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Makemake and MK 2
An artist’s conception shows the distant dwarf planet Makemake with its dark moon, MK 2, lurking to the right. (Credit: NASA / ESA / A. Parker / SwRI)

Chalk up another moon for the dwarf planets: Astronomers have sifted through imagery from the Hubble Space Telescope to find a tiny satellite circling Makemake.

Makemake (pronounced Mah-kay-mah-kay, like the Rapa Nui deity after which it’s named) is one of the five dwarf planets recognized by the International Astronomical Union, along with Pluto, Eris, Haumea and Ceres. It’s more than 50 times farther away from the sun than Earth is, which translates to a distance of 4.8 billion miles.

With a diameter of 870 miles, Makemake is the third-largest known solar system object beyond the orbit of Neptune, in a wide ring of icy material called the Kuiper Belt. (Planet Nine, a.k.a. Planet X, would change the order if it exists, but it hasn’t yet been found.)

Like Eris, the dwarf planet that stirred up all the fuss over Pluto’s planetary status, Makemake was discovered in 2005 by a team led by Caltech astronomer Mike Brown. Like Pluto, Makemake is thought to be covered in frozen methane.

Hubble view of Makemake and MK 2
This image from Hubble’s Wide Field Camera 3 shows Makemake as a bright spot and the moon MK 2 as the tiny speck indicated by the arrow. (Credit: NASA / ESA / A. Parker and M. Buie / SwRI)

For years, astronomers have been scanning Hubble imagery to figure out whether Makemake has a moon, using techniques similar to those that identified four of Pluto’s tiny moons. For years, they struck out. Now that Makemake’s moon has finally been spotted, in imagery that was captured a year ago, the astronomers think they know why they missed it before.

“Our preliminary estimates show that the moon’s orbit seems to be edge-on, and that means that often when you look at the system you are going to miss the moon because it gets lost in the bright glare of Makemake,” Alex Parker, an astronomer at the Colorado-based Southwest Research Institute who led the image analysis, said in a NASA news release.

Parker and his colleagues reported their discovery today in a Minor Planet Electronic Circular. The moon is provisionally known as S/2015 (136472) 1 and has been nicknamed MK 2. An official name will come later.

MK 2 was spotted when it was at a distance of about 13,000 miles from Makemake, and its diameter is estimated at 100 miles. It appears to be much darker than Makemake. Further observations could show whether the moon is in a circular or elongated orbit, and help astronomers come up with more precise estimates for the mass of Makemake as well as MK 2.

The discovery lends weight to the view that most dwarf planets have moons. Pluto, for example, has five. Haumea has at least two, and Eris has at least one. “This new discovery opens a new chapter in comparative planetology in the outer solar system,” team leader Marc Buie of Southwest Research Institute said.

Will MK 2’s discovery change Makemake’s planetary status? Technically, no. Potato-shaped asteroids are known to have satellites as well.

As far as the IAU is concerned, Makemake will remain a dwarf planet because it’s a roundish object in hydrostatic equilibrium that hasn’t “cleared the neighborhood around its orbit.” But MK 2’s existence does suggest that the planetary spectrum is broader and more varied than some scientists might think.

In addition to Parker and Buie, the discovery team includes W.M. Grundy and K.S. Noll. For more, check out “Discovery of a Makemakean Moon” on arXiv.org.

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