A Moon For MakemakeImagine, a frigid, distant shadow-region in the far suburbs our Solar System, where a myriad of twirling icy objects–some large, some small–orbit our Sun in a mysterious, mesmerizing phantom-like ballet within this eerie and strange swath of darkness. Here, where our Sun is so far away that it hangs suspended in an alien sky of perpetual twilight, looking just like a particularly large star traveling through a sea of smaller stars, is the Kuiper Belt–a mysterious, distant deep-freeze that astronomers are only now first beginning to explore. Makemake is a denizen of this remote region, a dwarf planet that is one of the largest known objects inhabiting the Kuiper Belt, sporting a diameter that is about two-thirds the size of Pluto. In April 2016, a team of astronomers announced that, while peering into the outer limits of our Solar System, NASA’s Hubble Space Telescope (HST) discovered a tiny, dark moon orbiting Makemake, which is the second brightest icy dwarf planet–after Pluto–in the Kuiper Belt.

The tiny moon–which for now has been designated S/2015 (136472) 1, and playfully nicknamed MK 2, for short–is more than 1,300 times dimmer than Makemake itself. MK 2 was first spotted when it was about 13,000 miles from its dwarf planet parent, and its diameter is estimated to be about 100 miles across. Makemake is 870 miles wide, and the dwarf planet, which was discovered over a decade ago, is named for the creation deity of the Rapa Nui people of Easter Island.

Discovered on March 31, 2005, by a team of planetary scientists led by Dr. Michael E. Brown of the California Institute of Technology (Caltech) in Pasadena, Makemake was initially dubbed 2005 FY 9, when Dr. Brown and his colleagues, announced its discovery on July 29, 2005. The team of astronomers had used Caltech’s Palomar Observatory near San Diego to make their discovery of this icy dwarf planet, that was later given the minor-planet number of 136472. Makemake was classified as a dwarf planet by the International Astronomical Union (IAU) in July 2008. Dr. Brown’s team of astronomers had originally planned to delay announcing their discoveries of the bright, icy denizens of the Kuiper BeltMakemake and its sister world Eris–until additional calculations and observations were complete. However, they went on to announce them both on July 29, 2005, when the discovery of Haumea–another large icy denizen of the outer limits of our Solar System that they had been watchingwas announced amidst considerable controversy on July 27, 2005, by a different team of planetary scientists from Spain.

Although the provisional designation of 2005 FY9 was given to Makemake when its discovery was made public, before that Dr. Brown’s team had used the playful codename “Easter Bunny” for this small world, because of its discovery shortly after Easter.

Makemake is about a fifth as bright as Pluto. However, despite its comparative brightness, it was not discovered until well after a number of much fainter KBOs had been detected. Most of the scientific hunts for minor planets are conducted relatively close to the region of the sky that the Sun, Earth’s Moon, and planets appear to lie in (the ecliptic). This is because there is a much greater likelihood of discovering objects there. Makemake is thought to have evaded detection during earlier searches because of its relatively high orbital inclination, as well as the fact that it was at its greatest distance from the ecliptic at the time of its discovery–in the northern constellation of Coma Berenices.

The Kuiper Belt

Dark, distant, and cold, the Kuiper Belt is the remote domain of an icy multitude of comet nuclei, that orbit our Sun in a strange, fantastic, and fabulous dance. Here, in the alien deep freeze of our Solar System’s outer suburbs, the ice dwarf planet Pluto and its quintet of moons dwell along with a cornucopia of others of their bizarre and frozen kind. This very distant region of our Star’s domain is so far from our planet that astronomers are only now first beginning to explore it, thanks to the historic visit to the Pluto system by NASA’s very successful and productive New Horizons spacecraft on July 14, 2015. New Horizons is now well on its way to discover more and more long-held secrets belonging to this distant, dimly lit domain of icy worldlets.

The Kuiper Belt is situated beyond the orbit of the beautiful, blue, and banded giant gaseous planet, Neptune–the outermost of the eight major planets of our Sun’s family. Pluto is a relatively large inhabitant of this region, and it was–initially–classified as the ninth major planet from our Sun after its discovery by the American astronomer Clyde Tombaugh (1906-1997) in 1930. However, the eventual realization among astronomers that the frozen little “oddball” that is Pluto, is really only one of numerous other icy bodies inhabiting the Kuiper Belt, forced the IAU to formally define the term “planet” in 2006–and poor, pitiful Pluto lost its lofty designation of “major planet” only to be re-classified as a mere minor one–a demoted dwarf planet.

