Astronomers have discovered a possible exosatellite the size of Jupiter
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- Astronomers have discovered a possible exosatellite the size of Jupiter
There may be satellites outside the Solar System that are comparable in mass to planets. Astronomers have discovered one such object: it orbits a brown dwarf, which, in turn, orbits a star. The unusual configuration makes it difficult to classify the find — scientists are still unable to determine unequivocally whether to consider it a moon or a planet. About what the researchers found and why the discovery can change the perception of space objects — in the material "Izvestia".
An unusual system of three objects
A possible candidate for exosatellites was found in the CD-35 2722 system. At its center is a star with about half the mass of the Sun. A brown dwarf orbits around it, an object that is more massive than a planet, but not massive enough to support the thermonuclear fusion of hydrogen typical of ordinary stars. The dwarf's mass exceeds 30 Jupiter masses.
The proposed satellite orbits a brown dwarf and, according to researchers, has a mass of at least the mass of Jupiter. Thus, the system consists of three levels: a star, a brown dwarf orbiting it, and another massive body orbiting this dwarf. The work is published on the website of the European Southern Observatory (ESO) and in the journal Nature.
Kevin Hoey, lead author of the study and an ESO student
It is quite difficult to describe this system using concepts familiar to us in the Solar System, such as "planet" and "moon".
How a possible satellite was discovered
To study the system, the scientists used the CRIRES+ instrument mounted on ESO's Very Large Telescope. The researchers applied the radial velocity method, the same approach they used to discover the first exoplanet in a Sun—like star.
The method allows detecting small changes in the motion of a celestial body under the influence of gravity of another object. When two bodies rotate around a common center of mass, their mutual attraction affects the movement of each of them. Even a relatively small satellite can cause noticeable changes in the velocity of a more massive object.
Analyzing the movement of the brown dwarf, the researchers found signs of the presence of another massive body in orbit around it. According to the publication, the proposed satellite has a mass of about 0.9 times that of Jupiter and orbits the brown dwarf in about 170 days.
At the same time, we are still talking about a possible discovery. Astronomers have yet to confirm the existence of the object and clarify its characteristics. According to researcher Alice Dzurlo, no matter how exotic this object may be, the system is unique and represents a breakthrough, as it is the first plausible detection of an exosatellite.
Planet or moon
The main feature of the find is its mass. The proposed satellite is comparable in this parameter to Jupiter, so it could be classified as a planet. However, it does not directly orbit the star, but rather orbits a brown dwarf, which itself is a satellite of the star.
Kevin Hoey
This exosatellite is massive enough to be considered a planet, but it does not orbit a star, although it does orbit an object that orbits the star. Since it turns out to be the third extra in this system, we want to call it the moon, although it does not look at all like the small rocky satellites of our system.
Dzurlo, in turn, described the object as a giant gaseous body orbiting a very massive companion, which itself is several times more massive than Jupiter. According to her, in the Solar System, the boundary between the planets and the Sun is clearly defined, so classifying satellites is relatively simple. In the CD-35 2722 system, the boundaries between stars, planets, and moons are blurred, which complicates the description of such objects.
Astronomers use the term "exosatellite" for a satellite outside the Solar System. The term "exoloon" is usually applied to a natural satellite of a planet or other celestial body outside our system, but there is no generally accepted definition of an exoloon yet.
The search for exomoons continues
Astronomers have been trying for many years to detect moons outside the Solar System, but no candidate has yet received sufficient confirmation. Despite the fact that scientists have discovered more than 6,000 exoplanets, data on possible exomoons remains limited.
Earlier in 2026, a research team led by Quentin Kral reported possible signs of a satellite in the HD 206893 system. Observations using the ESO Very Large Telescope Interferometer indicated the presence of the object, but it was not possible to definitively confirm its existence. In the future, astronomers expect to use ESO's Extremely Large Telescope, equipped with a mirror with a diameter of 39 m. It should expand the search for satellites beyond the Solar System, including smaller objects that are still difficult to detect.
Confirming the existence of exomoons will help scientists better understand how planetary systems form and evolve. The new observations will also make it possible to find out how common unusual configurations like the CD-35 2722 system are, and whether the usual classification of celestial bodies will need to be revised.
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