So, Is Pluto a Planet or Not?

Hubble photograph of Pluto and its three moons. Image credit: Hubble. Click to enlarge.
Unfortunately, the Solar System isn’t so simple. The case for Pluto’s planethood status has gotten a little eroded since its discovery, and there are further challenges facing it into the future.

The four gas giants are clearly planets. They dominate their respective orbits, and have clusters of moons, rings and all sorts of features that separate them from the rock and rubble of asteroids, comets, and other icy objects. Pluto, on the other hand, is nestled inside the Kuiper Belt; a vast population of ice bodies extending beyond the orbit of Neptune. There are an estimated 70,000 objects in the belt larger than 100 km (62 miles) across, and Pluto appears to just be a particularly large example.

As powerful observatories and space-based telescopes push out our understanding of the Kuiper Belt, many new objects have been discovered; several are close in size to Pluto. For every scientific measurement you can give Pluto: size, mass, moons, orbit, it ends up being a large Kuiper Belt Object. The brave members of the Bad Astronomy/Universe Today forum are giving this challenge their best attempt to define a planet.

And this controversy has been expanded with the discovery of 2003UB313 by the team of Michael Brown, Chad Trujillo, and David Rabinowitz. Also part of the Kuiper Belt, this object – code named Xena for now – is about 3000 km across. That makes it 700 km (430 miles) larger than Pluto! Its 557-year orbit is highly eccentric, varying between 38 and 98 astronomical units (the distance of the Earth to the Sun). Pluto, on the other hand, has an orbit that varies between 29 and 49 AU, and Neptune is 30 AU.

So there are times when Xena gets closer to the Sun than Pluto… and it’s bigger. Oh, and it probably has a moon too (code named Gabrielle). Is Xena a planet? If not, why does Pluto get to remain a planet, since it’s smaller, and sometimes orbits further from the Sun.

Objects have been unplaneted already. Before astronomers realized there were thousands of asteroids in the main asteroid belt, the first 4 discovered were considered planets for several decades: Ceres, Pallas, Juno and Vesta.

What’s a planet then? The International Astronomical Union has developed some definitions in 2001 for extrasolar planets, and modified them as recently as 2003, so we can start there.

Under their definition, planets are any objects orbiting stars or stellar remnants (like pulsars) which are below the limiting mass for thermonuclear fusion of deuterium. This sets an upper limit at about 13 times the mass of Jupiter.

What about a lower limit? Well, the IAU goes on to state that the minimum size/mass for an extrasolar planet should be under the same criteria for what’s used to define planets in the Solar System. This brings us right back to the beginning. When super powerful telescopes are developed that can detect objects as small as Pluto around other stars, whether or not they’re planets depends on Pluto’s planetary status.

Back to the beginning, then.

Mike Brown, one of the astronomers who original discovered Xena, has heard rumours that the International Astronomical Union is going to be discussing this dilemma at their upcoming meeting in Prague in August 2006. We could wind up with 8 planets (sorry Pluto), 9 planets (nothing changes), or 10 planets (welcome Xena and all future super-Plutos). And if the IAU extends this to 10 planets, will 11 be around the corner? Are you ready to memorize the 30 planets?

Brown states on his website:

• A special committee of the International Astronomical Union (IAU) was charged with determining “what is a planet.”
• Sometime around the end of 2005, this committee voted by a narrow margin for the “pluto and everything bigger” definition, or something close to it.
• The executive committee of the IAU then decided to ask the Division of Planetary Sciences (DPS) of the American Astronomical Society to make a recommendation.
• The DPS asked their committee to look in to it.
• The DPS committee decided to form a special committee.
• Rumor has emerged that when the IAU general assembly meets in August in Prauge they will make a decision on how to make a final decision!

Whatever they decide, NASA is going to see Pluto up close. New Horizons just launched earlier this year, and it will take 9 years to reach Pluto in 2015. Its Pluto/Charon encounter will begin in July, and last for more than 100 days, giving us our first close up look at this planet/big Kuiper Belt Object. By the time it arrives, we can only hope the IAU has made up their minds.

If the decision were up to me, I’d say Pluto is a planet. For starters we wouldn’t have to go back and edit all those astronomy textbooks, websites, sculptures, museum exhibits and PBS documentaries. Our Solar System just isn’t so simple; objects scale from the tiny to the huge, with all sizes in between. Any decision on Pluto’s planethood will be an arbitrary one, and the arbitrary decision I like is… Pluto’s a planet.

Written by Fraser Cain