As with many works of art, there is a bit of dramatization in the song. Eighty years ago today, 24-year-old Clyde Tombaugh did in fact discover Pluto, but it was not via direct observation on February 18, 1930, a cloudy night in Flagstaff, Arizona. On that afternoon, Tombaugh was blinking photographic plates taken on January 23 and 29 of that year, centered on the star Delta Geminorum, as part of the Lowell Observatory’s search for a trans-Neptunian planet, when he discovered a tiny dot that moved during the six-day interval.
Ironically, the astronomer who initiated the search, Percival Lowell, died in 1916 without realizing that he had in fact found the planet for which he was looking. He never recognized it because it was much fainter than the giant planet he believed was out there. Yet images of Pluto appear on photographic plates taken by Lowell in 1915 and 1916. And his search bequeathed us the Lowell Observatory, still a major research and public outreach center after 116 years.
So began the history of iconic Pluto, whose planet status was in doubt not since 2006 but since day one of its discovery. It wasn’t the giant planet expected. It wasn’t anything expected.
Yet that is the story with most discoveries, from Galileo’s finding of four “planets” circling Jupiter to William Herschel’s belief that he had discovered a comet in 1781. That object turned out to be the planet Uranus. When new objects are discovered, they are analyzed, studied, and labeled in terms of what we know. Before Galileo, no one “knew” of objects orbiting any planet other than Earth. Prior to 1781, no one could conceive of planets beyond Saturn. There was always a limit: here and no further.
And each time, new discoveries shattered that limit. Pluto, it turns out, is different from the solar system’s bigger planets, but it is equally different from tiny asteroids and Kuiper Belt Objects. It has an elliptical orbit, but it is not a comet, as comets are mostly composed of ice, and Pluto is 75 percent rock. It has features in common with Earth—a large moon formed by an impact; nitrogen in its atmosphere; geological differentiation into core, mantle, and crust.
For 80 years, astronomers have struggled to figure out where this little world belongs. Not a comet, not an asteroid, much bigger than most Kuiper Belt Objects—mysterious Pluto seemed to defy all attempts to classify it. Those who placed primacy on orbital dominance judged it to be not a planet at all.
Two major areas of discovery strongly suggest our classification schemes are based more on what we don’t know than on what we do know. One is the discovery of objects similar to Pluto—small planets in the Kuiper Belt like Haumea, Makemake, and Eris, and better understanding of Neptune’s moon Triton, which has proven very similar to these small outer planets. The other is the exciting and rapid discovery of exoplanets, planets orbiting other stars. If Pluto’s features and numbers are strange, they don’t even come close to those of these strange bodies, which include systems with two giant planets in a 3:2 orbital resonance, “hot Jupiters” orbiting extremely close to their parent stars, a giant planet with an orbit far more comet-like than Pluto, planets that orbit their stars backwards—and this is just the beginning.
All of this indicates we may need to rethink a good deal of what we previously understood regarding planets. Pluto was and is the tip of the iceberg, the first to indicate that there is far more diversity and strangeness among planets than anyone ever thought. No wonder some, who wanted to keep the number and concept of planet manageable, thought they needed to narrow the definition to one specific type of object, the type that is both spherical and gravitationally dominates its orbit.
Pluto, which Alan Boyle accurately describes as a “survivor,” has refused to go quietly or fit neatly into some lesser category that glosses over its similarities with the larger planets in favor of emphasizing its differences. Pluto makes some uncomfortable because we cannot quite understand it; we cannot fit it into any of the “boxes” we have set aside for understanding “planets.” Things that are poorly understood often end up in wastebasket categories or in some sort of scientific limbo. It is hard to admit that “we just don’t know.”
In his beautiful book Ten Worlds, astronomer Dr. Ken Croswell provides the most “fair and balanced” description of the solar system along with stunning photographs and data tables that include the largest moons of the solar system’s planets as well as Ceres, Vesta, Juno, and Pallas. Croswell, who discussed Pluto on KPFA ( http://www.kpfa.org ) this past Sunday, favors a compromise in which any spherical object orbiting the Sun and Pluto-sized or larger is deemed a planet.
