Discovery reopens old fight over what makes a planet a planet
A newly discovered cosmic object is not a planet, nor a moon – but a third thing for which we don’t yet have a name.
The object is about as massive as Jupiter and orbits a brown dwarf – a body that fuses deuterium in its core but is too small to burn hydrogen and shine like a real star. The brown dwarf, in turn, loops around a small star. This hierarchy of orbits means that the system, called CD-35 2722, the first with a known “exosatellite”astronomer Kevin Hoy and his colleagues report July 22 in Nature.
They invented this term because nothing else is quite true. She’s not exactly an “exomoon.” And “exoplanet” doesn’t seem acceptable either, says Hoy, of Diego Portales University in Santiago, Chile. “The structure of CD-35 is so different from that of the solar system that all the words we invented to describe the solar system don’t really work anymore.”
This discovery reignites a long-standing debate about what makes a planet a planet: an object’s intrinsic properties or its relationship to other bodies.
Hoy and his colleagues used the Very Large Telescope in Chile to observe the brown dwarf several times between October 2023 and February 2026. Periodic changes in the wavelengths of the brown dwarf’s light revealed a huge companion that pulled it back and forth – the same technique used finding the first exoplanets in the 1990s and hundreds since.
But this is the first time the technique has identified what Hoy calls a “moon thing”: something orbiting a larger something orbiting a larger something.
Does this nested arrangement make the object a moon? Hoy doubts it. “It’s so weird,” he said. “We’re probably not going to call it an exomoon.”
The official exoplanet definition also suggests no. Written in 2018 by the International Astronomical Union – the same organization that redefined Pluto as a dwarf planet in 2006 — without ambiguity classifies a planetary mass object orbiting a brown dwarf as an exoplanet. The brown dwarf doesn’t have to be the biggest thing there is.
“If you accept that it’s a brown dwarf, that forces the companion to be a planet,” says astronomer David Kipping of Columbia University, who searched for exomoons with a different technique for over a decade. “We can debate whether the IAU definition is wrong or right, but under the current IAU definition it is a planet.”
This doesn’t sit well with Hoy either. “Because of the configuration of the hierarchy of the system, it doesn’t look like what normal planets do.” And this system isn’t the only one that’s weird. In January, another group reported a tentative sign of a similar satellite around a brown dwarf in a different star system. If it holds up, this object is about half the mass of Jupiter.
Motion has defined the planets since the ancient Greeks coined the word “wandering star” to describe points of light that move against a backdrop of fixed stars. The Copernican Revolution shifted the focus to movement around the sun, but movement remained essential. A planet, the IAU authors write, is less about size or composition than one body moving around another “in a hierarchical configuration.”
A look at the solar system supports this idea. The largest moons, Saturn’s Titan and Jupiter’s Ganymede, are about the same mass and size as the smallest planet, Mercury. “We would be comfortable calling Mercury a moon if it orbits Jupiter and Ganymede a planet if it orbits a star,” says Hoy.
Ultimately, Kipping says, “nature doesn’t care how humans want to arrange things.” But finding objects orbiting other objects suggests that the discovery of real exomoons may not be far away.
“I think it’s exciting that we’re definitely going into the exomoon diet now,” Kipping says. “Even if she’s not an exomoon.”
