Dwarf planet puzzles astronomers

Dan Vergano, USA TODAY | USATODAY

Show Caption Hide Caption Dwarf planet 'occultation' observed by astronomers This animation shows the path of the shadow of the dwarf planet Makemake during an occultation of a faint star in April 2011. Several sites in South America saw the star briefly disappear as its light was blocked by Makemake.

Astronomers report a rare observation of the dwarf planet Makemake

Makemake resides 4.85 billion miles from the sun, farther than Pluto

The dwarf planet appears too cold to have an atmosphere

Out on the edge of the solar system beyond Pluto, an oblong dwarf planet poses a puzzle for astronomers. Makemake (Mah-kay-Mah-kay), a world some 890 miles across, orbits about 4.85 billion miles from the sun.

In 2011, a rare "occultation" of the dwarf planet occurred, where it passed in front of a distant star, which allowed astronomers to take a closer look at its shape. The results are reported in the journal Nature by a team led by Jose Ortiz of Spain's Instituto de Astrofísica de Andalucía. As expected, the results show the dwarf world has an oblong shape.

The surprise? The tiny world lacks any kind of atmosphere like the one draping its cool cousin, Pluto. A sharp drop-off in the starlight switched on and off by the occultation reveals no apparent nitrogen atmosphere on the planet, as the study authors say they expected, Nitrogen can remain a gas, unfrozen even at temperatures of -369 degrees.

Instead, any atmosphere on the dwarf planet likely froze out, similar to the more distant dwarf planet, Eris, draping the world with fairly reflective ice.

"The Eris, Pluto, Makemake trio make a great test case for understanding atmosphere in the Kuiper belt. They're all similar but with slightly different sizes and at very different distances from the sun," says Caltech astronomer Mike Brown, who led the team that found both Eris and Makemake in the last decade. Makemake is named after the creator god of Easter Island mythology, part of an astronomical tradition of naming worlds after deities.

"They didn't detect an atmosphere on Makemake. Should they have? I can't answer that yet," Brown says, by e-mail, suggesting that further analysis needs to confirm the finding. In any case, he adds that the study shows that astronomers can now experimentally probe the surfaces of the dwarf planets orbiting in the comet belt of the solar system.