moon Never-before-seen moon spotted around dwarf planet Makemake

[Reefa]

 

There’s another moon in the Kuiper belt. Observations with the Hubble Space Telescope have revealed a never-before-seen satellite around the distant dwarf planet Makemake – despite the fact that we’ve looked before.

 

“It’s been 11 years since Makemake was discovered, almost to the day,” says Alex Parker of the Southwest Research Institute in Boulder, Colorado. “For most of that time we thought it didn’t have satellites, despite having looked. It’s really that the moon spent most of its time hiding.”

 

Makemake was discovered in 2005, and named for a creation deity of the Rapa Nui people of Easter Island. It resides in the Kuiper belt, a collection of icy bodies that live beyond the orbit of Neptune, and is about two-thirds the diameter of Pluto, the largest object in the belt.

 

Other dwarf planets in the Kuiper belt, including Pluto and more distant Eris, have large satellites, so it stood to reason that Makemake should too. But despite several searches, none had shown up.

 

When Parker and his colleagues planned their observing campaign in April 2015, they thought they would have to look for even smaller, fainter moons than Pluto’s, which are up to 100,000 times fainter than Pluto itself.

 

So they were shocked when a moon just 1300 times fainter than Makemake appeared in a Hubble image taken on 27 April 2015.

 

“We were surprised to see a moon at all,” Parker says. “But we were also surprised because it’s not that faint.”

 

The moon was gone again two days later. That led Parker and his colleagues to conclude that the moon’s orbit is edge-on from our point of view, so during previous searches it was lost in Makemake’s glare.

 

The team can’t tell much about the moon yet, but it seems to be about 21,000 kilometres from Makemake and 160 kilometres across. If its orbit is circular, it probably completes a circuit around Makemake every 12 days. For now it is known as S/2015 (136472) or MK2, but Parker has ideas for a catchier name that may fit with the theme of Easter Island deities.

 

More observations will be needed to fully determine the moon’s orbit, but once that’s done the team can use it to figure out the system’s mass. If we’re lucky, Parker says, the two bodies might be in a period known as a mutual event season, during which they move across each other from the point of view of Earth. That could help astronomers make a rough map of Makemake’s surface, similar to the earliest maps of Pluto from the 1990s.

 

The moon is already helping to solve some mysteries about Makemake. Earlier observations of thermal emissions from Makemake with the Spitzer and Herschel space telescopes suggested that the dwarf planet was made up of two very different materials, one warm and dark, one cold and bright. The world could be mottled or covered in spots, but because its brightness doesn’t vary as it rotates, those spots would have to be small and evenly spaced.

 

“What the moon allows us to do is re-contextualise these observations and say that the dark material isn’t on Makemake,” Parker says. “Maybe they were actually seeing light coming from the moon, but weren’t recognising it.”

 

If that’s the case, then the moon would have to be dark as charcoal while Makemake is as bright as snow. That could suggest that the moon was an independent Kuiper belt object that Makemake captured when it went too close – unlike Pluto’s moons, which all seem to have been formed in a giant collision.

 

“This is a great result, a nice bit of observing,” says Mark Showalter of the SETI Institute in California, who discovered Pluto’s four smallest moons using a similar Hubble technique. “It also makes the Kuiper belt a richer and more interesting place for people like me who study dynamics. Every time we look, we find something new in this category of moons of KBOs, telling us more about how the Kuiper belt evolved over billions of years.”

 

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