Cosmic Matters: Planetary Astronomy: Exploring the Cosmic Neighborhood

Fall 2007

W. M. Keck Observatory

In this Issue:

By Mike Brown, Al Conrad, and Linda Copman

Image: The old solar system, traditional nine-planet model of the solar system. Image by Mike Brown, Caltech.

“It seemed clear to me that no one had ever really looked hard for large objects that might be distant and that there was a good chance that they would be out there. I started to do the first large-scale survey of the outer solar system since Clyde Tombaugh found Pluto in 1930. Since 1998, we have surveyed about 50 percent of the sky.” — Mike Brown

For the past eight years the adaptive optics (AO) system at Keck Observatory has provided fabulous opportunities to observe objects that are relatively bright. This includes most bodies in our solar system, which are well lit by the Sun. But as you move farther from the center of the solar system, you reach the point at which sunlight reflected off even small bodies is too faint to allow “locking the loops” of the Keck AO system. The Kuiper Belt Objects (KBOs) are, in fact, that distant. The KBOs that orbit the Sun at the distance of Pluto and beyond do not reflect enough sunlight to enable a traditional AO system to “lock” and correct for the blurring effects of the Earth’s atmosphere.

Image: Artist’s conception of the view from Eris with its moon Dysnomia in the background, looking back towards the distant sun. Eris was discovered using the Samuel Oschin Telescope, a smaller, robotic telescope at Mt. Palomar. Credit: Robert Hurt, IPAC.

Then, about two years ago, Keck Observatory pioneered its laser guide star adaptive optics (LGS AO) system, enabling astronomers to utilize the AO system while it was pointed at faint objects. This is done by using a laser to create a bright beacon at the top of the Earth’s atmosphere and locking the AO system on that brighter beacon, instead of on the faint target of scientific interest. Mike Brown, an astronomer at the California Institute of Technology, used the laser AO system at Keck Observatory to track the orbit of a small moon orbiting around the KBO named Eris. Eris is the larger-than-Pluto KBO that started the great planet debate.

The large telescope mirror at Keck Observatory, along with AO, are needed to follow up the discovery of KBOs with detailed studies to determine their composition, for example, or to look for moons. Brown discovered a moon of Eris using the NIRC2 instrument and adaptive optics with the Keck II Telescope. By following the orbit of this moon, named Dysnomia, Brown was able to estimate the mass of Eris. Several observations of the orbit were taken with both the Hubble Space Telescope and the Keck II Telescope.

“As soon as we find these objects, the first thing we always want to do is to get out to Keck to look at the spectrum of sunlight reflected from their surface to try to determine what they are made of. For the very largest objects, we can use the laser guide star adaptive optics system to see if they have satellites. This approach has been >very< fruitful.”
— Mike Brown

So what makes Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune planets, and Pluto and Eris not planets? The short answer to this question is provided by the International Astronomical Union (IAU) resolution, which was passed by a vote of 237 to 157 on August 24, 2006, and reads as follows:

“The IAU...resolves that planets and other bodies, except satellites, in our Solar System be defined into three distinct categories in the following way:

(1) A planet is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit.

(2) A ‘dwarf planet’ is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite.

(3) All other objects, except satellites, orbiting the Sun shall be referred to collectively as ‘Small Solar System Bodies.’”

What this means is that a planet must orbit the Sun by itself, and not just be hitch-hiking as a moon in orbit around some another body, and must be “big enough.” It is this “big enough” question that results in a gray area - and seems to get people’s dander up.

Early in their week-long meeting in Prague last year, IAU members thought they had hit upon a big-ness criteria that really captured “planet-ness.” The very small bodies in the solar system tend to be shaped like sea otters, potatoes, tennis shoes, dog bones, and other household items, but generally, they are not the nice, spherical shapes we associate with planets. This is because a solar system body must have a certain mass before its own gravity will overcome the internal, rigid forces that strive to retain whatever oddball shape resulted from the formation process. The effectiveness of the gravitational forces trying to make the body relax into a spherical shape, and the rigid forces resisting that process, both depend on things like density and composition, which vary from body to body. So it is not possible to simply specify a diameter, like say 1000 kilometers, above which a body is round and therefore a planet.

However, it turns out that on average, the right diameter is about 1000 km. As an example, of all of the million or so bodies floating in the asteroid belt between Mars and Jupiter, only one is round. This is Ceres, which is about 1000 km in diameter. All of the other million-minus-one asteroids in the belt are smaller than about 700 km and have oddball shapes.

Image: Taking the number of planets from 9 to 53 dramatically changes the look of the solar system. Here is a picture showing the old planets (black circles) and potential new planets (add red ellipses). Image and caption by Mike Brown, Caltech.

Early on in the meeting, the IAU had a simple definition for a planet: it must orbit the Sun and be round. The new body, Eris, satisfied both criteria, so it would become the 10th planet. Ceres meets both criteria, too, so that’s 11 planets. There’s also Charon (Pluto’s neighbor), so that makes 12. There were getting to be a lot of potential planets.

But where would that lead us? Many planetary scientists believe that, following future discoveries, Eris will be joined by many other large, round bodies. Eris is part of another large “neighborhood” in our solar system called the Kuiper Belt, which is like the asteroid belt but further away and colder. So we could potentially end up with 50, or even 100, “planets.”

