During observations with the Very Large Telescope of the European Southern Observatory ESO in Chile, an international research team discovered a curious “exosatellite”: The celestial body orbits a brown dwarf, and this in turn orbits a red dwarf star. The discovery shows that with constantly improving astronomical equipment, it should be possible in the near future to detect moons on planets that orbit other stars, the scientists write in the journal Nature.
Astronomers now know of over 6,000 planets that orbit not our sun, but other stars. Six of the eight planets in our solar system have moons. The differences are large: Earth only has one, while Saturn currently has almost 300. Celestial researchers therefore assume that moons also orbit around exoplanets, i.e. planets near other stars. But despite all their efforts, the researchers have not yet discovered such an “exomoon”. The team led by Kevin Hoy from ESO has now, for the first time, tracked down a companion around a brown dwarf, which in turn orbits a star.
Brown dwarfs are a type of prevented star: their mass is larger than that of planets, but smaller than that of “real” stars, in which energy is permanently generated by the fusion of hydrogen to helium. Only deuterium, also called “heavy hydrogen,” initially burns to form helium. But there is only a small amount of this nuclear fuel and so nuclear fusion goes out again and a brown dwarf slowly cools down.
“Planetary mass exosatellite” or “exomoon”?
This is also the case with CD-35 2722 b, a brown dwarf orbiting a red dwarf star 72 light-years away. It has around 30 times the mass of the planet Jupiter and takes around 870 years to orbit its star. However, as the observations of Hoy and his colleagues show, the brown dwarf “staggers” a little on its orbit. Apparently an external force is pulling on the brown dwarf: the gravity of another celestial body orbiting around the brown dwarf. The researchers' measurements indicate that this companion must be at least as heavy as Jupiter. The orbital period of this celestial body is about 170 days.
What should we call this celestial body? Hoy and his colleagues are cautious: They talk about the discovery of an “exosatellite with planetary mass”. The European Southern Observatory is less cautious, talking about the discovery of a so-called “exomoon” in the headline of its press release.
But the term “exomoon” was previously reserved for companions of planets and smaller celestial bodies. “Objects with planetary mass that orbit a brown dwarf,” the authors of the study also make clear, “are planets according to the current language of the International Astronomical Union.”
Is a new category of celestial objects needed?
“In the solar system we have a clear demarcation between the planets and the sun, which is why the definition of moons is simple,” explains astronomer Alice Zurlo from the Universidad Diego Portales in Chile, who was involved in the discovery. “In the CD-35 2722 system, in which the boundaries between stars, planets and moons are blurred, the description of the whole thing becomes much more complicated.” It is therefore necessary to define more clearly which celestial objects should count as moons and which should not.
Or a new category of celestial objects would have to be introduced for this purpose. A similar situation occurred in astronomy when more and more celestial bodies were discovered far out in the solar system that resembled Pluto, which had previously been considered a planet. To ensure that the number of planets in the solar system does not grow uncontrollably, astronomers then introduced “dwarf planet” as a new category - which now also includes Pluto.
Hoy and his colleagues also recommend a similar approach for companions of brown dwarfs. “With future observation possibilities, many similar systems are likely to be discovered,” the researchers write, “and the establishment of clear categories will therefore become increasingly important.” In any case, the system discovered is unique to date and represents a breakthrough in the search for exomoons.
