Jovian Moons

The planet Jupiter is intimately linked with the history of Galileo and the telescope, and is a fascinating topic in the field of astrobiology.

Modern astronomy — indeed, modern science — is often said to have begun with Copernicus's sun-centred solar system in 1543. But it was Galileo's groundbreaking use of the new telescope in 1609 which provided the crucial evidence that the old Earth-centred system may be wrong. A key part of Galileo's telescopic revolution was his discovery of the moons of Jupiter, now commonly known as the ‘Jovian Moons’. Not only were they four new worlds, previously unknown to man, but they were in orbit around Jupiter, nailing the lie that only the Earth was a focus of gravity.

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If we were to characterise our solar system then, roughly speaking, you could describe it as a two-body system: Jupiter and the Sun. The Jupiter system can be thought of as a planetary system in miniature, except that Jupiter was never massive enough for hydrogen to start burning at its core so it never became a ‘Sun’. It's way too small for that. Nonetheless, Jupiter is still bigger than all the other planets in the solar system put together — indeed, so big that it makes the Sun wobble slightly in its place at the centre of the solar system.

And that's a key point for the planet hunters who go looking for extra-solar planets, out in the local solar neighbourhood. They also look for that telltale wobble in local stars, for those stars too may have massive planets like Jupiter in orbit about them.

Planet hunters are keen to find other Earths but, given the fact that Jupiter-sized planets cause bigger wobbles, they're more likely to find other Jupiters first, since such systems (with their bigger wobbles) are more noticeable. Sadly, it does actually seem to be all about size.

Another factor that planet hunters keep in mind when they go looking for other Earths is something called the Habitable Zone. This is the region of space around a parent star that is most favourable for life. Not too hot, not too cold, so sometimes also called the ‘Goldilocks Zone’.

In our solar system, of course, Earth is slap bang in the middle of the Sun's Goldilocks Zone. But in exploring other systems, astrobiologists are mindful that planetary moons, as well as the planets themselves, may harbour life. After all, it's possible that a planetary moon may sit in a star's Goldilocks Zone, teeming with extraterrestrial life.

In our own system, there is such a candidate in Europa, one of Jupiter's moons. Even though it doesn't sit in the Sun's Goldilocks Zone, there may be life may in Europa's under-ice ocean, perhaps living in an environment similar to Earth's deep and dark ocean floors.