Did we just Discover the First Exomoon?

As of the time I'm making this video, there's currently no confirmed examples of any type of moon outside the solar system. Of the 6,000 exoplanets we know, so far, all of them are solitary, except for, in most cases, the stars they orbit. This isn't necessarily because exomoons are rare, it's just that they're very hard to detect, often because of their small size. It's still technically possible to find them with the normal methods of exoplanet detection, like transit and radial velocity, it's just usually going to be much more difficult. So far, most exomoon candidates have only been inferred based on the environments of the planets they orbit. For example, the hot Jupiter worlds WASP-49b and Tylos have evidence of moons around them based on clouds of sodium surrounding the planets, which could be created by volcanic eruptions from orbiting moons. You may have heard recently about a new exomoon candidate being discovered around the brown dwarf CD352722b. The paper discovering it will be in the description, but I will say right now that while it's not fully confirmed, the evidence that it does exist is decently convincing. And there's not one, but actually two candidate satellites here, though the second one seems unlikely, so I'll mainly just be talking about the first one. I will talk about the discovery itself in this video, but something I wanted to focus a little more on is what this object actually is. The answer to the question in the title of this video isn't actually about whether or not this object exists, though that is still a problem, but whether or not it should be called a moon. So let's first go over what this object could be like, before talking about why there's some weird problems with it. The first and obvious thing here is this new thing orbits a brown dwarf, somewhere around either 37 or 29 Jupiter masses. The brown dwarf itself is on a highly eccentric, wide orbit around its red dwarf star, averaging about 100 AU away from it, though gets much closer and much further. The star itself is only 5% as bright as the sun, which would make this a pretty cold object if it weren't for its large size allowing it to generate internal heat, as well as its young age around 150 million years old. So despite being so far away from a cold star, it has a temperature somewhere around 1500 Celsius, around 2700 Fahrenheit, though with a bit of uncertainty. This is also bright enough to allow it to be detected with direct imaging, which is how it was found in the first place. This object's exact orbit is pretty poorly constrained, mainly because it takes so long to complete, upwards of a few thousand years, but it is currently far enough away from its star to be studied independently of it. This means that it's possible to observe the object and discover things about it without significant contamination of the signal coming from the star. This also means that methods of exomoon detection similar to the ones we use for planets could be used for CD352722b, which is exactly what this paper did, searching for radial velocity signals around this brown dwarf. Radial velocity has been used to discover thousands of planets, and involves looking at a star's light to see how it changes due to the presence of a planet pulling on it, which will cause the star to move slightly forward and slightly back from our perspective. Since CD352722b emits its own light and is far enough away from the star to not be contaminated by it, radial velocity observations of just this object could be conducted. And they found something pretty interesting. The spectrum of CD352722b was being altered in a very similar way to stars with planets discovered with radial velocity. So there's strong evidence that there's an additional object orbiting this brown dwarf. This is where there's ambiguity in whether or not the object orbiting CD352722b is a moon or some other kind of object. The first important thing here is that CD352722b isn't a planet, it's a brown dwarf. And that brown dwarf's composition suggests it formed in a similar way stars do rather than planets, which gives it more reason to not be considered a planet. It's likely that this object was basically a star that didn't gain enough mass to actually become one rather than just being a particularly large planet. There's currently no consensus on whether or not objects orbiting brown dwarfs should be considered planets or moons. On one hand, most brown dwarfs form in the same way stars do, meaning anything around them should be considered planets given that the whole system should just be a scaled down version of a regular star system. Objects formed around brown dwarfs are meaningfully different from moons around smaller planets due to different formation histories of the parent object, and a distinction is at least worth considering. On the other hand, brown dwarfs aren't stars. The largest planets by mass overlap with the smallest brown dwarfs by mass. I'll talk about this more later, but for now I'll say the paper just sticks to calling this new object a satellite, which is what I'll call it for now. The second thing worth considering here is that this new object is at least 70% the mass of Jupiter. There's also no consensus on what to call it because there are different naming schemes for planets and moons. If you consider this thing a planet, it should be called CD352722BB, but if you think it's a moon, it would be CD352722B1. Personally, I think this object should be considered a planet, but because there's no consensus, I'm going to call it a secret third option for the rest of the video. A couple weeks ago, a group of astronomers were asked this question and actually voted to refer to this object as an exoweia, with weia being a Chilean slang word meaning thing. So, just for the purposes of this video, I'll be calling this new object