UCLA-Proposed Change to Scientific Definition of Planet Challenges Sun-Centric Orbit Criterion

Planetary scientists are proposing a new definition of a planet that challenges the current definition, which many researchers consider outdated and sun-centric. The current definition, established in 2006 by the International Astronomical Union (IAU), states that a planet must orbit the sun within our solar system. However, scientists have discovered thousands of planets orbiting stars outside our solar system, leading to a need for a revised definition.

According to Jean-Luc Margot, lead author of the article and UCLA professor of earth, planetary and space sciences and of physics and astronomy, the current definition is too specific and only applies to planets in our solar system. “We now know about the existence of thousands of planets, but the IAU definition applies only to the ones in our solar system,” Margot said. The proposed new definition aims to include celestial bodies that orbit 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 instance, it states that a planet has “cleared its orbit” without defining what that means. The proposed new definition provides quantifiable criteria that can be applied to define planets inside and outside our solar system.

The new definition states that a planet is a celestial body that:

• Orbits one or more stars, brown dwarfs, or stellar remnants
• Is more massive than 10^23 kg
• Is less massive than 13 Jupiter masses (2.5 x 10^28 kg)

Margot and his co-authors, Brett Gladman of the University of British Columbia and Tony Yang, a student at Chaparral High School in Temecula, California, analyzed the properties of objects in our solar system using a mathematical algorithm. They found that planets in our solar system share distinct qualities that can be used as a starting point for creating a taxonomy for planets in general.

For example, if an object has enough gravity to clear a path by accumulating or ejecting smaller objects nearby, it is considered dynamically dominant. “All the planets in our solar system are dynamically dominant, but other objects, including dwarf planets like Pluto, and asteroids, are not,” Margot said. “So this property can be included in the definition of planet.”

The requirement for dynamical dominance provides a lower limit on mass, while the upper limit is determined by the mass of 13 or more Jupiters, beyond which an object becomes a brown dwarf. The current requirement for a planet to be spherical is problematic, as distant planets can rarely be observed in enough detail to ascertain their shape with certainty. The authors argue that the shape requirement is difficult to implement and is effectively useless for definitional purposes.

However, the good news is that celestial bodies larger than 10^21 kg appear to be round in our solar system, so all bodies that satisfy the proposed lower limit on mass of 10^23 kg are expected to be spherical. The authors hope that their work will start a conversation that leads to an improved definition of a planet.

This proposed change is likely to take a few years to become official, but it is an important step towards a more inclusive and accurate definition of a planet.

Historical Context:

The current definition of a planet, established in 2006 by the International Astronomical Union (IAU), was a response to the discovery of Eris, a dwarf planet that was initially thought to be the tenth planet in our solar system. The IAU definition was designed to distinguish between planets and dwarf planets, and it stated that a planet must orbit the sun within our solar system, have sufficient mass to assume a hydrostatic equilibrium shape, and have cleared the neighborhood around its orbit. However, with the discovery of thousands of exoplanets, the definition has become outdated and sun-centric.

The proposed new definition aims to address these limitations and provide a more inclusive and accurate definition of a planet that can be applied to celestial bodies orbiting any star, stellar remnant, or brown dwarf.

Summary in Bullet Points:

• The current definition of a planet, established in 2006, is outdated and sun-centric, and does not apply to planets outside our solar system. • The proposed new definition states that a planet is a celestial body that: + Orbits one or more stars, brown dwarfs, or stellar remnants + Is more massive than 10^23 kg + Is less massive than 13 Jupiter masses (2.5 x 10^28 kg) • The definition includes a requirement for dynamical dominance, which means that an object must have enough gravity to clear a path by accumulating or ejecting smaller objects nearby. • The upper limit on mass is determined by the mass of 13 or more Jupiters, beyond which an object becomes a brown dwarf. • The shape requirement is problematic and difficult to implement, so the authors suggest that the definition focus on mass instead. • The proposed definition is expected to lead to a more inclusive and accurate definition of a planet, and it is likely to take a few years to become official. • The authors hope that their work will start a conversation that leads to an improved definition of a planet.