Home > detection, systemic faq, worlds > hd 20782 oct 20, 2006 (3.6%)

hd 20782 oct 20, 2006 (3.6%)

April 1st, 2006

As advertised in yesterday’s post, three newly published radial velocity data sets have just been added to the system menu of the Systemic Console, and to the www.transitsearch.org candidates list. The data set for HD20782, published by Jones et al. of the Anglo-Australian Planet Search, is definitely the most interesting of the trio. Let’s work the HD 20782 velocities over with the console, and see what they have to say.


First, fire up the console. (If you use Firefox on Windows, and you’ve had success getting the console to work with that particular line-up, please post a response in answer to Vincent’s comment on yesterday’s post. All of Aaron’s oklo.org Java development has been done on Mac OSX using Safari. Also, we’ve had many reports that the console works well with Internet Explorer on Windows, so if Firefox won’t run the Java, give IE a try. And could someone ask Mr. Bill G. to send me a check for that plug?)

At any rate, the HD 20782 radial velocity data set has one data point that sticks down like a sore thumb:


Activation of one planet and a little bit of fooling around with circular orbits shows that even when the discrepant point is ignored, the waveform of the planet is not at all sinusoidal. The points contain an almost sawtooth-like progression:

circular orbit fit

Because of the non-sinusoidal nature of the velocities, the periodogram (obtained by clicking the periodogram button) is rather uninformative. There’s a lot of power in a lot of different peaks, and it’s not immediately clear what is going on planet-wise:


Aaron has been working very hard on console development, and we will soon release an updated version with a number of absolutely bling features. Ever wondered what your fits sound like? One new feature is a “folding window”, which allows the data to be phased at whatever period one likes. The folding window is very useful for data-sets of the type produced by HD 20782. It quickly reveals that something like a 600 day periodicity brings out the overall shape of the planetary waveform:

folding window

Using 600 days as the basis for a 1-planet fit, activating eccentricity, and using a combination of slider work, 1-d minimization, and Levenberg-Marquardt, eventually produces excellent fits to the data that look like this:

fit to hd20872

Jones et al., for example, in their discovery paper, report an orbital period of P=585.86 days, an eccentricity, e=0.92, a mass (times the sine of the unknown orbital inclination) of Msin(i)=1.8 Jupiter masses, and a longitude of periastron of 147 degrees.

This planet is one bizzare world, and seems to be very similar to HD 80606 b (another oklo.org favorite). The orbital period is 1.6 years. The planet spends most of it’s time out at ~2.6 AU. In our solar system, this distance is out beyond Mars in the inner asteroid belt. Once per orbit, however, HD 20782 b comes swinging in for a steamy encounter with the star. The periastron distance is a scant 0.11 AU, roughly half Mercury’s distance from the Sun. The planet is likely swathed in turbulent white water clouds. Raindrops vaporize as the star looms larger and larger in the sky.

Stars that loom large in alien skies are good news for transitsearch.org, and in the case of HD 20782 b, we here on earth are particularly fortunate. HD 20782 b’s line of apsides lies within about 60 degrees of alignment with the line of sight to the Earth. This raises the a-priori geometric probability of having a transit observable from Earth to a relatively high 3.6%. (The a-priori probability of transit for a planet with a 1.6-year period and a circular orbit is only ~0.3%).

oribital figure

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