Webb Telescope Data Point To Six ‘Rogue Worlds’

RACHEL FELTMAN: This is Science Friday. I’m Rachel Feltman. Did you know that when you look up at the stars, almost every single one has at least one planet orbiting around it? That’s wild, right?

But here’s something even wilder. There are some celestial bodies that look a lot like planets but just float around freely in the cosmos, unattached to any particular star. They’re called rogue worlds. And with data from the James Webb Space Telescope, astrophysicists just identified six right here in our own Milky Way Galaxy.

So what can we learn from these rogue worlds? Can they teach us anything about how stars and planets are formed? Joining me to tell us more are two of the study’s authors, astrophysicist Dr. Adam Langeveld as well as Dr. Ray Jayawardena, a professor of physics and astronomy. Welcome to Science Friday.

RAY JAYAWARDENA: Good to be here, Rachel.

ADAM LANGEVELD: Yeah, thank you for having us today.

RACHEL FELTMAN: Thanks so much for being here. So, Adam, what exactly is a rogue world?

ADAM LANGEVELD: So a rogue world is kind of an object, a planetary mass object that kind of free floats in space, untethered to any star. There are kind of two ways that this can come about. The first is that these rogue objects can be formed similarly to stars, where there’s a kind of cloud of gas and dust that gravitationally contracts and kind of forms this orb, this planet star-like object. But it doesn’t reach the required mass to ignite fusion in its core to begin its life as a star, and then it can, if it doesn’t have enough mass, just drift in space alone behaving as a kind of a rogue floating object.

The other way is that it could be formed around another star, as planets do, like our solar system. And so a planet like Jupiter, for example, could form in the disk around a star that’s just formed as well. And then if it is in a kind of dense cluster, for example, a dense cluster of stars that are also forming, it could interact with the gravity of another star and get ejected out of its system and just end up just floating endlessly in space. So these rogue worlds are very interesting kind of single objects that we can now start to see.

RACHEL FELTMAN: So path one is that a cloud of dust and gas collapses, and path two is that a planet forms around a star but gets chucked out of its orbit. Is that right?

ADAM LANGEVELD: Yes, that’s correct.

RACHEL FELTMAN: So, Ray, tell me about the six rogue worlds that you found. Where are they?

RAY JAYAWARDENA: So these happen to be in a nearby star cluster that itself is rather young, just a couple of million years old, and it’s about a thousand light years from our solar system, which means it’s still very much in our neighborhood. And thanks to the incredible sensitivity of the Webb Telescope, particularly at infrared wavelengths, we’re able to probe deeper into this star cluster and identify objects fainter than ever before possible.

RACHEL FELTMAN: Do we have any idea how many rogue worlds or, in science speak, free-floating planetary mass objects there are in the Milky Way in total?

RAY JAYAWARDENA: The census of objects for this particular star cluster, NGC 1333, gives us the sense that maybe 10% of the members of that cluster have planetary masses. Our best estimate is that there are a couple of hundred billion stars in the galaxy. So if something like 10% of objects in the galaxy are planetary mass, that would get us into the billions.

RACHEL FELTMAN: Wow. Very cool.

So, Adam, with so many potential rogue worlds, what makes these six exciting?

ADAM LANGEVELD: So these six objects that we found in this recent study are particularly interesting since they’re some of the lightest objects that we’ve found of this type, and that is these free-floating objects in space. And that is thanks to the real improvement in sensitivity provided by James Webb compared to any of the other telescopes that we’ve been able to look at these regions before.

So what we find is six objects that are between 5 to 15 Jupiter masses. They’re kind of large gaseous objects. And what we don’t find is that there is no object below this mass, even though our experiment and our observations were designed to detect objects down to one Jupiter mass. And so we really think that we have found, at least in the region we are looking at, some of the lowest-mass objects that exist in this region, and that gives us a lot of important context for star and planet formation.

