First Exomoon Detection Challenges Cosmic Labels

Astronomers using ESO's Very Large Telescope have made the first plausible detection of an exomoon — a Jupiter-mass object orbiting a brown dwarf in the CD-35 2722 system, published today in Nature.

Astronomers using the European Southern Observatory's Very Large Telescope (VLT) in Chile have reported the first plausible detection of an exomoon — an object that challenges the very definitions of planet and moon.

The team, led by ESO student Kevin Hoy, discovered a Jupiter-mass object orbiting a brown dwarf that itself orbits the star CD-35 2722. Published today in Nature, the finding represents the strongest evidence yet for a natural satellite outside our Solar System.

"The satellite we report is a giant gaseous body orbiting a highly massive companion, itself several times the mass of Jupiter," said Alice Zurlo, director of the Millennium Nucleus of Young Exoplanets and their Moons. "We have a clear delineation between the planets and the Sun in the Solar System, so defining things like moons is simple. In the CD-35 2722 system, we are blurring the lines."

The team used the radial velocity method — the same technique that found the first exoplanet around a Sun-like star — to detect tiny wobbles in the brown dwarf caused by the orbiting object. The system, located roughly 80 light-years away, hosts a star about half the Sun's mass.

With over 6,000 exoplanets now confirmed, exomoons have remained stubbornly elusive. The discovery opens a new window into how planetary systems form and evolve. ESO's upcoming Extremely Large Telescope (ELT), with its 39-metre mirror, is expected to detect even smaller exomoons in the coming decade.

Sources: ESO Nature