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
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.