ONCE upon a time, 4.6 billion years ago, something was brewing in an unremarkable backwater of the Milky Way. The ragbag of stuff that suffuses the inconsequential, in-between bits of all galaxies - hydrogen and helium gas with just a sprinkling of solid dust - had begun to condense and form molecules. Unable to resist its own weight, part of this newly formed molecular cloud collapsed in on itself. In the ensuing heat and confusion, a star was born - our sun.We don't know exactly what kick-started this process. Perhaps, with pleasing symmetry, it was the shock wave from the explosive death throes of a nearby star. It was not, at any rate, a particularly unusual event. It had happened countless times since the Milky Way itself came into existence about 13 billion years ago, and in our telescopes we can see it still going on in distant parts of our galaxy today. As stars go, the sun is nothing out of the ordinary.
And yet, as far as we know, it is unique. From a thin disc of stuff left over from its birth, eight planets formed, trapped in orbit by its gravity. One of those planets settled into a peculiarly tranquil relationship with its star and its fellow planets. Eventually, creatures emerged on it that began to wonder how their neighbourhood came to be as it is - and could formulate the following six enduring mysteries of our familiar, and yet deeply mysterious, solar system.
How was the solar system built?
Looking at our neighbouring planets, you could be forgiven for thinking that if they do belong to the same family, it is by adoption rather than kinship. Not so: they are blood siblings
Why are the sun and moon the same size in the sky?
The sun is about 400 times as wide as the moon, but it is also 400 times further away. The two therefore look the same size in the sky - is it more than a coincidence?
Lurking in the solar system's dark recesses, rumour has it, is an unsighted world - Planet X, a frozen body perhaps as large as Mars, or even Earth
These cosmic apparitions have had humans pondering their nature for millennia, yet theories of where they originate still don't stand up
Since the first discovery of a planet orbiting another star in 1992, some 280 alien solar systems have been identified - but most look quite unlike ours
How will our solar system end?
Since the ructions that created the planets in the first 100 million years, nothing much has been happening. But something unpleasant is bound to shatter this comfortable calm
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