While I’m in Houston attending the 100 Year Starship Symposium (about which more next week), Andrew LePage has the floor. A physicist and freelance writer specializing in astronomy and the history of spaceflight, LePage will be joining us on a regular basis to provide the benefits of his considerable insight. Over the last 25 years, he has had over 100 articles published in magazines including Scientific American, Sky & Telescope and Ad Astra as well as numerous online sites. He also has a web site, www.DrewExMachina.com, where he regularly publishes a blog on various space-related topics. When not writing, LePage works as a Senior Project Scientist at Visidyne, Inc. located outside Boston, Massachusetts, where he specializes in the processing and analysis of remote sensing data.

by Andrew LePage


Like many space exploration enthusiasts and professional scientists, I was inspired as a child by science fiction in films, television and print. Even as a young adult, science fiction occasionally forced me to think outside of the confines of my mainstream training in science to consider other possibilities. One example of this was the 1983 film Star Wars: Return of the Jedi which is largely set on the forest moon of Endor. While this was hardly the first time a science fiction story was set on a habitable moon, as a college physics major increasingly interested in the science behind planetary habitability, it did get me thinking about what it would take for a moon of an extrasolar planet (or exomoon) to be habitable. And not “habitable” like Jupiter’s moon Europa potentially is with a tidally-heated ocean that could provide an abode for life buried beneath kilometers of ice, but “habitable” like the Earth with conditions that allow for the presence of liquid water on the surface for billions of years with the possibility of life and maybe a technological civilization evolving.

A dozen years later, the first extrasolar planet orbiting a normal star was discovered and a few months afterwards on January 17, 1996, famed extrasolar planet hunters Geoff Marcy and Paul Butler announced the discovery of a pair of new extrasolar giant planets (EGPs) opening the floodgate of discoveries that continues to this day. One of these new EGPs, 47 UMa b, immediately caught my attention since it orbited right at the outer edge of its sun’s habitable zone based on the newest models by James Kasting (Penn State) and his colleagues published just three years earlier. While 47 UMa b was a gas giant with a minimum mass of about 2.5 times that of Jupiter and was therefore unlikely to be habitable, what about any moons it might have? If the size of exomoons scaled with the mass of their primary, one could expect 47 UMa b to sport a family of moons with minimum masses up to a quarter of Earth’s.

I was hardly the first to consider this possibility since it was frequently mentioned at this time by astronomers whenever new EGPs were found anywhere near the habitable zone. But this realization did get me seriously researching the scientific issues surrounding the potential habitability of exomoons and I started preparing an article on the subject for the short-lived SETI and bioastronomy magazine SETIQuest, whose editorial staff I had recently joined. While working on this article, I started corresponding with then-grad student Darren Williams (Penn State) who, it would turn out, was already preparing a paper on habitable moons with Dr. Kasting and Richard Wade (Penn State). Published in Nature on January 16, 1997, their paper titled “Habitable Moons Around Extrasolar Giant Planets” was the first peer-reviewed scientific paper on the topic. They showed that a moon with a mass greater than 0.12 times that of Earth would be large enough to hold onto an atmosphere and shield it from the erosive effect of an EGP’s radiation environment. In addition, tidal heating could potentially provide an important additional source of internal heat to drive the geologic activity needed for the carbonate-silicate cycle (which acts as a planetary thermostat) for much longer periods than would otherwise be possible for such a small body in isolation.

I published my fully-referenced article on habitable moons in the spring of 1997. In addition to incorporating the results from Williams et al. and related work by other researchers, I went so far as to make the first tentative estimate of the number of habitable moons orbiting EGPs and brown dwarfs in our galaxy based on the earliest results of extrasolar planet searches: 47 million compared to the best estimate of the time of about ten billion habitable planets in the galaxy (estimates that are in desperate need of revision after almost two decades of progress). Since my research showed it was likely that habitable moons would tend to come in groups of two or more, I further speculated about the possibilities of life originating on one of these moons being transplanted to a neighbor via lithospermia. And since I did not have to contend with scientific peer-review for this article, I even speculated about the effects multiple habitable moons would have on a spacefaring civilization in such a system with so many easy-to-reach targets for exploration and exploitation.


Image: An artist’s conception of a habitable exomoon (credit: David A. Aguilar, CfA).

After SETIQuest stopped publication and I published a popular-level article on habitable moons in the December 1998 issue of Sky & Telescope, my scientific and writing interests lead me in other directions for the next decade and a half. But in the meantime, scientific work on exploring the issues surrounding habitable bodies in general and habitable moons in particular has continued. The current state of knowledge has been thoroughly reviewed in the recent cover story of the September 2014 issue of the scientific journal Astrobiology, titled “Formation, Habitability, and Detection of Extrasolar Moons” by a dozen scientists active in the field including one of the authors of the first paper on habitable exomoons, Dr. Darren Williams.

Even after 17 years of new theoretical work and observations, the possibility of habitable exomoons still remains strong. The authors show that exomoons with masses between 0.1 and 0.5 times that of the Earth can be habitable. A review of the available literature shows that exomoons of this size could form around EGPs or could be captured much as Triton is believed to have been captured by Neptune in our own solar system. Calculations also show that such exomoons, habitable or otherwise, are detectable using techniques that are available today, especially direct detection by photometric means like that employed by Kepler and by more subtle techniques such as transit timing variations (TTV) and transit duration variations (TDV) of EGPs with exomoons. As the authors state in the closing sentence of their paper:

In view of the unanticipated discoveries of planets around pulsars, Jupiter-mass planets in orbits extremely close to their stars, planets orbiting binary stars, and small-scale planetary systems that resemble the satellite system of Jupiter, the discovery of the first exomoon beckons, and promises yet another revolution in our understanding of the universe.

The fully referenced review paper is René Heller et al., “Formation, Habitability, and Detection of Extrasolar Moons”, Astrobiology, Vol. 14, No. 9, September 2014 (preprint).