In August 2006, the International Astronomical Union (IAU) voted on a definition of “planet”. To be considered a planet,

1. The object must be large enough to be spherical under its own gravitational force.  This is a general principle:  if a chunk of rock or a ball of gas has enough mass, it tends to become spherical just because that’s the way  gravity works.  Smaller objects, like pebbles or baseballs, can be spherical for other reasons, namely erosion or forces other than gravity, so some Solar System bodies lie on the boundary of this criterion.  Two examples of this are the asteroids Vesta and Pallas, which appear to be roughly spherical, but might not quite be massive enough to count.
2. The object must orbit the Sun directly.  Certain moons, like Ganymede (which orbits Jupiter) and Titan (which orbits Saturn), are actually more massive than Mercury, and Titan has a thicker atmosphere than Earth.  However, since they orbit planets rather than following a direct orbit around the Sun, they don’t count as planets in their own right.
3. The object must dominate its orbit.  In other words, the body must be the most massive object in its neighborhood.  It also, by virtue of its gravity, cleared most of the smaller objects out of its path, so that impacts won’t substantially change its orbit.
4. The object must be small enough that it doesn’t emit its own light via nuclear fusion.  Planets don’t emit their own light:  we see Venus, Jupiter, and the like by the light they reflect from the Sun.  Stars like the Sun, on the other hand, compress the hydrogen atoms in their cores so much that they fuse into helium, releasing a huge amount of energy in the form of light.  This criterion isn’t terribly important for our Solar System, since Jupiter (by far the largest planet) comes nowhere near the threshold for becoming a star.

Item 3 is the controversial one, for astronomers and non-astronomers alike.  For astronomers, the question is how to define the amount of domination?  Earth’s orbit crosses the orbit of many asteroids and smaller objects—impacts from some of these bodies in the past have caused mass extinctions (such as the Cretaceous impact that helped finish off the dinosaurs)—but we want to make sure the Earth counts as a planet despite this.  Earth’s planethood isn’t controversial, so we don’t need to worry about it.

However, the third  criterion is the one that changed the status of Pluto.  Pluto has not “cleared the neighborhood” of its orbit, since it is part of the Kuyper Belt, a region of space beyond the orbit of Neptune.  The Kuyper Belt is full of small icy objects, at least one of which—Eris—is larger than Pluto, and many of which are of comparable size to Pluto.  So astronomers were stuck with a new problem:  if Pluto is a planet, then Eris should be as well.  If Eris is a planet, why not Makemake or Quaoar, which are of comparable size to Pluto?  Astronomers needed to establish a rule to prevent us from having potentially dozens of new planets. We also need a definition of planet not based on sentiment or historical precedent:  if we say that Pluto is a planet because it has always been considered a planet, then we are making that decision unscientifically.  The Mousketeer role call of nine planets must fall, then:  either Pluto is a planet, and other bodies such as Eris must be included, or Pluto is not a planet.

So the IAU definition of 2006 declared that Pluto, Eris, and the asteroid Ceres are “dwarf planets”, objects that fulfill items 1, 2, and 4 from above, but fail to fulfill item 3.  To be very clear on this—nothing has changed about Pluto’s intrinsic character!  Some newspaper  articles made it sound like Pluto doesn’t exist anymore, or that something about Pluto had altered.  This is not true at all:  astronomers just settled for the first time in modern history on a definition of what a planet is.

To connect all of this with recent news, the original attendance list of dwarf planets has been expanded within the last two weeks to include the Kuyper Belt object named Makemake.  (The name refers to the god who created humanity in Polynesian mythology.) Makemake was first discovered in 2005, but it wasn’t officially named until this summer.  Although its mass is not known accurately yet, it appears to be about 2/3 the size of Pluto.  It’s a pretty safe assumption that astronomers will continue to find objects like Makemake, which will swell the ranks of dwarf planets.

So even though item 3 in the definition of planet above may be controversial, it seems like a good idea to have some way to distinguish the 8 major planets (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune) from the growing number of Kuyper Belt objects comparable in size to Pluto.  Unless of course you want to start teaching schoolkids that there are 11 planets… er, 12 planets, er…. as the number gets revised upwards.

Posted by Matthew R. Francis under Science Ideas & Science News |