If Neptune orbited Jupiter
What does the recent discovery of Kepler-1625b I tell us about the habitability of exomoons?
Star Wars fans might remember that the final battle to defeat the Empire took place on and above the forest moon of Endor, a natural satellite populated by the tough but lovable Ewoks. After speeder chases and tight combat between the trees, the Rebels destroyed the Imperial shield generator, eventually blowing up the Death Star, the famed superweapon that terrorized the Galaxy.
The moon — a bit more than one-third the diameter of Earth — orbited Endor, a gas giant probably comparable in size to Jupiter or Saturn. Habitable moons around gas giants have been a staple of science fiction, and they’ve come up a number of times on Worldbuilding. Yet they’ve remained largely hypothetical in our universe — until recently. In October, a pair of astronomers presented additional evidence for a Neptune-sized moon orbiting Kepler-1625b about ten times the mass of Jupiter. If confirmed, this object would be the first known exomoon — a moon orbiting an exoplanet. While they caution that these are only the first inklings of evidence for this candidate, it’s certainly an exciting object to speculate about. One question has been in many people’s minds: Does this mean exomoons could be habitable?
How they found it
To understand the prospects for finding future exomoons, it might be helpful to understand how this candidate was discovered. (Readers can skip this section if they want — I’ll get to the exciting bits later!) The Kepler-1625 system was one of many studied over a year ago by Alex Teachey and David Kipping, in an analysis of possible exomoon hosts observed by the Kepler space telescope. 283 other known exoplanets were discarded by their data analysis pipeline, but Kepler-1625b showed some strange signals that set off flags.
The astronomers were able to make follow-up observations last October using the Hubble Space Telescope. Over the course of 40 hours, they observed three transits of Kepler-1625b, where the exoplanet passes between us and the parent star. After completing the observations, they fit a couple of models to the light curves — graphs of the intensity of the star viewed over time. Kipping and Teachey found something called transit-timing variations (TTVs) in the data. In other words, the transits happened up to 78 minutes earlier than expected!
TTVs are often a sign of another exoplanet in the system, tugging on the observed planet by its gravity. In this case, while the presence of a second exoplanet hasn’t been ruled out, it seems unlikely, as it should have been found by now through other means. The pair instead found that a model featuring a massive moon orbiting Kepler-1625b fit the observations very well.
What makes an exomoon habitable?
The best-fit models gave the astronomers some information about the exomoon, designated Kepler-1625b I. It appeared to resemble Neptune in both size and mass, making it much larger than previously expected. Its orbit is very stable, and its equilibrium temperature is probably between 300 and 350 K, which is surprisingly hospitable. Plus, this was presumably lower by around 50 K earlier in the star’s lifespan, given that Kepler-1625, a Sun-like star, is 9 billion years old and is increasing its luminosity as it exits the main sequence. There’s also uncertainty in the properties of the star itself, as different studies have yielded different results.
In short, if Kepler-1625b I is real, it proves that massive exomoons can exist under conditions that might be conducive to life. If its orbit is stable, an Earth-mass exomoon would also enjoy a stable orbit, clearly surviving for many billions of years. This is an incredibly exciting prospect, as it means that even gas giants can harbor life in their moon systems.
Before we get too excited, it’s worth noting that there are a number of factors that influence habitability beyond mere mass and temperature. I’d be worried about a couple of things in particular:
- There could be really strong tidal forces, perhaps half a dozen times as dramatic as Earth’s.
- The atmosphere might be lost within tens of millions of years, in a worst-case scenario, unless it could be replenished.
- The gas giant’s radiation belts might be a slight problem.
Bearing this in mind, what might life be like for inhabitants of Kepler-1625b I? Given that we know very little about it — and given that even the parameters of its parent star and planet aren’t well-known — we have to do a bit of speculating. What we can say is that the moon could see temperatures similar to Earth’s, at that it would experience years of about 287 days, given Kepler-1625b’s orbital period. It might be tidally locked, which could have interesting consequences — and in fact I’d bet it probably is, after 9 billion years.
The surface gravity would be reasonable — depending on your definition of a surface in this case — and powered flight would be possible. Assuming the moon is gaseous, it would likely have an atmosphere composed of hydrogen and helium. In the case of an actual Neptune analog, water, methane and ammonia would be present in trace amounts, but an ice giant like Neptune seems unlikely to form so close to the parent star — unless, of course, the moon formed separately, further out, but developed an orbital instability, moved further in, and was captured through some complicated gravitational interactions.
Essentially, the orbital conditions are favorable enough for an Earth-like exomoon to be habitable, but Kepler-1625b I, likely a gas giant, would have all the normal giant planet habitability issues, like not having a surface like Earth’s.
How can we find more?
The total number of exomoons we know of is precisely 0. Kepler-1625b I has not been confirmed; the evidence is merely waggling its eyebrows a bit in that direction. We should be a bit excited that after two rounds of observations, the exomoon model still fits the data. However, we absolutely have paths forward to add to our sample size.
The holy grail of exomoon searching would definitely be to see the exomoon itself transit the star, which would show up as additional dips in the exoplanet’s light curve. Teachey and Kipping did see signs toward the end of the transits, which support the TTV data, but they did not investigate the dips in much detail. Now, as is the case with finding exoplanets, more massive exomoons should cause greater transit-timing variations and greater dips, meaning it would be harder to find an Earth-like moon. That said, the possibility certainly shouldn’t be discounted.
Like the astronomers, I’m very much cautious about declaring that Kepler-1625b I is real. More follow-up observations, hopefully by different groups, can shed new light on the transits and the TTVs. Even so, it provides us with an interesting starting point for worldbuilding, and maybe even a little extra inspiration.
