Finding exomoons using their host planet’s wobble

Exoplanets aren’t the only objects floating around other stars—they likely have comets and asteroids as well. Even some of the exoplanets themselves will have “exomoons,” at least according to our current understanding of the physics of planetary formation. However, we have yet to find any of these other objects conclusively, though there has been some hint at the presence of exomoons in the last ten years.

A new paper from astronomers at the European Southern Observatory (ESO), recently posted to the arXiv preprint server, suggests a way in which we might be able to finally detect the presence of an exomoon—using a technique that is also commonly used to find exoplanets themselves.

That technique is astrometry—measuring the “wobble” induced in an object when another object is orbiting it. When searching out exoplanets, that “wobble” is typically induced in a star, but in the case of exomoons, it would be induced in the planet itself.

Unfortunately, detecting such a tiny signal on top of what was already another tiny signal is exceedingly difficult. Back in 2018, astronomers from Columbia, including David Kipping of Cool Worlds fame, noted some weirdness in another exoplanet signal (admittedly a transit) that they thought could be caused by an exomoon. But the finding was never confirmed, along with many others that have lacked confirmation over the years.

That might begin to change though, as more and more powerful instruments come online. The paper focuses on a technology called optical interferometry, where several optical telescopes, spread over a slight distance, combine their signals to increase the resolution of the overall system. Arguably the most capable tool for this currently is GRAVITY, which, interestingly, is just the instrument’s name and not an acronym.

GRAVITY combines the signals from four different optical telescopes scattered around the Very Large Telescope Interferometer at the ESO’s campus in Chile. By doing so, it enables detection of wobbles as small as 50 microarcseconds—small enough to detect the induced wobble of relatively large moons (like the size of a gas giant) around an even larger planet that happens to be close by. In the near future, it will receive an upgrade known as PLANETES, which will enable 10 microarcsecond resolution, and thereby enabling the detection of smaller moons orbiting more distant worlds.

Perhaps most interestingly, a future, as yet theoretical instrument, known as the Kilometer-Baseline Interferometer, could reach as low as one microarcsecond precision. That would enable astronomers to find Earth-mass moons around much larger planets. The trope for space writers at this point is to compare that to Endor in the Star Wars series, but the point is that some of those giant planets the moons are orbiting could be in their star’s habitable zones, and while the stars themselves couldn’t support life due to being too massive, their moons potentially could.

One planet of particular interest is β Pictoris b, an exoplanet located about 64 light years away that is about 9 times the size of Jupiter. It has an unusual orbital tilt that many astronomers think might be caused by an as-yet-unseen moon. Detecting a moon of that size would require between 10 and 15 microarcsecond precision according to the authors—right in the sweet spot for the PLANETES instrument when it comes online, likely sometime later this decade.

That will be a good first test for the idea of using astrometry to detect a moon. But until then we can still search for some using GRAVITY.