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

Introduction

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Exo-moon discovery

1:40Welcome back to the Nature Podcast. This time, the exo-moon that isn't. And a transmissible cancer in Catfish. I'm Benjamin Thompson. And I'm Nick Patrick Howell.

2:06In Star Wars Episode 4, 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.

2:38In 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 the levels of fusion you see in stars. Together, our kind of moon and this brown dwarf are orbiting an actual star. 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.

3:11And 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 moon, and it's really hard for anyone to really draw lines at what counts as a moon or not. 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.

3:43And 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

4:16is by seeing if there is a faint dimming as one passes in front of a star. For a moon, that signal would be weaker, and they'll probably be going around a 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.

4:50They could also be candidates for where life exists, as moons can get warmer and cosier 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. The CD352722 system was just ranked one of the highest.

5:22I 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. 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.

5:56This system also ranked highly for probing with a specific technique called the Radial Velocity Technique. 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.

6:24And that same effect is, it's an effect you have with all waves related to moving objects, and it also applies to light. Due to this effect, Kevin and the team could see if the Brown Dwarf was moving away or towards them. As it would move closer, the frequency of light coming from the Brown Dwarf would be higher, while the opposite would be true as it moved away. If there was a satellite orbiting the Brown Dwarf, it would cause it to ever so slightly move, which would shift the frequency of light. It's this wobble that the team were looking for.

6:56This technique has been applied before to search for exoplanets, but it's been hard to use for exomoons. It needed a good candidate, like the Brown Dwarf Kevin and the team looked at. Using this radial velocity technique, Kevin and the team identified at least one object orbiting around the Brown Dwarf, the exosatellite. Previous claims of exomoon discovery have been met with scepticism due to the weak signals involved and possible interference from their stars.

7:26But Kevin and the team are pretty confident in their assessment. The main thing is that we just have to consider all of the other options for what could be inducing this signal, and the exact thing that we get doesn't seem to correspond to any of them. So I think that there is very, very strong evidence, I'm convinced that there is an object orbiting this Brown Dwarf. This is George Dransfield, an astrophysicist specialising in exoplanets who wasn't associated with the new study. Whether we're going to call it an exomoon or not

7:58is something that's very much up for debate. But I think that the radial velocity signal is quite clear. It seems very clear that there is a periodic signal here, there is something orbiting this Brown Dwarf. Well, George and Kevin's team may be sure about there being something orbiting the Brown Dwarf, even if they can't say it's a moon. There's a lot less known about exactly what this not-quite-moon is like. We don't know exactly for certain, for example, the mass of the object, because one of the features of the radial velocity technique

8:29is that you don't get the actual mass of the satellite, you only get a minimum mass. Like, we know it's not a second Brown Dwarf because we should have seen it from other mechanisms, but we don't. The minimum mass is about a Jupiter mass. If I had to put money on it, I would say the true mass is something like a few Jupiter masses, like two to three. And of course, the other thing that's uncertain is whether or not this object can be called a moon or not. I love the fact that they used exosatellite, because I think satellite is such a great way of talking about and introducing it. So generally, we think of satellites

9:00as things that are orbiting planets, but really a satellite is anything that's orbiting something slightly bigger. So, you know, a planet is a satellite, natural satellite of a star. So, you know, that fits perfectly, but we do immediately think of natural satellites and moons. So I think it's a really beautiful way of presenting it. For Kevin too, exosatellite seems to be the best term. When I go to conferences, I'll ask people on things and I've had a couple of polls and it seems like people are moving more towards there being kind of new term,

9:32because I don't think anybody is all that happy about having Jupiter sized moons is that just, it pushes against how we feel the moons should be so much. There are a few people who want to consider it just an exoplanet. Like the IAU definition right now specifically states that if you are a planet mass object and you orbit a star, a stellar remnant, or a brown dwarf, then you are a planet, no other considerations involved. So that doesn't take into account like kind of the hierarchical setup that we have in the system. For the most part, I would say the majority of people

10:02are on the new category side where they don't really feel that it's the same thing as an exoplanet. They also don't feel that it's the same thing as a moon. And so exosatellite has kind of been one of the main things people are settling on, at least for now. For George, whether or not this is an exomoon is not the most important part of this study. She's excited about the radial velocity technique that the team used to discover the object and hopes that many more exosatellites or even exomoons will be discovered with it.

10:32I think now this has kind of opened floodgates and in the next like five to ten years we'll see a lot more teams attempting this technique and so hopefully that will bring in now the next real exomoon candidates. So I think that the debate about what we're calling it should not overshadow that this is such an exciting new technique they've proven can work. So maybe researchers haven't found the first confirmed exomoon but as we keep peering out into space

11:02we're finding more and more odd systems like this one potentially revealing how much we don't know and can still learn about the universe. I think this discovery emphasizes something really really cool about astronomy and that's that there is plenty of room for surprises. There's lots of different systems that I'm sure we'll discover that will look very different to the solar system and any systems we've discovered before. people shouldn't be too surprised when we start finding new and even weirder things in the future. That was Kevin Hoy from Universidad

11:33Diego Portales in Chile. You also heard from George Dransfield from the University of Oxford here in the UK. For more on that story check out the show notes

Research highlights

11:42for some links. Coming up the discovery of an exceptionally rare form of cancer in brown bullhead catfish. Right now though it's time for the research highlights with Sharmini Bundel.

