How many moons does Jupiter have? The “official” total used
to be 67, but this has now been raised to 79, due to observations from Earth
(as opposed to from a spacecraft in orbit around Jupiter or passing by).
These observations were made almost by accident, given that
the observers based at the Carnegie Institution for Science were actually
looking for the mysterious “ninth planet” when they decided to train their
telescope on Jupiter because it was close to their line of vision at the time.
The new moons – none of which are more than three kilometres
in diameter – were first spotted in 2017 but needed further observation before
they could be confirmed as being in orbit around Jupiter as opposed to being
lumps of rocks that were simply passing by the planet at a safe distance.
Two of the new moons orbit Jupiter at a distance of around
11.5 million kilometres, but the rest are about twice as far away. By
comparison, Earth’s moon is never more than 405,000 kilometres from Earth.
Nine of the more distant moons orbit in the opposite
direction to Jupiter’s rotation, but the tenth is an oddity in that it orbits
in the same direction. This renegade moon, which has been unofficially named Valetudo,
is therefore in danger of colliding with one of the other moons – it is
travelling the wrong way down a one-way street.
It is almost certain that these tiny moons are what is left
after collisions of much larger objects in the remote past. However, it is
unlikely that those objects were themselves moons of Jupiter – it is far more
likely that their remains were captured by Jupiter after the planet and its
other moons formed. Had their origin been earlier, it is likely that there
would have been sufficient gas around to slow them down and cause them to
spiral into Jupiter itself.
What this implies is that the new moons are fragments of
material from the time of the formation of the Solar System and are therefore
worth closer study for understanding the early history of the Solar System.
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