RAY JAYAWARDENA: To build on what Adam said, in some sense, what’s most striking is what we didn’t find. We’re excited to have discovered a handful of new planetary mass objects that are free floating, not circling stars, down to about five times the mass of Jupiter in this star-forming region. But we did not find an abundance of even lighter objects, despite the Webb Telescope having sufficient sensitivity to do so. That suggests that if even lighter objects exist, they must be relatively rare in this young star cluster.

In other words, it’s intriguing to wonder if we are reaching the lowest-mass objects that might form the way that stars do, through the contraction of a cloud of gas and dust. So we’re getting at a very basic fundamental question. How low in mass can an object form the way that stars do?

And the idea, the star formation process, is quite astounding to think about. The most massive stars could be a hundred times the mass of the sun, and the lowest-mass object we found here in this cluster is about five times the mass of Jupiter. The fact that nature can produce objects ranging in mass by a factor of 20,000 through a set of physical processes is rather mind boggling to consider.

RACHEL FELTMAN: Yeah, and do you think that, because these objects are so low mass, that you might be witnessing them at the beginning of their formation?

RAY JAYAWARDENA: Well, given the very youth of the cluster, we are indeed looking at these objects relatively early in their lives, the same way that stars in the cluster show evidence of youth. In fact, the lightest object we found, the five-Jupiter-mass object among the handful, is surrounded by a disk of dust and gas itself.

RACHEL FELTMAN: Tell me more about that dusty disk that you discovered on one of the rogue worlds. How does it compare to rings we might be more familiar with, like the ones on Saturn?

RAY JAYAWARDENA: We would think of it as a scaled-down version of what our own sun started with, a protoplanetary disk out of which our solar system coalesced. What we see in the case of Saturn is not a primordial disk or primordial ring that we’re witnessing today. The rings of Saturn probably formed through collisions of larger bodies generating new particles. So here we are thinking about a planetary-mass object that was born with a disk of gas and dust surrounding it the way we assume our own sun was born and the way we do actually observe other young stars to be surrounded by circumstellar disks.

RACHEL FELTMAN: So I guess that’s another indication that we’re seeing something at a really early stage of its life. And just to make sure I have this right, you’re saying that because it has this disk, it probably formed like a star? I guess if it had been an ejected planet, any disk it might have had would have fallen apart when it got chucked out into space. Is that right?

RAY JAYAWARDENA: That seems likely. The kind of gravitational kick that would eject such an object would also likely disrupt a surrounding disk or at least a sizable disk from being carried away with it as it got ejected. So the evidence points to that five-Jupiter-mass object with a disk is more likely to have formed the way that stars do.

ADAM LANGEVELD: And given that the mass is so low or it’s the lowest-mass object that we found with a disk, again, it just points to the fact that this could be the potential limits or near the limits of how light of an object can form like a star, which is the big, overarching question that we were trying to investigate with this work.

RACHEL FELTMAN: So rogue worlds don’t fit neatly into the box of either planets or stars. And I’m curious, as scientists, do you find that frustrating or exciting?

ADAM LANGEVELD: For me, it’s a great curiosity and an exciting curiosity because every day we find things that don’t fit into those boxes. We find wonders that make us go, wow. There must be some different types of things out there that we’ve never even imagined.

It’s interesting itself because, as we kind of overviewed earlier, there is this overlap between those objects like this that form like planets and the objects that form like stars in terms of their masses. The masses at which these objects can be overlaps. And so trying to narrow that down and trying to narrow down their specific kind of type is very interesting for me.

RAY JAYAWARDENA: The blurring of boundaries is interesting, fascinating in itself, but it also means that the study of free-floating planetary-mass objects like these can shed light on both the star formation process but also the process of planet formation. That overlap is perhaps a little confusing at first blush, but it certainly scientifically is deeply interesting and potentially tells us about two separate processes in nature.

RACHEL FELTMAN: Yeah, very cool. Well, and I want to ask about the image from James Webb that accompanied your study. It’s so stunning. It’s the sea of orange and blue and purple with stars glittering everywhere. Adam, how did you react when you first saw that?

