What counts as a moon? Huge ‘exosatellite’ sparks debate

**Hannah Siddell** (0:04)
Nature, why is life so far?

**SPEAKER_2** (0:08)
Like, it sounds so simple.

**George Stransfield** (0:09)
They had no idea.

**Benjamin Thompson** (0:10)
But now the data's...

**SPEAKER_2** (0:12)
I find this not only refreshing, but at some level astounding.

**Nick Patrick Howell** (0:23)
Welcome back to the Nature Podcast. This time, the exomoon that isn't...

**Benjamin Thompson** (0:29)
And a transmissible cancer in catfish. I'm Benjamin Thompson.

**Nick Patrick Howell** (0:33)
And I'm Nick Patrick Howell.
In Star Wars Episode IV, In You Hope. Obi Wan Kenobi mutters the immortal words. That's no moon. And that may be the cry of millions of astronomers in response to a new Nature paper that details something that looks a lot like a moon and sounds a bit like a moon. But kind of isn't.
Now, moons are generally considered to be natural satellites orbiting something, usually a planet or a dwarf planet, which is itself orbiting a star. In this case, our moon-adjacent object is orbiting something else, a brown dwarf, a massive object that you can think of as kind of a failed star, as it does produce some light, but hasn't quite reached levels of fusion you see in stars. Together, our kind of moon and this brown dwarf are orbiting an actual star.

**Kevin Hoy** (1:42)
In our case, what we have is like a star that's about half the mass of the sun, that's orbited by a brown dwarf, which is like 35 times the mass of Jupiter, and then orbiting that brown dwarf is a Jupiter mass thing. So you kind of have the same kind of hierarchy set up, but the smallest component of that is still a Jupiter sized object, it's a very planet like object. And so it's hard for us to call that a I'm a moon- and it's really hard for anyone to really draw lines at what counts as a moon or not.

**Nick Patrick Howell** (2:09)
That's Kevin Hoy, one of the authors behind the New Nature Paper. Like he says, it's hard to pin down that observation as a moon. So he and the other authors refer to it as an exosatellite. And it's not just definitions that have been tricky in spotting exomoons, moons from beyond our solar system. There have been a few tentative hints of such objects, but they've been controversial, and none have been confirmed so far. The problem is that moons are often small, or at least smaller than whatever they're orbiting. So they're hard to detect, meaning that astronomers have disagreed on whether some telltale signs are really moons or not.
For instance, one key way astronomers spot exoplanets is by seeing if there's a faint dimming as one passes in front of a star. For a moon, that signal would be weaker, and they would probably be going around the planet, which further complicates things.
Which is a shame, as exomoons could give us insights into how star systems form. We know a lot about our own solar system, for example, by looking at its moons. Neptune has a moon that's a captured asteroid, while Earth's moon is thought to have formed when a Mars sized object collided with Earth's early state. They could also be candidates for where life exists, as moons can get warmer and cozier for life due to tidal heating. Essentially internal friction as the moon deforms due to the gravitational pull of the body it orbits.
Previously, astronomers have looked at a bunch of known star systems and ranked them in order of how likely it would be to spot an exomoon based on different techniques. And to Kevin and the team, there was one standout.

**Kevin Hoy** (3:58)
The CD352722 system was just ranked one of the highest. I think that's partially because the brown dwarf is so well resolved from the host star that we can get really good high quality data that looks just at the brown dwarf and we can mostly ignore the star. And so that helps a lot.

**Nick Patrick Howell** (4:14)
In fact, the brown dwarf orbits around CD352722 every 5,000 years in a very eccentric orbit, an orbit that's not very circular. Meaning that the brown dwarf is quite far away from the star, allowing Kevin and the team to look at it without any interference from the star itself.
This system also ranked highly for probing with a specific technique called the radial velocity technique.

**Kevin Hoy** (4:43)
So the radial velocity technique for finding astronomical objects relies on the Doppler effect. Usually the easiest way to explain it is with sound. So if you hear like an ambulance speeding by, as it's coming towards you, it'll sound a bit higher pitched than as it's going away from you.