S/2015 (136472) 1

• 1. Introduction

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  S/2015 (136472) 1, nicknamed MK 2 by its discovery team, is the only known moon of Makemake, a dwarf planet in the Kuiper belt. The team traced its presence back to images captured by the Hubble Space Telescope's Wide Field Camera 3 in April 2015. The discovery wasn't made public until the 26th of April 2016. Nearly a year passed between the sighting and the announcement.

  That gap raises an obvious question. Why did a moon orbiting one of the best studied dwarf planets in the outer solar system go unnoticed for so long?

  A second mystery follows close behind. Makemake had been giving off more heat than its brightness alone could explain, and astronomers already suspected something dark was hiding on or near it. Does MK 2 solve that puzzle?

  The moon still carries no real name, only a string of numbers and letters. What would it take to finally give it one?

• 2. A Dark Companion

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  At apparent magnitude 25 in visible light, S/2015 (136472) 1 is barely a smudge against the sky's darkness. The satellite is 1,300 times fainter than Makemake itself, a gap equal to a magnitude difference of 7.80. That faintness turned out to be a clue, not just an obstacle for observers.

  Before S/2015 (136472) 1 was found, the Spitzer and Herschel space telescopes had measured Makemake's far-infrared thermal emission. Makemake was giving off more heat than expected for how bright it looks in visible light. Astronomers reasoned that some extra dark surface area had to be responsible for that excess heat. Because Makemake's brightness barely changes as it rotates, the dark material had two possible homes. It could be spread evenly across Makemake's own surface, or sitting on a satellite instead. The discovery of this moon lent support to the second option: that Makemake's excess heat comes largely from dark satellites orbiting it.

  Assuming its surface is uniformly dark, S/2015 (136472) 1 has a geometric albedo of only between 2 and 4 percent, about as reflective as charcoal. That is strikingly low next to Makemake's own albedo of 82%. One explanation is that the moon's gravity is too weak to hold onto the bright volatile ices that sublimate off its surface into space. Based on this albedo and its visible brightness, astronomers estimate its diameter at somewhere between 175 and 250 km. The likeliest figure is 175 km, if the albedo sits at 4%. If the moon shares Makemake's density, it would add up to less than 0.2% of the whole system's mass.

  That darkness answers one mystery, but not why the moon evaded detection for so long. Its path around Makemake holds the rest of the answer.

• 3. Hidden In The Glare

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  S/2015 (136472) 1 orbits Makemake from a distance of 22250 km, completing a lap in just 18 days. The orbit is likely circular. Seen from Earth-based observatories, that orbit lies edge-on, so the moon appears to pass in front of or behind Makemake rather than beside it. That alignment makes the satellite easy to lose, since most of the time it disappears into Makemake's glare. That is why earlier telescope observations missed it.

  Around 7 years before or after January 2018, S/2015 (136472) 1 may have been eclipsing and occulting Makemake. That timing comes from its orbital inclination, as seen from Earth's line of sight. Astronomers still cannot say for certain whether the moon travels in the prograde or retrograde direction around Makemake.

  Nailing down its direction remains unfinished business. So does giving the moon an actual name.

• 4. Waiting For A Name

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  The designation S/2015 (136472) 1 works like a code. The S stands for satellite, 2015 marks its year of discovery, and the final 1 marks it as the first satellite found that year. The number in parentheses, 136472, is Makemake's own minor-planet number. Officially, the moon still has no name at all. Whenever it happens, a permanent name is expected to come from a figure within Easter Island's mythology.