Olbers' paradox - the apparent contradiction that the night sky is dark even though the universe is filled with luminous objects - is resolved in modern cosmology by proposing that the Universe is finite and ever-expanding. But current estimates of the number of icy fragments in the outer reaches of our solar system suggest that the night sky should be bright. Clearly this is not the case - and now two American astronomers have used the 400-year-old paradox to study these distant objects and how they shaped the early evolution of the solar system (S J Kenyon and R A Windhorst 2001 Astrophysical Journal Letters 547 L69).
Many icy bodies – ranging in diameter from under a metre to several kilometres – exist beyond the orbit of Neptune in a band known as the Kuiper belt. Current theories propose that the abundance of the objects grows exponentially as the size falls. But if very small objects are as common as this suggests, these Kuiper belt objects would violate Olbers’ principle – that is, they would make the night sky bright.
Scott Kenyon and Rogier Windhorst of the Smithsonian Astrophysical Observatory and Arizona State University realised that Olbers’ paradox imposed useful limits on the nature of Kuiper belt objects (KBOs). They analysed existing counts of larger KBOs and measurements of optical and infrared radiation from smaller objects to establish more accurately their size distribution. This in turn provides clues about evolution in the early solar system. “Our results suggest that – for the night sky to be dark – the size distribution of Kuiper belt objects must have two components”, Kenyon told PhysicsWeb. For large objects – that is, between 1 and 100 km across – Kenyon and Windhorst found that the abundance of objects towards the top of that range decreases sharply. But for objects less than a kilometre across, the distribution is much shallower – resulting in fewer very small fragments.
This discovery is consistent with classical theories of how particles collided and merged in the disk of gas and dust orbiting the Sun before the planets coalesced. “High speed collisions between smaller bodies produce smaller and smaller fragments. Low velocity collisions between the larger bodies produce larger and larger bodies,” said Kenyon. “The end result is two size distributions – a steep one for the larger bodies and a shallower one for the smaller bodies.”
The finding also lends weight to the idea that the KBOs formed around the same time as Neptune – if Neptune had reached its current size much earlier, it would have stirred up the small fragments before they had time to form the large KBOs we see today. Kenyon and Windhorst hope to use data from the Hubble Space Telescope to refine their results within the next year.