Comets are actually bright, streaking invaders from far, far away that carry within their mysterious, frozen hearts the most pristine of primordial ingredients that contributed to the formation of our Solar System about 4.6 billion years ago. This primeval mix of frozen material has been preserved in the pristine “deep-freeze” of our Solar System’s darkest, most distant domains. Comets are brilliant and breathtaking spectacles that for decades were too dismissively called “dirty snowballs” or “icy dirt balls”, depending on the particular astronomer’s point of view. These frozen alien objects zip into the inner Solar System, where our planet is situated, from their distant home beyond Neptune. It is generally thought that by acquiring an understanding of the ingredients that make up these ephemeral, fragile celestial objects, a scientific understanding of the mysterious ingredients that contributed to the precious recipe that cooked up our Solar System can be made.

Comets are really traveling relic icy planetesimals, the remnants of what was once a vast population of ancient objects that contributed to the construction of the quartet of giant, gaseous planets of the outer Solar System: Jupiter, Saturn, Uranus, and Neptune. Alternatively, the asteroids–that primarily inhabit the region between Mars and Jupiter termed the Main Asteroid Belt–are the leftover rocky and metallic planetesimals that bumped into one another and then merged together to form the four rocky and metallic inner planets: Mercury, Venus, Earth, and Mars. Planetesimals of both the rocky and icy kind blasted into one another in the cosmic “shooting gallery” that was our young Solar System. These colliding objects also merged together to create ever larger and larger bodies–from pebble size, to boulder size, to mountain size–and, finally, to planet size.

Brilliant, icy short-period comets invade the bright and toasty inner Solar System, far from their frozen domain in the Kuiper Belt. The Kuiper Belt is the reservoir of comet nuclei that is located closest to Earth. Short-period comets rampage into the inner Solar System more frequently than every 200 years. The more distant long-period comets streak into the inner Solar System’s melting warmth and comforting light every 200 years–at least–from the Oort Cloud. Because Earth dwells closer to the Kuiper Belt than to the Oort Cloud, short-period comets are much more frequent invaders, and have played a more important part in Earth’s history than their long-period kin. Nevertheless, Kuiper Belt Objects (KBOs) are sufficiently small, distant, and dim to have escaped the reach of our scientific technology until 1992.

Makemake is a classical KBO. This means that its orbit is situated far enough away from Neptune to remain in a stable stage over the entire age of our more than 4 billion year old Solar System. Classical KBOs have perihelia that carry them far from the Sun, and they are also peacefully free from Neptune’s perturbing influence. Such objects show relatively low eccentricities and circle our Star in a way that is similar to that of the major planets. However, Makemake is a member of what is referred to as a “dynamically hot” class of classical KBOs, which instead display a high inclination when compared to other classical KBOs.

Makemake, like Pluto, shows a red hue in the visible part of the electromagnetic spectrum. The near-infrared spectrum is marked by the existence of the broad methane absorption bands–and methane has also been observed on Pluto. Spectral analysis of Makemake’s surface shows that its methane must be present in the form of large grains that are at least one centimeter in size. In addition to methane, there appears to be large quantities of ethane and tholins as well as smaller quantities of ethylene, acetylene, and high-mass alkanes (like propane)–most likely formed as a result of the photolysis of methane by solar radiation. The tholins are thought to be the source of the red color of the visible spectrum. Even though there is some evidence for the existence of nitrogen ice on Makemake’s frozen surface, at least combined with other ices, it is probably not close to the same abundance of nitrogen seen on Pluto and on Triton. Triton is a large moon of the planet Neptune that sports a retrograde orbit indicating that it is a captured object. Many astronomers think that Triton is a wandering refugee from the Kuiper Belt that was captured by the gravity of its large, gaseous planet. It is possible that eventually the doomed Triton will plunge into the immense, deep blue world that it has circled for so long as an adopted member of its family. Nitrogen accounts for more than 98 percent of the crust of both Pluto and Triton. The relative lack of nitrogen ice on Makemake hints that its supply of nitrogen has somehow been depleted over the age of our Solar System.

It was on April 26, 2016, that the team of astronomers, using observations from the HST taken in April 2015, announced their discovery of the small, dark 160-kilometer moon circling Makemake at a distance of 21,000 kilometers. The Kuiper Belt is the frigid twilight home of several known dwarf planets, and some of these distant icy worlds have known moons–however the moon that belongs to Makemake marks the first discovery of a companion object to Makemake. Makemake is one of the quintet of dwarf planets recognized by the IAU.

