You'll find it in every astronomy textbook: the spherical cloud of a trillion lumps of rock and ice, most a few kilometres across, that forms the solar system's outermost boundary. The Oort cloud's distant edge could lie some 100,000 times further out from the sun than Earth, more than a third of the way to its nearest stellar neighbour, Proxima Centauri. Yet the textbook Oort cloud denizen is far too small for us to observe, and exists in almost total darkness. "From that distance, the sun appears so small that you could completely block it out with the head of a pin," says Brown. The only, rather circumstantial, evidence we have for its existence is the occasional passage of a "long period" comet - presumed to be an Oort cloud object knocked in our direction by the gravitational perturbations of other stars.
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