Astronomers may have detected an exomoon
by Rob Beschizza · Boing BoingAstronomers using the Very Large Telescope in Chile have found evidence of a moon-sized companion far beyond our own solar system, which might be the first exomoon to be detected (though it wouldn't be the first time we've been here). But doesn't seem to orbit a planet, so nobody is sure what to call it. The European Southern Observatory announced the detection July 22, timed to a paper in Nature led by Kevin Hoy, an ESO student based in Santiago.
This exosatellite or 'exomoon', a natural satellite outside our Solar System [1], is difficult to label, given the differences in this system compared to our own. Alice Zurlo, YEMS Director and collaborator on the study explains: "The satellite we report is a giant gaseous body orbiting a highly massive companion, itself several times the mass of Jupiter."
The object sits in the CD-35 2722 system, about half a solar mass at its center. Orbiting that star is a brown dwarf, too heavy to be a planet and too light to sustain fusion, at more than 30 Jupiter masses. Orbiting the brown dwarf is the new object, itself at least as massive as Jupiter. The team calls it an exosatellite. Hoy and coauthor Alice Zurlo of Universidad Diego Portales used the CRIRES+ spectrograph to measure small wobbles in the brown dwarf's motion — the radial velocity technique that turned up the first exoplanet around a sun-like star in 1995. Hoy described the arrangement as super weird next to our own, and said Solar-System vocabulary does not fit it. Zurlo characterized the find as a giant gaseous body circling a far more massive companion. Neither "planet" nor "moon" applies cleanly: the object is planet-massive but orbits something that orbits a star.
The naming problem is not merely rhetorical. ESO's own footnote to the release concedes that "there is no officially accepted definition for exomoon," an ambiguity with pros and cons for announcing the possible discovery of one.
More than 6,000 exoplanets have been confirmed, and even then the data is tentative. Earlier this year a team led by Quentin Kral reported hints of a satellite in the HD 206893 system using the VLT Interferometer, without a firm detection. The CD-35 2722 result sidesteps the transit method's sensitivity limits by watching the host wobble instead, which favors heavy satellites — one reason the first plausible detection is a gas giant rather than anything resembling Europa. ESO's Extremely Large Telescope, under construction at Cerro Armazones with a 39-meter mirror, is expected to spot such smaller objects along with providing higher-resolution evidence of exoplanets.
Wake me up when they discover the first exomoonmoon.