This is the point, mid-week of the IAU meeting, at which the debate really started heating up. In the end, the IAU resolved the problem by adding a clause to the definition, stating that a planet “has cleared the neighbourhood around its orbit.” This new clause takes care of the potential 50 to 100 yet-to-be-discovered Kuiper Belt objects; it takes care of Eris, Ceres, and Charon; and it also takes care of Pluto.

“Planets are the large dominant objects in the solar system. If you removed a planet, everything else would have to adjust to make up for the loss. Dwarf planets look like planets in that they’re round, but there are hundreds of them and no one would notice if you took away a few or added a few. There are millions and millions of other bodies (no one quite knows what the official term is these days….) that are so small as to not be round.”
— Mike Brown

Image: Planets from the first and second (final) IAU resolutions. Courtesy of NASA/JPL-Caltech, with caption and edits in red by Al Conrad, Keck Support Astronomer.

Click here for a video clip of one possible scenario for the evolution of our solar system (requires Quicktime player). The movie begins with the giant planets (Jupiter through Neptune) in circular orbits around the sun, with a dense belt of icy objects beyond (in green). As millions of years (Myr) pass, gaps appear in the proto-Kuiper Belt. The premise of this animation is that over the course of a billion years, continual gravitational encounters with small solar system bodies gradually altered the orbits of the outer planets. Learn more here.

Why has the recognition that Pluto is not really a planet resulted in such a cosmic shake-up? There are several reasons. First, the fact that the IAU itself did not reach a clear-cut consensus on the definition of a planet until the end of the meeting left many people confused. Second, some folks wonder whether the scientific community should even be messing with the more romantic notion of a planet. Third, there are now a tremendous number of text books, posters, wall maps, 3-D models, encyclopedias, and dictionaries that have to be revised. Educators are in a tough position.

Or maybe this is precisely what science is all about. West Hawai`i District Superintendent for the Department of Education, Art Souza, sees the demotion of Pluto as an opportunity to make science meaningful to young people:

“I wonder if Pluto becomes just a note in a textbook ‘correcting an error,’ or if it can it be the basis of deep and reflective conversations in classrooms across America? Pluto gives us an opportunity. Can we make this a larger moment, a teachable moment that allows kids to ‘think about’ the consequences of this decision in the framework of larger implications for research and scientific knowledge? Is it so important that Pluto has been downgraded? How is this relevant and how does it impact long held ‘knowledge-based beliefs’? How is this important in terms of what kids need to know about the universe and their role in it?”
- Art Souza

Quite a few astronomers and members of the public agree with Souza that the shake-up is good. Al Conrad, support astronomer at Keck Observatory, shares these thoughts: “The general population should not be hung up on a pigeon-hole definition, but should be more interested in our ever-changing understanding of the cosmos,” says Conrad.

Photo: Mike Brown and Emily Schaller working at Keck Observatory. Photo courtesy of Emily Schaller, August 30, 2006.

There exists a striking parallel between what happened with Pluto between 1930 and 2006, and what happened with Ceres between 1801 and 1850. Both were discovered and quickly identified as planets (in 1801 and 1930, respectively). They were later reclassified as non-planets about half a century later (in 1850 and 2006, respectively) because they were determined to be part of a larger, nearby population. Pluto, in the distant Kuiper Belt, just took about two hundred years longer to explore than Ceres did, because we needed larger telescopes, like those at Keck Observatory to do it. “This historical perspective of looking at what happened with Ceres goes a long way toward supporting the resolution passed by the IAU and validating where we stand today with the definition of a planet,” suggests Conrad.

One of Brown’s graduate students, Emily Schaller, has had the privilege of observing at Keck Observatory with her mentor roughly 55 times, “which is more than many astronomers could ever hope for in their lifetimes,” she says. Schaller is thrilled to be working on the cutting edge of planetary astronomy so early in her career. One particular evening stands out in her mind:

Photo: Mike Brown sporting 3-D glasses.

“In my third year of graduate school, after I had been coming to Keck for a while with Mike to observe objects in the outer solar system, I had the opportunity to be in charge of an observing run while Mike stayed home in preparation for the birth of his daughter. Late that night, Kris Barkume (another graduate student in our group) and I swung the Keck I Telescope around to look at a newly discovered dwarf planet around which our group had just discovered a moon using Keck LGS AO. Much to our surprise, we actually saw the moon with NIRC (under extremely good conditions), and, even though it was not what I was supposed to be doing, I decided that the best use of the time was to target the tiny moon itself to see what it was made of. When I called Mike the next day to tell him, he sounded grumpy, as if we had perhaps been wasting our time on something that was impossible, but within a few hours he called back and let us know that, in fact, our observations showed something remarkable: the satellite of the dwarf planet was unlike anything that had ever been observed in the solar system before. We now know that the moon formed when two dwarf planets collided at high speed, leaving one dwarf planet and a tiny moon. Knowing that I made a decision that led to this discovery is a great feeling.” — Emily Schaller, doctoral candidate at Caltech

To listen to Mike Brown’s lecture and view his Powerpoint presentation on “Pluto and Other Dwarf Planets: Discoveries in Our Solar System” visit Keck Observatory's Podcast page.

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