You may have heard that the Earth lies within what is called the ‘Goldilocks zone’. It is not so far away from the sun that everything freezes and so there would be no life. It is also not so close to the sun that everything would boil and there would be no life. It’s distance from the sun, is just right.

So when astronomers on the hunt for planets that could sustain life (or indeed have life on them) they look for stars similar to our own sun and then look for planets that might exist in a similar zone. As there are an estimated 20 billion sun-like stars in the Milky Way alone, then there’s a lot of potential earths out there. So the theory goes.

However, it turns out that just being at the right distance from a star so that water can form is not the only thing a planet needs if there is to be sustainable life. One of those things is, apparently, a large planet roughly the size of Jupiter.

And Jupiter is large. Really, really large. You could fit 1300 Earths inside Jupiter. If you like numbers it has ‘a diameter of 142,984 km (88,846 mi) at its equator'((https://en.wikipedia.org/wiki/Jupiter)). It has more than double the mass of all the other planets in the solar system combined. It’s a big, big planet. And the gas giant has a greedy 79 moons, three of which are larger than our own moon. It’s quite the thing.

You might be familiar with Jupiter having this big red spot on it, which if you had your own telescope you could probably see for yourself. Well, that big red spot is a storm and it’s quite a doozy. It’s possibly permanently, er, storming and it covers an area so large that the earth could fit inside it. Like I said, quite a doozy.

I learned the importance of Jupiter while I was reading Michio Kaku’s The Future of Humanity. Kaku writes,

The early solar system was full of cosmic debris that constantly rained down on the Earth. Fortunately, the gravitational field of Jupiter acted as a vacuum cleaner, either absorbing it or flinging it away. Computer simulations show that, without Jupiter, the Earth today would be bombarded with giant meteors, which would make life impossible. In the future, when considering solar systems to colonize, it would be better to look for those that have their own Jupiter, big enough to tidy up the debris.

Feel free to ignore the colonizing bit, as we ain’t going anywhere anytime soon. But be very grateful for our massive neighbour and its equally massive gravitational field. Jupiter gets pelted around 8,000 times more than we do.

Not only does Jupiter protect us from meteor bombardment but according to NASA, “But if Jupiter-like planets remain distant from their stars, they can serve as the gatekeepers to their planetary systems. They protect their fellow planets on inner orbits, allowing them to maintain nearly circular orbits that provide stable climates over extended periods of time.”

This is pretty important. We need Jupiter to have a stable orbit of its own. If it didn’t it would start to ‘wander’ and if it wandered towards the sun it would take us with it and basically cast us into the everlasting fires of the sun if it didn’t rip us to pieces before then. And as Jupiter neared the sun it would heat up and everything else would heat up so we’d basically be living between two giant fireballs. Not good. But Jupiter’s stable orbit also helps our own planet have a stable orbit and that means the conditions for life can be sustained.

Our solar system might not be the only way life can exist but it is so far the only example we know of. So how many other solar systems set up like ours might there be? Some astronomers think the answer is around 15%. That brings the billions down a fair bit to around a few hundred million, which is still a lot granted, but that number will get smaller still. It’s the size of the numbers that leads most scientists to hope or believe if you will, that there is life on other planets. But it is until we find conclusive evidence to the contrary just that, a belief. And as you read people write about this rare earth theory you also detect a hint of existential dread, that the idea that this planet is it has raises other uncomfortable possibilities.

I guess I could quote this verse a lot for a hole host of reasons but the Psalmist writes:

When I look at your heavens, the work of your fingers,
    the moon and the stars, which you have set in place,
what is man that you are mindful of him,
    and the son of man that you care for him?

Psalm 8:3-4

Based on the available evidence it seems to reasonable to say that, at the very least we’re either special or lucky and to all intents and purposes we are alone in the universe.

As it turns out this week, “We are witnessing the overture to a phenomenal astronomical moment known as a Great Conjunction of the largest planets in our solar system: Jupiter and Saturn. For the first time in nearly eight centuries — yes, not since 1226 — this “greatest of great conjunctions”* will be observable Monday, Dec. 21, 2020, coinciding, by chance, with the winter solstice.”

So if you happen to take a look up into the heavens and see Jupiter and Saturn, take a moment to thank God for Jupiter because if it wasn’t there then we wouldn’t be here.