How to Define a Planet – The Sequel

Hold your breath: astronomers are re-evaluating their definition of a planet. Spoiler: it won’t bring Pluto back into the family.

At the 2006 Prague meeting of the International Astronomical Union (IAU), a badly organized and poorly attended vote of the IAU General Assembly on a hastily formulated definition demoted Pluto to the status of dwarf planet, leaving just eight “regular” planets in the solar system.

The 2006 definition reads as follows: “A planet is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit.”

The sticking point was the third condition. The eight major planets all dominate their parts of the solar system, accompanied only by Trojan asteroids (mainly in the case of Jupiter) and moons. But Pluto is one of many Kuiper Belt objects orbiting beyond Neptune in more or less similar paths.

Many astronomers hate the IAU definition; some consistently keep calling Pluto a planet. “It has led to much confusion and bad publicity for the IAU,” says planetary scientist Jean-Luc Margot (UCLA). Together with Brett Gladman (University of British Columbia, Canada) and Tony Yang (Chaparral High School, California), he has come up with a new and much more precise definition. But their proposal won’t change the status of Pluto.

At the IAU’s 32nd General Assembly in Cape Town, South Africa, Margot told a meeting of the organization’s Division F (Planetary Systems and Astrobiology) that the third condition was on the right track. The team’s “unsupervised” clustering algorithm divides solar system objects into two distinct groups, based on the software’s assessment of the objects’ dynamical dominance – that is, whether or not they share their orbits with other bodies. “Nature has provided a huge gap in dynamical dominance,” says Margot. “It’s clearly essential.”

However, the current IAU definition doesn’t quantify what is meant by clearing the orbit’s neighborhood, and even if it were more specific, this would depend on a planet’s distance from the Sun. The phrase “nearly round” is also pretty vague. Moreover, the 2006 definition says a planet needs to orbit the Sun, so it excludes exoplanets, which is weird. “The IAU definition is inaccurate,” comments Joseph Nuth (NASA Goddard), the president of one of the commissions of Division F. “And if something is broke, you have to fix it.”

That’s exactly what Margot, Gladman, and Yang are trying to do. In their convoluted proposal, described in detail in a recent paper in The Planetary Science Journal, they include parts of the IAU’s definition for exoplanets, such as a maximum mass of 13 Jupiter masses – objects above that limit undergo thermonuclear fusion of deuterium (heavy hydrogen) in their cores and are known as brown dwarfs.

Next, they precisely quantify dynamical dominance, using an awkward formula with fractional exponents that includes the mass of the central star and the size of the planet’s orbit.

Finally, since an exoplanet’s mass is much easier to determine than its shape, they suggest a minimum mass of 10²¹ kilograms in order for an object to be called a planet. As Margot explains, any object more massive than 10²¹ kilograms is in hydrostatic equilibrium (i.e., it’s spherical, or ellipsoidal due to rapid rotation) as a result of self-gravity, independent of its composition.

At the meeting, Margot admitted that the proposed new definition is not free of certain quirks and limitations. “Perfection is not attainable,” he and his coauthors write in their paper. However, they add, “[w]e hope that these considerations will help start the conversation about making planetary taxonomy both quantitative and useful.”

Incidentally, Margot also presented an alternative, much simpler definition, one purely based on mass: “A planet is a celestial body that (a) orbits one or more stars, brown dwarfs, or stellar remnants; (b) is more massive than 10²³ kg; and (c) is less massive than 13 Jupiter masses (2.5 × 10²⁸ kg).” Here, the minimum mass is the mass that approximates the large gap in dynamical dominance, he explains. (For comparison: Pluto’s mass is 1.3 × 10²² kilograms – large enough for hydrostatic equilibrium, but too small for dynamical dominance.)

This simpler definition would of course be much easier to communicate to the public at large. However, an online survey among a small group of planetary scientists revealed a 70% preference for the more complicated, physics-based proposal, says Margot. “If this preference is representative of the IAU membership, I anticipate that the mass-based proposal will promptly fall into oblivion.”

Contrary to the haphazard process in 2006, Margot hopes that astronomers will now have ample time and opportunity to discuss the new proposals, to formulate a new IAU resolution (a job for the organization’s Commission F2), and to conduct an informed vote during the 33rd General Assembly in August 2027 in Rome, Italy. “It can and should be done properly,” says Nuth.

Not everyone agrees that a new definition is necessary. Gonzalo Tancredi and Julio Fernández (both at the University of the Republic, Uruguay) were involved in crafting the 2006 resolution. At the IAU meeting in Cape Town, they suggested astronomers not start a new discussion. “It wouldn’t change much,” says Tancredi. “It might be better to add a bit more interpretation to the existing definition.”

But according to Margot, most of his 25 survey respondents agreed that an improved definition is desirable. “Our community and the public deserve better definitions for such important astrophysical terms as ‘planet’,” he and his coauthors write in their paper.

Margot, Gladman, and Yang also have a take-home message for those planetary researchers who are opposed to any definition that excludes Pluto from the planetary family. “Readers who are chagrined that smaller bodies are not recognized as planets should take comfort in the fact these these bodies are no less worthy of exploration,” they write. “In other words, a taxonomic classification in one group or another is not an indicator of scientific importance.”

This story was made possible by a grant from the Dutch VWN Trip-fund.