Sizing up Pluto is no small matter!

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 so bright and far away that he was sure it was bigger than Pluto. “Guaranteed,” Brown said when he announced the discovery, in July 2005. 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. 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.

What exactly defines a planet?

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?

“Maybe,” Brown said, although as he tells in his 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. Not everyone is convinced that Eris is smaller than Pluto. Brown is perplexed. The occultation measurement seems to demonstrate that Eris’ diameter is less than 2,360 kms, or 1,466 miles, Brown said.

That is smaller than earlier estimates of 3,000 kms, based on infrared light from Eris, and 2,400 km, based on Brown’s observations with the Hubble Space Telescope. But he notes that conflicting figures for the size of Pluto appear on websites.

Wikipedia, citing a 2006 paper, puts the diameter at 2,306 kms, give or take 20 kms. And now Pluto is bigger than Eris, “because, um, 2,306 kms is greater than 2,360 kms?” Brown asked, rhetorically on his blog. 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 joked that Pluto would disappear in 1984. 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.

The surface of Pluto can reach minus 360 degrees Fahrenheit, warm enough for methane and nitrogen ices to evaporate and create an atmosphere, which bends light. “Pluto’s atmosphere is 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.

Young reanalysed the stellar occultation data and found Pluto to be bigger, with the data compatible with a diameter as large as 2,400 kms. Eris is about three times as far from the sun as Pluto, much colder and nearly devoid of any atmosphere to distort an occultation. Astronomers may now know Eris’ size more accurately than Pluto’s. A precise measurement of Pluto will finally come in 2015 when NASA’s New Horizons spacecraft is to fly past.

Even if Eris is slightly smaller than Pluto, it is still the heavyweight of the Kuiper Belt. 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.

Kuiper Belt and beyond

Other Kuiper Belt objects that Brown has discovered include Haumea, a dwarf planet. Then Brown and his collaborators discovered two moons orbiting Haumea. Later, they found a dozen more pieces of Haumea, orbiting not around Haumea but around the sun.

Beyond the Kuiper Belt, more mysteries await. Astronomers want to find more objects like Sedna, which Brown discovered in 2003. Sedna is currently about three times as far from the Sun as Neptune, 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.