Join EarthSky Deborah Byrd and astronomer Michael Brown LIVE at 12:15 p.m. CDT (17:15 UTC). Will the definition of a planet change? Brown is a professor of planetary astronomy at the California Institute of Technology. His 2012 book is called “How I Killed Pluto and Why It Deserved It.” He helped discover our solar system’s Kuiper Belt, which is similar to the asteroid belt but contains up to 200 times more material. In early 2016, Brown and a colleague, Konstantin Batygin, also of Caltech, described how the similar orbits of six small bodies in the outer solar system could be explained by another major planet in our solar system. They call it Planet Nine. And they’re still hoping to find it. Click here for the “Notify Me” button.
- When Pluto was demoted from planet status in 2006, It was because the International Astronomical Union definition of planet had changed. The change created an uproar.
- Since the mid-1990s, scientists have discovered more than 5,000 exoplanets.or worlds orbiting distant stars. Meanwhile, the IAU definition applies only to planets within our solar system.
- A new definition of planet proposed includes exoplanets. Specifies that “planets” can orbit one or more stars, brown dwarfs or stellar remnants. It sets mass limits that its creators say should apply to planets everywhere. But it does not restore Pluto to its full planetary status.
UCLA originally published this statement on July 11, 2024.
The current definition is “sun-centered.”
Planetary scientists are proposing a new definition of a planet to replace one that many researchers consider heliocentric and outdated. The current definition, established in 2006 by the International Astronomical Union (IAU),UIA), the organization that officially names objects in space, specifies that to qualify as a planet, a celestial body must orbit the Sun within our solar system.
But scientists know that celestial bodies orbiting stars outside our solar system are fairly common, and an upcoming paper in the Journal of planetary science A new definition of a planet is advocated, one that includes no limitation to our solar system. The proposal also provides quantitative criteria to further clarify the definition of a planet.
Jean-Luc MargotThe paper's lead author and a professor of Earth, planetary and space sciences and of physics and astronomy at UCLA, will present the proposed new definition at the IAU General Assembly in August 2024.
Proposed change adds exoplanets and mass limits
By the current definition, a planet is a celestial body that orbits the Sun, is massive enough that gravity has forced it into a spherical shape and has pushed other objects out of its orbit around the Sun. Margot explained why this is not adequate:
The current definition specifically mentions orbit around the Sun. We now know of thousands of planets, but the IAU definition applies only to those in our solar system. We propose a new definition that can be applied to celestial bodies orbiting any star, stellar remnant or brown dwarf.
The authors argue that while the requirement to orbit the Sun is too specific, other criteria in the IAU definition are too vague. For example, it says a planet has “gone out of orbit” without specifying what that means. The proposed new definition contains quantifiable criteria that can be applied to define planets inside and outside our solar system.
In the new definition, a planet is a celestial body that:
- orbits one or more stars, brown dwarfs or stellar remnants and;
- is more massive than 1023 kg and;
- It is less massive than 13 Jupiter masses (2.5*1028 kg).
A high school student participated in the redefinition of the planets
Margot and her co-authors Brett Gladman of the University of British Columbia and Tony Yang, a student at Chaparral High School in Temecula, California, ran a mathematical algorithm on the properties of objects in our solar system to see which objects clustered together. The analysis revealed groups of distinct qualities that the planets in our solar system share and that can be used as a starting point for creating a taxonomy of the planets in general.
For example, if an object has enough gravity to clear a path by accumulating or ejecting smaller objects nearby, it is said to be dynamically dominant. Margot explained why this is important:
All planets in our solar system are dynamically dominant, but other objects, including dwarf planets like Pluto and asteroids, are not. Therefore, this property can be included in the definition of a planet.
The dynamical domain requirement provides a lower limit on mass. But potential planets can also be too large to fit the new definition. Some gas giants, for example, are so large that thermonuclear fusion of deuterium occurs and the object becomes a substar It is called a brown dwarf and is therefore not a planet. This limit has been determined to be the mass of 13 or more Jupiters.
Do planets have to be round?
On the other hand, the current requirement that planets be spherical is more problematic. Distant planets can rarely be observed in sufficient detail to determine their shape with certainty. The authors argue that the shape requirement is so difficult to implement that it is useless for definitional purposes, even though planets are generally round. Gladman explained how this helps:
The fact that the definitions are based on the easiest quantity to measure (mass) eliminates arguments about whether or not a specific object meets the criterion, which is a weakness of the current definition.
The good news is that in the solar system, celestial bodies larger than 10twenty-one kg appear to be round. Therefore, all bodies that satisfy the proposed lower limit for mass of 1023 Kg are expected to be spherical.
While any official change to the IAU's definition of a planet is likely to be years away, Margot and her colleagues hope their work will start a conversation that will result in an improved definition.
Bottom line: Astronomers will vote on a new definition of a planet in August 2024. The update would include exoplanets and set mass limits.
Read more: Scientist proposes another new definition of planet
Read more: Exoplanets are worlds that orbit other stars
Leave feedback about this