No confirmed moon has yet been discovered around any planet beyond the Solar System. Noise in telescope detectors and spots on the surface of the host star were thought to be the reason. They supposedly created an insurmountable sensitivity limit for detecting small bodies. A new study disproves this assumption. The obstacle can be overcome by repeatedly observing the same planet.
Record Sensitivity Limit
Astronomer David Kipping of Columbia University analyzed data from twelve transits of the planet LP 890-9 c across its host star. The result was negative. With 95% confidence, there are no moons around it larger than 0.1 Earth radii. This is the most sensitive search of its kind in the history of extrasolar observations. The preprint of the study on arXiv has not yet undergone formal peer review.
The radius of any possible moons of this planet would not exceed approximately 640 kilometers. Our Moon is almost three times larger, so a body of its size around LP 890-9 c would almost certainly have been detected.
Red Noise and a Single Transit
Previous attempts relied on just a single transit of a planet in front of its star. This was how astronomers searched for a moon around the gas giant Kepler-167 e. Computer models produced a false result because of so-called “red noise.” Under such conditions, detecting anything significantly smaller than Earth was impossible.
The nature of this interference is quite ordinary, as Universe Today writes. The detector slowly heats up, the telescope’s pointing drifts slightly, and spots appear on the surface of the star itself. All of this creates smooth, false fluctuations in brightness.
David Kipping explains why exomoons have not yet been found. Recording from 2024. Source: Fraser Cain / Universe Today Podcast
The Effect of Averaging Data
A moon’s motion follows the laws of celestial mechanics. The body appears in a predictable place at a predictable time, rather than where an instrumental failure happened to occur. Instrumental interference and stellar spots repeat differently each time.
Therefore, averaging several observations gradually smooths out the interference. One of the twelve observing sessions of LP 890-9 c was noticeably noisy. After adding data from just one clean transit, the sensitivity improved substantially.
The planet completes one orbit around its star in 8.46 days, so twelve consecutive transits take about one hundred days. For targets with longer orbital periods, the same series would stretch over years.
An Orbit That Is Too Close
The distance from LP 890-9 c to its ultracool red dwarf is 0.04 astronomical units. This is approximately one-tenth of the distance between the Sun and Mercury. Tidal forces at such a distance are extremely powerful.
Over billions of years, tidal forces either strip a moon away and fling it into interstellar space or, conversely, pull it inward, where the body breaks apart into rings. David Kipping himself says that he did not expect to find a moon there.
A Question of Observing Time
The purpose of the study was to demonstrate the principle rather than to hunt for the first exomoon. The main conclusion concerns the instrument itself. James Webb does not have a noise floor that prevents the search for small moons, although astronomers had assumed the opposite for the past several years.
Thomas Winterhalder discusses how exomoons will be searched for in the coming years. Recording from 2025. Source: Fraser Cain / Universe Today Podcast
The practical limitation lies elsewhere. Time on this telescope is one of the scarcest resources in astronomy, and a series of twelve observing sessions is expensive. Such proposals will now compete with other programs without being able to cite technical impossibility as the reason.
