Astronomers Have Detected an Exomoon for the First Time

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An international research team has discovered evidence of a moonlike body. It marks the first time such an object has been detected outside our solar system—though just how close it comes to being a moon is a question researchers are still grappling with.

Although more than 6,000 exoplanets have been discovered to date, there’s been a lack of definitive evidence of moons in other planetary systems. The new findings challenge that by focusing on an object orbiting the brown dwarf CD-35 2722 B in a system 73 light-years away.

A brown dwarf is a celestial body that lies between a planet and a star that lacks the mass required to become a star. CD-35 2722 B has a mass about 37 times that of Jupiter and orbits the star CD-35 2722, which has about 40 percent of the sun’s mass; they are separated by an apparent distance of about 2.8 arc seconds.

The research team observed CD-35 2722 B over a total of 23 nights between October 2023 and February 2026 using the CRIRES+ infrared spectrograph mounted on the European Southern Observatory’s Very Large Telescope . Using a technique known as the Doppler method, the researchers captured how CD-35 2722 B wobbles as it is slightly pulled by the gravity of another celestial body orbiting nearby.

The technique uses the Doppler effect observed in a celestial body’s spectrum to record changes in radial velocity—essentially looking for those—to estimate an exoplanet’s mass, orbital period, and orbital eccentricity. This method was also used in 1995 when the first exoplanet orbiting a sunlike star was discovered.

Since CD-35 2722 B is sufficiently distant from its host star, it was possible to obtain its spectrum directly with virtually no interference from the host star’s light. Similar attempts have been made with other brown dwarfs and planets. In this study, the combination of an instrument with higher wavelength resolution and minimal spectral contamination enabled measurements that were up to 100 times more precise than previous ones.

A view of the vicinity of the brown dwarf CD-35 2722.

The analysis, published in Nature last month, revealed a strong signal indicating the presence of a large moon orbiting over a period of approximately 170 days and having a mass of at least 0.9 times that of Jupiter. In terms of mass ratio, this body accounts for about 2.5 percent of the mass of the brown dwarf it orbits, which is a significantly higher value compared with moons in our solar system. The system with the highest mass ratio among the planets and their moons in the solar system is Earth and the moon (approximately 1.2 percent); the mass ratios of the moons orbiting Jupiter and Saturn are much smaller.