Size matters...sort of--Eris and Pluto

"The War of the Worlds, Round 2"

by

Kenneth Chang

January 10th, 2011

The New York Times

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.

The smaller size of Eris would actually make it more interesting, Dr. Brown said. With less surface area, it would have to reflect almost all the light that hits it to explain how bright it is. And to explain the shininess, it must have a thin layer of methane and nitrogen frost — the remnants of a thin atmosphere that froze as Eris moved along its elliptical orbit away from the Sun.

“There is no other plausible explanation,” Dr. Brown said.

Eris is currently almost nine billion miles from the Sun, but its elliptical, 557-year orbit takes it as close as 3.5 billion miles. Pluto’s distance from the Sun varies between 2.7 billion and 4.6 billion miles.

Even if Eris is slightly smaller than Pluto, it is still the heavyweight of the Kuiper Belt — 27 percent more massive than Pluto. The orbital period of Charon put a precise figure on Pluto’s mass, and Eris’ mass is similarly well measured because it too has a moon, named Dysnomia. That means Eris must have a much larger rocky core than Pluto, perhaps indicating that Pluto and Eris did not form in quite the same way.

Other Kuiper Belt objects that Dr. Brown has discovered in the past decade have also shown the outer system to be more curious — and violent — than many would have expected. One of those is Haumea, a dwarf planet that is highly elongated and spins very fast, once every four hours.

“The only thing we could think that would cause it is if it got smacked, really hard, by something else early in the history of the solar system,” Dr. Brown said, “and everyone kind shook their head and said, well, no, that makes no sense, the probability of that happening is nearly zero.”

Then Dr. Brown and his collaborators discovered two moons orbiting Haumea that looked like shards knocked off by an impact. Still, the skeptics doubted.

Later, they found a dozen more pieces of Haumea, orbiting not around Haumea but around the Sun.

“At this point, there is no question this thing suffered a giant impact,” Dr. Brown said. “I think we should have called it Humpty Dumpty instead of Haumea — we can actually put this thing back together again.”

Other large Kuiper Belt objects also have tiny fragment moons, suggesting that giant impacts were not uncommon. Many of them are also rockier than expected; impacts could have knocked away outer layers of ice. Indeed, a giant, high-speed impact in Eris’ past could be the reason it has less ice and more rock than Pluto.

Recent measurements indicate that the other large Kuiper Belt objects are also, like Eris, smaller and shinier than had been thought. The one known as Makemake now appears to be only 1,200 kilometers in diameter, not 1,500. Another, Quaoar, might be only 900 kilometers wide, compared with earlier estimates of 1,200 kilometers.

That leaves a sizable gap between Pluto and Eris and the next largest ones. “It’s a really strange distribution of sizes,” Dr. Brown said. “I don’t think it’s caught anybody’s attention yet.”

Perhaps the biggest surprise since the discovery of Eris is that there has not been another Eris.

Dr. Brown’s sweep of the Kuiper Belt turned up a series of larger and larger objects — up to Eris. After Eris, the search found nothing else of note. There was a possibility of another large Kuiper Belt object lurking in the Southern Hemisphere sky, out of view of the California telescope that Dr. Brown was using.

But Scott S. Sheppard of the Carnegie Institution of Washington just completed a survey of the remaining sky, and he, too, found nothing large.

“It is highly unlikely there are other Pluto-size objects” in the Kuiper Belt, Dr. Sheppard said.

Beyond the Kuiper Belt, more mysteries await.

Astronomers want to find more objects like Sedna, which Dr. Brown discovered in 2003. Sedna is currently about three times as far from the Sun as Neptune, which is already outside of the Kuiper Belt. But at the other end of its 11,800-year elliptical orbit, Sedna will be 32 times as far out as Neptune.

And beyond Sedna is the Oort cloud, an even more distant collection of bodies astronomers have not yet found, but which they are absolutely certain exist. (Some of the comets that pass by the Sun originated in the Oort cloud but were gravitationally nudged to dive toward the inner solar system.)

And one of those more distant objects could turn out to be larger than one or more of the remaining eight planets, which could reignite the debate over what should be called a planet.

That could help resolve a sore point in the Brown household.

Dr. Brown originally wanted to name Eris after his daughter, Lilah, who was born three weeks before the discovery was announced. He imagined that when someone asked, “Were you named after the planet?” Lilah could retort, “No, the planet was named after me.” His wife, Diane, dissuaded him.

Dr. Brown eventually settled on Eris, the Greek goddess of strife. Dysnomia, the moon, is a demon of lawlessness and the daughter of Eris. Dr. Brown also chose that name because the first syllable is pronounced like the first syllable of Diane.

Lilah, now in kindergarten, is one of the people upset with her father over Pluto. “Killing is bad, and she knows I killed Pluto and therefore I’m a bad person,” Dr. Brown said. “And she would like me to do something about it.”

Dr. Brown explained to Lilah why Pluto was no longer a planet. “And she’s fine with that,” he said. “She has a solution, which is an interestingly creative solution.”

Lilah told him, “Why don’t you find another planet and name it Pluto?”

“That’s why I keep looking,” Dr. Brown said.

Lilah still does not know she almost had a planet named after her.