A Moon For Makemake

The observations of April 2015, that unveiled Makemake’s tiny moon, were made with HST’s Wide Field Camera 3. HST’s ability to observe faint objects close to bright ones, along with its sharp resolution, enabled the astronomers to spot the moon that was being masked by Makemake’s glare. The announcement of the dim little moon’s existence was made on April 26, 2016 in a Minor Planet Electronic Circular.

The team of astronomers used the same HST technique to observe the little moon as they did for discovering the small moons of Pluto in 2006, 2011, and 2012. Several earlier hunts around Makemake had not succeeded in spotting it. “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,” commented Dr. Alex Parker in an April 28, 2016 Hubble Press Release. Dr. Parker, who led the image analysis for the observations, is of the Southwest Research Institute in Boulder, Colorado.

The discovery of a moon can lead to a treasure chest filled with valuable information about the dwarf-planet system. This is because, by measuring the moon’s orbit, astronomers can then go on to calculate a mass for the system and gain an important insight into its evolution.

Discovering the little moon also reinforces the theory that most dwarf planets have moons.

“Makemake is in the class of rare Pluto-like objects, so finding a companion is important. The discovery of the moon has given us an opportunity to study Makemake in far greater detail than we ever would have been able to without the companion,” Dr. Parker continued to explain.

The discovery of Makemake’s little moon increases the parallels between Pluto and Makemake. This is because both of the small icy worlds are already known to be well-coated in a frozen shell of methane. Furthermore, additional observations of the little moon will readily reveal the density of Makemake–an important result that will indicate if the bulk compositions of Pluto and Makemake are similar. “This new discovery opens a new chapter in comparative planetology in the outer Solar System,” Dr. Marc Buie commented in the April 26, 2016 Hubble Press Release. Dr. Buie, the team leader, is also of the Southwest Research Institute.

However, the astronomers will require more HST observations in order to obtain accurate measurements in order to determine if the moon’s orbit is circular or elliptical. Preliminary estimates suggest that if the moon is in a circular orbit, it finishes a circle around Makemake in 12 days or longer.

Determining the shape of the moon’s orbit will help resolve the question of its mysterious origin. A tight circular orbit would indicate that MK 2 is likely the result of a collision between Makemake and another KBO. Conversely, if the moon is in a wide, elongated orbit, it is more likely to be a captured object from the Kuiper Belt. In either case, the event would have probably occurred several billion years ago, in our primeval Solar System.

The discovery of a moon for Makemake may have solved one perplexing puzzle concerning this distant, icy object. Earlier infrared studies of the dwarf planet showed that while Makemake’s surface is almost entirely frozen and bright, some areas seem to be warmer than other areas. Astronomers had suggested that this discrepancy may be the result of our Sun warming certain dark patches on Makemake’s surface. However, unless Makemake is in a special orientation, these mysterious dark patches should cause the ice dwarf’s brightness to vary substantially as it rotates. But this amount of variability has not been observed.

Earlier infrared data did not have sufficient resolution to separate MK 2 from Makemake’s veiling glare. The astronomers’ reanalysis, however, based on the more recent HST observations, indicates that much of the warmer surface spotted earlier in infrared light may simply be the dark surface of the companion MK 2.

Several possibilities could provide an answer as to why the moon would have charcoal-black surface patches, even though it is circling a dwarf planet that is as bright as freshly fallen snow. One theory that has been suggested proposes that, unlike larger objects such as Makemake, its own little companion moon is so small that it cannot gravitationally keep a grip onto a bright and icy crust, which then sublimates, undergoing a sea-change from solid to gas under the melting influence of warming sunlight. This would make the little moon akin to comets and other KBOs, many of which are well-coated with very dark material.

When the American astronomer James Christy discovered Pluto’s largest moon Charon back in 1978, astronomers were quick to calculate the mass of the system. Pluto’s mass was hundreds of times smaller than the mass originally estimated for it when it was first discovered in 1930. With Charon’s discovery, astronomers suddenly acquired a new understanding that something was fundamentally different about Pluto.

Dr. Parker noted in the April 26, 2016 Hubble Press Release: “That’s the kind of transformative measurement that having a satellite can enable.”

Judith E. Braffman-Miller is a writer and astronomer, whose articles have been published since 1981 in various magazines, journals, and newspapers. Although she has written on a variety of topics, she particularly loves writing about astronomy, because it gives her the opportunity to communicate to others the many wonders of her field. Her first book, “Wisps, Ashes, and Smoke,” will be published soon.