Our knowledge of the outer solar system has changed dramatically in the last decade and we now know of several hundred small bodies that exist there. These objects range in size from 50 to 2000 km across and have remarkably diverse orbits. However, none of these objects have properties that are as extreme as that of “Sedna”, which was recently discovered by Mike Brown and co-workers at the California Institute of Technology (Caltech). This object is currently three times further away than Neptune, making it the most distant body ever observed in the solar system, but how it got there is puzzling astronomers.
Most of the known objects in the far reaches of the solar system – including Pluto – seem to be confined to trajectories with low eccentricities that lie just beyond Neptune. The rest spend most of their time a significant distance away and take several millennia to orbit the Sun.
The principal constituent of these transneptunian objects is undoubtedly ice, and they should be regarded as inactive comets due to their enormous distances from the Sun. Indeed, the Caltech team thinks Sedna, which is named after an Inuit sea goddess, could be a member of the inner Oort cloud – a hypothetical collection of comets orbiting the Sun far beyond the orbit of Pluto (Astrophys. J. at press).
In the May issue of Physics World Brian G Marsden at the Harvard-Smithsonian Center for Astrophysics in Cambridge in Massachusetts in the US describes this latest discovery in more detail.
