Our dictionary is bird-shotted, our synonymicon is swiss-cheesed…and what’s it all got us? Nottadernthing!!!
—George Herriman, Krazy Kat (1926)
Most of the objects in the Asteroid Belt are pretty small, but a few are of substantial size. Ceres (which is one of the official dwarf planets) and Vesta particularly stand out; the Dawn space probe is currently orbiting Vesta and will visit Ceres in 2015. Despite their relative closeness to Earth, these asteroids are small enough that they are hard to study even with powerful telescopes, so a lot of our knowledge about them is still sketchy. Based on flybys, many asteroids appear to be “rubble piles”: rather than being solid objects, they consist of chunks of rock held together by mutual gravity. Larger asteroids don’t fall into that category, but exactly what they are like on the inside is a lot harder to determine.
However, at the Lunar and Planetary Science Conference in Texas last week, the research team analyzing data from the Dawn space probe announced that Vesta may be very like the terrestrial planets. Based on a variety of studies, Mercury, Venus, Earth, and Mars all seem to have similar interiors: a core of solid or molten iron, a mantle that can deform under pressure (very like plastic, albeit made of rock), and a solid crust. (Earth’s Moon also shares this structure.) The details vary—as I reported in Ars Technica, Mercury’s interior seems particularly weird—but the basic core-mantle-crust differentiation seems like a pretty standard blueprint. Based on Dawn’s study of Vesta’s gravitational field, the asteroid also appears to have a solid iron core that takes up a lot of the interior, with a mantle making up the remainder of the interior. Vesta also has a magnetic field, a feature it shares with Earth and Mercury.
At first glance, Vesta isn’t very planet-like at all, or at least not like what we are conditioned to think of as “planet-like”: it isn’t even approximately spherical. It’s also quite small: only 578 kilometers (359 miles) across at its widest, with a correspondingly small mass (0.004% of Earth’s mass and 0.4% of the Moon’s mass). However, the strongly differentiated interior structure indicates that Vesta has a great deal in common with the terrestrial planets, possibly including the basic outline of how it was formed billions of years ago.
Being me, I started thinking. I’ve written several posts about the meaning of “planet” (including one even titled “What is a Planet?“), so I guess you could title this post “Matthew can’t leave well enough alone”. Not that wisdom has ever stopped me before, so here are some untutored thoughts about planets, as sparked by Vesta.
The debate over what should be called a “planet” usually centers around Pluto for historical reasons, though it’s important to note that after their discovery, the first observed asteroids—including Vesta—were called planets. After their numbers grew until they outnumbered the “classical” planets they were reclassified, but the old nomenclature still survives in some places where you’ll sometimes see them referred to as minor planets. Little was known about their structure and shape until much later: the name “asteroid” refers to them being “starlike”, meaning astronomers only saw them as points of light in their telescopes.
Pluto is obviously not in the same category as the asteroids, but it also doesn’t resemble the classical planets either: it’s tiny (though still bigger than the largest asteroids), icy, and generally odd. Well, it was odd until the discovery of other objects in the outer reaches of the Solar System. Eris, Haumea, Makemake, Quaoar (my favorite name), Sedna, Orcus, etc. show the true nature of the trans-Neptunian region, revealing that Pluto isn’t alone. Obviously some big questions remain: for example, Eris is significantly more massive than Pluto even though both appear to be nearly the same physical size, and some of the iceballs are highly reflective while others are highly unreflective. For extra oddness, Haumea is ellipsoidal rather than spherical. We’re just learning about the objects Out There, and while the New Horizons probe will help out, I imagine it’s going to be some time before we’ll know as much about the outer Solar System as we do about regions closer to home.
So what do we think about Vesta’s decidedly terrestrial character? We’re at an interesting juncture in planetary science: we have quite a few objects in our census, but an inefficient vocabulary. Perhaps the best comparison lies in taxonomy (the arrangement and naming of species), where there are fashions in lumping together organisms into larger groups by common characteristics, or splitting them into smaller groups according to fine variations.
