Geophysical definition of planet

The International Union of Geological Sciences is the main organization that helps scientists agree on names and classifications in Earth sciences. However, this group has not created an official definition for the word "planet". Because of this, many scientists, especially those who study the physics of Earth and other planets, use different ways to define what a planet is. These geophysical definitions vary among experts. Some choose to follow one of these geophysical definitions rather than the one created by the International Astronomical Union, the main group that decides how to name planets and other objects in space. This difference in definitions shows how scientists sometimes have unique ways to describe the same thing based on their area of study.

Definitions

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

Another definition was suggested by planetary scientists Alan Stern and Harold Levison in 2002. They said a planet must be big enough for its gravity to shape it into a round form, but not so big that it can create energy through nuclear fusion. This helps scientists understand planets better.

They also described different types of planetary bodies, like planets that orbit stars, large 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 believed this geophysical way of classifying objects was reasonable and did not conflict with the idea of what a planet is. For them, a planet is a special kind of world that follows certain movement rules.

They also noted that deciding if something is round can be tricky because we can't always see it clearly. The roundness of an object depends on its material and temperature, making it hard to classify. For example, the icy moon Mimas is round even though it is smaller than the rocky asteroid Vesta, which is not round despite being larger. Because of these challenges, they suggested that while there might be some uncertainty in calling something a "world," we can more easily determine if it is a planet by looking at its mass and how long it takes to go around 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 depends on the definition used and the details known about faraway objects. In 2006, scientists thought that icy bodies needed to be about 400 kilometers wide to be considered round and planetary. But by 2010, they learned that some icy moons up to 1,500 kilometers wide, like Iapetus, aren't perfectly round. Despite this, many scientists still 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 massive 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 quite 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 did not agree with the International Astronomical Union (IAU) decision about what a planet is, especially because it meant Pluto was no longer called a planet. Some of these people signed a petition to protest the decision, though not all of them had another idea for what should count as a planet.

Others thought the IAU's idea was not good for all kinds of science and wanted to make new rules just for studying planets from the inside, like earth scientists do. For example, astronomers Stern and Levinson offered a different idea about what a planet is, which they said 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, and sometimes small rocky worlds called asteroids have been called "minor" planets, though 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 understand what counts as an exoplanet — a planet orbiting a star far from our Solar System. These definitions focus on the shape and balance of an object, but measuring these features for distant planets is very hard. Scientists usually know only the mass, size compared to its star, 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 discussing the best ways to define what a planet truly is.