Between Asteroids and Planets: A New Category?

Our current classification schemes for celestial objects are becoming more and more inadequate as new discoveries are being made. An article published in the October 9, 2009 issue of Science presents compelling evidence that Pallas, located in the asteroid belt between Mars and Jupiter and considered an asteroid since its mid-19th-century demotion from planethood, is not an asteroid at all, but an object of an intermediate category between asteroid and planet, described by some as a "protoplanet," a "planetary embryo," and/or a "baby planet."

Protoplanets are usually thought of as the precursors to full planets during the early formation of solar systems. They are objects in the process of accreting in a protoplanetary disk around a protostar (a star that has not yet ignited through hydrogen fusion). Our early solar system was much more active and violent than today's, as objects in the protoplanetary disk regularly impacted one another and some began the runaway growth that would eventually transform them into full blown planets.

The asteroid belt between Mars and Jupiter is composed of objects that never accreted into a single large planet, mostly because Jupiter, with its strong gravitational field, gobbled up most of the material in that region for itself.

Researchers including Britney Schmidt, who did a very thorough presentation on Vesta and Pallas at the Great Planet Debate, used the Hubble Space Telescope to study Pallas in depth and found that unlike most asteroids, this body is not simply a loosely-held together rubble pile. Measuring 165 miles in diameter, Pallas has surface features and color variations that indicate it experienced thermal evolution and had the potential of growing into a full blown planet.  Pallas may even have some degree of geological differentiation. The researchers theorize Pallas formed from water-rich material and began to experience the same geological differentiation seen on planets, with heavy elements sinking to the core and lighter ones rising toward the surface.

In fact, Pallas, like Vesta, may once have been spherical. An impact crater examined by the researchers indicates Pallas was impacted by a large object some time in its past.

In the words of the "Discovery Space" web site, which can be found at http://dsc.discovery.com/space/big-pic/hubble-pallas-asteroid-protoplanet.html , "The Hubble Space telescope has taken a look at the large asteroid 2 Pallas and realized that this isn't just another large rock with a crater in it. Pallas is a protoplanet."
Opponents of using the criterion of hydrostatic equilibrium as an identifying feature for objects to be designated as planets often raise the question of "borderline" objects about which it is difficult or impossible to tell whether hydrostatic equilibrium has been attained. What is interesting here is that Pallas is almost but not quite spherical, just short of being in hydrostatic equilibrium. That makes it, like Vesta, one of those "borderline" objects skeptics often raise in their arguments.

While Pallas had the geology that put it on the path of becoming a planet, including liquid water and geological processes, the process was stopped and frozen in place early in the lifetime of the solar system due to Jupiter's gravitational influence. Yet Pallas, which researchers say "is closer to a planet than to a typical asteroid," remained largely intact in spite of early impacts, making it--and Vesta--unique solar system objects, essentially "fossils" representing a state all planets went through on their journey to becoming planets.

In his October 8, 2009 column, Alan Boyle states: "The bottom line is that Pallas is, well, right on the line when it comes to the important features dividing the solar system's big planets and dwarfs (and, for that matter, roundish natural satellites such as our moon) from irregular objects such as small asteroids and comets." See http://cosmiclog.msnbc.msn.com/archive/2009/10/08/2092402.aspx

What this means, is that any classification system for this and other solar systems, if purporting to be accurate, essentially requires a new, intermediate category between asteroid and dwarf planet (I'm using dwarf planet as a subclass of planet for objects that orbit stars and are in hydrostatic equilibrium but do not gravitationally dominate their orbits--Stern's "unter planets"). Assigning Vesta and Pallas to the asteroid category does not do them justice because it tells us nothing about their advancement beyond the state of most asteroids, erroneously placing them in the same grouping as large rubble piles that never came close to being shaped by their own gravity. We can call this intermediate category "protoplanets," "planetary embryos," "baby planets," or something else, but for the sake of accuracy and thoroughness, astronomers must designate a new and separate category for them.

We may discover some borderline Kuiper Belt Objects that also fall into this protoplanet category. This would provide an answer to the skeptics who ask about those objects on the "borderline."  Incidentally, Pluto, Eris, Haumea, and Makemake do not fit into this category, as their being in hydrostatic equilibrium is not in doubt.

Should protoplanets be considered a subclass of planets? Maybe the answer is to establish a spectrum that reflects what is really out there instead of neatly putting things into categories, resulting in unlike bodies thrown together in classifications such as the IAU's "small solar system bodies." We have such a spectrum for stars, the Herszprung-Russell Diagram, and it has been modified with the addition of extra categories to account for brown dwarfs, a class of objects on the bottom of the stellar category, massive enough to fuse deuterium (an isotope of hydrogen) for a short time but not massive enough to ignite as full-fledged stars. Protostars, "baby stars" in the process of formation, stars that have not yet "turned on" and begun hydrogen fusion, still have a place on the Herszprung-Russell Diagram. Why not set up a similar diagram for planets with "protoplanets" at the bottom, just above asteroids. This seems far better and more comprehensive than throwing a very diverse group of objects together under a broad wastebasket category called "small solar system bodies."

Creating such a system should not be the big deal it has turned out to be. And it does not have to be done by the IAU. Instead, how about planetary scientists getting together and working on a useful classification system? It's about time.