It's Time to Demote Pluto... Again

Originally published in the California Tech, page 3.

In 2006, the International Astronomical Union (IAU) voted to reclassify Pluto, stripping it of its status as a planet while establishing a new class of celestial bodies: dwarf planets. While it makes sound scientific sense to demote Pluto from its planetary status, I argue that we should go even further by destroying the designation dwarf planet — a category which is both scientifically useless and pedagogically confusing. Instead, we should call Pluto what it really is: a Kuiper Belt Object.

So why, according to the IAU, isn’t Pluto a planet? The IAU has established three criteria for evaluating whether or not an object is a planet or dwarf planet. According to resolution B5¹, a planet is “a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to … [assume] a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood [sic] around its orbit.” The first criterion is fairly straightforward, separating the notion of planet from the notion of a natural satellite or moon. The second is a statement about the size of an object — a planet must be big enough for gravity to be the dominant force sculpting its shape. And the third is, well, confusing. Unfortunately, the third criterion is critical. It is the qualification Pluto fails and it establishes the difference between a planet and a dwarf planet. Planets have “cleared their neighborhood” and dwarf planets haven’t.

What the “clearing its neighborhood” criterion comes down to is gravitational influence. Planets, especially large ones like Jupiter, have gravitationally dominated their orbits. Anything that passes too close to Jupiter will either crash into the planet or be ejected from the solar system. In this way, Jupiter clears its neighborhood. The same process works for the other seven planets as well. However, Pluto exists in a belt of similar objects — the Kuiper Belt — where its diminutive size is enough to make it round, but not enough to clear its orbital path.

Because this process is inherently gravitational, it means that the IAU criteria establish two separate size thresholds, both of which must be passed in order to be a planet. Dwarf planets, only passing the “roundness” criterion, exist in a sort of in-between size category, a poor consolation prize to satiate angry Plutophiles. The dwarf planet distinction fails on two counts: it groups together objects with very little in common (other than roundness) and it fails to group together objects that share important physical properties and histories.

There are five objects in the solar system that qualify as dwarf planets: problematic Pluto, our own Mike Brown’s Eris, Ceres (the largest object in the asteroid belt) and the two obscure additions of Haumea and Makemake. Pluto, Eris, Haumea and Makemake are all residents of the Kuiper Belt, which lies beyond Neptune’s orbit, while Ceres is located much closer to the sun as the largest resident of the asteroid belt, which is between the orbits of Mars and Jupiter. While the Kuiper Belt Objects (KBOs) in this group have much in common with each other, they are vastly different from Ceres, both in composition and history. Ceres is primarily rocky; the KBOs are icy. Ceres, as a member of the asteroid belt, has primarily been influenced by Jupiter, while the KBOs’ histories are heavily shaped by the influence of Neptune.

Lumping Ceres together with these other objects has real consequences: it leads to the impression that Ceres is located in a completely different region of the solar system. As an astronomy outreach educator, I have encountered several aspiring amateur astronomers who mistakenly believed Ceres orbited beyond Neptune. The category dwarf planet, then, is misleading, a term that obscures truth.

A categorization that makes more sense is to group Ceres with objects that share its composition and history — the asteroids. Ceres may be an exceptionally large member of the asteroid belt, but this does not warrant the distinction of “dwarf planet.” Similarly, Pluto, Eris and their lesser known cousins should be classified alongside the rest of the Kuiper Belt Objects, with which they have more in common than with outlier Ceres.

Superstar astrophysicist Neil deGrasse Tyson advocates for a similar zone-like division of the solar system², separating asteroids and Kuiper Belt Objects. He even goes as far as splitting the planets into two categories — inner planets and outer planets — a division that again reflects the shared composition and history of the rocky planets and gas giants. He implemented this categorization in his design for the solar system exhibit in the Hayden Planetarium before the discovery of Eris, before Pluto’s demotion was even up for discussion.

