VLT’s GRAVITY Instrument Detects ‘Tug’ from Colossal Exomoon; Could Be Largest Natural Satellite Ever Found

Astronomers observed a subtle wiggle in the orbit of a massive exoplanet. They concluded that it may be hiding a huge moon. The massive exoplanet known as HD 206893 B is 28 times more massive than Jupiter. It has a back-and-forth motion which is an indicator of the probable presence of a giant moon half the mass of Jupiter. The giant moon is thousands of times more massive than other large moons in our Solar System. It can easily force a re-definition of what is actually a moon.

A Wobbling Giant and Its Enormous Moon

According to the new research, astronomers have employed the Very Large Telescope's technology, known as GRAVITY and have detected a wobbly motion. HD 206893 B is a significant, massive gas giant with a mass of about 28 times that of Jupiter and is approximately 133 light-years away from us. The wobbles take nine months.

A moon about one-fifth the distance of the sun and the Earth, or about 0.2 AU away, has been found Its mass would weigh about 40% as much as Jupiter's mass—about nine times as massive as the planet Neptune, and would dwarf the Solar System's moon mass.

Redefining Moons and the Search for Exomoons

A moon this massive challenges our definitions, and exomoons are notoriously hard to detect – none has been unambiguously confirmed yet. Astronomers note that any object orbiting a planet or brown dwarf is generally called a moon (no strict definition exists), so at nearly half Jupiter's mass, this companion blurs the line between a moon, a double-planet system, or a sub-brown-dwarf.

Confirming it would likely be the first official exomoon discovery, highlighting how exomoon science is evolving – as techniques improve, our concept of what constitutes a “moon” may change.