For years I have been fascinated by the search for exomoons—that is, moons orbiting planets in other star systems. These worlds are exciting for a whole host of reasons: they could tell us a great deal about the processes that formed our solar system and others, and whether we share a common formation history with the estimated hundreds of billions of planets in the Milky Way. There's reason to think moons may play an important role in the habitability of their host planets, as some scientists think our Moon has influenced the evolution of life on Earth. And of course, if moons are habitable in their own right, they stand to expand dramatically the amount of real estate in the Universe where life could arise and flourish. The moons in our Solar System are truly remarkable worlds, astonishingly different from one another, and there's every reason to think exomoons could be equally diverse and exotic.
But so far, no one in the scientific community has been able to produce an unambiguous detection of an exomoon. It's not for lack of trying; there are a handful of us around the world actively seeking these objects, and some have dedicated many years of work to the problem. But exomoons are spectacularly tricky to find. They tend to be much smaller than planets, making their transits—small dips in the intensity of the starlight as they pass in front of the star from our point of view—quite shallow, and "lost in the noise." And every time their host planet transits, the moons show up in a different place, making them particularly difficult to detect. Ingenious indirect methods, like looking for the moons' gravitational influence on their host planet, are now routinely employed as part of the search. But this work has been computationally demanding and requires exceptionally careful analysis.
Even so, I've long felt that the first genuine detection of an exomoon is right around the corner, and I have been thrilled to participate in this race. It's been my great privilege working as a graduate student here at Columbia under my advisor, David Kipping, who has been one of the pioneers in this field.
This morning David and I put out a paper on the arXiv that represents the culmination of years of work searching for the signature of exomoons in the population of stars examined by the Kepler Mission. After carefully analyzing an ensemble of the highest quality planetary transit signals, we have determined that exomoons appear to be quite rare in the inner regions of star systems (regions of space close to the host star). This finding was both remarkable and, frankly, a bit disappointing.
We had hoped and expected to see a significant moon signal in the data, showing us for the first time that moons really are common elsewhere in the galaxy, and that these worlds could be ripe for future observation. Instead we found that the moons we seek are not present in any significant way in the data, meaning that they will remain elusive for the time being, likely hiding in the outer regions of these star systems where Kepler was unable to gather much data. This is a fascinating result on its own, but we had hoped for something else. That's the way science goes, though: you have to follow the facts wherever they lead you.
In any case, there was another result in our paper that may make a considerably larger splash, which you may have even read about in the news, or soon will: We announced our identification of a single exomoon candidate, indeed the strongest candidate we've seen in the five year history of the Hunt for Exomoons with Kepler (HEK) collaboration. The system, Kepler-1625 b, has withstood a host of preliminary tests aimed at ruling out the presence of a moon, and our proposal to observe this system with the Hubble Space Telescope was recently accepted. We are thrilled to get the chance to observe with Hubble, and hope that the observation will confirm our suspicion unambiguously that this is a genuine exomoon detection, which would be the first of its kind.
But we want to make it very clear what this is, and what it isn't. At this point what we have is an exomoon candidate, which is very different from an exomoon detection. While we are optimistic about this candidate, and certainly hope to be credited with the discovery of the exomoon if that is in fact what it is, it remains simply a candidate. Furthermore, we require the follow-up observation on Hubble precisely because we feel the data from Kepler are simply inconclusive. The evidence is tantalizing, but it's just not enough to claim a discovery at this point.
And because a discovery of this sort would be a big deal, we have proceeded so far with an abundance of caution, as we have seen so many times how a sensational scientific discovery announced in the media can evaporate under further scrutiny. Let's be clear: we're not just trying to save ourselves from embarrassment; the announcement and subsequent retraction of potentially ground-breaking results has the effect of eroding public trust in science over time, and we are chiefly concerned with not contributing to that problem.
So why did we put out our paper online now, before a proper peer review and the Hubble observation on which this discovery hinges? Well, it turns out all Hubble observing proposals are publicly available online even before the observation is made, and we found out that at least one media outlet was going to publish the fact that we will be observing our exomoon candidate with Hubble. It remains to be seen how much attention this will get, but we worried that getting the public excited about this object before we really know much of anything for sure is just bad for science. And of course we also worried that it would be very easy for another scientist to come along and try to claim the discovery of this moon using what we feel is really insufficient data. To avoid this, and to be absolutely clear on what we've done and what's left to do, we put out our paper in its current form, which is submitted, but not yet accepted, to the Astrophysical Journal.
Peer review is a critical part of the scientific process, and we are not terribly comfortable putting out our results before they have been examined by a qualified referee. Unfortunately, we feel the circumstances have forced us to make our results freely available to the public before such a review, so that everyone may see for themselves what we are claiming and what we are not. While David and I are both big proponents of engaging with the public and boosting interest in the incredible things happening every day in astronomy, we have serious concerns about the potential for sensational headlines misleading the public into thinking a discovery has been made when it is really too early to say that for sure.
The irony is not lost on me that I'm writing this blog post saying we don't want this to get too much media attention. But I hope I've been able to state our position clearly. And when we've got something real, we'll let you know!