Pluto's Dwarf Planet Family Could Get Bigger

Although Pluto was kicked out of the planetary club by the International Astronomical Union (IAU) four years ago, the passionate "Pluto debate" rumbles on.

But the fact remains that even before discovery of Eris in 2005 by a Palomar Observatory team headed by Caltech's Mike Brown, Pluto's ranking as a planet in the solar system was tenuous at best. Eris was measured to be nearly 30 percent larger than Pluto; did that make Eris a planet too? No, but its discovery began a series of events that changed our understanding of the solar system forever.

According to a paper presented at the Proceedings of the 9th Australian Space Science Conference, the "dwarf planet club" should start accepting applications from smaller members and according to Brown - discoverer of over 100 Kuiper Belt objects (or KBOs) - this is no bad thing for the beleaguered Pluto.

The solar system currently has six bona fide dwarf planets: Eris, Makemake, Haumea, Quaoar* and Pluto in the Kuiper Belt and Ceres in the asteroid belt (between the orbits of Mars and Jupiter).

However, in this proposal, smaller wannabe dwarf planets could be considered for promotion.

For 50 or so small rocky and icy bodies in the Kuiper Belt, Pluto's loss could be their gain; where Pluto was demoted, they could be promoted. Who said there were no winners to come out of Pluto's re-classification?

"Small Solar System objects are irregularly shaped, like potatoes," said Charley Lineweaver of the Australian National University in Canberra, who headed the research. "If an object is large enough that its self-gravity has made it round, then it should be classified as a dwarf planet."

Indeed, one of the criteria for a celestial body to be considered a planet is that it must be round (i.e. it must be massive enough to achieve "hydrostatic equilibrium" - in other words it must have enough gravity for its surface to be rounded, rather than chunky or potato shaped). But this criterion also extends to dwarf planets, and there are plenty of rounded large asteroids that would make happy dwarf planets.

The 2006 IAU definition of a dwarf planet states that such a body should be massive enough to achieve hydrostatic equilibrium, it must orbit the sun, but it cannot "clear its own orbit." In the latter case, planets (like Earth) are able to fulfill this criterion as they have a stronger gravitational pull, "sweeping" their orbit clear of debris. Dwarf planets do not have this kind of gravitational oomph, so their orbits are often filled with asteroids, other dwarf planets and even planets.

Considering all the criteria for dwarf planets, plus the rule that they must be of certain brightness, dwarf planets have a minimum radius of 420 km (260 miles). But the Australian astronomers want this minimum size to be decreased, down to 200 km (124 miles) for icy bodies and 300 km (186 miles) for rocky bodies.

Why? Well, this is the limit at which small solar system bodies go from being "potato-like" to a more dwarf planet-looking spherical shape. It is therefore dubbed the ‘potato-limit.'

The nicest thing about this proposal is that it sets a physical size of what it takes to be a small dwarf planet. Until now, the IAU definition has been rather arbitrary. But that's science for you, continually developing and refining our understanding of the celestial zoo on our interplanetary doorstep.

But what does this mean for Pluto? Is it a bad thing that its dwarf planet family could face a baby boom in the future?

"I'm sure Pluto is happy with this," Mike Brown told me via email. "Instead of just being the second largest of a small number of dwarf planets, it is now the second largest of a huge population of dwarf planets."

Also, when asked whether this development was a surprise, Brown (who wasn't part of the Australian research) pointed out that the Australian Space Science Conference paper "basically says all the same things that we've been saying since 2005 so I tend to agree with all of it."

Although the demotion of Pluto originally caused uproar (how dare those megalomaniac astronomers disrespect a school textbook favorite in such a way!), the Pluto issue needed to be addressed. After all, it has always been the misfit of the solar system.

*NOTE: As commented by Alan Boyle (of MSNBC's Cosmic Log) Quaoar isn't technically a bona fide dwarf planet, it is a candidate dwarf planet.

Image: An artist's impression of the view of Pluto and Charon from one of Pluto's smaller moons (either Nix or Hydra). Credit: NASA, ESA and G. Bacon (STScI).

Original source: Australian Geographic. Special thanks to Mike Brown.