Astronomers Report a Possible Moon Orbiting a Brown Dwarf Beyond Our Sun

A team of astronomers reports evidence of what could be the first moon found outside our solar system, if confirmed: a roughly Jupiter-mass body that appears to orbit a brown dwarf, an object too massive to be a planet but too small to be a star. The study was published online in the journal Nature on July 22.

The brown dwarf, called CD-35 2722 B, orbits the young star CD-35 2722, which has about half the mass of the Sun, according to Phys.org. The brown dwarf itself has roughly 30 to 37 times the mass of Jupiter, depending on which figure in the coverage is used. The paper in Nature, titled “Planetary-mass exosatellite detected around the substellar companion of a star,” is led by Kevin Hoy, a student at the European Southern Observatory in Chile who is also affiliated with Universidad Diego Portales, with Alice Zurlo of that university among the collaborators, Phys.org reported.

The team used the radial velocity method on spectra from the CRIRES+ instrument on ESO’s Very Large Telescope in Chile. The method looks for tiny periodic wobbles in an object’s motion caused by something orbiting it. Scientific American reported that the signal repeats about every 170 days, and put the satellite’s mass at at least about 90 percent of Jupiter’s; Centauri Dreams gave a minimum mass of about 0.74 Jupiter masses, and ESO’s release says at least as massive as Jupiter, so the mass is uncertain and the true value could be higher.

The researchers and outside commentators are cautious. Phys.org, relaying the ESO release, quoted the team as calling the result strong evidence, and Zurlo as calling it the first plausible detection of an exosatellite. Astronomer David Kipping told Scientific American, “It’s a candidate object, but it probably is real.” Scientific American noted that earlier Keck Observatory data were inconclusive. Whether such an object counts as a moon at all is unsettled, and lead author Hoy proposes the neutral term “exosatellite.”

Phys.org noted that no exomoon has yet been confidently detected, and that a team led by Quentin Kral reported hints of a satellite around HD 206893 a few months earlier but no firm detection.

The result matters because moons are common in our solar system and may shape the conditions on planets and brown dwarfs, yet none has been confirmed around another star. Finding one would show how such systems form.

What happens next depends on more data. Additional observations over longer stretches would help show whether the roughly 170-day signal keeps repeating. Phys.org noted that the 39-meter Extremely Large Telescope, which ESO is building, should be able to detect smaller exomoons in the future.

The study was reported as a research finding, not a confirmation. Until further observations bear it out, the signal remains a promising lead.