Stacking 12 JWST transits pushes an exomoon search down to 0.1 Earth radii

Detectors drift as they warm, telescope pointing wanders, and starspots rotate into view; each leaves slow structure in a light curve that can imitate the dip of a small moon ^[8]. A moon has no such freedom. Its orbit fixes where it must be and when, so its signal returns transit after transit while the detector's does not ^[9]. Add enough visits and the instrumental part averages down toward a clean view of the star ^[10].

Kipping, an astronomer at Columbia and host of the Cool Worlds YouTube channel, posted the analysis as an arXiv preprint titled "JWST Excludes Exomoons Down to 0.1 Earth Radii Around a Rocky, Temperate Exoplanet" ^[3][21]. The demonstration inside it is smaller than the headline exclusion. One of his 12 transits of LP 890-9 c carried noticeable red noise, and pairing it with a single clean transit raised the sensitivity substantially ^[11]. That pair is a sixth of the data he had ^[23]. The write-up describes the gain as significant without quantifying it ^[11].

Earlier exomoon searches each rested on one transit ^[6]. JWST's look at the gas giant Kepler-167 e ran into red noise that left the models unable to detect any moon much smaller than Earth ^[7]. A transiting body blocks starlight in proportion to its cross-section, so a limit at 0.1 Earth radii sits about a factor of 100 deeper in blocked light than an Earth-sized floor ^[22].

LP 890-9 c orbits 0.04 AU from its star, less than a tenth of the Sun-Mercury distance ^[15]. Over billions of years, tides at that separation either strip a moon away into deep space or pull it inward until it becomes rings, and Kipping did not expect a detection ^[16][14]. He used the planet because JWST had already observed it a dozen times, and the paper is written as a proof of concept ^[17][4].

Kipping's claim about the instrument is that JWST has no noise floor stopping it from finding relatively small moons, an assumption the phys.org account says many astronomers have held for the past several years, and that moons the size of ours are detectable when observers commit to multiple transits ^[18][19]. Time on the telescope is among the scarcest commodities in astronomy, and the same pool pays for peering back to the earliest epochs of the universe ^[20].