Oort cloud


It’s incredible to think that a black hole can be so big that it would take light four days to cross from end to end. But far, far out beyond Neptune’s orbit is something much bigger: the Oort cloud. It’s an enormous region of space encapsulating the planets that stretches 50,000 AU from the Sun to around 100,000 AU in diameter at its outer boundaries: from one side to another it’s about two light years long.

It’s made of water, ammonia and methane ice in the form of icy particles and trillions of larger bodies. It’s suspected that many of the Solar System’s comets were born here and some trans-Neptunian objects (objects that orbit the Sun at a greater average distance than Neptune) are Oort cloud members too. It’s divided into two distinct regions, the inner and outer Oort cloud, containing several trillion comets larger than one kilometre (0.62 miles) in diameter. Considering the size of the Oort cloud (it would take our current fastest spacecraft launch, New Horizons, around 20,000 years to reach its outer edge at 58,536 kilometres per hour/36,373 miles per hour), it isn’t very massive, just a fraction of the 100 or so Earth masses of material ejected from the centre of the Solar System.

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