Could Kepler find something closer to home?Posted: December 9, 2009
You might have heard about Kepler and NASA space mission to find planets around other stars. But recently this paper came out recently showing how it could be used to probe unknown distant reaches of our own solar system.
One of the successful methods in the rapidly developing field of discovering worlds around other stars over the last decade has been the transit method. Put simply, the planet that orbits the star gets in the way, blocking out a bit of the light from the the star’s surface. Hence for a brief period the star appears slightly dimmer. Detecting this requires staring at a star on and off for a long period and making very precise brightness measurements, what Kepler is designed to do, stare at lots of stars and look for these dips in brightness. But couldn’t something else get in the way too? Yup.
Comets are collections of ices (frozen water, carbon dioxide etc.) that occasionally pass through the inner solar system on their orbits around the Sun. These appear to be made of two separate populations, one of comets with short orbital periods that seem to orbit in the same plane as the other planets in the solar system and one of longer period comets which have orbits with random inclinations. It’s thought that these two populations have two separate places of origin. The short period comets are thought to come from a disk of objects extending from 30AU (1AU is the distance from the Earth to the Sun) to maybe 100AU. Longer period comets seem to come from much further away. The idea of a distant spherical cloud of icy bodies as an origin for long period comets was first thought up by my second favourite Estonian astonomer Ernst Opik (Brits who recognise the surname may know his grandson, cheeky boy MP Lembit) and later resurrected by the Dutchman Jan Oort. This is now known as the Oort Cloud, icy bodies in a spherical shell extending from a few thousand AU to tens of thousands of AU. To put that upper bound into context, the nearest star to the Sun is only 260,000AU away.
Unfortunately there are no definite Oort Cloud members known. Their distance and small size make direct detection difficult. However a paper out this week by astronomers in the US and in Israel has suggested that the Kepler mission could detect them by chance. The principle is
the same as the detection of planets by transits. An icy body in the Oort Cloud passes in-front of a background star and thus blocks out some of its light thus dimming it. The rate at which these events happen will depend on the number of objects in the Oort cloud and how close to the Sun it’s inner boundary is. The study finds that Kepler could detect occultations (when a solar system body passes in-front of a background star) of up to one hundred 10km+ in size Oort Cloud objects. The precise detection rate could allow astronomers to constrain the dimensions and density of the Oort Cloud by observations for the first time.
Eran O. Ofek, & Ehud Nakar (2009). Detectability of Oort cloud objects using Kepler Submitted to ApJL arXiv: 0912.0948v1