Just a few weeks ago I wrote about stellar interactions, taking note of a concept advanced by scientists including Ben Zuckerman and Greg Matloff that such stars would make for easier interstellar travel. After all, if a star in its rotation around the Milky Way closes to within half a light year of the Sun, it’s a more feasible destination than Alpha Centauri. Of course, you have to wait for the star to come around, and that takes time. Zuckerman (UCLA), working with Bradley Hansen, has written about the possibility that close encounters are when a civilization will attempt such voyages.

I have a further idea along the lines of motion through the galaxy and its advantages to explorers, and it’s one that may not require tens of thousands of years of waiting. We’d like to get to another star system because we’re interested in the planets there, so what if an interstellar planet nudges into nearby space? I’ll ignore Oort Cloud perturbations and the rest to focus on a ‘rogue’ or ‘free-floating’ planet as the target of a probe, and ask whether we may not already have some of these in nearby space.

After all, finding free-floating planets – and I’m now going to start calling them FFPs, because that’s what appears in scientific papers on the matter – are hard to find. There being no reflected starlight to look for, the most productive way is to pick them out by their infrared signature, which means finding them when they’re relatively young. This is what Núria Miret Roig (University of Vienna) and team did a couple of years ago, working with data from the Very Large Telescope and other sources. Lo and behold, over one hundred FFPs turned up, all of them infants and still warm.

Image: The locations of 115 potential FFPs [free-floating planets] in the direction of the Upper Scorpius and Ophiuchus constellations, highlighted with red circles. The exact number of rogue planets found by the team is between 70 and 170, depending on the age assumed for the study region. This image was created assuming an intermediate age, resulting in a number of planet candidates in between the two extremes of the study. Credit: ESO/N. Risinger (skysurvey.org).

But young FFPs are most likely to be found in star-forming regions, two of which (in Scorpius and Ophiuchus) were subjected to Miret Roig and team’s searches. What’s likely to amble along in our rather more sedate region is an FFP with enough years on it to have cooled down. The WISE survey (Wide-Field Infrared Survey Explorer) showed how difficult it is to pin down red dwarfs in the neighborhood, although it can be done. But even there, when you get down to L- and T-class brown dwarfs, uncertainty persists about whether you can find them. With planets the challenge is even greater.

Sometimes FFPs are found through microlensing toward the galactic core, but I don’t think we can rely on that method for finding a population of such worlds within, say, half a light year. Nonetheless, Miret Roig is not alone in pointing out that “there could be several billions of these free-floating planets roaming freely in the Milky Way without a host star.” Indeed, that number could be on the low side given what we’re learning about how these objects form. Given the excitement over ‘Oumuamua and other interstellar interlopers that may appear, I’m surprised that there hasn’t been more attention paid to how we might detect planet-sized objects near our system.

The ongoing search for Planet 9 demonstrates how difficult finding a planet outside the ecliptic can be right here at home. While pondering the best way to proceed, I’ll divert the discussion to rogue planet formation, which has always been central to the debate. Are the processes rare or common, and if the latter, do most stellar systems including our own, have the potential for ejecting planets? The last two decades of study have been productive, as we have refined our methods for modeling this process.

Recent work on the Trapezium Cluster in the Orion Nebula shows us how the catalog of FFPs is growing. The Trapezium Cluster is helpfully located out of the galactic plane, and there is a molecular cloud behind it that reduces the problems posed by field stars. I was startled to learn about this study (conducted at the European Space Agency’s ESTEC facility in the Netherlands by Samuel Pearson and Mark J McCaughrean) because of the sheer number of FFPs it turned up. Some 540 FFP candidates are identified here, ranging in mass from 0.6 to 13 Jupiter masses, although the range is an estimate based on the age of the cluster and our current models of gas giant evolution.

Image: A total of 712 individual images from the Near Infrared Camera on the James Webb Space Telescope were combined to make this composite view of the Orion Nebula and the Trapezium Cluster. Credit: NASA, ESA, CSA/Science leads and image processing: M. McCaughrean, S. Pearson, CC BY-SA 3.0 IGO.

