In the ever-evolving universe of astronomy, planet definitions must sometimes catch up with new discoveries and understandings.
The term “planet”, as defined by the International Astronomical Union (IAU), is currently under review.
The old definition describes a planet as a celestial body that orbits the sun, has enough mass to be forced into a spherical shape by gravity, and has cleared other objects from its orbit.
However, this IAU definition only applies to celestial bodies within our solar system, sparking debate among scientists.
Expanding the definition of planets
We inhabit a vast universe, where celestial bodies orbiting outside our solar system are frequently discovered.
This reality raises a question to ponder: Shouldn’t the definition of a planet be extended beyond the boundaries of our solar system?
An article soon to be published in a well-known scientific journal argues just that. The authors of the study propose a new definition of a planet – one unconstrained by the boundaries of our solar system and reinforced by quantitative criteria.
An outdated definition of planets
The pioneers of this proposal are scientists from the University of California, Los Angeles (UCLA). They are advocating a replacement for the IAU definition, which they see as sun-focused and outdated.
“The current definition specifically mentions the orbit of our sun. It only applies to the planets in our solar system,” said the study’s lead author, Professor Jean-Luc Margot. He added that the team’s proposal would apply to celestial bodies orbiting any star, stellar remnant or brown dwarf.
The researchers argue that the IAU’s requirement that a planet orbit our sun is too specific, while other criteria are too vague.
They present a clear definition, filled with measurable criteria applicable to planets inside and outside our solar system.
Characteristics of the planets
According to the new proposal, a planet is a celestial body that orbits one or more stars, brown dwarfs or stellar remnants. It has a mass exceeding 1,023 kg, but less than 13 Jupiter masses (2.5 X 1,028 kg). Providing such specific size limits is a key component of the proposal.
Scientists used a mathematical algorithm to study the properties of objects in our solar system and identify distinct attributes shared by our planets.
The analysis served as a basis for developing a universal classification for these celestial bodies, introducing critical elements such as dynamic dominance.
Mass and gravitational influence
An object is called dynamically dominant if it has enough gravity to open a path by accreting or ejecting smaller nearby objects.
Interestingly, all the planets in our solar system exhibit this feature, but others, such as dwarf planets such as Pluto and some asteroids, do not. Consequently, this feature has been cited as an essential addition to the definition of planet.
While dynamical dominance sets a lower limit on mass, mass also serves as an upper limit, especially when celestial bodies become so massive that they initiate thermonuclear fusion of deuterium and transform into dwarfs known as brown dwarfs.
In particular, the requirement that a planet be spherical is more complicated to implement because of the difficulty in observing the shapes of distant planets.
Therefore, the authors propose definitions based on mass, which is easier to measure, thus avoiding debates about whether or not a celestial body meets the spherical criterion.
Planets and dwarf planets
A revision of the official IAU definition of a planet may not be imminent, but the researchers hope their work will spark a discussion leading to an improved definition.
According to the proposal, the distinction between planets and dwarf planets becomes clearer.
Dwarf planets, such as Pluto, which lack dynamic dominance, are part of a special category. This ensures that the definition includes exclusively bodies that fit the strict criteria of mass and dynamic dominance.
The understanding of exoplanets also plays a crucial role in the discussion. Exoplanets—planets located outside our solar system—require a definition that transcends the limitations of the solar system’s core.
Focusing on mass and gravitational influence, the new definition provides a robust framework, making classification easier and more accurate.
The study was published in Journal of Planetary Science.
—
Do you like what you read? Subscribe to our newsletter for engaging articles, exclusive content and the latest updates.
Check us out at EarthSnap, a free app brought to you by Eric Ralls and Earth.com.
—