What the hell are these things?

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You know the drill, I find a paper detailing some wonderful discovery, waffle a bit of background and then give a glib summary of the conclusions. This is a bit different, this is the scientific process snow leopard caught outside it’s den, looking for a kill. Because the answer here is, “We don’t know”.

You might have heard of the Kepler telescope, it’s up there, looking for planets around other stars and maybe other things. It searches for planets by staring at a field with a lot of stars in it. If a planet orbiting one of the stars has an orbit with just the right alignment it can (once per orbit) get in the way of some of the light from the star, causing it to appear dimmer. Conversely, when the planet is on the other side of its orbit, the star can eclipse it, but given planets are much dimmer than stars, the effect on the measured brightness is small.

However, today this paper came out detailing two very strange discoveries. These are two objects, orbiting around two stars where the smaller object going behind the star causes a bigger dip in brightness than the smaller object passing in-front of the star. This means the smaller object must be pretty bright and as the authors calculate, pretty hot too. Using the data from the light curves of the star-object systems, they are able to calculate estimates for the size and temperature of these objects. One is between a fifth and a quarter of the Sun’s radius and about 9000°C, orbiting the parent star every 5 days. While the other is about a twelfth of the Sun’s radius, 10000°C and goes round its parent object every 23 days. To put these numbers into context, Jupiter is about a tenth of the Sun’s radius and a typical gas giant planet in the sort of orbits these objects are in would have temperatures below 2000°C.

So these objects are much, much hotter than they should be. So what are they? Well the authors speculate that they could be the hot remnants of stars stripped of their outer atmospheres. When stars age they puff up. If stars are in binary systems that are close enough, the binary companion can remove the outer layers of the star, leaving a small hot core. Perhaps this is one of these objects.

Strange, seemingly new classes of objects such as this will attract a lot of follow-up studies. The primary goal will be to work out what mass these objects are, the authors produce an estimates of the masses based on interactions between the objects, but an RV mass will be much more accurate. This will require measurements of the parent stars’ radial velocity.

So the conclusion of the paper, don’t know. Possibly the most interesting conclusion you can have in science.

Jason F. Rowe, William J. Borucki, David Koch, Steve B. Howell, Gibor Basri, Natalie Batalha, Timothy M. Brown, Douglas Caldwell, William D. Cochran, Edward Dunham, Andrea K. Dupree, Jonathan J. Fortney, Thomas N. Gautier III, Ronald L. Gilliland, Jon Jenkins, David W. Latham, Jack . J. Lissauer, Geoff Marcy, David G. Monet, Dimitar Sasselov, & William F. Welsh (2010). Observations of Transiting Hot Compact Objects Submitted to ApJL arXiv: 1001.3420v1


Could Kepler find something closer to home?

This post was chosen as an Editor's Selection for ResearchBlogging.org

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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