Planets, Dwarf Planets, and Moons

Before Galileo raised his telescope to the sky, the only known Solar System bodies were the ones that could be seen with the naked eye (and which share their names, in most Latin-based languages, with the days of the week): the Sun and Moon, Mercury, Venus, Mars, Jupiter and Saturn. Galileo added Jupiter's four large moons to the list (and came close to adding a planet, but more about that later). His successors, using better and better telescopes, added Uranus, Neptune, Pluto, and a host of additional moons.

For a long time, we saw the planets merely as dots of light in the sky, possibly with some imagined influence on our lives. As we began to recognize them as worlds, they became the settings for our fantasies of exotic creatures and civilizations. With the advent of advanced telescopes and robotic spacecraft, the planets became real places to us. We have better maps of Venus and Mars than of much of Earth. We've taken multi-year tours of Jupiter and Saturn. We've even flown by Uranus and Neptune, getting a close look at their rings and their strange, tilted magnetic fields.

The moons have become worlds in their own right, every bit as fascinating as the planets they orbit. We've cruised by volcanoes erupting on Io, hydrocarbon lakes on Titan, and shepherd moons tending the rings of Saturn. In fact, aside from Mars, the moons—in particular, Jupiter's Europa and Saturn's Enceladus—have become the most promising places in the Solar System to look for extraterrestrial life. (See the article on Jupiter, the September "Go Observe" feature.)

Now our view of the Solar System has taken a new twist. The discovery of a large population of small worlds at the edge of the known Solar System has prompted us to reconsider how we think of the planets, sparking an uproar among fans of Pluto.

Snow White and the 200 Dwarfs

Pluto's troubles began in earnest in January 2005, when a team of astronomers led by Mike Brown of Caltech discovered an object far beyond the orbits of Neptune and Pluto, ultimately named Eris. It was spherical and it was pretty big—slightly larger and more massive than Pluto. And it was just the largest of a slew of spherical objects beyond Neptune that were discovered within a few years of Eris, including Quaoar (pronounced "Kwa-war"), Haumea, Makemake, Orcus, and an object that Brown dubbed "Snow White," but which is likely ultimately to trade its name for that of a mythological creator deity like the other newly discovered objects beyond Neptune's orbit.

In all, Brown and his colleagues discovered 14 of these things among the Kuiper belt comets, plus one called "Sedna" which orbits the Sun far beyond the Kuiper belt and may be the first observed resident of a hypothesized inner Oort cloud. Combined with the findings of other astronomers, Brown estimates that about 70 such objects have been observed, although as yet, the International Astronomical Union recognizes only five. All together, he estimates that there are some 200 objects beyond Neptune that are massive enough to pull themselves into spheres under their own gravity.

At these staggering distances, however, our best telescopes are unable to resolve these objects into the spheres they are presumed to be. Their shape is inferred from their apparent mass, which astronomers estimate from measurements of the light they reflect and the heat they emit. "To be fair," Brown said, "we don't really know if anything other than Ceres (the largest object in the asteroid belt) and Pluto are round. We just know that it would be hard to keep them from being round."

When is a planet not a planet?

So are all of these spherical objects planets? If they're not, and if at least one of them is bigger than Pluto, then what is Pluto? Has the number of planets in the Solar System jumped from 9 to 13 or 79 or 200 or more? What exactly do we mean by "planet"? Astronomers realized that this question hadn't been formally addressed since the ancients used the word to describe the celestial lights that wandered across the background stars from one night to the next.

But informally, we have been distinguishing planets from moons for a very long time. And we've been doing so based not only on what these objects are, but also on what they do. If an object orbits something significantly bigger than itself as it circles the Sun, it's a moon regardless of how big and round it may be. If Titan and Mercury switched places, then Titan would be a planet and Mercury would be a moon.

In 2006, the International Astronomical Union decided to take this approach a step further. In order to qualify as a planet from now on, it decided, not only must an object be massive enough to pull itself into a sphere, and not only must it not orbit anything other than the Sun, it must also dominate its region of space. It must either capture or toss aside the bulk of any asteroids, comets, or other debris in its path, as all the planets from Mercury to Neptune have done. It must unquestionably own its orbit.

By this definition, nothing in the asteroid belt or Kuiper belt can be considered a planet because nothing in those zones has cleared away the asteroids and comets. And the Kuiper belt is where Pluto lives. So now, after 76 years of being the ugly duckling of planets (scrawny and strangely behaved, though beloved by many), Pluto has emerged as a swan in a new category of heavenly bodies: the dwarf planets. Keep in mind, though, that this is not necessarily the end of the story. Scientists always reserve the right to reclassify objects based on further discoveries, thought, and discussion.

But for now at least, Neptune is considered the last of the Solar System's true planets, and the news comes just in time for a milestone in its career. At 30 times Earth's distance from the Sun, Neptune takes 164 Earth years to complete one orbit. And so in 2010, it completes exactly one trip around the Sun since its official discovery in 1846. Just one Neptunian year since that planet was first seen by the eye of man.

But wait! It turns out that when Galileo tracked Jupiter, he noted a "star" in the vicinity that moved across the sky like a planet. He evidently didn't realize what he was observing, but one of the "stars" diagramed in his notebooks is very close to where astronomers calculate that Neptune would have been at that time, and it doesn't match the position of any known real stars. So it is very likely that Galileo discovered Neptune more than two centuries before its official unveiling.

Galileo unlocked the door to the Solar System. The scientists and engineers who followed him opened it wider than Galileo perhaps could have imagined. So what message would someone like Mike Brown, who has glimpsed the furthest reaches of the Solar System, send to Galileo across the centuries if he were able?

Prof. Brown considered the question for a moment and replied, "You were right!"





Mars Reconnaissance Orbiter image of Mars.

Thanks to spacecraft like Mars Reconnaissance Orbiter, which took this picture during its mapping orbit, the planets have become real places to us. A person standing on the Martian surface would be barely visible in this image.

Jupiter's moon Europa.

In our solar system, there are more moons than extraterrestrial planets that are considered promising places to look for life. Jupiter's moon Europa, seen here, is a leading contender.

Eris, with its satellite Dysnomia.

Eris, seen here with its satellite Dysnomia, was found to be slightly larger and more massive than Pluto, prompting a new look at what it means to be a planet.

Mercury and Titan.

It’s not just what you are that makes you a planet, it’s also what you do. Mercury (left) is a planet and Titan (right) is a moon. But if they switched places and Mercury orbited Saturn while Titan orbited only the Sun, then Titan would be a planet and Mercury would be a moon.

Diagram of the "new" solar system.

In this diagram of the “new” solar system, the black circles represent the orbits of Jupiter, Saturn, Uranus, and Neptune. Mercury, Venus, Earth, and Mars lie within the innermost black circle. The red lines indicate the orbits of objects thought to be dwarf planets. Credit: Mike Brown, Caltech.


Neptune, imaged by NASA’s Voyager 2, the only spacecraft so far that has flown close to the planet. It is now considered the most distant planet from the Sun.

Infant stars

Artist's rendering of the LCROSS separation from spacecraft Centaur. LCROSS is scheduled to impact the lunar South Pole Oct 9, 2009 at 4:30 a.m. PDT, in search of water on the Moon.