Nemesis meets its, uh, nemesis

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Interesting fact of the day: examining the fossil record suggests that mass extinctions on Earth occur approximately once every 26 million years (Myr). One possible explanation for this is a companion dwarf star to the Sun on a 26 Myr orbit. Every time is passes by, the theory goes, it messes up the Oort cloud and throws comets our way, greatly increasing the possibility of a collision (and all the nasty effects that could go with it). This potential star was christened Nemesis for obvious reasons.

Artist's impression of the arrival of Nemesis

If Nemesis exists then its orbital period (the time it takes to complete one orbit) is predicted to vary by a few Myr every orbit, because of the influence of other stars in the Galaxy. Remember this – it will be important later!

The theory that the mass extinction periodicity is caused by the Nemesis companion star was first suggested in 1984. Since then the quality of the fossil record datasets has improved significantly which has led two scientists, Adrian Melott and Richard Bambach to re-examine the hypothesis using these new modern data. They found strong evidence that mass extinctions have been occurring every 26.8 Myr, stretching back over 500 Myr (interesting aside: that’s approximately as far back as they could go in the fossil record apparently, as that’s how long things with hard body parts, which make good fossils, have existed).

Their results seem at first glance to support the existence of Nemesis. However, the mass extinctions they see are too regular – if they were being triggered by the visits of Nemesis the gap between them should vary more, given the predicted variation in the length of the star’s orbit.

If Nemesis isn’t responsible for these apparently regular extinctions then what is? The alternative explanations range from oscillations in the galactic plane to geological instabilities in the Earth itself, and if someone publishes new research on any of them I’ll maybe try and explain them too. One final thing to note: if Nemesis is out there after all, the new all-sky surveys to be carried out by Pan-Starrs, WISE or the planned LSST should see it.

ResearchBlogging.orgAdrian L. Melott, & Richard K. Bambach (2010). Nemesis Reconsidered MNRAS arXiv: 1007.0437v1

Could Kepler find something closer to home?

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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