Pluto is one of the ten large, trans-Neptunian objects that all have moons John R. Foster/SPL

Everybody gets a moon! With the discovery of a small moon orbiting the third-largest dwarf planet, all the large objects that orbit beyond Neptune now have satellites.

Trans-Neptunian objects (TNOs) spend most or all of their orbits beyond Neptune. Last April, the dwarf planet Makemake became the ninth of the ten TNOs with diameters near or above 1,000 kilometres known to have a moon.

So when dwarf planet 2007 OR 10 was found to be rotating more slowly than expected, it was suspected that a moon might be the culprit. To try to find it, John Stansberry at the Space Telescope Science Institute in Baltimore, Maryland, and his colleagues went back to the Hubble Space Telescope archives and found eight images of the world from 2009 and 2010.


“We basically just stretched the images a lot harder than the people who originally took the data, and there was a moon,” says Stansberry. The moon was in every image. The team presented these results at a planetary sciences meeting in October and now in a paper.

Dwarf planet 2007 OR10 with its newly discovered moon circled NASA, STSci, Wesley Fraser, Gábor Marton et al

The discovery of moons around all the largest TNOs gives us a window, not just into the objects themselves, but also into our solar system’s history. TNOs are relics from the era of planet building, so they present an opportunity to peer into the past.

A moon of one’s own

The moons around each of these relatively small worlds probably formed when a large rock collided with the parent body and the debris coalesced in orbit. The fact that every large TNO has a moon points towards a crowded, chaotic past.

“Today, those objects are fairly rare and them hitting each other is improbable,” says Scott Sheppard at the Carnegie Institution for Science in Washington DC. “That means the environment there must have been much more dense: there must have been 10 or 100 times more objects out there in the past than there are now.”

Astronomers hope to take more images of 2007 OR 10 to catch further glimpses of its satellite, as measuring the moon’s orbit can tell us more about the dwarf planet and the early solar system.

“Knowing the mass and density tells you something about how much rock and ice is in the interior, which can tell you things about the gas chemistry in the protosolar disc,” says Stansberry.

Journal reference: arXiv, DOI: arXiv.org/abs/1703.01407