Did we just find a planet around Tabby's Star?

Boyajian's star, often called Tabby's Star online, is a pretty famous object in astronomy you may have heard of before. This is mainly because of the star's unusual dimming, which was discovered by Tabitha Boyajian in 2015. It was proposed by some that these dimming events, which will be explained in more detail soon, could have been caused by some kind of alien megastructure like a Dyson swarm, though for somewhat obvious reasons this is considered unlikely and the evidence doesn't actually support it. Regardless of the validity of alien claims, something weird is happening with Boyajian's star that is worth investigating. Because some natural explanations for this dimming event are still really interesting, with some potential causes being planetary collisions, moons stripped away from their planets and being destroyed, planets surrounded by a swarm of Trojan objects, and a lot more. And this is all made more complicated by a recent paper that came out finding a candidate exoplanet orbiting Boyajian's star. This unconfirmed planet, which I'll be calling Boyajian B for the purposes of this video, could be very helpful in determining once and for all what is happening with this system. So what is this planet candidate like and what does it mean for Boyajian's star? Before we talk about the planet, we should also talk about the star and its system to get a better idea as to why the system is so odd. Boyajian's star doesn't currently have an official name, but goes by many designations, like KIC 8462852. It was given the nickname Tabby's Star in 2015 when the astronomer Tabitha Boyajian noticed its unusual dimming events. However, this name is somewhat unprofessional and breaks the established convention for naming stars. We don't call Barnard's Star Edward's Star, because stars named after people usually take their last names. Calling the star Tabby's Star instead of Boyajian's Star, as it should be called, is pretty weird, especially because this never happens with stars that are named after men. Boyajian herself also prefers using the name Boyajian's Star, so that's the name I'll be using for this video, and the name I suggest using for anyone else who wants to talk about this star. The name Tabby is used in the title of this video because it's more well-known online and people aren't going to click on this if they don't know what star I'm talking about, but Boyajian should usually be used in place of it. Anyways, the reason the dimming events around Boyajian's Star are so weird is because they're irregular and don't seem to be following any kind of pattern. This is different from normal stars that vary in brightness, which often have stellar cycles that cause them to dim and brighten on regular intervals, but what's happening at Boyajian's Star seems to be largely random. This means that whatever is causing the brightness changes, it's unlikely to be caused by normal stellar activity, leaving the only really plausible explanation as something passing in front of the star to block the light. However, this is also a problem because of the scale of the brightness changes. In 2011 and 2013, the star dimmed by 15% and 22% respectively, which is far too large of a dip for a transiting exoplanet to be the cause. For comparison, a planet the size of Jupiter transiting the star would only cause it to dim by about 1%, meaning whatever object passed in front of it was extremely large. And again, because this dimming was not happening at regular intervals, it can't be a planet on a regular orbit anyway, it would have to be several objects transiting at different times. However, something we can say for certain is that the observations of the star in various wavelengths of light show that these dimming events are consistent with clouds of dust passing in front of the star. This may make it sound like the mystery is solved, it's just dust, but it doesn't end there. Because again, the dips in brightness aren't regular, meaning this isn't just a transiting asteroid belt or something on a regular orbit, or a young star surrounded by a protoplanetary disk. Something unusual still must have happened here to produce these odd dust clouds. Something else we can be pretty confident is that this is not some kind of alien megastructure. This is for two reasons. The first one I've already said, observations of the star are strongly consistent with this being dust clouds, and opaque objects such as a large planet or a Dyson swarm are disfavored. Also, a Boyajian star having a Dyson swarm wouldn't even explain the dimming, because a Dyson swarm or other type of megastructure would still produce periodic transits with a detectable pattern, which we don't see. So not only is there evidence directly against an alien megastructure, even if there wasn't, an alien megastructure is still inconsistent with the observations in the first place, and the only reason it's ever been proposed as an explanation is because a few natural ones don't work. But anyways, let's get to the actual topic of this video, the recent planet candidate found around Boyajian's star and what it means for the system. I say recent, but the data suggesting there might be a planet here dates back to 2019, but this analysis of the data was only recently released. There's not much information about Boyajian b, but from what I can tell, multiple teams have independently detected the transit, which does provide some pretty good evidence it exists. So let's talk about what it may be like. Boyajian b, if it exists, would be a large gas giant on a wide orbit around Boyajian's star, taking about 1200 days to complete, and it transits. I should note here that the transit of Boyajian b would dim the star slightly, however this planet alone is not enough to explain all of the star's dimming events. Essentially this is one normal periodic dimming event surrounded by many other abnormal aperiodic dimming events, because the system isn't complicated already. This transit suggests Boyajian b has a radius roughly 70% larger than Jupiter, but doesn't constrain its mass. Boyajian b's mass estimates come from radial velocity observations, though there's a high amount of uncertainty. The paper in the description suggests the planet may be around 9.4 Jupiter masses, though with a lot of uncertainty. It may be as small as 5.3 Jupiters, or as large as 14.3, and could even be larger. However, they're somewhat confident the object