JuMBOs: a newly discovered cosmic formation
The JWST is helping solve all kinds of cosmic mysteries. But there are still plenty of them out there. And very recently, within the past few weeks even, has come the discovery of a new cosmic formation. Jupiter Mass Binary Objects, or JuMBOs, are the latest mystery to come out of images from the JWST. As the name suggests, these are binary objects: two objects orbiting one another in close proximity. They’re too small to be stars, but also don’t orbit a star like a conventional planet would. So what on Earth, or in space, is going on? The University of Exeter’s Matthew Bate…
Matthew - The thing about the James Webb Space Telescope is it works in the infrared and also very high resolution. This was pointed at the Orion Nebula cluster, which will be very familiar to any amateur astronomer. And these JuMBOs, they're very young though they're relatively hot because they've just recently formed. The assumption is that these JuMBOs are also around a million years old, but some of them still have temperatures below about a thousand Kelvins. It's like 700 degrees Celsius. And so these are primarily in the infrared, which James Webb is perfectly designed to spot. And the surprising aspect is that about 42 objects appear to be binary Jupiter mass objects. So several Jupiter mass planets orbit around each other, but not in orbit around a star. And this has never been seen before.
Will - They're being described as objects rather than, you know, a star or a planet. What about them is making it so difficult for them to be classified?
Matthew - So these come right at the boundary where they could form a little bit like stars form, but then that'd be extremely low mass compared to stars. Or they could form like planets, but then they're quite high mass compared to most planets, which means they potentially could form either way. And we don't really know which way they are forming. So stars we believe form in the clouds of mostly hydrogen and helium gas with a bit of dust, a bit of hippie elements. These clouds when they are dense enough, they collapse under their own gravity, get denser and denser and eventually form an object, a protostar, which may initially only be a few Jupiter masses material, but then it accumulates more and more gas from the surrounding cloud and grows to higher and higher masses. Below about 75 Jupiter masses. These objects are not massive enough to fuse hydrogen. So when they form they're very hot, but then they just cool down because they don't have a hydrogen fusion power source. And so these are known as brown dwarves. And when you get to below 13 Jupiter masses, then they have no fusion at all. So the other possibility of course is that they form like planets. So we believe planets form in discs of gas and dust surrounding a star. But the interesting thing about these JuMBOs is they're not orbiting stars. So if they formed as planets, they then would've had to be ejected from the star that they formed around. But they're also binaries, they're orbiting each other. So that means you'd have to eject two, several Jupiter mass planets simultaneously and yet still have them bound together in a wide orbit.
Will - It seems putting all this together, from what we know of them. Our conventional way of categorising things, well it might be able to explain a freak event, but these are very common seemingly.
Matthew - Exactly right. So to form low mass brown dwarfs the same way as stars. I've worked a lot on theories where you might form a few objects, like 4, 5, 6 objects, in a collapsing gas cloud and they will interact with each other gravitationally. And maybe you can throw some of these out of the cloud when they still have just a few Jupiter masses of material so they don't then accrete up to become a proper star. But how do you get binaries chucked out of the gas cloud? That's not very clear. If you had one or two of these binaries, you could say this is just a freak event, which, you know, two objects happened to get thrown out about the same time they were weakly bound together. But the issue is with these observations, they've spotted 42 of these objects in a cluster where there's around 400 free-floating Jupiter mass objects. And so this isn't a freak way of forming them, whatever the mechanism is, it's forming quite large numbers of these.
Will - Obviously it's very early days, but would you like to speculate on any of the theories as to how they're forming then?
Matthew - To be honest, I don't think any of our theories really explain this. Again, they might explain one or two, they just don't explain how we've got so many of these in a single cluster. One of the interesting things now will be to use James Webb to look at other young star forming regions and to see whether you see more of these JuMBOs in every young star forming region that you look at, or whether this is unique to Orion. If it's unique to Orion, then it may have something to do with the fact that you've got massive stars there. So massive stars emit very strong radiation fields and these can destroy the molecular clouds in which stars are forming. And so it's possible that in Orion you had a whole lot of stars that were just about to form and collapse to form very low mass objects that then would've gone on to accrete more material. But the cloud was then overwhelmed by this radiation from the massive stars, blown away, and you just leave these low mass objects which don't have the opportunity to accrete to higher masses anymore. But again, why is it only objects with a few Jupiter masses? Why does this seem to be happening just around maybe one to five Jupiter masses? We don't really understand it. I think.
Will - And as a final point, just to ease everyone's mind, we are talking about these unknown giant entities flying, untethered through the galaxy. We're not expecting to see one anywhere near Earth soon.
Matthew - Well, that's a good question. The older ones would be quite cold and so they'll be hard to spot, right? Remember, the only reason we can see these with James Webb is that they're very young and they still have temperatures of maybe a thousand Kelvin. So they're emitting very strongly in the infrared. But these will cool quite quickly and as they cool, they become much, much fainter. And so they're harder and harder to spot. And so if there are large numbers of these things around and they're quite old, they'd have to be quite close to us in order for infrared telescopes to spot them. So it will be interesting to try and work out how close you'd expect the closest of these to be. But there could be some hiding out there because they could be old and very faint and we might not have detected them.