Six years ago this month, Michael E. Brown, a professor of planetary astronomy at the California Institute of Technology, spotted an object in the night sky that was so bright and so far away that he was sure it was bigger than Pluto.“Guaranteed,” Dr. Brown said when he announced the discovery, half a year later, in July 2005.
Well...maybe not, after all.
In November, that object, now known as the dwarf planet Eris, passed in front of a dim, distant star. Astronomers led by Bruno Sicardy of the Paris Observatory measured how long the star disappeared behind Eris and, from that, calculated the width of Eris. “It’s clearly smaller,” said Alain Maury, who observed the brief disappearance, or occultation, of the star at the San Pedro de Atacama Celestial Explorations Observatory in Chile.
For now, Drs. Maury and Sicardy decline to say exactly how small Eris is, because they first want to publish the results in the journal Nature. But they say that even accounting for the uncertainties in the observations, the largest possible Eris is smaller than the smallest possible Pluto.
The news raises the question of what might have happened if Eris’ true size had been known from the beginning. Dr. Brown’s discovery of Eris — and the presumption that it was bigger than Pluto — was the falling domino that pushed the International Astronomical Union to come up with a new definition of “planet” that excluded Pluto. Pluto and Eris were downsized to “dwarf planets” — roundish objects that do not gravitationally dominate their orbits.
If astronomers had believed Pluto to be larger than Eris — even slightly — might they have kept the solar system at nine planets and sidestepped the ensuing kerfuffle?
“Maybe,” Dr. Brown said, although as he tells in his unapologetically titled book “How I Killed Pluto and Why It Had It Coming,” he thinks the International Astronomical Union got it right.
The occultation measurement — which means Eris is not just smaller than expected but also incredibly shiny — is the latest surprise of the Kuiper Belt, a ring of icy debris beyond Neptune. That belt turns out to be even stranger than astronomers thought a few years ago.
And not everyone is yet convinced that Eris is definitely smaller than Pluto. Dr. Brown, for one, is perplexed. The occultation measurement seems to demonstrate convincingly that Eris’ diameter is less than 2,360 kilometers, or 1,466 miles, Dr. Brown said. That is smaller than earlier estimates of 3,000 kilometers, based on infrared light from Eris, and 2,400 kilometers, based on Dr. Brown’s observations with the Hubble Space Telescope.
But he notes that a number of conflicting figures for the size of Pluto appear on various Web sites. Wikipedia, citing a 2006 scientific paper, puts the diameter at 2,306 kilometers, give or take 20 kilometers.
And now Pluto is bigger than Eris, “because, um, 2,306 kilometers is greater than 2,360 kilometers?” Dr. Brown asked, rhetorically and quizzically, on his blog.
Delving further to make sense of the numbers, Dr. Brown did not question the yet-to-be-published Eris measurements by Dr. Sicardy’s group, but rather concluded, “I have to say: there is something fishy in the size of Pluto.”
More than 80 years after Clyde W. Tombaugh spotted Pluto, astronomers still have not pinned down exactly how big, or small, it is.
For decades, Pluto was the magically shrinking planet. It was first thought to be about as large as Earth — nearly 8,000 miles wide — but subsequent measurements had it smaller and smaller.
In 1980, Alexander J. Dessler, now at Texas A&M University, and Christopher T. Russell of the University of California, Los Angeles, published a graph of the mass estimates through the years and jokingly predicted that Pluto would disappear entirely in 1984. “Those of you interested in observing Pluto should hurry,” they wrote.
Needless to say, the size of Pluto stabilized. Between 1985 and 1990, the orbit of Pluto’s moon, Charon, was edge-on, as seen from Earth, and eclipses enabled astronomers to measure the diameters of Pluto and Charon more directly. Pluto has also passed in front of a few stars, too, just as Eris did in November.
But that was still not the end of the story. The surface of Pluto can reach a relatively balmy minus 360 degrees Fahrenheit, warm enough for some methane and nitrogen ices to evaporate and create an atmosphere, and the atmosphere bends light.
“Pluto’s atmosphere is kind of a like a crummy convergent lens,” said Eliot F. Young, an astronomer at Southwest Research Institute’s space studies department in Boulder, Colo., who was an author of the 2006 paper. “Each ray is bent toward the center of Pluto.”
Thus, the true diameter of Pluto remains uncertain.
Dr. Young reanalyzed the stellar occultation data and found Pluto to be bigger, with the data compatible with a diameter as large as 2,400 kilometers.
But he said, “I don’t think it’s that big.” The minimum diameter, according to his calculations, is about 2,300 kilometers, leaving a sizable uncertainty of 100 kilometers. “This is embarrassing for me to talk about,” Dr. Young said.
Eris is about three times as far from the Sun as Pluto, much colder and almost completely devoid of any atmosphere to distort an occultation. So astronomers may now know Eris’ size more accurately than Pluto’s.
Still, the range of possible Pluto sizes seems to overlap the possible Eris sizes. “If you looked at the two of them right next to each other sitting in space, they would look to be exactly the same size,” Dr. Brown said. “You couldn’t tell by eye until you took out your really, really big ruler.”
Drs. Maury and Sicardy point to another estimate that finds a larger Pluto, based on the absorption of light by methane in Pluto’s atmosphere.
That analysis, by Emmanuel Lellouch of the Paris Observatory, and his collaborators, who included Dr. Sicardy, said that to explain the patterns they saw, Pluto had to be at least 2,360 kilometers wide.
But Dr. Young, while lauding the methane measurements, said that too much was still not known about the structure of Pluto’s atmosphere to make that confident a conclusion about its size.
A precise, direct measurement of Pluto will finally come in 2015 when NASA’s New Horizons spacecraft is to fly past.
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