11:55Engineers have come up with a new design for earthquake-resistant buildings and it all depends on having floors that move.

12:07Earthquakes can cause tall buildings to sway dangerously or even collapse so many modern towers contain what's called a tuned mass damper. This is a large weight often suspended on springs or using fluid to cushion movements. It moves out of sync with the building's natural vibrations and so reduces the overall motion. They're big hard to install and tuned to specific vibrations which may not be useful during an unpredictable earthquake. Now researchers

12:37have tested a design which uses parts of the building itself as the mass damper. The top floors of the tower are connected to the core with springs and an energy absorbing mechanism. When vibrations hit these floors can move sideways and cushion the swaying of the building. The new idea has been tested but only in a scale model. The engineers took a tower to a wind tunnel and found that their damping system resulted in much less motion. Move over to Nature Communications to read more about that research.

13:09When worn they're invisible to the eye and imperceptible to the touch but they can still stick to your face and measure your brainwaves. These are wearable sensors designed to go unnoticed. In the past decade scientists have created numerous transparent wearable sensors but it's usually been pretty obvious if you're wearing one. Now researchers report the development of electrodes that are fully invisible when attached

13:40to your skin. The new electrodes have two layers a thin adhesive film embedded with titanium dioxide nanoparticles that reduced the film's visibility and a conductive layer of silver nanowire. The researchers who designed them were able to monitor volunteers brainwaves eye movements and facial muscle activity with accuracy similar to other devices. The sensors were described by the wearers and by volunteers observing them as imperceptible to the touch. However the data recording device

14:11used to monitor electrical signals was still visible. Being able to hide those components could bring these invisible sensors closer to commercial feasibility. Keep a close eye out for that paper in Science Advances.

14:27Next up on the show this week I've been finding out

Transmissible cancer

14:30about a vanishingly rare form of cancer that's recently been identified in a particular species of fish. Lake Memphra Magog a large body of water split between Quebec in Canada and Vermont in the northeast US is a good place to catch brown bullhead catfish that live on the bottom of the lake.

14:57But back in 2012 anglers started seeing something strange. Many of the catfish were covered in these odd black lesions. They were finding these catfish that had inky black and sometimes crusty tumors on them. A study showed that these lesions were malignant melanomas skin cancers and surveys suggested that roughly one-third of brown bullhead catfish sampled

15:28had them. There were understandably concerns both ecologically and in terms of public health. Could an environmental contaminant or a virus have been the cause? Scientists searched for clues. Scientists that included Julie Dragon. My name is Julie Dragon and I am an associate professor at the University of Vermont. Initially Julie and her colleagues were doing studies looking to see whether something like a cancer causing virus could be the culprit. The team didn't find anything but

15:58eventually after sequencing the genomes of individual fish they found something else. Once the sequencing technology was good enough and cheap enough that we could do whole genome sequencing that's when we discovered that the tumor cells were not as much like their hosts as they were like each other. You'd normally expect a cancer cell to be to broadly look like the organism it was taken from although with a mutation or series of mutations that caused it to become cancerous.

16:29But in this case the cancers were genetically similar to each other but not the fish they were taken from. This was a strange and perplexing finding. It's one of those crazy stories in science I think. We had no inkling that this was going to be the case. We were still certain that it was going to be some sort of mutation in a tumor suppressor gene or something suppressing the immune system that allowed this cancer to come up individually in each fish.

17:01And when we saw what we saw we sort of scratched our heads and then we started asking around other colleagues and found someone who had actually discovered this transmissible cancer in bivalves. He's on our paper as well and he said hey I think you have another transmissible cancer. Transmissible cancers are rare. Like seriously rare. They've only ever been seen previously in dogs, Tasmanian devils in Australia and as Julie says bivalves. Several species in fact.

