Imagine finding another moon, not around a planet orbiting a distant star, but around a rogue planet drifting alone in the vast cosmic ocean! That's the tantalizing possibility driving cutting-edge research using the James Webb Space Telescope (JWST). Scientists are on the hunt for 'exomoons' – moons orbiting planets outside our solar system – specifically around free-floating planets (FFPs). Why? Because these FFPs, untethered to any star, offer a unique laboratory to study planetary formation and the potential for moon-hosting environments.
JWST is uniquely positioned to tackle this challenge. It's collecting detailed 'light curves' – essentially brightness measurements over time – of several FFPs to analyze their atmospheric conditions. But here's the exciting part: these same light curves can also reveal the presence of exomoons! If a moon passes in front of its host planet (a 'transit'), it causes a tiny dip in the planet's brightness. JWST's unparalleled sensitivity makes it possible to detect these subtle dips.
This particular study focuses on WISE J085510.83-071442.5, affectionately nicknamed WISE 0855. This is a planetary-mass 'Y dwarf' – a very cold and faint object – and its relative proximity to us (only 2.3 parsecs, or about 7.5 light-years) makes it a prime target. WISE 0855 has a temperature ranging from 250 to 285 Kelvin (around -23 to 12 degrees Celsius) and a mass estimated at 6.5 times that of Jupiter (with a margin of error of plus or minus 3.5 Jupiter masses). The scientists analyzed 11 hours of data acquired by JWST's Near-Infrared Spectrograph (NIRSpec).
The team used sophisticated techniques, including Gaussian process (GP) modeling, to tease out any potential exomoon transit signals from the natural variability of WISE 0855 itself. And this is the part most people miss: distinguishing a tiny moon transit from the 'noise' of a planet's own changing atmosphere is incredibly difficult! These FFPs can have dynamic weather patterns, much like Jupiter, which can cause fluctuations in brightness that mimic the signal of a transit.
So, did they find an exomoon? The short answer is no. They didn't detect any statistically significant evidence of a transit in the data they analyzed. But that's not the end of the story. The researchers then performed 'injection and recovery' tests. This involves adding artificial transit signals of varying depths to the data and seeing if their analysis methods could successfully detect them. This allowed them to determine the size of exomoon they could have detected. They found that they could reliably detect transits with depths of 0.5% or greater 96% of the time. For WISE 0855, this corresponds to a moon with a mass ratio similar to that of Titan and Saturn. That's pretty impressive!
But here's where it gets controversial... The team calculated that if every FFP had a Titan-sized moon in a system similar to Jupiter's Galilean moons (Io, Europa, Ganymede, and Callisto), there would be a 91% chance of detecting it after 18 such observations. This implies that JWST, with more observations of FFPs, could provide meaningful insights into how common exomoons are around these solitary worlds.
This study represents the first concrete demonstration that JWST possesses the capability to detect moons around FFPs with masses similar to the Galilean moons orbiting Jupiter.
Authors of the study include Mikayla J. Wilson, Mary Anne Limbach, Andrew J. Skemer, Johanna M. Vos, Brittany E. Miles, Melanie J. Rowland, Andrew Vanderburg, Adam C. Schneider, Caroline Morley, Brooke Kotten, Andrew Householder, Roxana Lupu, James Mang, and Richard Freedman. Their findings are detailed in a 19-page paper with 18 figures, accepted for publication in The Astronomical Journal. The research falls under the categories of Earth and Planetary Astrophysics, Instrumentation and Methods for Astrophysics, and Solar and Stellar Astrophysics.
Now, stepping back, consider this: if we don't find many exomoons around FFPs, what does that tell us about how moons form? Could it be that moons are more likely to form in the chaotic environments around young stars, rather than around solitary planets? And if exomoons are rare around FFPs, does this impact the likelihood of finding habitable moons elsewhere in the galaxy? What kind of moons do you think we'll find first, and where? Let's discuss in the comments!
