What is the Oort Cloud?

Hypothetical enormous icy shell thought to surround the whole solar system, source of long-period comets.

Can you see it?

Not as one thing, no — the Oort Cloud is a place rather than a sight, a vast shell all round the Sun far beyond the planets. Nothing in it has ever been seen out there; what reaches us are the comets that fall in from it.

Worth knowing about the Oort Cloud

Why it stands out: Proposed by Jan Oort in 1950 (Ernst Öpik had suggested it in 1932), from the orbits of comets.; How we know: Telescopes cannot see objects so small and so far away. Scientists infer it exists because it would explain where long-period comets (ones that take more than two hundred years, and often millions, to go round once) come from. Rung 3 of the cosmic ladder — how the solar system is arranged

The numbers

Distance from the Sun	estimated 2,000 to 100,000 AU

How anyone knows this

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