Tuesday was a busy day: two press conferences, a science session on the Wide-Field Infrared Survey Explorer (WISE), two invited talks, and a session about planet definition.
In one of the press conferences, University of California at Berkeley astronomers Edward Wishnow (pictured at right) and Charles Townes announced that they’ve measured Betelgeuse’s diameter. This red supergiant seems to be shrinking. Its diameter is some 15 percent smaller than it was in 1990 — that’s a pretty significant change. However the astronomers do acknowledge that the star has a pretty fuzzy “surface,” which can affect the measurements.
If you read our news page, you can read about all the stuff I’ve heard about in the press conferences. But something that’s not on that page is the evening session about planet definition. It was 3 hours of presentations and discussions regarding planet definition and, of course, Pluto. With such heavyweights as Neil deGrasse Tyson, Alan Stern, and Mark Sykes, it was well worth the time!
During the session, seven scientists spoke about their theories regarding planet classification. Each one was different, and each had convincing reasons. Because you weren’t able to be there, I’ve included a summary of each of the speakers in this blog.
Tyson gave an overview of “Planet X,” Pluto’s classification over the past 76 years, and then commented on the International Astronomical Union (IAU) planet definition. One of his main points was that while the IAU says the planet must have cleared its orbit of dust and debris, “cleared” really isn’t the right term. The question is whether the planet dominates its orbit, and Pluto does not. (The point that location matters — and variations of it — came up often during the evening.) Tyson also reminded us that classification schemes have been around for a long time in astronomy (think of Hubble’s galaxy tuning fork, and how that evolved as de Vaucouleurs classification), so why should a planet definition be any different? By the way, if you ever have a chance to see Tyson speak, you should definitely go!
Stern followed with the main argument that an object’s attributes are more important than its location. He discussed a reason why the IAU’s definition is flawed: It requires that a planet has “cleared the neighborhood” around its orbit but neither Earth nor Jupiter have “cleared” their orbits (Earth rams through a whole lot of dust each day). Stern felt that the Geophysical Planet Definition is better than the IAU’s. This definition says a planet is a celestial body that has sufficient mass so that it can assume a hydrostatic equilibrium shape (round) due to gravity, but is not massive enough to sustain fusion at its core. This definition would mean our solar system has some 30 orbiting planets.
Charles Beichman of Jet Propulsion Laboratory spoke about brown dwarfs and where (and how) one draws the line between a brown dwarf and planet. His main point is whether the mechanism of formation should be part of the planet definition. Beichman explained that stars and brown dwarfs form by collapse of material. While planets near to their home stars (within 10 Astronomical Units) form by accreting disk material, those farther out form by material collapsing. So where do you classify these planets that form through collapse? This is a characteristic of stars and brown dwarfs.
Getting back to the location issue, Renu Malhotra of the University of Arizona discussed her research that the solar system’s planets likely weren’t born in the same orbit they’re currently in — they migrated out. As Neptune’s orbit extended out, some objects beyond its orbit could be trapped into periodic gravitation influence on each other. This explains Pluto’s orbital resonance, ellipticity, and inclination. This theory also predicted that ultra-Neptunian objects (the Kuiper Belt objects [KBOs]) would be trapped in resonances. Observations have confirmed that a number of KBOs pile up at resonances with Neptune’s orbit. In this theory, Neptune must have migrated out by 8 AU to account for this.
University of California at Los Angeles astronomer Jean-Luc Margot argued that geophysically similar planetary bodies may belong to different taxonomic classes depending on the dynamical environment. Think of magma vs. lava (same thing, but different name depending on if it’s underground or above ground) and meteoroid vs. meteorite (same thing, but different name depending on if it’s on Earth or soaring through space). Margot’s main focus was that planets must be dynamically dominant, meaning they dominate their orbit. In his classification scheme, location is crucial. He proposed that scientists label any round body as a “world.” This classification would cover planets, round satellites, dwarf planets, and free-floating planets. If you label all round bodies as worlds, then some worlds are planets, but others are not.
Mark Sykes of the Planetary Science Institute takes a similar stance to Stern, saying that from a geophysical perspective, planets are best defined as gravitationally round things. He also said that dwarf planets may have subsurface oceans. How wild would it be if life ends up being more possible on dwarf planet Ceres than Jupiter’s moon Europa?
The last speaker, David Weintraub of Vanderbilt University, gave an overview of the history of planet classification. In 1780, scientists defined planets by what they were and what they were not. Over the centuries the definition has had to change with new discoveries (such as Uranus and Neptune). He ended with the thought that we’ve discovered a new part of the solar system (KBOs), but we don’t understand it. We’re nowhere near being able to close this debate because we need more information.
After these seven experts spoke, they joined each other on stage and discussed questions from the audience. Even though they mostly disagreed on the definition of a planet, they did tend to agree on a few things. (1) The IAU needs to come up with a better definition. (2) It’s a concern that we as a society feel that schoolchildren need to memorize a number. The issue is that the solar system is changing and composed of different types of objects, not that scientists assign a number to our planetary system. (3) This discussion is premature. Scientists need to discover hundreds or thousands more KBOs before any final definition can be created.
And even though no one broke out in outright arguments, it still was a fascinating session.
Order in the picture (left to right)
Steve Maran, AAS press officer
Renu Malhotra
Mark Sykes
Alan Stern
David Weintraub
Jean-Luc Margot
Charles Beichman
Neil deGrasse Tyson