Humans discovered the first exoplanet in 1992 and since then they have discovered about 6,000 other planets outside our solar system. However, those researchers never found a moon orbiting any of those planets. That may have changed with the recent discovery of the first moon-like object in another solar system.
The findings were published in a recent study. Kevin Hoy, lead author of the study and a PhD student at the Instituto de Estudios Astrofísicos, says this potential moon, known as CD-35 2722 B, orbits a brown rod instead of a traditional planet. It is also about 90% the size of Jupiter and larger than Saturn. This moon-like object takes 170 days to fully orbit its host.
Hoy and other researchers used an old trick to find this moon: radial velocity analysis, which is used to detect wobbles in stars and is an indicator that an exoplanet is nearby. Planets shrink as they move away from their host star, and exoplanets are discovered by observing changes in their appearance.
This is the same method that scientists used to find the first exoplanet, and it was used to find this exosatellite, which is the name given to objects that have not yet been described as exoplanet or exomoon.
“This is the first time, to our knowledge, that this method has produced evidence of satellites orbiting a black dwarf,” Hoy said in the study.
Most people understand that a moon is something that orbits something other than a star, like Earth’s moon or various other moons in the solar system.
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Schrödinger’s moon?
The findings of this study are not without their controversy. Of course, the big question is whether this is really a moon. Brown dwarfs, often called failed stars, are much larger than the average planet but lack the chemical reactions in their cores needed to become a star. The exosatellite orbiting this dark brown is also the largest non-planet in our solar system and larger than most exoplanets discovered by researchers to date.
What makes things difficult is that no one really knows what the moon is. There is no standard definition from the International Astronomical Union, the authority that usually comes up with such definitions. Most people understand the moon as a naturally occurring object that orbits a larger object, like our moon orbits the Earth.
There are some rules for this. The host object cannot be a star because the orbiting object would be a planet. It will also not be man-made, or considered a satellite. The definition has expanded as astronomers discovered more and more distinct types of moons. One example is Zoozve, a quasi-moon that may look like a moon on the surface of Venus but is actually a celestial body that orbits the sun and is close enough to look like a moon.
In CD-35 2722 B, Hoy told me that one problem is a special type of brown dwarfs. Brown dwarfs are considered dwarf objects because they are too large and dense to be a planet but lack the hydrogen fusion necessary to become a star. They exist somewhere in the middle between the two, so it is not clear whether the object orbiting the little brown object is to be considered a planet or a moon.
That’s not the only roadblock. “The object itself is at least the mass of Jupiter, which is much larger than the rocky and icy moons of the solar system,” Hoy said. “It’s not clear that something that big should be considered the moon.”
Hoy says the currently accepted IAU definition is a small brown star cluster, in that an object orbiting a small brown object of sufficient size would be considered a planet. However, this brown dwarf orbits its host star like a planet and CD-35 2722 B orbits a brown moon; therefore, it fulfills both the lunar behavior and the planetary meaning.
“Maybe when the definition of exomoons is written, things like this will meet the definitions of planets and moons, but we will have to see,” said Hoy. “Those names don’t match the solar system, so I wouldn’t be surprised if they wrote this definition in a way to preserve that.”
Until that day, no one is sure what to make of CD-35 2722 B, and until the IAU opens the metaphorical box and comes up with an official definition of what a moon is, CD-35 2722 B can truly be considered both a moon and a planet.
