20 centimeters per second
HD 69830. What a difference a year makes. Last June, HD 69830 languished in the obscure backwaters of the Henry Draper Catalog. Now, however, like its buddy HD 209458, the star is a star. Google “HD 69830″, and the search returns 660 entries (and growing daily). Google “HD 69831″ and (until the crawlers manage to find this post) your search does not match any documents.
In Thursday’s post, we gave an overview of the HD 69830 planetary system, which contains Neptune-mass planets in 8.67, 31.6, and 197 day orbits. Perhaps the most astonishing thing about this discovery announcement is the tiny radial velocity amplitude of the 197 day planet in the model. This object induces a radial velocity amplitude of 2.2 meters per second, with a reported error of only twenty centimeters per second. That’s about the speed your finger moves if you trace it quickly across the title of the discovery paper. This detection required a very quiet star and an extraordinary technique. The Swiss seem to have broken through to the next level.
I wonder what that outer planet looks like. Over at transitsearch.org, I have a Fortran cron job that processes all of the known exoplanets every night to produce updated transit ephemeris tables. In order to predict transit depths, the code needs an estimate for the planetary radius, which in turn requires an estimate of the effective surface temperature. The transitsearch model reports 262 K, just below the freezing point (273 K), suggesting a brilliantly reflective orb swathed in white water-based clouds.
In an upcoming post, I’ll delve into some responsible (and also some irresponsible) speculations about the world beneath those clouds. A responsible viewpoint has planet “d” forming at a larger orbital radius than it currently occupies, and then migrating in to position. In this formation-followed-by-migration scenario, there were plenty of ices available during planetary assembly, and the planet will have a structure (and size) very close to those of Neptune.
An irresponsible, more provocative scanario has planet d forming in-situ, out of refractory silicate and metallic materials. The final product in this case is a super Earth, smack in the sweet spot of the habitable zone, and endowed with ten Hubble times worth of geothermal activity. But more on that in the upcoming post.
A driving goal at oklo.org is to get our readers beneath the headlines and critically examining what the radial velocities themselves have to say. To do this, we need the data. The Lovis et al. discovery article in Nature contains a link to supplementary material, but when you click on the link you get a .pdf article about hydrothermal vent tubeworms:
Hmmm. Even if we adopt the most optimistic giant-Earth-like structural models for HD 69830 d, this supplementary material seems to be jumping the astrobiological gun. With tubeworms obscuring the radial velocities, we were compelled to resort to Dexter to extract as many velocities as we could from Figure 2 of the .pdf version of the paper. Eugenio managed to scrape 53 data points off the graph. We were busy using the console to work up fits to these dextered velocities when Darin Rogozzine at Caltech managed to guess the correct link and supplied oklo.org with the url.
In the next post, we’ll have a go at the rv’s. If you want an advance crack at them, they’re now on the web-based version of the console. We’ll get them on the downloadable version tomorrow.