What stopped me cold about this work is that among the 540 candidate FFPs, 40 are binaries. Two free-floating planets moving together without a star, and enough of them that we have to learn a new term: JuMBOs, for Jupiter-mass binary objects. How does that happen? There are even two triple systems in the data. Digging into the paper:

…we can compare their statistical properties…with higher-mass systems. The JuMBOs span the full mass range of our PMO [planetary-mass object] candidates, from 13 MJup down to 0.7 MJup. They have evenly distributed separations between ∼25–390 au, which is significantly wider than the average separation of brown dwarf-brown dwarf binaries which peaks at ∼ 4 au [42, 43]. However, as our imaging survey is only sensitive to visual binaries with separations > 25 au, we can not rule out an additional population of JuMBOs with closer orbits. For this reason we take 9% as a lower bound for the PMO multiplicity fraction. The average mass ratio of the JuMBOs is q = 0.66. While there are a significant number of roughly equal-mass JuMBOs, only 40% of them have q ≥ 0.8. This is much lower than the typical mass ratios for brown dwarfs, which very strongly favour equal masses.

That last line is interesting. Our FFP binary systems tend to have planets of distinctly different masses, which implies, according to the authors, that if the JuMBOs formed through core collapse and fragmentation – like a star – “then there must be some fundamental extra ingredient involved at these very low masses.” But the binary systems here go well below the mass where this formation method was thought to work. That opens up the ‘ejection’ hypothesis, with the planets forming in a circumstellar disk only to be ejected by gravitational interactions. So note this:

In either case, however, how pairs of young planets can be ejected simultaneously and remain bound, albeit weakly at relatively wide separations, remains quite unclear. The ensemble of PMOs and JuMBOs that we see in the Trapezium Cluster might arise from a mix of both of these “classical” scenarios, even if both have significant caveats, or perhaps a new, quite separate formation mechanism, such as a fragmentation of a star-less disk is required.

Ejection is a rational thing to look at considering that gravitational scattering is a well-studied process and may well have occurred in the early days of our own system. On the other hand, in star-forming regions like Trapezium the nascent systems are so young that this scenario may be less likely than the core-collapse model, in which the process is similar to star formation as a molecular cloud collapses and fragments. The open question is whether a scenario like this, which seems to work for brown dwarfs, is also applicable to considerably smaller FFPs in the Jupiter-mass range.

In any case, it seems unlikely that binary planets could survive ejection from a host system. As co-author Pearson puts it, “Nine percent is massively more than what you’d expect for the planetary-mass regime. You’d really struggle to explain that from a star formation perspective…. That’s really quite puzzling.”

All of which triggered a new paper from Fangyuan Yu (Shanghai Jiao Tong University) and Dong Lai (Cornell University), which takes an entirely different tack when it comes to formation of binary FFPs:

The claimed detection of a large fraction (9 percent) of JuMBOs among FFPs (Pearson & McCaughrean 2023) seems to suggest that core collapse and fragmentation (i.e. scaled-down star formation) channel plays an important role in producing FFPs down to Jupiter masses, since we do not expect the ejection channel to produce binary planets. On the other hand, (Miret-Roig et al. 2022) suggested that the observed abundance of FFPs in young star clusters significantly exceeds the core collapse model predictions, indicating that ejections of giant planets must be frequent within the first 10 Myr of a planetary system’s life.

Yu and Lai look at close stellar flybys as a contributing factor to FFP binary formation. If we’re talking about dense young star clusters, encounters between stars should be frequent, and there has been at least one study advancing the idea that bound binary planets could be the result of such flybys. Yu and Lai model two-planet systems to study the effects of a flyby on single and double-planet systems. Will an FFP result from a close flyby? A binary FFP? Or will the flyby star contribute a planet to the system it encounters?

These numerical experiments yield interesting results: The production rate of binary pairs of FFPs caused by stellar flybys is always less than 1 percent in their modeling, even when parameters are adjusted to make for tightly packed stellar systems. Directly addressing the JWST work in Trapezium and the large number of JuMBOs found there, Yu and Lai deduce that they cannot be caused by flybys, and because ejection scenarios are so unlikely, they see “a scaled-down version of star formation” at work “via fragmentation of molecular cloud cores or weakly-bound disks or pseudo-disks in the early stages of star formation.”

The matter remains unresolved, producing much fodder for future observations and debate. And while we figure out how to detect free-floating planets that may already be far closer than Proxima Centauri, we can create science fictional scenarios of journeys not just to a single rogue planet, but to a binary or even a triple system cohering despite the absence of a central star. I can only imagine how much Robert Forward, the man who gave us Rocheworld, would have enjoyed working with that.

The paper is Pearson & McCaughrean, “Jupiter Mass Binary Objects in the Trapezium Cluster” (preprint). The Miret-Roig paper is “A rich population of free-floating planets in the Upper Scorpius young stellar association,” published online at Nature Astronomy 22 December 2021 (abstract). The Fangyuan Yu & Dong Lai paper is, “Free-Floating Planets, Survivor Planets, Captured Planets and Binary Planets from Stellar Flybys,” submitted to The Astrophysical Journal (preprint).