What is a Planet?

(For some reason, I’ve been getting repeated Ask.com hits for “What is a planet?” even though the Demote Pluto entry has slid off the front page. Askers, this entry is for you.)

The word “Planet” comes from the Greek “planetos,” meaning “wanderer,” for how they seemed to move independently of the stars. In a sense, we can loosely term anything revolving around the sun a “planet,” from a huge gas giant to a fragment of rocket debris which has left earth and entered into a solar orbit. More specifically, however, we reserve the term “planet” for those larger bodies we have all grown up knowing: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and until recently, Pluto. (By the way, I’ve never needed any “My Very Educated Mother” mnemonics. We were taught the order of the planets straight out in kindergarten, and I memorized them on the spot — although I would occasionally get Uranus and Neptune reversed for a few years.)

Pluto’s planet status has come under fire in recent years thanks to the discovery of more and more similar objects in its vicinity. (If the vastness of the outer solar system could correctly be termed a “vicinity.”) These objects, along with Pluto, are now known to be sizeable representatives of what we call the “Kuiper Belt,” a loose disc of small icy bodies pushed to the outer solar system by the gravitational interactions of Jupiter and Neptune. To prevent the confusion of what could be hundreds or thousands of large Kuiper Belt Objects being termed as planets, astronomers of the International Astronomical Union recently voted on a contentious series of resolutions to clarify the term “planet.” The following requirements were defined for a body to be considered a planet:

  • Is orbiting the sun.
  • Has sufficient mass for its gravity to pull it into a spherical shape.
  • Has not so much mass that nuclear fusion has started in its core.
  • Has “cleared its orbital neighborhood.”

That last requirement has been the eye of a recent semantic storm, as it is the rule which disqualifies Pluto from full planethood, yet is phrased to make it seem that a planet must have “cleared” its “neighborhood” by accreting other matter into itself such that it is the only concentration of mass in its orbit. This is, of course, a faulty understanding of the concept, as the solar system is a crowded place, and planets must share their orbits with various moons and asteroids. As astronomer Michael Brown explains, the concept of clearing” refers to a planet’s ability to gravitationally dominate other bodies in its orbit, either by accreting them, capturing them into orbit or at stable LaGrange points, forcing them into an orbital resonance, or causing them to leave their orbit.

In this sense, planets with moons can be said to have “cleared” the moons by keeping them in secondary orbits. Planets with co-orbital asteroids — including Earth — “clear” these asteroids by collecting them in gravitational calm spots called LaGrange Points, which revolve in positions synchronized to the planet. Earth and Jupiter, for example, both have moons and co-orbital asteroids which are regarded as “cleared.” In response to those who use the faulty interpretation of “clearing” to argue that Neptune cannot be a planet by the IAU definition because it has not “cleared” Pluto, it should be noted that Neptune’s gravitational influence has locked Pluto into a 3:2 orbital resonance. This establishes Pluto as first among a special class of Trans-Neptunian objects called “plutinos,” all of which have orbits in the area of Neptune’s, whose cycles have also been locked into the same 3:2 resonance by interactions with Neptune’s gravity.

Pluto, on the other hand, has not cleared its orbital neighborhood of Neptune, nor does it exercise significant gravitational influence over other Kuiper Belt Objects or Trans-Neptunian Objects outside of its own binary partner, Charon. It does, however, have sufficient mass to attain a spherical shape — as do other large KBOs recently discovered, like Quaoar, Sedna, and 2003 UB313 — which is why the IAU has assigned to them the status of “dwarf planet.”

This demotion — actually a kind of promotion, really, as it boosts Pluto from “smallest planet” to “one of the largest spherical KBOs” — has caused significant outrage, as a lot of non-astronomy-minded people out there seem to identify sentimentally with Pluto. Perhaps they project their own personalities onto Pluto as a lonely planetary underdog floating in the outer reaches of its environment, and the semantic demotion of Pluto can be seen as a cosmic confirmation of a deeply rooted societal outcast complex?

Ah, but this is not psychology or sociology, this is astronomy, and as Michael Brown says in his NY Times op-ed piece: “After all, it’s not a great idea to let cultural attachments dictate scientific categories.”

Hence the Demote Pluto Campaign, now a defense of the IAU decision. Granted, the “planet” definition is still one that needs refinement and clarification, and a demotion to “dwarf planet” status is certainly no reason to cease all Pluto-directed exploration initiatives; but we are on our way to a deeper understanding of the kinds of things that float through outer space, and they will most likely continue to defy our feeble attempts to pigeonhole them into neat, tidy categories. The best we can do is keep exploring, researching, and learning. The worst we can do is insist that our nostalgic feelings for a cartoon character large Kuiper Belt Object control how we approach the incomprehensible vastness of the universe.



  1. I’d expect this brazen assault from a PC (planetary-correctness), inside- the-beltway-kind-of-guy. ;)

    Out here in the hinterlands of SW Oklahoma, we certainly pull for the underdog planets. We also appreciate icy rocks, as we’ve had an unbearable hot summer.

    And who’s this Kuiper feller, and where did he get his belt?

    Pluto Lives!

    Chairman, Plutonian Restoration Movement