You鈥檒l 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鈥檚 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 鈥渓ong period鈥� comets聽鈥� bodies that pass us less than once every 200 years聽鈥� is the only evidence we have for the Oort cloud鈥檚 existence, and that is circumstantial to say the least.
The Oort cloud might even come in two parts. In 2003, of the California Institute of Technology in Pasadena and his colleagues, observed a dwarf planet, dubbed Sedna, swinging on a highly elliptical orbit reaching out to about 1000 times the Earth-sun distance. That gave succour to the idea that within the cloud鈥檚 spherical outer shell lies a disc of objects in the plane of our solar system, sometimes known as the Hills cloud.
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That鈥檚 still a long way from a direct spot. The typical denizen of the Oort cloud is probably just a few kilometres across, far too small for us to observe in its native habitat. What鈥檚 more, these objects exist in almost total darkness. 鈥淔rom that distance, the sun appears so small that you could completely block it out with the head of a pin,鈥� says Brown.
Oort cloud objects are thought to be stuff left over from when the planets formed, and getting an idea of how many of them there are at different sizes could help us understand how that process occurred. One idea involves a technique that exoplanet hunters use: looking for a dip in light as an orbiting planet 鈥渢ransits鈥� across the star鈥檚 face. In 2009, Eran Ofek of Caltech and Ehud Nakar of Tel-Aviv University in Israel showed that the then newly launched Kepler Space Telescope could theoretically detect Oort cloud objects a few tens of kilometres across, .
In practice, however, it鈥檚 hard to be sure that a single fleeting dip in starlight is not a detector glitch 鈥� and besides, Kepler is pointing high above the plane of the solar system, away from the densest concentration of objects.
A specially designed Oort cloud hunter, called Whipple, failed to get off the ground in 2011. In the absence of direct evidence, however, the constant influx of new comets 鈥� such as comet ISON, which spectacularly broke up as it came too close to the sun in December 2013 鈥� seals the deal for most astronomers. 鈥淲e can be quite confident the Oort cloud exists even though we have never actually imaged an object,鈥� says of the Carnegie Institution for Science in Washington DC.
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