Astronomers report a possible first moon beyond the Solar System

How was the candidate detected?

Hoy, Zurlo and their colleagues used the CRIRES+ instrument on ESO’s Very Large Telescope. They applied the radial velocity method, which measures extremely small changes in an object’s motion and previously helped discover the first exoplanet orbiting a Sun-like star.

Two orbiting bodies pull on each other through gravity, causing both to move around a shared center of mass. Even a smaller companion can leave a detectable wobble in the larger object’s motion, known as its shared orbital pivot.

By examining subtle changes in the brown dwarf’s motion, the researchers identified what they consider strong evidence for another massive body. Its estimated mass, size and composition resemble those of a planet, but its position makes it a candidate for a different classification.

Is this really the first exomoon?

Astronomers have searched for years for moons around objects beyond the Solar System, but no candidate has yet met the standard required for a confirmed discovery. More than 6,000 exoplanets have been identified, while only a handful of possible exomoons have emerged from observations, usually with limited supporting evidence.

Earlier in 2026, a team led by Quentin Kral reported possible signs of a satellite in the HD 206893 system. Observations from ESO’s Very Large Telescope Interferometer suggested that a moon-like companion might be present, but the researchers did not establish a definitive detection.

The CD-35 2722 candidate offers a separate opportunity for confirmation, supported by measurements of how the suspected satellite affects its larger companion’s motion. Neither candidate has yet produced an undisputed discovery.

What counts as an exomoon?

A satellite is any object that travels around another object. Satellites can be natural, such as the Moon, or artificial, such as a spacecraft. An exosatellite is a satellite outside the Solar System.

The term exomoon generally describes a natural satellite orbiting a planet or another object beyond the Solar System, although astronomers have not agreed on an official definition. That distinction matters here because the CD-35 2722 candidate could qualify as a planet under some classification systems.

In our Solar System, planets orbit the Sun and moons orbit planets, providing a clear dividing line between the categories. CD-35 2722 instead blurs the boundaries among stars, planets and moons, making the satellite’s orbital relationship as important as its mass.

What could follow the finding?

Confirming moons beyond the Solar System could help researchers understand how planetary systems form, how their components interact and why some develop structures unlike ours. Future observations would also show whether massive satellites such as this candidate are rare exceptions or part of a broader population.

ESO’s upcoming Extremely Large Telescope is expected to advance that search. Its 39-meter mirror and advanced instruments could detect smaller exomoons that current observatories cannot reach.

As astronomers identify more satellites in distant systems, they may also find that the familiar distinctions among stars, planets and moons do not apply universally.

The Nature paper, titled “Planetary-mass exosatellite detected around the substellar companion of a star,” lists Kevin Hoy, Alice Zurlo, Pablo A Peña R, Jana Köhler, Silvano Desidera, Raffaele Gratton, Cecilia Lazzoni, Simon Petrus, Florian Rodler, Jonathan Smoker, Valentina D’Orazi, Ilaria Carleo and Ilaria Giovannini. It appeared in volume 655, issue 8124, on page 865, with DOI 10.1038/s41586-026-10751-w.