17:32Things like clams and mussels. Their name rather gives an idea as to what they are. A cancer that can be transmitted between individuals. There are no microbes involved. A cancer cell is the thing transmitting the disease and once it's passed to another host somehow it avoids being targeted by the immune system and goes on to cause disease. And it seems that Julie and her colleagues may now have identified a fourth example of a transmissible cancer and they put forward their evidence in

18:03a paper in Nature this week. This evidence comes from comparing the genetic sequence of tumours and regular tissue from the fish they came from along with tissue from uninfected fish. The team looked at both the DNA found within mitochondria and the DNA contained within cell nuclei. Tumour cells from different fish were essentially clones of each other. Their DNA shared changes or variants that weren't seen in non-cancerous cells. These could be small changes to a single DNA

18:33letter or big changes, for example, where a section of the genome was deleted or duplicated. And in the nuclear DNA taken from cancer cells, there was a lot of similarity in these variations. The number of changes as we add more fish in, but hundreds of thousands of shared variants in these tumours. And if you look at human melanomas, you know, as you compare any two humans, they'll share some variants in their tumours, but as you add more

19:03people and compare, there'll be fewer and fewer that they actually share with each other. The pattern is the opposite with the fish. For every fish we added in, the tumours shared more and more the same variants. As well as analysing fish collected in 2023, the team looked at fish samples and data collected in previous years, enabling them to assess how the cancer might have changed over time. It seems to have stayed relatively stable and we sampled from coves that are quite far apart.

19:35It's unlikely the fish are crossing these coves and interacting with each other, so we don't really know how they all share this, but that's what the evidence says. Julie says that the team's results point towards this being one of these very rare transmissible cancers, the first identified in a fish. Hannah Siddle, from the University of Queensland in Australia, works on immunology, animal disease and genetics. She was impressed by the way that the research team went about coming to this conclusion. Really, I don't think you could

20:06have come to this discovery and they could have made this discovery without that level of detail in the genomics that they've done. It's only with that level of detail that you can actually draw that conclusion. But although this catfish cancer may have joined a very short list of other transmissible cancers, there is a lot to learn about it. Although they've done a lovely job, I think there's a couple of things where they could go further in the future. One, which is how are these cells transmitting?

20:36I think that's a really key question. Whether there's other strains of the tumours that exist in other bodies of water. I have an interest in immunology, so for me as well, understanding something about how these tumour cells are able to evade the immune system. These are some of the things that Julie and her colleagues are working to find out. Tumour cells are spread between Tasmanian devils, mainly via biting. For dogs, it's sexually transmitted, while bivalves are thought to ingest cancer cells that

21:07are just floating through the water. Julie thinks that it could be passed on when the catfish come together to spawn, but that's a theory that needs investigation. And there are many more things to figure out besides. Like, what are these tumours doing to the fish? Transmissible cancer in dogs can be fought off by the immune system, while transmissible cancers found in Tasmanian devils are usually fatal. We don't know what the effect is. That is one of the questions we hope to address in the future because we have seen fish that we have

21:38aged, and based on their weight and their age, it seems like the tumours are not impacting their ability to eat or survive, but we don't know for sure. And the questions keep on coming. Where did this cancer come from? Although this work has its beginnings with Lake Memphra Magog anglers back in 2012, there is a paper from 1925 that describes black tumours on brown bullhead catfish in a different area of the northeastern U.S. That's one of my favourite parts of this

22:09story, is we found this paper, and this researcher from the Ohio State University, Oswald, was looking on the cape because he had gotten reports from a doctor in Falmouth, Massachusetts, about these brown bullhead, which they called pouty, with these inky black tumours. And he was pointed to references, actually, from Henry David Thoreau in his journals from the 1850s to fish

22:40with the same condition. He wanted to investigate and started conducting a study, and he actually took the cells from one fish, cultured them in a dish, and injected them into another fish, and they grew. Canine transmissible cancer is a disease that's thousands of years old, while in Tasmanian devils, it only came about a few decades ago. So, what's the situation here? It could be that it's been around for a long time in the catfish, but suddenly, and for whatever reason, its

23:10incidence has shot up in this location. Julie and the team have cast their net wide to look for it in other places, and they do have some evidence that they found it in other locations.

23:23Julie's paper appears to have solved the mystery of what is causing widespread melanomas in brown bullhead catfish in Lake Memphra Magog, but it has spawned a number of other mysteries that now need to be solved. Right now, you could count the number of examples of transmissible cancers on one hand, but they may be more common than we think, says Hannah, who you heard from earlier. When the Tasmanian devil tumor was first defined back in the early 2000s, I think we did think this was just kind of a weird

23:55thing, right? But I think, you know, what we have to consider now, we've had another one arise in the Tasmanian devil independently. We've got multiple in the bivalves, and now we have an example in fish as well. And so I think we really need to consider that actually this is something that happens more often than we probably think. There's probably more out there that we just don't know about because we're not sampling in the right places. So I think we really have to consider that these things are more common and maybe shaping population dynamics in

24:27ways that we're not expecting because we're not observing it. So I think that's something we need to consider. Hannah Siddle from the University of Queensland in Australia there. You also heard from Julie Dragon from the University of Vermont in the US. To read Julie's paper, look out for a link in the show notes.

Conclusion and ads

24:49And that's all for this time. If you'd like to stay in touch with us, you can. We're on email, podcast at nature.com, or you can find us on social media, at naturepodcast. I'm Nick Badge-Ciao. And I'm Benjamin Thompson. Thanks for listening.

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