Finding moons around extrasolar planets is an invigorating quest. After all, at least three moons around gas giants right here in our own system — Europa, Enceladus and Titan — are considered of high astrobiological interest. What about gas giants in the habitable zone of some distant star? The image below shows what a moon of such a planet one might look like, as imagined by astronomer Dan Durda (Southwest Research Institute). Could such worlds be?


As we learn more, bear in mind that the hunt for ‘exomoons’ has already begun. The CoRoT spacecraft is searching for transit timing variation signals (TTV) — variations in the time it takes a planet to transit its star — as described by Sartoretti and Schneider in a 1999 paper. David Kipping (University College London) has been developing a second method called transit duration variation (TDV), which works in conjunction with the first. The TDV signal is brought about by velocity changes as the planet/moon ‘system’ is observed over time, the result of both planet and moon orbiting a common center of mass.

Dr. Kipping now offers a further take on these two effects, which should be able to detect and characterize an exomoon when used in tandem. In the new paper, the astronomer breaks transit duration variation itself into two parts, one based on velocity (V), the other on what he calls the transit impact parameter (TIP).

…an exomoon around a transiting exoplanet should induce a transit duration variation effect with two dominant components. One of these components is due to the moon altering the velocity of the host planet, which we label as the V-component. The second constituent is due to the impact parameter of the transiting planet varying as a result of the moon’s presence, which we label as the TIP-component.

The analysis of these combined effects should allow astronomers to tell the difference between moons in a prograde or retrograde orbit, because the TIP component acts constructively with the V-component in prograde situations and destructively with it in retrograde orbits. The effect is to boost the exomoon detection method via TDV by about ten percent in magnitude. All this helps us understand more about how the moon might have been formed and tells us something about the stability of its orbit.

The key point is that we have signals thrown by these effects that should be observable today, and will certainly be so as our instrumentation improves:

We therefore propose that it should be possible for future observations to not only detect an exomoon and determine its mass, but also provide a confident deduction of the sense of its orbital motion. Although this determination will likely require photometry at the limit of planned missions, it seems likely that once an exomoon is detected a more in-depth investigation would be able to answer the question of orbital sense of motion conclusively.

There are caveats here, including the fact that the calculations are effective for a planet-moon system in which the plane of the two objects’ orbits is aligned with the star-planet orbital plane — large exomoon inclinations would be disruptive. But it’s interesting to note that in Kipping’s view, exomoons at low inclination angles should be observable in the lightcurve during any planet-moon eclipse. That would be an exciting confirmation of the first detection of a moon around a distant extrasolar planet.

The paper is Kipping, “Transit Timing Effects due to an Exomoon II,” accepted for publication in Monthly Notices of the Royal Astronomical Society and available online.