Moon and Jupiter 
Our solar system has seven large moons -- satellite worlds that are similar in size to Earth's Moon. And a single planet -- giant Jupiter -- has four of them.
Now, astronomers in Japan and Colorado think they know why.
They simulated the birth of moons around Jupiter and Saturn, the two largest planets in the solar system. The scientists assumed that big moons formed in disks of gas and dust that encircled each planet. As big moons were born, interactions with the disk made the moons move inward, until they crashed into their planet. But gas and dust that were orbiting the young Sun replenished the disks, so they kept making moons.
Because Jupiter is so massive, it cleared a gap in its orbit around the Sun. When its current moons formed, they used up the material in the planet's encircling disk. And with no new material from the disk around the Sun, the moons were saved. That gave Jupiter the four big moons we see today.
And if you have binoculars or a telescope, you can see these big moons yourself. They look like tiny stars arrayed on either side of Jupiter. This evening, Europa is to the left of Jupiter, with Ganymede, Io, and Callisto to the right. But their configuration changes from hour to hour, so the show always looks a little different.
And the entire Jovian system is easy to spot tonight because Jupiter is below the Moon as darkness falls, and remains close to the Moon throughout the night. Jupiter looks like a brilliant star.
Script by Ken Croswell, Copyright 2010