ADAM LANGEVELD: I thought it was very, very beautiful. It was a step-by-step process. So we got the initial images that need to be then kind of filtered and colored and kind of put together because they’re taken in seven parts. But once those images came together, it really was a magnificent sight and something that definitely made us motivated to continue to investigate and look deeper into what we see in the data, in the spectra that we get as well alongside it.

RACHEL FELTMAN: Yeah. And could you explain a little bit more about how these images get put together? I’m always fascinated by the creation of these breathtaking images that are maybe not what I’d see if I just took a rocket ship there and definitely not what each individual instrument sees. But yeah, how does that all come together?

ADAM LANGEVELD: Yeah, so firstly, the image that has been kind of released in the news articles is a mosaic of several kind of squares that we take in this region. So firstly, those have to be kind of merged together.

Secondly is that we take this same image in multiple filters. That is to take the image looking at two different wavelengths such that objects appear brighter in different wavelengths. And by doing this, we can get the kind of color gradient. We can apply a color to the lower wavelength and a different color– for example, red– to the higher wavelengths. And by then combining those two together, we get this nice gradient in color that shows so vividly and so nicely in the final image that we see.

RAY JAYAWARDENA: And remember, these images are taken at infrared wavelengths, not in the visible part of the spectrum. And the Webb Telescope is particularly capable of giving us a good infrared view of the universe.

The beauty of the nearest instrument on Webb that we used for this project is that not only are we able to obtain these stunning images of a star-forming region but we also get a spectrum of every object in our field of view. That means in one fell swoop, we can identify, out of hundreds of point sources in the field, which are the most interesting, which handful of faint objects are most relevant when we are exploring the lowest-mass free-floating objects in this new cluster.

RACHEL FELTMAN: Yeah, well, and speaking of the James Webb Space Telescope, Ray, I know that you worked on the James Webb. So how does it feel to work with data from the system you helped build?

RAY JAYAWARDENA: It’s thrilling. I’ve been involved with a team that developed the nearest instrument for Webb for two decades now, and this particular project of targeting a young star cluster to probe the planetary mass regime in it has been in the works for 10 years. And I remember watching with some mix of excitement and trepidation the launch of Webb on Christmas Day of 2021 with my two kids with so much riding on it, the work of thousands of engineers, scientists, and others over decades, and also so much potential for scientific discovery. And it’s truly gratifying and thrilling to see the Webb Telescope performing so spectacularly, and we’re able to do science that simply was not within the reach of humanity until now.

RACHEL FELTMAN: Wow. Yeah. So my last question for both of you is, now that you’ve found these rogue worlds, what’s next? What else are you hoping to find out?

RAY JAYAWARDENA: So we are, in particular, trying to learn more about these free-floating planetary-mass objects. We are targeting a handful of similar objects with evidence of dusty disks around them so we can learn about those disks, determine if they’re sizable, which might provide a clue as to whether such objects formed in situ the way stars do or whether, In some cases, they formed as planets and were later ejected.

In fact, we have an observing program with Webb that’s underway now. The very first data for that follow-up project were taken just a week ago.

ADAM LANGEVELD: One thing that I would like to add to that and something that I’m very curious and interested in myself is that the nature of these objects that we do see in this region, I would like to try to characterize their composition, for example, with the nearest data that we got the instrument on JWST. We only obtained the spectrum over a very narrow wavelength range, which gives us nice information but relatively limited information about their composition.

And so it would be great to really observe these in more detail over a much wider wavelength range with other instruments on the JWST to see if we can really narrow down their composition. For example, we might expect them to have molecules such as water, carbon monoxide, and maybe methane, which would really also give us a nice insight into their formation conditions as well and their comparison to both exoplanets and stars and the lowest-mass stars. So that, for me, is also very interesting.

RACHEL FELTMAN: Well, I think that’s all the time we have. Adam and Ray, thank you both so much for joining me.

ADAM LANGEVELD: Thank you for having us. Yeah, it was very nice to chat.

RAY JAYAWARDENA: It’s been a pleasure.

RACHEL FELTMAN: Dr. Adam Langeveld is an astrophysicist at Johns Hopkins University. Dr. Ray Jayawardena is a professor of physics and astronomy, also at Hopkins.

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