Rare Apparition of Dwarf Planet Makemake Reveals a Largely Airless World

Scientific American

A new look at the dwarf planet Makemake, one of the more recent additions to the known solar system, has pinned down some of the object's most basic—and important—attributes.

Astronomers took to observatories across South America in April 2011 to catch a rare glimpse of the dwarf planet—or at least its shadow—as Makemake (pronounced "mah-kee mah-kee") crossed in front of a faint background star and dimmed the star for about a minute. The duration of the occultation, as such celestial conjunctions are called, allowed the astronomers to more precisely estimate Makemake's physical size. The researchers reported their findings in the November 22 issue of Nature. (Scientific American is part of Nature Publishing Group.)

Perhaps more important than the new size measurement are the implications for a Makemakean atmosphere. The astronomers detected a sudden drop in starlight when Makemake’s occultation began, as if someone had suddenly switched the star to a lower wattage rather than dialing down a stellar dimmer switch. The sharpness of the occultation suggested that the dwarf planet lacks a significant global atmosphere, which would lend a fuzzy edge to its shadow. "If the decrease is abrupt, you see that there is not an atmosphere," says study co-author Noemi Pinilla-Alonso, a postdoctoral planetary scientist at the University of Tennessee Knoxville. By contrast, when Pluto passes in front of a star, "the decrease is gradual, which shows that there is an atmosphere."

Makemake, discovered in 2005, is one of five recognized dwarf planets, along with Ceres, Eris, Haumea and Pluto. Currently Makemake is near aphelion, the point farthest from the sun along its 307-year orbit in the Kuiper belt, a ring of comets and other icy objects beyond Neptune. Makemake's distance from the sun is presently more than 50 times that of Earth, placing the dwarf planet in an intermediate regime between the locations of Pluto and the more distant Eris. Whereas Pluto hosts a significant atmosphere, Eris's atmosphere appears to have frozen out and collapsed into a reflective surface layer.

Makemake may exist in an intermediate state, with only localized clouds of methane vaporized from the dwarf planet's sunward-facing side. "It doesn't have a global atmosphere, but it could have some local atmosphere," Pinilla-Alonso says. The researchers suspect that Eris, Pluto and Makemake all cycle through stages—from localized clouds to full-fledged atmosphere to atmospheric collapse—as the worlds move closer to, and then farther from, the sun.

Makemake does have methane frost on its surface, which should provide a ready supply of gas at the dwarf planet's warmer locales. "It absolutely has an atmosphere," says astronomer Mike Brown of the California Institute of Technology, who discovered Makemake in 2005, along with Chad Trujillo of Gemini Observatory in Hawaii and David Rabinowitz of Yale University. "It’s just a question of how much."

Makemake is only the latest of the dwarf planets to be carefully measured during an occultation. When Eris eclipsed a background star in 2010, astronomers found it to be a near-twin to Pluto in size (both are somewhat larger than Makemake). But opportunities to make detailed observations of Makemake during an occultation are few and far between. The dwarf planet passes in front of three or so background stars per year, but often the shadow from the occultation falls on a sunlit or cloudy part of Earth, or on a remote region without telescopes. The April 2011 occultation, which had been predicted the year before, cut right across South America, exposing Makemake to view from some of the world's premier observatories, including two European Southern Observatory facilities in Chile: the Very Large Telescope atop Cerro Paranal and the New Technology Telescope at La Silla in the Atacama Desert. According to José Luis Ortiz of the Institute of Astrophysics of Andalusia in Spain, lead author of the new study, such opportunities arise only about once a decade. In all, the astronomers witnessed Makemake's occultation from seven telescopes across South America, which allowed for an unprecedentedly precise estimate of the object's diameter.

"This is the right way to measure sizes," says Brown, who also co-discovered Eris in 2005. (That same year, Brown clashed with Ortiz over the discovery of Haumea, alleging that the Spanish researchers had improperly accessed his observing records to scoop the Americans and claim credit for the find.)

The new look at Makemake found that the dwarf planet is 1,430 kilometers across if it is spherical and 1,430 kilometers by 1,502 kilometers if it is elliptical. The study’s authors favor the latter possibility, but Brown disagrees. "Even that small bit of not round is really strange," he says, adding that Makemake would have to be rotating rapidly or composed of low-density material to become so oblate. "I’m fairly certain that that's just wrong," he adds.

All the same, Brown is pleased to see Makemake in the spotlight. Its discovery was initially overshadowed by that of Eris, which outweighs Pluto and was originally trumpeted as the 10th planet in the solar system. "Makemake has always been sort of the unloved large Kuiper belt object," he says. "We were so swamped with Eris and all the interesting stuff with Haumea that people didn't really give Makemake the attention it deserved."

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