from New Scientist

The Oort cloud surrounds our solar system – why can’t we see it?

It’s a giant sphere of a trillion rocks encircling us that occasionally sends comets slinging our way. That’s a convincing story – but we’d love some direct evidence

Oort cloudOort cloud: out of the darkness Credit: Jon Lomberg/SPL

You’ll find it in every astronomy textbook: the spherical cloud of a trillion lumps of rock and ice that forms the outermost boundary of our solar system. 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.

Out there, the gravitational pull of other stars and even of the Milky Way itself outweighs that of the sun. These influences can sometimes knock an Oort cloud object off course and in our direction, becoming what we know as a comet. Indeed, the need for a source for “long period” comets – bodies that pass us less than once every 200 years – is the only evidence we have for the Oort cloud’s existence, and that is circumstantial to say the least.

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