Six thousand exoplanets have been cataloged across the galaxy, yet not a single exomoon has ever been confirmed. That drought may have just ended.
A team led by Kevin Hoy at Universidad Diego Portales in Chile has detected a planetary-mass object orbiting the brown dwarf CD-35 2722 B, a substellar companion roughly 100 light-years from Earth. The candidate, described in Nature, has a minimum mass of 0.9 Jupiter masses and completes one orbit every 170 days.
What makes the detection unusual is the technique: radial velocity monitoring of a directly imaged brown dwarf. The team used ESO’s VLT/CRIRES+ spectrograph to measure tiny shifts in the brown dwarf’s spectrum as the unseen satellite tugged it gravitationally. The same method that found the first exoplanet around a sunlike star in 1995 has now been applied to a substellar host for the first time.
“We find what appears to be the periodic signal of at least one orbiting satellite,” the authors write. “This is the first time this technique has produced evidence of satellites around a companion brown dwarf.”
The object sits at an awkward intersection of classification schemes. It orbits a brown dwarf rather than a star, is nearly as massive as Jupiter, and occupies a gray zone where the formal definition of “exomoon” has never been written. The International Astronomical Union has not established criteria for what qualifies as a moon outside the solar system, leaving the discovery in terminological limbo.
“Although it is uncertain whether this exosatellite will fulfill the presently undefined criteria for qualifying as an exomoon, it is a marked step toward that first uncontroversial detection,” the researchers note.
Brown dwarfs straddle the boundary between planets and stars, too massive to be planets but not massive enough to sustain hydrogen fusion. CD-35 2722 B, an L-type dwarf, was first identified in 2011 as a companion to an M dwarf star. Applying RV monitoring to such an object required exceptional precision: the team achieved roughly 3 meters per second accuracy in the infrared K-band after developing a dedicated telluric modeling pipeline.
Stability simulations using the MEGNO chaos indicator and REBOUND N-body code show that the one-satellite configuration remains dynamically stable over at least 3,000 orbits of the brown dwarf. A two-satellite model was tested but not statistically preferred, and a secondary 14-day signal appears to be an artifact.
The discovery opens a new frontier in exoplanet science. As instrumentation improves, the same technique could be extended to fainter and less massive hosts, potentially revealing a population of exomoons around exoplanets themselves. For now, the field has its first credible candidate.