An ExoMoon discovery may have given astronomers their first confirmed look at a moon beyond our solar system. Scientists using the European Southern Observatory's Very Large Telescope in Chile have reported evidence of a massive exosatellite orbiting a brown dwarf about 71 light-years from Earth.

The finding, published in Nature, could represent a major milestone in the search for moons beyond our solar system. Unlike exoplanets, which have been detected by the thousands, exomoons have remained elusive because of their size and the difficulty of distinguishing their signals from those of their much larger host worlds.

Astronomers detected the object in a system known as CD-35 2772. The discovery is particularly unusual because the object described as the moon is enormous, with a mass at least comparable to Jupiter.

A Moon Orbiting Something That Is Not a Planet

The system presents an unusual problem for astronomers because the host of the newly identified exosatellite is not technically a conventional planet.

CD-35 2772 is a brown dwarf, an object larger than a planet but not massive enough to sustain the nuclear fusion that powers an ordinary star. The brown dwarf in this system has about 33 times the mass of Jupiter.

Kevin Hoy, the lead author of the study and an ESO student and researcher at Chile's Universidad Diego Portales, described the brown dwarf as occupying a mass range between conventional planets and the smallest stars.

The relationship between the two objects also challenges familiar definitions. The proposed moon is roughly Jupiter-sized, while its host is substantially more massive. That makes the system very different from Earth's relationship with its comparatively small moon.

How Astronomers Found the ExoMoon

Finding an exomoon is substantially more difficult than simply pointing a telescope at the sky.

Most exoplanets are not directly visible because the light from their host stars overwhelms the faint light reflected or emitted by the planets. Astronomers instead rely on indirect detection techniques.

One of the best-known methods is the transit technique. When a planet passes in front of its star, it blocks a tiny amount of starlight. Sensitive instruments can measure the resulting dip in brightness.

Another technique is radial velocity. Astronomers look for the gravitational wobble created when an orbiting object pulls on its host. A larger gravitational influence produces a larger measurable movement.

Researchers used the radial-velocity method in their search for the proposed exomoon. Instead of looking for the moon directly, they examined the motion of the brown dwarf and detected a wobble that suggested another massive object was orbiting it.

The researchers then used the size of the wobble to estimate the companion's mass.

The First Exomoon

The discovery raises questions extending far beyond a single distant solar system.

Our solar system contains hundreds of known moons, while astronomers have identified thousands of planets orbiting other stars. Yet no comparable population of exomoons has been established.

The researchers caution that it is too early to determine whether the CD-35 2772 system represents a common type of planetary system or an unusual exception.

ExoMoon Discovery Marks a Milestone in Astronomy

The discovery could nevertheless provide astronomers with a new target for studying how moons and planets form and evolve.

It also raises an intriguing question about astronomical classification. When a Jupiter-sized object is orbiting a brown dwarf rather than a star, how should astronomers define the smaller body?

Hoy acknowledged the unusual nature of the system, saying the object is being called a moon because of its position in the system even though it differs dramatically from the relatively small moons found around planets in our solar system.

ELT Telescopes Could Find More

The search for exomoons is expected to accelerate as more powerful telescopes become available.

The European Southern Observatory's Extremely Large Telescope, or ELT, is expected to begin operations later this decade. The facility is designed to collect substantially more light than the existing Very Large Telescope, potentially allowing astronomers to examine distant planetary systems in much greater detail.

Researchers hope the next generation of telescopes will help determine whether massive exosatellites such as the candidate around CD-35 2772 are rare curiosities or part of a much larger population.

Hoy and his colleagues are already investigating another potential exosatellite candidate, according to the research team.

A New Chapter in the Search for an Exomoon

The discovery of thousands of exoplanets has fundamentally changed astronomers' understanding of planetary systems beyond our own. The possible identification of an exomoon adds another layer to that picture.

If confirmed through additional observations, the CD-35 2772 system could become an important laboratory for studying how massive planets, brown dwarfs and their companions form.

For now, the finding offers another reminder that the universe frequently defies familiar categories. A brown dwarf that sits between a planet and a star, accompanied by a Jupiter-mass object that may qualify as a moon, is an unusual arrangement — and potentially the beginning of a much larger astronomical story.

Additional Information

The original research is available through Nature's study on the suspected exomoon.

Readers can learn more about the European Southern Observatory's Very Large Telescope and the organization's plans for the Extremely Large Telescope.

NASA provides additional background on exoplanets and the search for worlds beyond our solar system as well as information about the planets and moons in our own solar system.

For background on the techniques astronomers use to identify distant planets, readers can explore NASA's exoplanet detection methods.

For related astronomy reporting by Jackie Allen, readers can visit her USA Herald author page.

Additional Jackie Allen coverage at USA Herald includes Total Eclipse of the Blood Red Moon and Skywatchers: Tuesday Brings a Total Eclipse of the Blood Moon.

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