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SYFY WIRE Bad Astronomy

Astronomers find a moon for a distant, frigid world

By Phil Plait

To be honest, there’s not a huge amount we know about the distant solar system object provisionally named (225088) 2007 OR10. But now we know something new: It has a moon.

That’s actually very cool. And terribly important. Or it will be, soon.

Let me sum up. No, it is too important. Let me explain.

2007 OR10 is a member of our solar system, a world orbiting well past Neptune. And by well, I mean well: The closest it ever gets to the Sun is about 5 billion kilometers, roughly the same distance out as Pluto. But its orbit is very elliptical, and at its furthest, it gets 15 billion kilometers from the Sun. That’s a long, long way.

Which to be fair, is why we don’t know much about it. Right now it’s about 13 billion km out. That’s a tremendous distance, making it difficult to observe. Heck, it was only discovered in 2007.

orbit of 2007 OR10

Besides its orbit, we know it’s very red; observations of it using different filters (as well as spectra; that is, breaking it’s light up into individual colors) indicate it’s much better at reflecting red light than blue. This happens a lot with worlds out in the distant solar system: The simple molecule methane breaks down when exposed to the ultraviolet light from the Sun, then build back up into more complex molecules called tholins, which can be various shades of red and brown. We see this on Pluto, for example, and OR10 is the right size and distance from the Sun to undergo this as well.

Its size can be estimated by its brightness ... though that’s not easy. If you use visible light it’s hard; it could be small and shiny or bigger and dark. However, this can be nailed down by looking in the infrared, where warm objects emit light in well-understood ways. From those observations, OR10 was previously found to be about 1535 km (960 miles) across, about half the diameter of Earth’s moon. That makes it the third largest object in this region of the solar system known; only Pluto and Eris are known to be bigger.

Interestingly, every object in the solar system out past Neptune (called Trans Neptunian Objects or TNOs) bigger than 1000 km in size has been found to have at least one moon. It would actually be rather weird if OR10 didn’t. Not only that, but careful measurements of its brightness over time showed it had a rotation period (a “day”) of about 45 hours, which is much longer than average for most TNOs (which are usually around 24 hours). A moon can interact gravitationally with its primary to slow the spin via tides, so astronomers got suspicious. Does OR10 have a moon?

That’s why a group of astronomers looked through archived observations of OR10 made by Hubble back in 2009 and 2010. Careful examination of the data revealed that yes, OR10 has a moon, so faint it was missed in previous analysis (Note: the moon was actually announced in 2016 at a conference; this news announcement coincides with the release of the official scientific paper)!

The moon is about 240 km (150 miles) across, so it’s pretty big compared to OR10...assuming it’s about the same composition and reflectance (that is, red). If it’s more reflective it could be smaller. I’ll note that Pluto’s moon Charon is much darker than Pluto, so it’s possible the calculated diameter of OR10’s moon will change with further observations.

illustration of dwarf planets

Unfortunately, the moon is only seen in a handful of images (there’s one set of observations made in 2009 and another in 2010). That’s not enough to determine its orbit, and that’s the most critical aspect of all this! Why?

Once you can calculate the orbit of the moon, you can also find the mass of both objects. That’s huge! If we know the size and the mass, that means we can calculate the density, and that tells you (at least in part) what the objects are made of. Water ice, for example, is less dense than rock. A mix of the two would have a density somewhere in between. So seeing the moon move around enough to determine the orbit leads to great things.

I find that amazing; if you see an object with a moon and observe it for a while, you can determine their distance, orbits, separations, masses, and even guess at their composition! That’s so cool.

We’ve found over the years that objects out there past Neptune are pretty diverse. Makemake, for example, is about the same size as OR10 but much more reflective. Other TNOs with moons show variations in reflectivity as well, so there’s clearly a complicated and dynamic history to these objects. Moons are probably the product of collisions between bodies out there; the debris ejected coalesces to form a moon or moons. This isn’t hugely well understood right now, so the more of these we find the more clues we have for their origins.

And one last note. 2007 OR10 is the largest known object in the solar system that doesn’t have a name. Astronomers who find these worlds get the honor of naming them, generally giving them a handle that reflects their nature in some way; TNOs are typically named after native culture gods of the underworld (following the path of Pluto, named for the Roman god). Hopefully, given this new discovery of a moon, we won’t have to wait too much longer before we can call 207 OR10 by something a bit less prosaic.

Image Credit: NASA, ESA, C. Kiss (Konkoly Observatory), and J. Stansberry (STScI)