Astronomers find first Jupiter-sized exosatellite

International researchers have found the first convincing evidence of a natural companion outside the Solar System, ERR reported. The Jupiter-sized object orbits the brown dwarf CD-35 2722 B, which itself circles a parent star about 73 light-years from Earth.

The unusual system blurs the boundary between planets and moons. Because the object orbits a brown dwarf that orbits a star, astronomers could be tempted to call it a moon. However, it does not resemble the rocky moons found in the Solar System, and researchers currently prefer the term “exosatellite”.

“It is very difficult to describe the relationship between such a giant gaseous body and its massive host using current terminology,” said Alice Zurlo, a co-author of the study and an astrophysicist at Diego Portales University in Chile.

Under the International Astronomical Union’s definition, a planet is an object that orbits a star or brown dwarf and cannot independently initiate nuclear fusion. Brown dwarfs have properties between those of planets and true stars. They are several times more massive than even gas giants, but not massive enough to ignite permanently as stars.

Astronomers had previously searched for satellites around brown dwarfs, but the glare of the parent star interfered with spectrometer observations. CD-35 2722 B is much farther from its parent star, at a distance of about 2.8 arcseconds. It takes nearly 5,000 years to complete one orbit around the star and follows a highly elongated orbit.

This allowed researchers to study the brown dwarf’s light spectrum without interference from the star and measure the Doppler effect almost 100 times more precisely than in earlier attempts.

The team used the Viper programme to extract the radial-velocity signal from the data. The software creates a template from each collected spectrum, allowing researchers to estimate the object’s tiny back-and-forth movements. These rhythmic movements occur when the gravity of an invisible companion periodically pulls the brown dwarf towards it.

A clear signal emerged from observations collected over 26 nights. Calculations published in Nature showed that the satellite completes one orbit around its host in about 171 days.

The companion has a mass of at least 0.9 times that of Jupiter, accounting for 2.5 percent of the host body’s total mass. In the Solar System, the Earth and Moon have the highest comparable mass ratio, at just 1.2 percent. Other planets and moons in the Solar System fall well below those figures.