weia to avoid picking an actual side for what to call it, and I get to use an actual name instead of a scientific designation, which I always like. Anyways, as I've said already, weia is at least 70% the mass of Jupiter. However, what mass it actually is depends on its unknown orbital inclination, a problem that's common with objects discovered with radial velocity. The authors of this paper consider it unlikely for weia to be another brown dwarf and significantly larger than the minimum mass, though they give a different minimum depending on whether or not the second satellite exists, because this paper does suggest the existence of a two-satellite model, which does explain the data just as well as a one-satellite model. The second satellite would be closer to CD352722b than weia on an 87-day orbit rather than weia's roughly 170-day orbit. However, stability analysis of the system has suggested that a two-satellite model would be unstable, as the two objects would eject each other over a period of a few hundred years, and even if they were in an island of stability where they didn't do that, gravitational interactions with CD352722b's star would probably do it anyway. So while its object having two satellites does fit the data, it's unlikely, as such an arrangement is unlikely to be stable. So, weia takes about 170 days to orbit CD352722b, about 0.2 AU away from it. This is about half the distance between the Sun and Mercury, making the system again more similar to a planetary system than a moon system. Its orbital eccentricity is also suspected to be similar to Mercury's, with an eccentricity of 0.27 compared to Mercury's 0.21. Unfortunately, this is all we know about it at the moment, but we can make some guesses about its environment. This of course is all assuming weia actually exists, which while the evidence is decently strong, is not actually confirmed at the moment. If it does exist, it's possible that weia is locked to CD352722b, though because it's pretty far away from it, its days would be about 6 months long if that's the case. Because the brahndorf is so relatively dim, weia would receive a significant amount of light from the star, and it would experience pretty large seasonal variations in temperature over the course of its parent's highly eccentric orbit. The system is also pretty young, and it's a possibility that weia could have some excess internal heat of its own from its formation, but that highly depends on its currently unknown true mass. The one satellite model in the paper suggests a different minimum mass for weia of about 0.9 Jupiter masses, making it very similar in size to Jupiter itself. Though because this object was so recently discovered and isn't even confirmed to exist yet, there's not much speculation we can do about its environment other than this. But the existence of weia is a perfect example of why I dislike the current definition of what makes a planet. The IAU definition of a planet completely falls apart when trying to classify an object like this. This is also why I prefer the geophysical planet definition, which throws out any orbital requirements and defines a planet solely on its physical characteristics, and under that definition, weia would be a planet. It's also worth noting that this isn't even the only planetary mass object found orbiting a brahndorf. There's many other candidate objects orbiting other brahndorfs, like one that's poetically named 2MassJ08350622plus1950350, as well as a few others with similar names, which have been called planets by the discoverers. So CD352722b and its satellite candidates aren't even the first pair of its type to be suspected to exist, it's just the strongest evidence of such a system yet. And as more objects like this get discovered, it's going to become increasingly clear that the current planet definition doesn't really work. The IAU's definition simply can't handle edge cases like this yet, and while there have been attempts to solve it, like defying a where a moon has to be a specific percentage smaller than its planet to be called a moon, as far as I can tell, these attempts also don't have any kind of consensus. There's also the question of why a Jupiter mass object is being called a moon, when if you moved it just a little bit and put an orbit around the star, it would be called a planet. I think the planet definition needs less rules, not more, to be more flexible and allow for the inclusion of edge case objects like weia without having to arbitrarily add stuff like mass limits. Under the geophysical definition, if an object has never done any kind of nuclear fusion, is massive enough to be rounded by its own gravitational force, and is differentiated into layers based on density, it's a planet. Under this definition, weia would be a planet, which is one of the reasons why I think it should be considered one rather than a moon. Finding the first exomoon would be pretty cool, but I think weia is too abnormal of a candidate to be considered a moon, and should instead be classified as a really interesting exoplanet candidate. But that's just my own opinions, and my purpose in including this section of the video is to just show that there's currently a bunch of problems with classifying objects like this, and it needs a solution. But until then, using the word exosatellite or the new word exoweia, invented entirely just for this object, works as a bandaid solution until a more permanent one can be created. Until then, this is currently all we know about the new satellite candidate orbiting the ground dwarf CD352722b. Hopefully we can learn more about it soon and create a definition for what makes a planet to settle the debate of whether or not this object should be called a planet, a moon, or some weird third option. Thank you for watching. If you enjoyed, check out my other videos about exoplanets and space exploration.