But this division makes pedagogical sense. Simply memorizing a list of planets is not instructive and leaves learners with a rigid and inflexible understanding of science, as the backlash against Pluto’s reclassification demonstrates. Teaching the solar system as a collection of different classes of objects opens up a more flexible understanding of science and leads naturally to scientifically relevant questions. Why, for instance, is it that the inner planets and outer planets are separated by belt of asteroids? Why are inner planets rocky and outer planets gassy? Such inquiries are more reflective of the true nature of science. Science is not simply a list of facts, but a systematic way of asking questions and organizing knowledge. Shouldn’t we, as scientists, strive for terminology that accurately reflects the exciting, ever-changing processes by which we discover it in the first place?

***

I originally wrote this piece for En/Wr 84, Caltech's science writing course. I plan on posting more material from that class soon.

¹ You can read about the IAU definition here.
² See Tyson's The Pluto Files

Blinky-Blinky: How to Discover Kuiper Belt Objects

The Remote Observing Facility doesn't look special from the outside. It's one of many featureless white doors on the maze-like first floor of Caltech's Cahill Center for Astronomy and Astrophysics. Tonight, though, it's the only door propped open, and from down the hallway, you can hear the voices of its occupants, busy setting up for the next twelve hours of work. When work begins for the night, it's 8 pm, Pacific Time--that's 6 pm, in Hawai'i, where the Keck Telescope is located.

Inside the windowless room, there are three digital clocks. The first gives the current Greenwich Mean Time, a number that is dutifully recorded at the beginning of each exposure of the telescope. Another gives the time in Hawai'i, keeping track of sunset, sunrise, and twilight on the distant island. Finally, the clock in the middle keeps track of the local time in Pasadena, the only real link to the rhythms of daily life outside of the strange limbo-like atmosphere of the office.

The first step is to turn on and calibrate the instruments and run through a series of checklists for the telescope. Under the row of clocks, a webcam whirs to life, and three panels on the monitor below it blink on. The first is dark--later, when observing starts, it connects us to the telescope operator. She sits at the summit of Mauna Kea, moves the telescope into position, and sets up guidance and tracking so that the telescope stays pointed in a fixed direction as the Earth rotates beneath it. The second shows the telescope technician, located at the base of the mountain and acts as IT for the night. The last one is an image of me and the other occupants of the ROF.

Observing officially begins at the end of astronomical twilight. The targets are potential Kuiper Belt Objects¹, identified by another telescope on a previous night and picked out by a computer as likely candidates. The idea behind these detections is what researcher Mike Brown calls "blinky-blinky," observe a patch of sky at two different times, and see if anything has moved. Look a third time, just to make sure the apparent movement wasn't caused by random noise in the instrument. If the object is seen in three different places, along a straight line, there's a good chance what you've found is real.

This is the same method Clyde Tombaugh used to discover Pluto, only he did it by literally blinking between two images. Today, Pluto-killer Mike Brown has autonomous programs to do it for him. For any given candidate object, the computer even spits out potential distances and orbits. Intuitively, this makes sense: objects that are farther away appear to move more slowly across the sky per unit time.

Looking at follow up targets several months later allows for a confirmation of candidates' existence, as well as a narrowing down of orbital properties. Observations fall into a particular routine. Every two minutes, we move onto a new target. We press a button to expose the telescope--it's essentially the same process as taking a long exposure image with a digital camera--and note the time. Two minutes later, we reposition the telescope, get an "okay"from the summit, and expose again. Every so often, the telescope gets re-aligned with a guide star of known position, and observations resume. Research continues like this for the next eight hours. Once we get two exposures of the same object, we can do our own makeshift "blinky-blinky" to get a first look at the data as it arrives.

Luckily for me, finals week means I leave the ROF early after only half a night of observing. Mike Brown² and the rest of his team stay up until dawn, searching the clear Hawai'ian skies for distant worlds.

¹ Kuiper Belt Objects are icy bodies that orbit near Neptune and beyond. When you think about Kuiper Belt Objects, think about Pluto-like objects.
² Mike Brown also does some neat research on Europa.