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 good telescope, it’s the only natural satellite you can see with your own eyes. As a result, most people have a distorted perception of what the 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 city-sized space rocks to massive rounded bodies that are potentially large enough to be considered planets in their own right.
So exactly how much solar system have we found the moons? The answer, it turns out, depends on your definition of a moon.
The International Astronomical Union (IAU) officially recognizes 288 planetary moons orbiting the eight worlds of the solar system, according to NASA. But there are another 473 “small-body satellites” — moons of asteroids and dwarf planets — listed by NASA Jet Propulsion Laboratory. If we count both types, as most astronomers would, this brings the total number of natural satellites of the solar system to 761 (as of June 2024).
But this is likely only 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 past few years alone, and technological advances are likely to 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] it would be an object that is in orbit around a larger, non-stellar object,” Ashton told Live Science. “But that’s not the full answer.”
For example, there are currently thousands of man-made satellites in orbit around the Earth that meet the above definition but are not considered moons because they are not natural. These spacecraft also have previously limited lifespans falling back to Earth and burning up in our atmosphere.
Some natural satellites, such as quasi-moons and mini-moons, are also temporary and do not actually orbit planets.
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There is also an issue 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 “moons,” also have “vague definitions” and are not considered true moons, Gladman added.
Even among the known moons, there are still points of disagreement. For example, planetary moons can be divided into two groups: regular moons, which are usually larger and have small circular orbits near 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 regular moons, about 20 are also considered large 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, the nearest neighbors of our home star, mercury AND Venus, have no true moons due to their proximity to the giant ball of gas, which would have removed any possible moons from the planets long ago. Venus has a known quasi-moon, Zoozve, but that doesn’t count because it actually orbits the sun, not Venus.
Moving on, we come to Earth. our the native planet has only one large moon. However, it also has at least seven quasi-moons AND occasionally earns additional mini moons for about a year at a time. These moon cheaters don’t even count. However, some scientists think we could use these space rocks as temporary bases help us become an interplanetary species.
The other is Marchwhich has two true moons: Phobos AND Deimos. Both are only a few miles wide and orbit very close to the Red Planet. Phobos is slowly falling towards Mars and is predicted to crash into the world’s surface one day – if not snapped first.
As we move on to the giant planets, things start to get more interesting. The first gas giant, Jupiterthere are 95 moons, including four large moons – Callisto, europeIo and Ganymede, the largest moon in the solar system. But Saturn there are more: There are at least 146 Saturnian moons, including six major moons, such as Titan, Mimas AND Enceladus.
Ice giants Uranus AND Neptune have 28 and 16 moons respectively and seven large moons between them.
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But these numbers have changed significantly only in recent years. As of early 2023, astronomers, including Ashton and Gladman, have found at least 62 irregular new moons around Saturn AND 12 Jovian new moonsas well as one pair of Neptunian moons and a single moon around Uranus.
Advances in technology may 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 will likely continue to increase significantly over the coming years. For example, Ashton revealed that he has already discovered several extra planetary moons, but is still awaiting 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. Such a world is elusive Planet Nine — a hypothetical giant planet that could be hiding in the far reaches of the solar system. If this planet exists, researchers have already hypothesized that it may be surrounded by many moons.
Researchers have also speculated that other distant worlds trapped in interstellar space by the gravitational pull of the sun, known as rogue planetsthere might even be a moon.
Small body satellites
NASA’s number 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 may also have multiple satellites that are difficult to distinguish from each otherAshton said.
The IAU also estimates that there may be “more than 100” additional dwarf planets waiting to be found in the outer solar systemall of these can have moons.
“Like the planet’s giant satellites, we have yet to discover the vast majority of them,” Ashton said. In total, there will probably be “about as many small-body satellites as planetary moons,” he added.
However, other astronomers, such as Gladman, are less certain about how many small-body satellites there might be. “So many of the ‘parent’ population [of host objects] it is not yet known that it is difficult to address this question”, he said.
How many moons can 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 increase significantly in the future.
Past studies have shown there may be hundreds, if not thousands, of small planetary moons that could be detected 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 does not prevent researchers from being able to make a guess.
“I think there are probably about 10,000 moons in the solar system,” Ashton said.
But there’s no telling how long it might take us to find them all.