How many moons does Jupiter have?
As of 2 February 2012, we know of 67 moons around Jupiter. We will probably continue to find more and more, as we keep looking, since Jupiter is large enough that it constantly captures passing asteroids and comets, turning them into moons. The smallest ones are so hard to see, and have such big irregular orbits, that astronomers sometimes lose them, until they can be rediscovered years or decades later! The four largest moons are Io, Europa, Ganymede and Callisto, and can be seen through binoculars if you have good eyesight. They were discovered by Galileo through his primitive telescope in 1610, and the fact that they were visibly orbiting Jupiter was an important part of his argument that the Earth is not at the center of the Universe.
Update:
Two new moons were announced on 2 June 2017, and 5 June 2017. These moons, provisionally named S/2016 J 1 and S/2017 J 1, were discovered on 8 March 2016 and 23 March 2017, respectively. The announcements only came out this month because the International Astronomical Union insists on confirmation that a new body actually exists, by other astronomers, before going public. It took so long to verify their existence because these moons are absolutely tiny, at probably no more than 2km across. Incidentally, astronomers have actually lost some of the earlier moons. They’re tiny, so they’re easily buffeted by the gravity of other passing objects, and they’re hard to see so we don’t always spot when that happens!
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