Intriguing data from an event in 2007 hints at an exomoon forming around a giant planet in a youthful star system 420 light years from Earth.
Moons are a big deal. In our own solar system we've discovered 176 natural satellites (even asteroids have them). Some, like Ganymede or Titan, are comparable in size to the planet Mercury. Many harbor vast coatings of frozen water and could also hold interior oceans. Tweak the orbital circumstances and it seems entirely possible that moons could exist with terrestrial-type surface environments - temperate and potentially as lush as our science fiction fantasies of Endor or Pandora. It's also reasonable to imagine that these might represent a vast population of new worlds across the cosmos, a segment of the planetary menagerie that we've simply not yet seen.
But finding exoplanets is hard, and finding exomoons is even harder. One approach is to mine the database of a mission like NASA's Kepler space telescope, looking for signs of moons perturbing the transits of planets across the face of their parent stars. A planet with moons orbits its common center of mass with those satellites, all the while orbiting the star. The upshot is that the timing and duration of the planetary transit will vary a tiny amount, depending on the properties of the moons - like their masses and distances from the mother world.
Another possibility is that an exomoon can be spotted through gravitational microlensing. And indeed there have been claims of the signature of a planet and moon in such data - although perhaps as a free-floating pair, not themselves in orbit around a star anymore.
Giant planets with moons offer one additional signpost. We know that worlds like Saturn or Jupiter also harbor ring systems, disk-like spreads of very small bodies and particles that can also produce distinctive transit signals if the parent world passes in front of a star. We also think that when gas giant planets like these were young they were likely surrounded by 'rings on steroids' - circumplanetary disks of orbiting material akin to a scaled down version of the circumstellar disks that make planets.
Back in 2007, the SuperWASP project monitored the light coming from a youthful star, barely 16 million years old, called (for short) J1407. Some 420 light years from us, this star started to show a series of eclipse, or transit, events - its light dimming and brightening in ways that indicated something was moving across the face of the star from our point of view. These events lasted for 56 days - an absolute lifetime for planetary transits.
What was happening around J1407?
The first analysis, by Mamajek and colleagues in 2012, suggested that we had witnessed the effect of ring system around a planetary companion to the star. Now a new analysis by Kenworthy and Mamajek, improves on that study with a more detailed attempt to clean, model, and analyze the data. Their conclusions are that a large planet, or substellar object, with a set of some 37 rings did indeed transit J1407.
The configuration is on a grand scale. The planetary body is probably between 10 and 40 times the mass of Jupiter - suggesting it could be a brown dwarf. It's most likely orbiting this young star once every 13 years or so - we've yet to see the next transit. The ring system has to be about 0.6 astronomical units (AU) in radius - by comparison, Saturn's rings extend a paltry 0.0009 AU above its atmosphere.
But it's the gaps in the rings that are perhaps the most intriguing features. The deepest of these could correspond to a 4 million kilometer wide empty swathe. One way to make such a gap is for a forming exomoon to sweep this space clear - a satellite world that could be 80% the mass of the Earth, taking nearly 2 years to circle its parent body.
So has an exomoon been spotted?
Perhaps. The overall explanation for the 56 days of complex variation in stellar light makes good sense, and we fully expect that ring systems or circumplanetary disks should exist in these kind of circumstances - in a youthful, forming system.
The catch is that the parent body of this putative exomoon seems to be in the borderlands between what we typically consider a planet (objects less than about 13 times the mass of Jupiter, where sustained fusion of deuterium is not possible in the core), and a star (more massive than 65-80 Jupiters). In other words, this system - comprising J1407, which is 90% the mass of the Sun, together with a possible ringed brown-dwarf - might be better classified as a type of binary star system. The exomoon might be an exoplanet with a substellar parent.
Except this characterization could also be rather unfair. One could equally see this as an example of something wonderfully new - a set of circumstances that sit outside our still narrow view of planetary systems. We simply don't have the right words, the right labels, for all that we're discovering. It mirrors some of the recent debate over the (still) controversial definitions of what are, or aren't, planets in our own solar system. Nature, as is so often the case, easily outraces our imaginations.