Geophysical definition of planet

The International Union of Geological Sciences helps scientists decide on names and groups in Earth sciences. But this group has not made an official meaning for the word "planet". Because of this, many scientists who study the physics of Earth and other planets use different ways to say what a planet is. These geophysical definitions are different for each expert. Some scientists choose to use one of these geophysical definitions instead of the one made by the International Astronomical Union, the main group that decides how to name planets and other objects in space. This shows how scientists sometimes describe things in different ways based on what they study.

Definitions

Some scientists use the definition of a planet made by the International Astronomical Union in 2006. This says a planet is an astronomical object that goes around the Sun, is big enough to be rounded by its own gravity, and has cleared the neighbourhood around its orbit.

Another idea about what a planet is was suggested by planetary scientists Alan Stern and Harold Levison in 2002. They said a planet must be big enough for its gravity to make it round, but not so big that it can make energy by nuclear fusion. This helps scientists learn more about planets.

They also talked about different kinds of objects in space, like planets that go around stars, big moons, and planets that move between stars.

Other names for geophysical planets

In 2009, Jean-Luc Margot and Levison suggested calling round objects in space "worlds." They thought this was a good way to group objects without arguing about what a planet is. For them, a planet is a special world that moves in a certain way.

They said that deciding if something is round can be hard because we can't always see it well. The roundness of an object depends on what it is made of and its temperature. For example, the icy moon Mimas is round even though it is smaller than the rocky asteroid Vesta, which is not round even though it is larger. Because of these difficulties, they suggested that while it might be tricky to call something a "world," it is easier to tell if it is a planet by looking at its mass and how long it takes to orbit the Sun.

Geophysical planets in the Solar System

See also: List of gravitationally rounded objects of the Solar System

The number of geophysical planets in our Solar System changes with how we define them and what we learn about faraway objects. In 2006, scientists thought icy bodies needed to be about 400 kilometers wide to be round and considered planets. But by 2010, they found that some icy moons up to 1,500 kilometers wide, like Iapetus, aren't perfectly round. Still, many scientists include such bodies when talking about planets.

Today, the eight major planets are widely accepted as geophysical planets. These are ☿ Mercury, ♀ Venus, 🜨 Earth, ♂ Mars, ♃ Jupiter, ♄ Saturn, ⛢ Uranus, and ♆ Neptune. There are also nine dwarf planets that many scientists consider true planets, including Ceres, Orcus, Pluto, Haumea, Quaoar, Makemake, Gonggong, Eris, and Sedna.

In addition, there are nineteen large moons that are big enough to be considered planetary-mass moons. These include Earth's Moon, Jupiter's Io, Europa, Ganymede, and Callisto, Saturn's Mimas, Enceladus, Tethys, Dione, Rhea, Titan, and Iapetus, Uranus's Miranda, Ariel, Umbriel, Titania, and Oberon, Neptune's Triton, and Pluto's Charon.

Comparison to IAU definition of a planet

Some geophysical definitions of a planet are similar to the official definition set by the International Astronomical Union (IAU). Others are close to just part of what the IAU says about planets.

One newer idea from 2018, created by astronomer Alan Stern, does not require a planet to orbit a star or to clear its neighborhood of other objects. Because of this, it considers dwarf planets and large moons to also be planets.

The IAU currently recognizes five bodies as dwarf planets: Ceres, Pluto, Eris, Haumea, and Makemake. Some astronomers also think four more objects — Quaoar, Sedna, Orcus, and Gonggong — should be counted as dwarf planets, but this hasn’t been proven yet.

Reaction to IAU definition

Many people disagreed with the International Astronomical Union (IAU) decision about what a planet is. This was because the decision meant that Pluto was no longer called a planet. Some people signed a petition to show they did not agree with the decision. Not everyone who signed had a different idea about what should be called a planet.

Others felt the IAU's idea did not work well for all kinds of science. They wanted new rules just for studying planets from the inside, like earth scientists do. For example, astronomers Stern and Levinson suggested a different idea about what a planet is. They thought this idea was better for their work. Some earth scientists agree with the IAU, while others support Stern and Levinson's idea. Both ways of thinking about planets have been used for many years. Sometimes, small rocky worlds called asteroids have been called "minor" planets, but people do not always agree on this.

Main article: definition of what is and is not a planet

Applicability to exoplanets

Geophysical definitions help us know what counts as an exoplanet — a planet that orbits a star far from our Solar System. These definitions look at the shape and balance of an object. But measuring these for faraway planets is very hard. Scientists usually know only the mass, size, or brightness of an exoplanet.

Some very small exoplanets, like Kepler-1520b and WD 1145+017 b, are much lighter than Earth. Others, like SDSS J1228+1040 b, might be very tiny in size. Because of these challenges, scientists keep studying and talking about the best ways to define what a planet really is.