NANTES, France—Makemake may well have the most exotic name of the dwarf planets, and it now looks to be just as unusual on its surface.
One of five recognized dwarf planets in our solar system, Makemake (pronounced MAH-kee MAH-kee) was discovered in 2005 in a broad, elliptical orbit outside the realm of Neptune. Its location places it in the family of bodies called trans-Neptunian Objects, or TNOs, and it qualifies as a dwarf planet because it is large enough to have settled gravitationally into a roundish shape.
Since then, distant Makemake has been subject to investigation by astronomers using the best tools at their disposal, from the Hubble Space Telescope to the Keck telescopes in Hawaii. Now a new set of observations made by the space-borne European Herschel telescope, which launched in 2009, is showing Makemake’s surface to be a puzzling arrangement of hot and cold spots.
“I believe that in our TNO sample with Herschel of about 140 targets, it’s the most exotic object we have,” Thomas G. Müller of the Max Planck Institute for Extraterrestrial Physics in Garching, Germany, said Tuesday at a joint meeting here of the American Astronomical Society Division for Planetary Sciences and the European Planetary Science Congress.
Müller and his colleagues used Herschel to make thermal measurements of the dwarf planet, which is roughly 1,500 kilometers in diameter. What they found stood in sharp contrast to infrared measurements at wavelengths of 24 microns from the Spitzer Space Telescope. Herschel’s long-wavelength data, ranging up to 500 microns, indicated a much colder, more reflective object than would be expected from Spitzer data alone. The Herschel data demand an extremely high reflectance, or albedo, of greater than 90 percent, indicating that Makemake is extremely icy.
But the Spitzer data appear solid—indeed, the two observatories agreed in their observations of Makemake at 70 microns—so Müller tried his best to accommodate the observations from both spacecraft in his thinking about the one dwarf planet. “There must be darker terrains to explain this [Spitzer] point and brighter terrains, higher-albedo ones, to explain this [Herschel data] here,” he said. What emerges is a world with icy patches that reflect almost all incident light and dark patches that absorb light and heat up with great efficiency. “It’s impossible to fit it with a single albedo,” Müller said. “There’s no way.”
But Makemake reflects the same amount of light throughout its rotation, so the bright and dark spots cannot be distributed in broad, odd-shaped patches, as appears to be the case for Pluto. One possibility is that bright and dark surfaces are confined to zones that look the same throughout Makemake’s rotation from Earth’s viewpoint. For instance, if astronomers are viewing Makemake equator-on, the equator could be dark and the upper latitudes bright.
“But when you look at these physical properties you need to model it, it’s very extreme,” Müller said. The equator would have to be above 50 Kelvin (–223 degrees Celsius), while the hemispheres would be below 30 Kelvin (–243 degrees C), and somehow those regions would have to coexist next to each other. At more than 50 astronomical units (AU)—that is, more than 50 times as far from the sun as Earth is—Makemake’s inferred temperatures push the bounds of the possible.
“I believe that out there at 52 AU, that’s the most extreme temperatures you can produce on a body, and we have them on a single body next to each other,” Müller said. “So that’s why I believe it’s the most exotic object we have.”
Makemake’s bright and dark patches could be mixed together on the surface, as long as the patches are small and evenly distributed, so that the overall balance of dark and light remains the same throughout the dwarf planet’s rotation. Müller floated a few fanciful ideas for what such a world might look like, including a striped rendition that he dubbed “zebra-make.” But given that he shares a name with star German footballer Thomas Müller, he prefers an alternate pattern resembling a familiar object comprising light and dark blocks: “soccer-make.”