is less than 28 Jupiter masses, which likely puts it in the very large gas giant or small brown dwarf range. This is unfortunately all we can say about it for now, as it's still unconfirmed, and follow up observations to characterize the planet have not yet been conducted. However, we may be able to confirm or rule out this planet's existence fully by the end of the year, as the Gaia Space Telescope observed about a billion stars, Boyajian's being one of them, over several years with astrometry. The fourth data release from Gaia, releasing later this year, will have data precise enough to detect an object of this size and orbit around Boyajian's star, so within a few months we will definitively know whether or not this planet exists. I'll make an update when that happens. So, Boyajian b is likely a pretty big planet, or brown dwarf, on a roughly 3 year orbit around its star, and was detected by transit, but does not explain all the other weird transiting dust clouds orbiting Boyajian's star. However, what Boyajian b, if it exists, can do is help us constrain potential explanations for this star's dimming events. Over the over a decade of observations of this system, a ton of explanations for it have been proposed, and it's pretty hard to keep track of which ones are disfavored and which ones aren't, since there's just a lot to look through. A deep dive into everything related to Boyajian's star deserves its own video, but for now I'm going to be focusing on the explanations that specifically require a planet, or are significantly helped by a planet being present. This is because if Boyajian b does actually exist, then the explanations that require a giant planet messing things up, of which there are a surprising amount, has suddenly gained some more credibility now that we know there may actually be a planet here for those explanations to use. One proposal that requires a planet suggests that the dimming of Boyajian's star is caused by a large ringed planet which has an orbit filled with swarms of Trojan objects at its L4 and L5 Lagrange point, which could explain a specific dimming event in May 2017. However, this explanation would require a planet 4.7 times wider than Jupiter in radius, and to the best of my knowledge we've never seen a planet that large before, and Boyajian b's suspected radius is only about 1.7 Jupiters. The planet would also have to orbit at 5.9 AU, which is further out than Boyajian b's suspected distance. For another example, a 2019 study suggests that the dimming events could be explained by a disrupted exomoon. In this scenario, a moon was somehow ejected away from its planet and fell closer to the star, where it began evaporating, explaining the lack of a pattern and the origin of the dust. However, in many of these scenarios, the requirement is actually that a planet fell into the star and its moon survived on a somewhat normal orbit, which happened in about 10% of simulations run by the paper suggesting this. Again, Boyajian b wouldn't really be useful here, because in this scenario, the planet the moon came from no longer exists. However, for a planet to collide with a star, it would have to migrate all the way there, and Boyajian b, being such a massive object, could in theory at least be a cause of such migration. So these explanations aren't directly supported by the potential existence of Boyajian b, because they require different planets on different orbits, but that doesn't mean a planet is ruled out as part of the explanation. One likely explanation for Boyajian's star is that the star is surrounded by a large disc of comets, or maybe asteroids, which was disrupted by some kind of large object and is now obscuring the star. This would seem to produce the results we see, though what kind of object would be capable of producing it is still up for debate. It's hard to see how a star could have so many comets that 22% of its light can be blocked at once, though a scenario where some large asteroids embedded in dust clouds may be more likely. Either way, some have proposed that these comets or asteroids or planetesimal fragments could have been disrupted by a passing star, which seems to be supported by the fact that Boyajian's star does actually have a small red dwarf orbiting around it, and is a binary star system. The companion star is currently about 885 AU away from the main star. However, because the star is so far out, it seems unlikely that it could disturb objects so close to the main star. We know there aren't any other stars closer in, because they would be detectable, leaving a giant planet as the potential explanation. And the paper discovering this candidate does actually say that the existence of Boyajian b does support the hypothesis that the dimming events are being caused by asteroids and comets being disrupted by a giant object, in this case this very large planet. So it seems like a explanation for Boyajian's star is starting to materialize. The existence of Boyajian b is disrupting an asteroid or comet belt of some kind, sending objects toward the star and causing the dimming events. However, there's still a ton of unanswered questions. To the best of my knowledge, an asteroid or comet belt hasn't actually been detected around this star, so this does still remain speculative. We also don't know how big the belt would have to be and where it would have to be in relation to the planet to produce these results, and if such an arrangement of objects actually exists around Boyajian's star or not. While the hypothesis that a planet is disturbing cometary objects does seem to be decently likely, with how much is unconfirmed, for now, it still remains one possibility of many. So, the mystery around Boyajian's star is still largely unsolved. That doesn't mean it's aliens, but it does mean that, so far, we don't have enough evidence to decide which natural explanation is the most likely, since there are many candidates. However, the unconfirmed existence of the giant planet Boyajian b does seem to allow for the existence of a giant object around the star capable of disrupting smaller objects, which is necessary in many hypotheses trying to explain this system. Within the next few months, we should definitively know whether or not this planet exists, which will help a lot in finally determining what exactly is happening in this weird system. Thank you for watching. If you enjoyed, check out my other videos about exoplanets and space exploration.