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Wednesday, January 11, 2012 / Duncan Forgan

PlanetWatch: The Hunt for Exomoons


Over the Christmas break, I saw this paper pop up in my Twitter feed, which caught my eye. The Hunt for Exomoons with Kepler (HEK) is an ambitious attempt to do something that has never been done before – detect a moon orbiting an exoplanet.

Now, let’s put the audacity of this venture in perspective. Exomoons are pretty much invisible in radial velocity (or Doppler Wobble) measurements, unless you have a superhuman spectrograph that’s absurdly sensitive to minute shifts in velocity. The best way to detect them is via transits (hence the current attempt to use Kepler). You’re unlikely to “see” the moon itself. What you’re more likely to see is the host exoplanet’s transits “wobbling”, as the moon tugs on the planet (see pic below).

Diagram of the star-planet-moon system (moon not shown, taken from Kipping 2009)

The first type of “wobble” is referred to as Transit Timing Variation (TTV), where the period of the transit (i.e. the time interval between two transits) changes with a well-defined sine curve. The second is Transit Duration Variation (TDV), where the length of time a single transit takes changes.

TTV is pretty good, but it is degenerate: this means that it can only measure the exomoon mass and orbital radius as a combined entity, not separately. TDV is also degenerate in a similar way, but measuring both signals together allows scientists to disentangle mass and orbital radius. Transit variations can be used to detect non-transiting planets in the same system as transiting planets, but this has never been achieved for exomoons.

HEK will perform an automated analysis of all planet candidates found by Kepler (over 2300 at last count), and search for positive detections. Even the null detections will allow an estimation of what fraction of exoplanets have a massive moon, which will be an important value for planet formation theorists to understand and explain.

The HEK project is likely to be our best shot of achieving exomoon detection. We expect Kepler to be able to detect exomoons down to about 0.2 Earth Masses. These are pretty massive moons compared to the Solar System (for comparison, Jupiter’s most massive moon, Ganymede, is 0.025 Earth masses), but detecting such a tiny object at such large distances would be a triumph of modern astronomy.

Lastly, I should note that the HEK paper is headed up by none other than David Kipping, who is a friend and colleague (we began our PhDs in the same year). I look forward to seeing his name pasted across the papers when the first exomoon is detected!

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