The International Astronomical Union’s criteria for planets is an example of splitter thinking (see my earlier post for details): they were essentially designed to cut off the list of planets before it grew too big. After all, if Pluto is a planet, so is Eris, and many of the others I mentioned previously would have to be included as well. If you don’t mind including all those other objects, you definitely fall into the lumper mentality. Some people (call them the Extreme Lumpers) propose a broader definition for planets: if it’s massive enough to be spherical under its own gravity, it’s a planet. (The slightly convoluted wording is so that Haumea can be included: it’s big enough to be spherical, but due to its interesting history it’s ellipsoidal instead.) Extreme Lumping like this will make for a lot of planets—perhaps hundreds.
Extreme Splitting (a position I’m sympathetic toward) begins by noting that the Jovian planets—Jupiter, Saturn, Uranus, and Neptune—don’t resemble the terrestrial planets very much. They are chemically and structurally different, and likely formed in a different way, at least as I understand it. Neither the Jovian nor terrestrial worlds are like Pluto and its cousins, which based on density have a high proportion of ice in their structure. Without the kind of data New Horizons will provide, we can’t know more about the details of Pluto’s interior (e.g., is it differentiated like Earth), but it seems safe to conclude that it won’t turn out to be much like the terrestrial worlds on the inside or outside. (It might be somewhat like Titan, on the other hand, which is an intriguing possibility!)
With Extreme Splitting, we seem to have at least three types of planet-like objects in the Solar System: rocky bodies, Jovian worlds, and icy bodies. We can split even more by distinguishing between rubble-pile asteroids and their more solid cousins, and perhaps dividing between Eris-like and Pluto-like frozen worlds due to the different mix of rock and ice. Jupiter and Saturn have noticeable differences from Uranus and Neptune, and thus some people divide “gas giants” from “ice giants”. But why stop there? A lot of planet-like objects are moons: Europa, Enceladus, Titan, and so forth each have features that are interesting. Earth’s Moon is more like a terrestrial planet, Phobos and Deimos are asteroid-like (asteroidoid?), and maybe Neptune’s moons resemble Kuiper Belt objects. Extreme Splitting lets us get at the fine details of what characteristics objects share, and how they differ.
Of course if you carry Extreme Splitting too far, you end up with each object lying in its own category. However, I’m advocating Extreme Splitting for a different reason: if we break all our categories down far enough, we can begin to lump them in new and different ways. We have gotten it into our heads that “planet” is a sacrosanct notion, but in many ways that’s a carry over from pre-scientific times. Our categories serve us, not vice-versa: a one-size-fits-all definition for planet may be something we need to abandon. Vesta is like Earth in some ways, and not in others, so for some purposes it may suit us to lump them together, but split them in other ways. (In taxonomy, it may suit scientists to lump dinosaurs and birds together for some purposes, but distinguish them at other times; arguing over whether Archaeopteryx is a bird or dinosaur may be like arguing over whether Pluto is a planet in that sense.)
Rather than making Big Lumps our goal, maybe we should aim to make our categories more fluid. This has an added advantage for those who teach or care about science education: it breaks free of the attitude that there is a list of planets to memorize. Instead, by focusing on the broader ideas revealed by Extreme Splitting, we are freed to lump objects together in different and dynamic ways, which leads students into a deeper understanding of how science works. Additionally, many exoplanets don’t fit neatly into our existing lumps: “super-Earths”, “hot Jupiters”, and “waterworlds” are already terms in use. If nothing else, maybe we can stop splitting hairs over whether a potential planet candidate “clears its orbit”.
Much of the wonder of science is in studying objects for their own sake. Pluto and Vesta are interesting whether they are called planets or not, after all. My thinking has evolved on this issue, and likely will continue to do so, but as I have been reading about Vesta and Mercury, I feel we should use them as an opportunity to change the way we think. The diversity of planet-like objects in our Solar System and beyond should encourage us to find ways of expressing that diversity, and lead us to create new categories, rather than getting stuck with our old ones.