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September 2nd, 2006

Image Source.

Wow! The American Scientist and Sky and Telescope articles are clearly getting the word out. We’ve been seeing a significant increase in traffic on the oklo.org site, both in terms of visits (yellow bars) and bandwidth and page views (green and blue bars). The bandwidth increase is especially gratifying. It reflects the fact that many users are registering on the back-end, downloading the console, and submitting fits. As I write this, new and interesting fits for a variety of different radial velocity data sets are rolling in to the star catalog. Our goal of fostering original, public-participation exoplanet research is starting to be realized, and I want to thank everyone who’s lending a hand.

Late August stats.

If you’re a first-time visitor to the Systemic Project website, please read the blog entries that were posted prior to this entry. They contain the information you need to start participating, and they give an overview of the current project status. If you are a return visitor, please have a look at the updated back-end. Stefano has made a number of code and design improvements that streamline the workflow and make the site easier to navigate.

On to some planet issues. The Mu Ara (HD 169061) system, which contains four known planets, is shaping up to have significant implications for the systemic project. Intense interest in the system has been spurred by a recent paper from the Swiss group (Pepe et al. 2006) that presents a self-consistent 4-planet model. Pepe et al.’s orbital fit (given in their Table 1) provides an excellent match to the radial velocity data sets, but when they carried out a long-term integration of the system, they found that the gravitational interactions between the planets lead to catastrophe after 76 million years. The parent star Mu Arae has an estimated age of 6.4 billion years, so clearly we don’t yet have a full understanding of what’s going on with this system.

The discord within the Pepe et al. model is provided by the two middle planets, one of which has a 310 day orbit, and the other which orbits in 643 days. The planets are on the edge of the 2:1 mean motion resonance, with the practical consequence that they experience a strongly chaotic orbital evolution. The orbits change eccentricity and orientation on a timescale of only decades:

I’ve made a movie that tracks the evolution of the orbits over 528 years. Here are links to a .mov version (288 kB) and an .mp4 version (1.5 MB). It’s clear from the movie that the interaction is both complicated and unpredictable. The planets display no catastrophic excursions on the 500 year timescale of the movie, but eventually, they experience orbit crossings leading to a likely ejection of the inner 0.5 Jupiter mass planet.

The Mu Ara dataset HD169061_B06P06CH on the console back-end combines both the Pepe et al. data as well as the most recent data from Butler et al. 2006. I’m hoping that someone can get a stable, self-consistent, low chi-square fit to this combined data set. Such a fit would give the best available view of what’s going on with the system, and would underscore the scientific relevance of the systemic project.

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  1. andy
    September 4th, 2006 at 20:07 | #1

    Would the system be more stable if the planets were in the 2:1 resonance rather than on the edge of it?

  2. greg
    September 4th, 2006 at 20:22 | #2

    A mean motion resonance such as the 2:1 resonance can be either stabilizing or destabilizing. Given that the system exists, however, its presence in the 2:1 resonance would indicate that the resonance is stabilizing, and the system would be in much better shape. I think its definitely worthwhile to look for fits that are in the resonance…

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