Forgotten Planet

October 18, 2009

There’s another dwarf planet to add to the list of solar system bodies that  share minor league status with Pluto.

Newly published Hubble Space Telescope pictures show that the large asteroid Pallas is nearly spherical. In other words the body has enough gravity to pull itself into ball where all surface features are essentially the same distance from the core.

This is one criterion for a planet according to the International Astronomical Union (IAU). Hubble’s sharp view can resolve the disk of Pallas and shows that it is slightly egg-shaped, and roughly the width of West Virginia.

Pallas is the third most massive asteroid following Ceres and Vesta. Serveral years ago Hubble showed that Ceres too is a sphere. But poor Vesta got ripped off. The southern pole was lopped off by a titanic impact that left Vesta distinctly non-spherical. So, by the IAU rules, it fails at planethood. The IAU purists might get hung up on semantics and argue that Pallas isn't a perfect sphere either. But frankly neither is Earth, it is pear-shaped.

The precise physical measurements of Pallas can be used to calculate a density that falls midway between it being a ball of all rock, or a ball of all ice. This means the Pallas probably formed from water-rich materials, like its bigger brother Ceres.

This all implies Pallas is made from ice and rock and differentiated because it is big enough to have  a hot core of radioactive debris from one or more nearby supernova explosions that preceded our sun’s birth.

The Hubble team also thinks they see a large bowl shaped crater  about 9 miles deep. This shouldn’t be a surprise, considering the pockmarked appearance of other asteroids. The absence of craters would worry me that maybe Pallas is the Star Wars Death Star.

Pallas in fact enjoyed planet status when discovered in 1802. By the mid 1800’s astronomers decided there were simply too many objects swarming in the vast 300 million mile wide gulf between Mars and Jupiter. so they demoted Pallas and other so-called minor planets to “asteroids” (for star-like). The demotion back in the 1800s didn’t cause all the fuss that Pluto’s demotion from major planet status did in 2006.

Therefore, a rational way to categorize the solar system is that it contains four classes of planets: terrestrial, gas giants, icy dwarfs, and rocky dwarfs. This is likely the standard makeup of typical planetary systems scattered across the galaxy.

If Pallas turns out to have a water-ice mantle, it is a potential abode of life. The asteroid would be an easier place to land on than Mars because of its much weaker gravitational field and lack of atmosphere. Unfortunately, it's hard to get to becasue its orbit is far out of the ecliptic plane where the other planets and most asteroids reside.

 

Comments

[Bob]

Since childhood I have called these large asteroids 'planetoids'. So, 100-years after the change, text books still had them as minor planets.
If we have decided to call these large objects in the asteroid belt 'asteroids', why are we not calling the large objects in the Kuyper belt 'Kuyperoids'? Why are we calling them 'Plutoids'? Will it take more than 100-years too shed the notion of a planet Pluto?