved mirrors and lenses. Some, however, collect radio emissions (energy from a different portion of the electromagnetic spectrum) through a network of antennas.

Venus: The second planet out from the sun, it has a rocky core, just as Earth does. Venus lost most of its water long ago. The sun’s ultraviolet radiation broke apart those water molecules, allowing their hydrogen atoms to escape into space. Volcanoes on the planet’s surface spewed high levels of carbon dioxide, which built up in the planet’s atmosphere. Today the air pressure at the planet’s surface is 100 times greater than on Earth, and the atmosphere now keeps the surface of Venus a brutal 460° Celsius (860° Fahrenheit).