A newly found cosmic object is just not a planet, not a moon — however a 3rd factor we don’t have a reputation for but.
The thing is about as huge as Jupiter and orbits a brown dwarf — a physique that fuses deuterium in its core however is simply too small to burn hydrogen and shine as a real star. The brown dwarf, in flip, loops round a small star. That hierarchy of orbits makes the system, referred to as CD-35 2722, the primary with a recognized “exosatellite”, astronomer Kevin Hoy and his colleagues report July 22 in Nature.
They coined that time period as a result of nothing else is kind of proper. It’s not precisely an “exomoon.” And “exoplanet” doesn’t really feel proper both, says Hoy, of the College Diego Portales in Santiago, Chile. “The construction of CD-35 is so completely different from the construction of the photo voltaic system that every one the phrases we invented to explain the photo voltaic system simply aren’t actually working anymore.”
The discover reignites a longstanding debate about what makes a planet a planet: an object’s intrinsic properties or its relationship to different our bodies.
Hoy and his colleagues used the Very Massive Telescope in Chile to watch the brown dwarf a number of instances from October 2023 to February 2026. Periodic shifts within the wavelengths of the brown dwarf’s gentle revealed an enormous companion tugging it back and forth — the identical approach used to seek out the primary exoplanets within the Nineties and lots of since.
However that is the primary time the approach has recognized what Hoy calls a “moonish thingy”: one thing orbiting an even bigger one thing orbiting an even bigger one thing.
Does that nested association make the article a moon? Hoy doubts it. “It feels so off,” he says. “We’re most likely not going to get away with calling it an exomoon.”
The official definition of exoplanet additionally suggests not. Written in 2018 by the Worldwide Astronomical Union — the identical physique that redefined Pluto as a dwarf planet in 2006 — it unambiguously categorizes a planet-mass object orbiting a brown dwarf as an exoplanet. It doesn’t require the brown dwarf to be the most important factor round.
“If you happen to settle for that this can be a brown dwarf, that sort of forces the companion to be a planet,” says astronomer David Kipping of Columbia College, who has looked for exomoons with a special approach for greater than a decade. “We are able to argue about if the IAU definition is flawed or proper, however by the present IAU definition it’s a planet.”
That doesn’t sit proper with Hoy both. “Due to the system hierarchy setup, it doesn’t really feel the identical as what regular planets are doing.” And this technique isn’t the one oddball. In January, one other group reported a tentative signal of an identical satellite tv for pc round a brown dwarf in a special star system. If it holds up, that object is about half Jupiter’s mass.
Movement has outlined planets because the historic Greeks coined a phrase for “wandering star” to explain the factors of sunshine that transfer in opposition to the background of fastened stars. The Copernican revolution shifted the main focus to motion across the solar, however movement was nonetheless key. A planet, the IAU authors wrote, is much less a query of measurement or composition than of 1 physique shifting round one other “in a hierarchical configuration.”
A take a look at the photo voltaic system helps this concept. The biggest moons, Saturn’s Titan and Jupiter’s Ganymede, are roughly the identical mass and measurement because the smallest planet, Mercury. “We’d be comfy calling Mercury a moon if it orbited Jupiter and calling Ganymede a planet if it orbited a star,” Hoy says.
Finally, Kipping says, “nature doesn’t give a rattling about how people wish to neatly bucket issues.” However discovering issues that orbit different issues that orbit different issues suggests a discovery of true exomoons will not be far off.
“I believe it’s thrilling that we’re positively getting into the exomoon regime now,” Kipping says. “Even when this isn’t an exomoon.”
