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Image Source.

Last week, I wrote a post introducing HD 17156 b, a Jovian planet on a highly eccentric 21.2-day orbit around a V=8.17 solar-type star lying 250 light-years away in Cassiopeia (RA=2h 50m, Dec=72 deg).

A photometric check for transits by HD 17156 b was reported in the discovery paper, but due to the nearly three-week orbital period, it was only possible to rule out about 25% of the transit window. Given the highly favorable geometry of the planetary orbit, this means that there’s an impressive ~11% chance (8.25% if you take the discount) that the planet can be observed in transit. The expected transit depth is a very respectable 1%, and given the bright parent star, it’s a straightforward detection for small-telescope observers everywhere in the Northern Hemisphere.

What’s it worth to catch HD 17156 b in transit? From a crass cash-money standpoint, one can estimate a dollar value. Because the planet has a long period and an eccentric orbit, it would be the first transiting example of its kind, and would thus be expected to generate a fairly large number of citations. From a career standpoint, an ADS citation is worth at least $100 (see, e.g. here). Based on the citation count for the TrES-1 discovery paper (144 citations in three years) it’s reasonable to expect that at one decade out, a HD 17156 b transit would garner of order 200 citations, for a conservative total value of 20K. Given the 10% probability of the transit coming through, the resulting expectation value is equivalent to having twenty Benjamins floating down from the black velvet of the night sky.

I used the systemic console’s bootstrap utility to generate a set of orbital fits to the published radial velocities for HD 17156. Each orbital fit describes a unique sequence of central transit times. For a particular transit opportunity, the aggregate of predicted central transit times from the different fits can be plotted as a histogram. Here’s the resulting plot for the transit opportunity that’ll occur next Monday (HJD 2454353.68):

The uncertainty in the time of central transit is ~0.3 days. A window this narrow is rare for a planet that hasn’t yet been thoroughly checked. In fact, as far as Transitsearch.org opportunities are concerned, it doesn’t get much better than this. Extending our opportunity cost analysis, the expected monetary return for observations within the 1-sigma transit window is an impressive $114 per hour. (Only rarely does the expected return per hour exceed minimum wage for existing transit opportunities.)

Scientifically, a transit by HD 17156 b would certainly be very exciting. The planet should be heating up very rapidly during its periastron passage, which should spur the generation of hemispheric-scale vortices and an 8-micron light curve that’s detectable with the Spitzer telescope. Observation of the secondary eclipse (assuming it occurs) would allow for a measurement of the global planetary temperature near the orbital apastron.

The frame above is from a hydrodynamical study of HD 17156 b that Jonathan Langton has just finished computing. If all the talk of dollars, ephemerides, opportunity cost, and expectation value is leaving you stressed out, then just kick back with this fat 1.0 MB .mov of the simulation and get your groove on.

Image Source.

August 1st marked the most recent ‘606 day, which came and went without wide remark. Perhaps this was because in late Summer, HD 80606 rises and sets in near-synch the Sun, and is thus lost from the Earth’s night skies.

At the moment, HD 80606b is headed back out toward apastron.

The global storms and shockwaves that were unleashed at the beginning of August are dissipating rapidly, and the flux of heat from the planet is likely fading back down to the sullen baseline glow that arises from tidal heating.

HD 80606’s next periastron passage occurs on November 20th, and the Spitzer Space Telescope is scheduled to observe the whole event (details here). It’s going to be a big deal. Spitzer can only observe HD 80606 during two three-week windows each year, and fortunately, the Nov. 20th Periastron passage occurs during one of these windows. It’s literally the only opportunity to catch HD 80606 b’s big swing before Spitzer’s cryogen runs out in 2009.

The orbital geometry of the periastron passage looks like this:

Each marker of the orbit is separated by one hour. The prediction for the pseudo-synchronous rotation of the planet is also indicated. The planet should be spinning with a period of 36.8 hours. Jonathan Langton’s hydrodynamics code predicts what the temperature distribution on the planet should look like at each moment from Spitzer’s viewpoint in our solar system:

Transitsearch.org observers have covered a number of the HD 80606 b transit opportunities, and it seems pretty certain that the planet doesn’t transit. This isn’t surprising. The geometry of the orbit is such that when the planet crosses the plane containing the line of sight to the Earth, it’s quite a distance away from the star. Not so, however, for the secondary transit. There’s a very respectable 15% chance that Spitzer will detect a secondary transit centered two hours prior to the periastron passage.

Even if the planet doesn’t transit, we should be able to get a good sense of the orbital inclination from the shape of the light curve. If the orbit is nearly in the plane of the sky, then we should see a steady rise followed by a plateau in the 8-micron flux coming from the planet. For more nearly edge-on configurations, the flux peak should be clearly discernable. The observations are scheduled to start 20 hours prior to periastron and end 10 hours after.

Image Source.

HAT-P-2b. The name doesn’t exactly ring of grandeur, but this planet — a product of Gáspár Bakos’ HAT Net transit survey — is poised to give the Spitzer Space Telescope its most dramatic glimpse to date of a hot Jupiter.

HAT-P-2b’s orbit is remarkably eccentric for a planet with an orbital period of only 5.6 days, and by a stroke of luck, periastron is located almost exactly midway between the primary and the secondary transits (as viewed from Earth). The strength of the stellar insolation at periastron is nine times as strong as at apastron, which more than guarantees that the planet will have disaster-movie-ready weather.

On June 6th, Josh Winn and his collaborators used the Keck telescope to obtain 97 radial velocities for HAT-P-2. The observations were timed to occur before, during, and after primary transit, and the Rossiter-McLaughlin effect is clearly visible in their data (preprint here):

The symmetry of the Rossitered points indicates that the angular momentum vector of the planetary orbit is aligned with the spin pole of the star:

This state of affairs also holds true for the other transiting planets — HD 209458b, HD 149026b, HD 189733b — for which the effect has been measured. The observed alignments are evidence in favor of disk migration as the mechanism for producing hot Jupiters.

With its apparent magnitude of V=8.7, the HAT-P-2b parent star is roughly ten times brighter than the average planet-bearing star discovered in a wide-field transit survey. The star is bright enough, in fact, to have earned an entry in both the Henry Draper Catalog (HD 147506) and the Hipparcos Database (HIP 80076), but with its surface temperature of 6300K (F8 spectral type) it was too hot to have been a sure-fire “add” to the ongoing radial velocity surveys. Prior to this May, it had been entirely ignored in the astronomical literature (save a brief mention in this paper from 1969).

HAT-P-2’s intrisic brightness and its planet’s orbital geometry mean that in a relatively compact 34-hour observation, Spitzer can collect on the most interesting features of the orbit with high signal-to-noise. In particular, there is an excellent opportunity to measure the rate at which the day-side atmosphere heats up during the close approach to the star. The planet, in fact, presents such a remarkable situation that a block of Director’s Discretionary time was awarded so that the observations can be made during the current GO-4 cycle. They’ll be occurring soon.

Both HAT-P-2b and HD 80606 b will provide a crucial ground truth for extrasolar planetary climate simulations. Jonathan Langton’s current model, for example, predicts that that the temperatures on HAT-P-2b will range over more than 1000K. At the four times shown in the above orbital diagram, the hemisphere facing Earth is predicted to show the following appearances:

Spitzer, of course, can’t resolve the planetary disk. It measures the total amount of light coming from the planet in chosen passband. At 8-microns, the planet’s light curve should look like this:

The temperature maps only hint at the complex dynamics of the surface flow. A better indication is given by the distribution of vorticity,

which we’ll pick up in the next post…