Why planets have moons

One of four planet-size moons in orbit around the planet Jupiter , Europa has an almost glassy surface of ice that scientists say may cover an ocean that is 31 miles 50 kilometers deep. Tidal forces between Jupiter and Europa are believed to generate enough heat to keep the ocean liquid. Observations with the Hubble Space Telescope also indicate Europa has a tenuous atmosphere of oxygen.

The combination of water, a heat source, and an atmosphere raises the possibility that Europa harbors life. The best known moon is the one in orbit around planet Earth. It is Earth's only natural satellite and the only extraterrestrial body that humans have visited. Gravity on the moon is one-sixth of Earth's gravity, allowing astronauts to take giant leaps on its surface.

The moon lacks an atmosphere, but spacecraft have found water ice at both poles, deposited from impacting comets. According to a leading theory , a Mars-size body smacked Earth about 4.

But unlike Earth's moon, many moons formed from the same material that glommed together and gave rise to the body they orbit.

Other moons are asteroids captured into orbit by a larger body's gravity. Only the dwarf planet Pluto's moon Charon is thought to have formed from a collision like the one that gave rise to Earth's moon. No matter how they form, the moons are many. Only Mercury and Venus are moonless. Earth has one, Mars two. Neptune has 13 and Uranus The gas giant Jupiter has 63 known moons.

Saturn has at least 60—and 42 of those have been discovered since The dwarf planet Pluto has three moons.

Eris, another dwarf planet, has one moon. Any large object striking a planet would have thrown material off the world into orbit, allowing it to coalesce into a moon. Snapshot : ALMA spots moon-forming disk around distant exoplanet.

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Strong aurorae dazzle astronauts on space station. How would a human body decompose on Mars? First look at the total solar eclipse. That's how they wound up standing on Galileo's shoulders. In , the great astronomer Galileo Galilei noticed four heavenly bodies that appeared to revolve around Jupiter. Named Io, Europa, Ganymede and Callisto, these are Jupiter's biggest moons by far — and they were the first to be discovered.

As stargazing technology grew more sophisticated, it became clear that the quartet had lots of company. Sheppard's team just brought the total number of identified Jovian moons i. Jupiter had always had the most moons in the solar system, until the summer of That's when astronomers discovered that Saturn had another 20 moons , bringing its total to Uranus boasts 27 and Neptune has Mars, our beloved next-door neighbor, possesses two satellites: Deimos and Phobos.

And if that's making you feel insecure about Earth's lonely moon, at least you can take solace in the fact that Mercury and Venus are totally moon-less. There's a reason why Jupiter has so many satellites while other planets — ours, for instance — have so few. It all comes down to gravity. Astronomers divide the planets within our solar system into two categories. Mercury, Venus, Earth, and Mars are the so-called "terrestrial" or "inner" planets while Jupiter, Saturn, Uranus, and Neptune have been classified as "gas giants," also known as "outer planets.

The size gap between those factions is quite considerable; Although Uranus is the smallest outer planet, it's still 15 times more massive than Earth, the largest of the inner planets. None of the other planets can compete with Jupiter in terms of sheer bulk, however.

You'd need more than duplicates of our puny home world to equal Jupiter's colossal mass. It's an absolute monster. Now, as Isaac Newton observed, there's a positive correlation between the mass of an object and the strength of its gravitational field. Because the gas giants are so massive, they're able to attract more satellites. But that's not the only reason why planets like Jupiter have such large moon collections.

Our solar system's gas giants are relatively far away from the sun. In contrast, some stars have massive, Jupiter-like planets called "hot Jupiters.

Imagine if Saturn switched places with Mercury.