Beta Pic b is a promising target for exomoons, because (i) its nearby and relatively bright and (ii) its orbit is almost edge on to our line of sight. If the exomoons formed in the disk surrounding the planet, chances are that the exomoon orbit is close to edge on as well, maximising the signal.
We used the infrared spectrograph CRIRES+ on the VLT to measure the radial velocity of this self-luminous young exoplanet over several months, and were able to get to a precision of about 160 metres/second - enough to be sensitive to larger exomoons. The Hill sphere is the region of gravitational influence of a planet orbiting around its star. Radial velocity sensitivity drops with increasing distance from the planet, but another technique, astrometry, can indirectly detect an exomoon by deviations in the expected orbit of the planet.
The astrometry on Beta Pic b was reported by Macias, Jenkins and Vanderburg who added their limits into this paper. By combining these two results, we can rule out any exomoon greater than three Saturns in mass around Beta Pic b - not too constraining in itself for moon formation theory, but it’s the first time we do this for the whole Hill sphere of an exoplanet. Another season of observations with CRIRES+ would take us down to exomoon masses of a few Earths, and test moon capture theories suggesting wide separation Neptune mass exomoons on longer orbital timescales.
Exomoons are in a similar position to exoplanets in 1994 - we see multiple moons around all the gas giants in our Solar system so they certainly should be around exoplanets: but we still have to get that first confirmed detection - maybe in the next few years? Fingers crossed!
I would like to say huge thank you to all my collaborators, this was a group effort in the best possible sense - I’d like to highlight Rico and Dario working on the RV measurements, and Isabella, Sydney and Andrew for kindly sharing their astrometry and the whole team for their input and patience with me :)