The language in the book is appropriate for children and adults; it does not “dumb down” descriptions of the solar system’s worlds, yet is filled with scientific data and explanation that is accessible to all ages. Significantly, Dr. Croswell chooses to use the word “worlds” rather than planets to avoid the planet definition debate entirely.
In spite of its title, the book covers far more than the ten largest bodies orbiting the Sun. The large moons of the planets, asteroids, comets, meteors, even the solar system’s birth, all are presented alongside the most up-to-date photos of these objects. This is not an older generation’s simple eight or nine planet solar system; it is one with multiple, complex objects that don’t fit neatly into categories. For example, Triton is a moon of Neptune; Pluto and Eris are either planets or dwarf planets or both, yet these three objects are strikingly similar.
Dr. Croswell also distinguishes between gas giants Jupiter and Saturn and ice giants Uranus and Neptune, in contrast to others who blur the distinction between the two subclasses and simply list all four as gas giants.
Croswell commemorates the 80th anniversary of Pluto’s discovery with an article in the February 2010 issue of Highlights for Children titled “How Cool Is Pluto?” Here again, he admirably gives equal credence to both sides of the debate. Children and adults can weigh in by casting their own votes here http://www.highlightskids.com/Survey/plu
Significantly, the back cover of Ten Worlds notes that Croswell became interested in astronomy when his first grade teacher taught his class about the planets of the solar system. Can there be any better illustration of how much the way we teach the solar system to children matters? At the end of his discussion of Eris, which he views as the tenth planet, he shows his respect for students of all ages by writing, “You decide!”
Alan Boyle, author of The Case for Pluto, quotes several astronomers in his column here http://cosmiclog.msnbc.msn.com/archive/2
Naturally, Mike Brown says he cannot imagine another major reclassification of dwarf planets unless a big planet is found in the outer reaches of the solar system beyond Pluto. He states his “suspicion” that the current definition will stick not just for the next five years, but for the next 80.
But in astronomy, those things we can barely imagine do happen and are happening more and more frequently. My personal prediction is that the IAU definition, which is hardly “sticking” even now, will not survive the next decade except as possibly one of several classification systems existing side by side with equal credibility.
Ninety-seven-year-old Patricia Tombaugh, Clyde’s widow, wouldn’t be surprised if the debate continues for another 80 years. My wish for her is that she stick around for many of those years, especially to see the close up photos of Pluto New Horizons will give us in 2015.
Of course, I’m betting on the assessment of Dr. Alan Stern, who sees the tide turning “dramatically” in Pluto’s favor.
Several commemorations of this special anniversary are taking place today. At 1 PM EST, Alan Boyle will discuss Pluto on WKXL 1450 radio in Concord, New Hampshire, on the program “Looking Up with Mal and Dave.” You can listen online at http://www.wkxl1450.com/site/ .
Gresham, Oregon, is celebrating the anniversary officially as “Pluto Day.” Read more about this celebration plus see one of the latest Hubble photos of Pluto here: http://www.oregonlive.com/gresham/index.s
In New Mexico, once again, legislation has been introduced in support of Pluto’s planet status. Rep. Joni Marie Gutierrez introduced a House Memorial declaring February 18, 2010 as “Pluto Is A Planet In New Mexico Day.” Read more about it here: http://insidethecapitol.blogspot.com/201
Croswell has an op-ed column in The Wall Street Journal supporting Pluto’s planetary reinstatement, which, in a pleasant surprise to me, actually makes mention of this blog. That letter can be found here: http://online.wsj.com/article/SB10001424
It’s quite clear that rumors of the “fall” or “death” of the little planet discovered 80 years ago today have been greatly exaggerated. Dr. Stern says it best when he predicts that New Horizons data will have us “surprised,” and “shocked,” with “something wonderful.”
We should expect no less from the little planet that could. Happy 80th anniversary, Planet Pluto!