If you look up on a clear night, the brightest and largest object in the sky will probably be the moon. And unless you have a decent telescope, it is the only natural satellite you can see with your own eyes. As a result, most people have a warped perception of what a moon is and how common these natural satellites really are.
In reality, there are hundreds, if not thousands, of natural satellites, in our cosmic neighborhood, ranging from irregular town-size space rocks to massive rounded bodies that are potentially big enough to be considered planets in their own right.
So exactly how many solar system moons have we found? The answer, it turns out, depends on your definition of a moon.
The International Astronomical Union (IAU) officially recognizes 288 planetary moons orbiting the solar system’s eight worlds, according to NASA. But there are also a further 473 “small-body satellites” — the moons of asteroids and dwarf planets — listed by NASA’s Jet Propulsion Laboratory. If we count both types, as most astronomers would, this brings the total number of natural solar system satellites to 761 (as of June 2024).
But this is likely just “the tip of the iceberg,” Edward Ashton, an astronomer at the Academia Sinica Institute of Astronomy and Astrophysics in Taiwan, told Live Science in an email. Astronomers have found dozens of new planetary moons and small-body satellites in the last few years alone, and technological advancements will likely accelerate the rate at which they can spot more in the coming years, he added.
What is a moon?
“The simplest definition [of a moon] would be an object that is in an orbit around a larger, non-stellar object,” Ashton told Live Science. “But that is not quite the complete answer.”
For example, there are currently thousands of human-made satellites in orbit around Earth that satisfy the definition above but are not considered moons because they are not natural. These spacecraft also have limited life spans before falling back to Earth and burning up in our atmosphere.
Some natural satellites, such as quasi-moons and minimoons, are also temporary and do not truly orbit planets.
Related: Will Earth ever lose its moon?
There is also a question of size, Brett Gladman, an astronomer at the University of British Columbia in Canada, told Live Science in an email. For example, ring particles — the tiny fragments of rock that make up the rings of planets like Saturn and Uranus — individually orbit their host planet but are not considered moons, Gladman said.
Objects smaller than a few hundred feet, often referred to as “ring moons” or “moonlets,” also have “fuzzy definitions” and are not considered true moons, Gladman added.
Even among recognized moons, there are still points of contention. For example, planetary moons can be split into two groups: regular moons, which are typically larger and have small, circular orbits close to the equator of their host planet; and irregular moons, which can be much smaller and have larger, more elliptical orbits around their host planets, Ashton said.
Among the regular moons, around 20 are also considered major moons — meaning they are large enough to have a rounded shape caused by their gravity, according to The Planetary Society.
Planetary moons
Starting closest to the sun, our home star’s nearest neighbors, Mercury and Venus, have no true moons because of their proximity to the giant ball of gas, which would have ripped away any potential moons from the planets long ago. Venus does have one known quasi-moon, Zoozve, but this does not count because it actually orbits the sun, not Venus.
Moving on, we come to Earth. Our home planet has just one major moon. However, it also has at least seven quasi-moons and occasionally gains additional minimoons for around a year at a time. These lunar imposters do not count, either. However, some scientists think we could use these space rocks as temporary bases to help us become an interplanetary species.
Next up is Mars, which has two true moons: Phobos and Deimos. Both are just a few miles wide and orbit very close to the Red Planet. Phobos is slowly falling toward Mars and is predicted to crash into the world’s surface someday — if it is not ripped apart first.
As we move on to the giant planets, things start to get more interesting. The first gas giant, Jupiter, has a whopping 95 moons, including four major moons — Callisto, Europa, Io and Ganymede, the largest moon in the solar system. But Saturn has even more: There are at least 146 Saturnian moons, including six major moons, such as Titan, Mimas and Enceladus.
The ice giants Uranus and Neptune have 28 and 16 moons, respectively, and seven major moons between them.
Related: Where does the solar system end?
But these numbers have changed significantly in the past few years alone. Since the start of 2023, astronomers, including Ashton and Gladman, have found at least 62 new irregular moons around Saturn and 12 new Jovian moons, as well as a pair of Neptunian moons and a single moon around Uranus.
Advancements in technology can explain the sudden increase in moon discoveries. More powerful telescopes can spot smaller moons — particularly irregular moons — that are abundant around giant planets, Ashton said.
As a result, the number of moons is likely to continue to rise sharply over the next few years. For example, Ashton revealed that he has already discovered some additional planetary moons but is still waiting for follow-up observations to confirm their existence before submitting them to the IAU.
The number of planetary moons may also increase if we find more planets in the solar system. One such world is the elusive Planet Nine — a hypothetical giant planet that may lurk in the far reaches of the solar system. If this planet does exist, researchers have already hypothesized that it could be surrounded by multiple moons.
Researchers have also speculated that additional distant worlds captured from interstellar space by the sun’s gravitational pull, known as rogue planets, could also have moons.
Small-body satellites
NASA’s count of 473 small-body satellites is even more uncertain than the number of planetary moons because we are finding more asteroids all the time. Some asteroids can also have multiple satellites that are hard to distinguish from one another, Ashton said.
The IAU also estimates that there could be “more than 100” extra dwarf planets waiting to be found in the outer solar system, all of which could have moons.
“Just like the giant planet satellites, we are yet to discover the vast majority of them,” Ashton said. In total, there will probably be “around as many small-body satellites as planetary moons,” he added.
However, other astronomers, like Gladman, are less sure about how many small-body satellites there may be. “So much of the ‘parent’ population [of host objects] is still not known that it is difficult to address this question,” he said.
How many moons could there be in total?
There are more than 700 known natural satellites in the solar system. However, as we have seen, this number is likely to grow significantly in the future.
Past studies have shown that there could be hundreds, if not thousands, of tiny planetary moons that could be discovered with more powerful telescopes, Gladman said. And as we have already seen, there is great uncertainty about how many small-body satellites are really out there.
But this uncertainty doesn’t stop researchers from being able to venture a guess.
“I think that there are probably about 10,000 moons in the solar system,” Ashton said.
But there is no telling how long it could take us to find them all.
