A Proposed Simpler Definition of Planet

On August 24^th, 2006, The International Astronomical Union (IAU), which has the last word on all things astronomical, redefined the word “planet.” Technically, they gave the first actual definition of a planet. Prior to that, “planet,” Greek for “wanderer,” meant any celestial object whose position in the sky changed relative to the background stars. Humans knew of seven planets since long before written history: Mercury, Venus, Mars, Jupiter and Saturn, but also two other perhaps surprising planets, the sun and the moon. They, too, changed positions relative to the background stars. That’s not so much a definition as a description.

In more modern times, since the invention of the telescope, three other objects joined the ranks of planets: Uranus, Neptune, and Pluto, this last one added in 1930. That’s the way things stood until January 5, 2005. On that date, Cal Tech astronomer Mike Brown and his colleagues Chad Trujillo and David Rabinowitz discovered a Trans-Neptunian Object, TNO, they called Zena, but whose name would officially be changed to Eris. The rest of the world learned about Eris on July 29^th that same year when the astronomers announced their discovery. TNO’s exist in the region of our solar system beyond Neptune, also called the Kuiper Belt, the realm of Pluto.

Early analysis indicated the Eris was about 10% larger than Pluto and thus, if Pluto deserves the moniker planet, Eris did, too. But Brown and his colleagues began finding other not-quite-so-large TNO’s in the Kuiper Belt and eventually decided that Pluto was just one among many objects roughly equal in size and so, their thinking went, represented a large class of similar objects that don’t fit with the other planets. Also, astronomers had begun to regularly discover plant-sized objects orbiting other stars, known as extra-solar planets. They didn’t wander in our skies, so astronomers needed a physical definition of planet.

Partly due to Brown’s team’s discovery of Eris, the IAU meeting in Prague, Czechoslovakia, two years later redefined the word planet. A resolution passed on the final day of the conference, after 3/4ths of the delegates had already left, created a new definition:

      A Planet:

1. Is in orbit around the Sun but is not a satellite (moon) of another object,
2. Has sufficient mass to assume hydrostatic equilibrium (shape is determined by gravity overcoming the rigidity of the body of the object), and
3. Has "cleared the neighborhood" around its orbit.

Because Pluto orbited in the Kuiper Belt, it couldn’t satisfy provision #3. Pluto became the first of a new class of objects called Dwarf Planets, and Eris would be classified one as well. Almost immediate controversy followed the vote, particularly regarding the last point. Jupiter, the largest planet in the solar and indisputably a planet, has thousands of asteroids, known as Trojan asteroids, at the L4 and L5 points in its orbit around the sun. Even Earth has asteroids that are near to or cross its orbit. In a sense, no object in our solar system fits that part of the definition. Also, the clause that states “A planet is in orbit around the sun” excludes all the known extra-solar planets.

Speculation mounted that many of those voting didn’t want the number of planets to blow up to unreasonable numbers. Our solar system would suddenly contain dozens or hundreds of objects called planets, to the chagrin of some astronomers. Since then, several persons or groups have proposed new definitions of planet. None of the proposed counter-definitions include a “clear the neighborhood” clause.

Most start with a statement “In orbit around a star (not necessarily our sun).” Many proposals also add something to the effect that it can’t itself be a star, an object which sustains nuclear fusion. I believe that needs further clarification. A brown dwarf is a star-like object much larger than a planet and often called a failed star, and is large enough to have initiated limited fusion reactions. Our suns, as do virtually all stars in the sky, generate energy by fusing four hydrogen atoms to create one helium atom, neutrinos, and energy, essentially the same energy source tapped by a hydrogen nuclear bomb.

Prior to the initiation of this hydrogen fusion stage, also known as a star’s main sequence stage, all stars contain some limited amount of lithium. It’s considerably easier to fuse than hydrogen and most brown dwarfs spend a brief amount of time sporting lithium fusion. That process essentially demarcates brown dwarfs from large gas giant planets, like Jupiter. Brown dwarf is a in a classification distinct from stars and planets.

All planet definitions include the “hydrostatic equilibrium” clause, and many stop there. That would mean our and every other round or nearly round moon in our solar system becomes a planet. Along with the largest asteroids, that makes the total planet count in our solar system over 100. While I abhor making emotional statements like “That’s just too many planets” to become a part of a scientific definition, I think it muddies the planetary waters.

Some planet definitions include the “not a satellite (or natural moon) of another object,” but that also lacks precision. What makes something a satellite of another object? That lack of precision leads me to propose this definition of natural satellite: when two non-stellar objects co-orbit each other, if the barycenter (the center of mass) is within the body of the more massive one, the smaller one is a moon. The barycenter of the Earth-moon system is only 2,900 miles from the center of Earth, barely half way to Earth’s surface. Our moon is truly a moon.

Charon, the largest object orbiting Pluto is so massive, the barycenter of the Pluto-Charon, while close to Pluto, is in space between the two, making Pluto and Charon a double planet. No other Moon in our solar system meets that criteria.

So I propose this definition of “planet:”

     A planet:

1. Is in orbit around a star (an object capable of supporting on-going fusion of hydrogen or heavier elements) or originally formed around a one,
2. Is not itself a star, regular or brown dwarf,
3. Is not a satellite (moon) of another object (see definition of moon),
4. Has sufficient mass to assume hydrostatic equilibrium.

This definition brings Pluto back as a planet and adds Charon, Eris and all the currently recognized Dwarf Planets, of which there four others, and a few other asteroids. That definition also lets us unequivocally define the planet status of those 4000+ know extra-solar planets. The extra clause “or originally formed around a one” also allows us to also include the many millions of Rogue Planets that don’t orbit a star because they were gravitationally torn from their parent by the gravity of a close encounter with another star.

In order to makes the classification simpler, I propose we divide planets into types. Terrestrial planets include Mercury, Venus, Earth, and Mars, because they’re like Earth, mostly composed of rock and metals, and not frozen volatiles, although if too close to their star the rock/metal could be molten. Gas Giant planets, like Jupiter and Saturn are composed almost entirely of gasses. Although we usually group Uranus and Neptune in that category, they and extra-solar planets like them will be classed as Ice Giant planets, as their interiors include a large proportion of ices. Finally, those planets composed of mostly frozen volatiles, like Pluto and Ceres, will be classed as Ice Dwarf planets. Although in our solar system, the classification seems to follow the distance from the sun, that’s not necessarily true elsewhere. Our list of known extra-solar planets contains a large percentage of “hot Jupiters,” gas giant planets in close proximity to their parent star. It’s virtually impossible that they formed there, but due to their stellar system dynamics, they migrated inward towards their star. One of these categories should apply to all rogue planets, too, despite their orphan status.

Welcome back, Pluto.