Exomoons

Moons are everywhere in the Solar System. Among the thousands of planets found around other stars there are almost certainly moons too, but as of 2026 not a single one has been confirmed. A Nature paper published in July 2026 says it plainly: no exomoon has ever been confidently detected.

How you would find one

Most exoplanets are found by the transit method: a planet passes in front of its star and dims it slightly. A moon would leave two traces. It could add its own small dip to the light, and its pull would make the planet arrive a little early or late. Both effects are tiny, and both can be mimicked by noise, by spots on the star or by the way the data are processed.

A moon the size of Neptune

The most famous candidate orbits Kepler-1625b, a Jupiter-sized planet around a Sun-like star about 8 000 light-years away in Cygnus. In 2017 Teachey, Kipping and Schmitt found a hint of a moon in Kepler’s data. The Hubble Space Telescope then watched one transit, in October 2017. The planet arrived 77.8 minutes early, and the light curve showed a second, shallow dip. In 2018 they reported a candidate moon about 4 times Earth’s radius, as big as Neptune, circling at about 40 times the planet’s radius. Nothing like it exists in the Solar System.

Others were not convinced. In 2019 Kreidberg, Luger and Bedell processed the same Hubble data again and found that a planet with no moon fitted just as well. They called the moon’s dip likely an artifact of the data reduction.

In 2022 Kipping’s team reported a second candidate, Kepler-1708 b-i, about 2.6 Earth radii across, the only survivor of a search of 70 cool giant planets. It rests on just two transits. In late 2023 Heller and Hippke concluded that neither planet is likely to have a large moon, blaming the fitting of the star’s edge for one signal and the star’s own activity for the other. Kipping and colleagues replied that both remain viable.

What the newest telescopes see

The James Webb Space Telescope has joined the search. In 2025 it watched a 60-hour transit of Kepler-167e, a cold Jupiter-like planet. Seven of twelve ways of analysing the data showed a strong moon signal, but a starspot crossing explained it equally well, and the team asked for another look at the next transit, in October 2027. In 2026 twelve Webb transits of the rocky planet LP 890-9c ruled out moons down to 0.1 Earth radii, small enough to exclude a twin of Europa.

The strongest new case, announced in July 2026, does not orbit a planet at all. Using ESO’s Very Large Telescope, Hoy, Zurlo and colleagues found a body of at least 0.9 Jupiter masses circling a brown dwarf, itself over 30 times Jupiter’s mass, every 170 days. ESO called it a possible first exomoon, “if confirmed”, while noting that the word “moon” may not really fit.