Humans discovered the first exoplanet in 1992 and have since discovered about 6,000 other planets outside our solar system. However, these researchers have not once found a moon orbiting one of these planets. This may have changed with the recent discovery of the first moon-like object in another solar system.
The results were published in a recent study. Kevin Hoy, lead author of the study and a doctoral student at the Instituto de Estudios Astrofísicos, says this potential moon, known as CD-35 2722 B, orbits a brown dwarf 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 completely orbit its host.
Hoy and other researchers used a fairly old trick to discover this moon: radial velocity analysis, which is used to detect the wobbles of stars and indicates that an exoplanet is nearby. Planets become dimmer as they move away from their host star, and exoplanets are discovered by observing changes in spectral shift.
This is the same method that scientists used to detect the first exoplanet, and it was used to detect this exosatellite, which is the name given to objects that have not yet been defined as an exoplanet or exomoon.
“This is the first time, to our knowledge, that this technique has produced evidence of satellites around a companion brown dwarf,” Hoy said in the study.
Most people understand that a moon is an object that orbits another object that is not a star, such as Earth’s moon or the various other moons in the solar system.
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Schrödinger’s Moon?
The study’s findings are not without controversy. Perhaps the biggest question is whether it really is a moon. Brown dwarfs, often called failed stars, are much larger than a standard planet, but lacked the chemical reactions in their core needed to become a star. The exosatellite orbiting this brown dwarf is also larger than all but one of the planets in our solar system and larger than most exoplanets discovered so far by researchers.
What complicates matters is the fact that no one actually knows what a moon is. There is no standard definition from the International Astronomical Union, the authority that usually offers such definitions. Most people understand that a moon is a natural object that orbits a larger object, like our moon orbits the Earth.
There are a few rules to this. The host object cannot be a star because then the orbiting object would be a planet. It also cannot be made by humans, otherwise it is considered a satellite. The definition has expanded as astronomers discover additional and unique types of moons. One example is Zoozve, a quasi-moon that would look like a moon when viewed from the surface of Venus but is actually an asteroid that orbits the sun and is close enough to look like a moon.
For CD-35 2722 B, Hoy told me that one problem was the special nature of brown dwarfs. Brown dwarfs are considered substellar objects because they are too large and too dense to be a planet, but lack the hydrogen fusion needed to be a star. They exist halfway between the two, so it’s unclear whether something orbiting a brown dwarf would be considered a planet or a moon.
This is not the only obstacle. “The object itself has at least the mass of Jupiter, which is much larger than the solar system’s rocky and icy moons,” Hoy said. “It’s not clear that something this big should be considered a moon.”
Hoy says that currently accepted IAU definitions group brown dwarfs with stars, to the extent that an object orbiting a brown dwarf 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 the brown dwarf like a moon; therefore, it fulfills both the behavior of a moon and the definition of a planet.
“Perhaps when a definition of exomoons is written, objects like this will meet both the planet and moon definitions, but we will have to see,” Hoy said. “These terms are mutually exclusive in the solar system, so I wouldn’t be surprised if they wrote the definition in a way that preserved that.”
Until that day comes, no one really knows 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 could realistically be considered both a moon and a planet.
