What Is a Moon?

A moon is a natural object that orbits a planet, just like Earth’s Moon orbits our planet. Scientists call moons “natural satellites” because they travel around planets the same way satellites do. Not every planet has a moon — Mercury and Venus have none at all. As of 2023, scientists have discovered over 290 moons orbiting planets in our solar system. New moons are still being found, especially around the giant outer planets.

Moons of Mars

Mars has two small moons called Phobos and Deimos. These moons are much smaller than Earth’s Moon and have rough, lumpy shapes instead of being round. Phobos is the larger of the two, measuring about 22 kilometers across, which is tiny compared to Earth’s Moon at about 3,474 kilometers across. Scientists think Phobos and Deimos may be asteroids that were captured by Mars’s gravity long ago. Phobos orbits so close to Mars that it circles the planet three times every single day.

Jupiter’s Many Moons

Jupiter has more known moons than any other planet in the solar system, with over 95 discovered so far. The four largest moons — Io, Europa, Ganymede, and Callisto — were first spotted by the astronomer Galileo Galilei in 1610 using an early telescope. Ganymede is the biggest moon in the entire solar system and is even larger than the planet Mercury. Europa is especially exciting to scientists because it has a thick layer of ice covering a vast ocean of liquid water underneath. Many researchers believe Europa’s hidden ocean could be one of the best places to search for life beyond Earth.

Saturn’s Many Moons

Saturn has over 140 known moons, giving it the most moons of any planet. Its largest moon, Titan, is the second-biggest moon in the solar system and the only moon known to have a thick atmosphere. Titan’s atmosphere is mostly nitrogen, similar to Earth’s, and its surface has lakes and rivers made of liquid methane instead of water. Another notable moon is Enceladus, which shoots huge plumes of water ice into space from cracks near its south pole. Scientists have found that Enceladus also has a salty ocean beneath its icy surface, making it another promising place to look for life.

Moons of Uranus and Neptune

Uranus has 28 known moons, many of which are named after characters from the plays of William Shakespeare. Its largest moon, Titania, is about 1,578 kilometers across and has deep canyons and cliffs on its surface. Miranda, another moon of Uranus, has one of the most unusual surfaces in the solar system, with giant cliffs up to 20 kilometers tall. Neptune has 16 known moons, and the largest is Triton, which orbits Neptune in the opposite direction of the planet’s spin. Triton is one of the coldest objects in the solar system, with surface temperatures around minus 235 degrees Celsius.

Record-Breaking Moons

Some moons in the solar system hold notable records. Ganymede is the largest moon, while Deimos, one of Mars’s moons, is one of the smallest at just about 12 kilometers across. Io, one of Jupiter’s moons, is the most volcanically active body in the solar system, with hundreds of volcanoes erupting constantly. Triton is one of the coldest places ever measured in the solar system. Titan is the only moon where a spacecraft has actually landed — the Huygens probe touched down on its surface in 2005.

How Moons Form

Moons can form in several different ways. Some moons formed at the same time as their planet from the same cloud of gas and dust that swirled around the young Sun. Other moons are captured objects — asteroids or comets that wandered too close to a planet and got trapped by its gravity. Earth’s Moon is thought to have formed when a Mars-sized object crashed into the young Earth about 4.5 billion years ago, sending debris into orbit that eventually clumped together. Some of the smaller, irregularly shaped moons of the outer planets were likely captured from the asteroid belt or the Kuiper Belt.

Why Moons Matter

Studying moons helps scientists understand how the solar system formed and changed over billions of years. Moons like Europa and Enceladus, with their hidden oceans, are top targets in the search for life beyond Earth. NASA’s Europa Clipper mission, launched in 2024, will study Europa’s ice shell and ocean in detail. Titan’s thick atmosphere and methane lakes give scientists a chance to study a world with weather and liquid on its surface, somewhat like a frozen version of early Earth. By exploring these moons, we learn not just about them but about the possibilities for life throughout the universe.