An exomoon is essentially just a moon around an extra-solar planet.  To date, no-one has ever discovered an exomoon but astronomers are closing in!

a short article on exomoons

david kipping

Artist’s impression of a habitable exomoon around a Saturn-like exoplanet, courtesy of Sylvain Girard.

The easiest way to look for an exomoon is generally considered to be through closely watching the motion of the host planet, because the moon is too small to see directly.  For example, as the Earth orbits the Sun, it exhibits a slight wobble due to the presence of our moon.  In the animation below, you can see a Jupiter-like planet with an Earth-like moon.  Notice how the Jupiter-like planet wobbles about a common centre.

So now imagine throwing this wobbling motion into a solar system and see what happens...

Notice that now we do not show the moon because as far as modern telescopes are concerned, the moon is invisible to us.  But even though we cannot see the moon directly, we can see the effect it has on the host planet.

Now imagine that this planet transits across its host star.  Basically, this means the planet passes in front of the star once every orbital period and thus causes a dip in the amount of observed star light.  Transits are fantastic tools for exoplanet astronomers.  With the wobbling animation in mind, the basic idea is that every time we do a transit, the planet’s wobble makes the transit appear slightly differently.  These differences allow us to infer the presence of a moon.

The wobbling causes two changes : 1) change in planet’s position 2) change in planet’s velocity.

The change in position means that the transit lightcurve seems to shift about, in what is known as transit time variation (TTV), as first predicted by Sartoretti & Schneider in 1999 [1].

The duration of a transit is inversely proportional to the velocity of the planet.  So since velocity is changing then transit duration is changing.  This manifests itself with the transit duration variation (TDV) effect, as first predicted by myself during the course of PhD in 2008 [2, 3].

The really beautiful thing about these two timing effects is that they are best mates.  TTV and TDV always exhibit a 90-degree phase shift and this essentially gives astronomers a very unique signature to identify exomoons.  If you try to use TTV by itself, you will run into the problem that a plethora of different physical phenomenon can cause TTV, not just moons.  By using TTV and TDV together, you can finally say, that signal is definitely a moon.

Another very nice thing about combining the two is that it allows you to calculate both the mass of the moon and the orbital distance of the moon.  It is impossible to work out either of these properties from TTV or TDV alone!

So hopefully I’ve been able to give you some idea as to how astronomers will be looking for exomoons over the coming years!  We can find Earth-mass exomoons with current telescopes and will very shortly be able to go down to Titan mass moons!

A final thought I’d like to leave you with.  We know of over 300 exoplanets today.  30 of these are in the habitable zone of their host star.  Unfortunately, they are all gas giants which means they are not great places to look for life.  But if just one or two of these 30 planets has a moon, then we may already know another place in the Universe where conditions for life could exist! (See the Articles section for some discussion on these points).

Photo credit: NASA, ESA, RAS, UCL, G. Bacon (STScI), BSN, Andy McLatchie and Sylvain Girard

For those interested in more technical details, including the equations and derivations, please see the original papers on the subject: paper 1 and paper 2.

references

[1] Sartoretti, J. & Schneider, J. 1999, A&AS, 14, 550

[2] Kipping, D. 2009, MNRAS, 392, 181

[3] Kipping, D. 2009, MNRAS, 396, 1797