Wladimir Lyra
·4 min read
The year was 2015. After traveling billions and billions of miles through the Erebus, New Horizons encounters the boatman, and the somber lord of the departed. The spacecraft could have been called Orpheus.
The story of Pluto is a fascinating chapter in the history of astronomy. Discovered in 1930 by Clyde Tombaugh (later illustrious Las Cruces resident, and founder of NMSU’s Astronomy program), Pluto was found accidentally while searching for something much bigger. When Uranus was discovered in 1781, astronomers set to calculate its orbit to predict its future location. Yet, Uranus was never where it should be, even after accounting for the gravitational pull of all known planets. This led to the suggestion that another planet existed beyond Uranus, whose gravity was influencing its orbit. In 1846, French astronomer Urbain Leverrier calculated the position of this unseen planet, and sent the coordinates to another astronomer, Johann Galle, in Berlin. Galle found Neptune the same night he received Leverrier's letter, very close to the predicted position ― a dramatic validation of celestial mechanics, and one of the most remarkable moments of 19th century science.
Astronomers believed they could use this method to discover more planets: find a planet, calculate the orbit, measure the difference between the observed and predicted positions, ascribe the difference to the pull of an unseen planet, and use the difference to locate the planet. Indeed, even after accounting for Neptune’s gravity, a small error persisted in Uranus’ calculated position, prompting the prediction of a planet beyond Neptune. When Clyde found Pluto, its orbit was close to the predicted orbit of X. It seemed again a remarkable validation of the theory.
Pluto soon became America’s favorite planet, especially after Walt Disney named one of his characters, a charismatic dog, after the newly found planet. Plutomania caught the country – and the world. Children around the world memorized “the nine planets”.
Yet, something was odd about Pluto. While Planet X was estimated to be about six times more massive than Earth, Pluto’s mass is only 20% that of the Moon, itself 1% of the Earth’s mass. Pluto is much too small to gravitationally perturb Uranus or Neptune. As it turns out, Planet X doesn’t exist. The remaining errors in Uranus’ orbit were resolved after the spacecraft Voyager 2 flew past Neptune in 1989 and measured its mass more precisely. Looking for a ghost, Clyde found something real.
Planet X or not, Pluto existed. Smaller than the Moon, but inhabiting the realm of the giant planets, it defied our notions regarding the architecture of the Solar System. The tension heightened in 1992, when UCLA astronomer Dave Jewitt discovered Albion, a small object in a similar orbit to Pluto. Since then, over 2400 “trans-Neptunian objects” have been discovered. Pluto is the largest object in its region, but it shares the orbit with thousands of other bodies. The final blow came in 2005 with Caltech astronomer Mike Brown’s discovery of Eris, a more massive object than Pluto (though denser and smaller) beyond Neptune.
The discovery of Eris spurred the International Astronomical Union (IAU) to define “planet”. The definition that was eventually accepted requires that a planet must 1) orbit the Sun, 2) be round and 3) have cleared it orbit. The third criterion excluded Pluto, as it shares its orbit with other objects. Objects that are round but do not clear the orbit are classified as “dwarf planets”, a new category. The definition also explicit states, confusingly, that dwarf planets are not planets.
The IAU definition has been highly controversial. The New Horizons mission, launched in 2005 to explore Pluto, found itself en route to an ex-planet. The definition was criticized for several reasons. First, it was the result of a vote, suggesting the strange notion that science advances democratically -- should we vote on the value of the gravitational constant? Second, a ridiculously small number of astronomers cast a vote – the turnout was about 4%. Third, some say planetary scientists, not astronomers, should have made the decision.
Alternatively, some advocate for a definition of planet based only on intrinsic characteristics. The argument is that “clearing the orbit” depends on location: if Mercury were where Pluto is, it wouldn’t clear its orbit; if Pluto were where Mercury is, it would. Why should a planet's status depend on its location? Planethood should depend on intrinsic characteristics: there are rocks, which are not big enough to be round; planets, which are round; and stars, that shine on their own. Mass is the key factor. Indeed, geophysically, there's little difference between the Moon and Mercury: they're both planets. This would make the Earth-Moon system a binary planet. Stars can orbit stars. Why can’t planets orbit planets?
The debate over Pluto’s status highlights the evolving nature of scientific classification. Definitions should reflect our growing understanding rather than arbitrary criteria. As we continue to explore the cosmos, our definitions and categories must adapt to accommodate new discoveries and insights. The story of Pluto reminds us that science is a dynamic process, ever refining and expanding our knowledge of the universe.
Wladimir Lyra is an associate professor of astronomy at New Mexico State University. He can be reached at @LyraKstellation (Twitter).
This article originally appeared on Las Cruces Sun-News: Is Pluto a planet or not?
