Back on The Old Blog™ in 2022 I wrote about an astonishing Hubble Space Telescope observation of a galaxy so far from us it took its light an incredible 12.9 billion years to reach Earth. Normally it would be too faint to see, but it so happens that a galaxy cluster lies between us and it, and the gravity of that cluster acts like a lens, magnifying and amplifying the more distant galaxy’s light. 

But the truly amazing thing is that the galaxy had a tiny blob of light in it that looks very much like a single star! Nicknamed Earendel, it was thought to be the most distant single star ever detected. 

But is it? To test that idea, a team of astronomers analyzed JWST spectra of the star as well as a star cluster seen in that galaxy, comparing them to determine if maybe Earendel is actually a compact cluster of stars instead of a lone star [link to journal paper].

What they found is that the results are consistent with it being a cluster, but cannot rule out it being a single star either. I’ll admit to being a little skeptical at first when I read the first paper back in 2022, though by the end they presented a decent case for it being a single star. These new results do cast some doubt on that.

That’s a little disappointing, but hey, that’s life. And science. I’d rather know the truth then accept a cool concept that’s wrong. And either way we can’t be sure about Earendel right now anyway. It’s an important topic, too, since whether it’s a cluster or a single star, it’ll help us understand what conditions were like when the Universe was less than a billion years old. So whatever it is it’s still very interesting and something I hope astronomers continue to study.

Tip o’ the lens cap to AAS Nova for their article about this, where they go into details if you want more.

Quaoar (pronounced “KWAH-oh-ar”) is a Kuiper Belt Object, a body of rock and ice that orbits the Sun past Neptune at a distance of about 6.5 billion kilometers. There are millions of such objects, but Quaoar is one of the biggest. It’s just over 1,000 km wide, about half the size of Pluto.

It was discovered in 2002, and a few years later astronomer (and my friend) Mike Brown discovered a moon orbiting it, which is named Weywot. Even cooler, in 2023 astronomers discovered Quaoar has rings! That makes it one of only four minor planets known to have rings.

The two rings were discovered via the occultation method: as seen from Earth, Quaoar passed in front of a distant star, blocking its light. These occultations, as they’re called — think of them as like mini-eclipses — are important because we can use them to measure the size of objects too small to be seen as anything more than dots in a telescope (if we know the orbital speed, and can time how long the occultation takes, that tells us how wide the object is, or at least how wide the part is that passed in front of the star). But the astronomers were surprised to see dips in the star’s light before and after the main occultation, indicating the presence of a ring.

Well! On June 25, 2025, the ring was predicted to pass over another star as seen from the Monterey Institute for Research in Astronomy’s Oliver Observing Station in California. Two astronomers observed the event, and unfortunately didn’t see any dip in the light from that ring [link to journal paper].

But what they did see is way better: a single deep dip that completely blocked the starlight, lasting for about 1.2 seconds, that occurred 2.4 minutes before the predicted ring graze.

What could have done that? The astronomers ruled out terrestrial sources like birds, airplanes, and drones. It’s not likely to be a main belt asteroid either.

They don’t think it could be a third ring either, because a ring that opaque would’ve been detected in other occultations. That leaves one likely candidate: a moon.

They don’t give a size for it, but if it’s in the same plane as the rings (which is probably true) it’s 5,760 km from Quaoar, which is interesting: that puts it in a 7:2 resonance with the moon Weywot. That means it goes around Quaoar seven times for every two times the other moon does. This simple ratio, called a resonance, is very common in multi-moon systems. Moons can shift their orbital size due to the influence of gravity of other moons, and once they fall into a simple resonance the gravity works to keep them there (if it tries to pull ahead or lag behind to break the resonance, the other moon tugs on it to keep it there). So that’s indirect evidence this is in fact a moon.

It’s really faint (they get a magnitude of about 28, which, dang) so it would take a really big telescope to see it. Perhaps if JWST is used to look in the infrared it might be possible to spot it. I don’t know if this would be considered a high enough priority to use it though. Time on JWST is precious.

Anyway, this is pretty nifty. If it really is a moon, the coincidence involved is astonishing; the chance of such a small object happening to pass in front of a star is slim. 

I love occultations, and I love that they are more than just cool things that happens. They actually aid discovery and can be used to measure planets and moons and rings. The more we observe distant objects like Quaoar the more we learn how complex and interesting they are, so I hope we get many more opportunities like this one!

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