systemic

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As the academic quarter draws to a close, it gets harder to keep up a regular posting schedule. This year, certainly, the difficulty has nothing to do with a lack of exciting developments associated with extrasolar planets.

A few unrelated items:

It appears that the HD 80606b Spitzer observations went smoothly, and that the data has been safely transmitted to Earth via NASA’s Deep Space Network. It is currently in the processing pipeline at the Spitzer Science Center. When it clears the pipeline, the analysis can start.

Back in September, I wrote a post about Bruce Gary’s Amateur Exoplanet Archive. This is a web-based repository for photometric transit observations by amateurs. With the number of known transits growing by the month, there’s a planet in transit nearly all of the time. Over 90 light curves have been submitted to the archive thus far. For transiting planets such as HD 189733b or HD 209458b, which have significant numbers of published radial velocity data, it’s very interesting to take the transit center measurements from Bruce’s archive and use them as additional orbital constraints within the console. The September post gives a tutorial on how to do this.

It really is turning out to be a banner year for extrasolar planets. As we head into December, this year is averaging more than one planet per week. The detection rate is more than double that of the previous four years.

The plot above gives a hint that Saturn-mass planets might wind up being fairly rare, as one might expect from the zeroth-order version of the core accretion theory. (For more information, this series: 1, 2, 3, 4, 5, 6, and 7 of oklo posts compares and contrasts the gravitational instability and core accretion theories for giant planet formation.)
Also, if you give talks, here’s a larger version of the above figure.

Another interesting diagram is obtained by plotting orbital period vs. year of discovery:

It’s possible that this diagram might be hinting that true Jupiter analogs are relatively rare. Could be that the disks around metal-rich stars are able to form Jovian mass planets and then migrate them in, while stars with subsolar metallicity form ice giants beyond the ice line. In this scenario, our solar system lies right on the boundary between the two outcomes.

It could also be the case that there are a whole slew of true-Jupiter analogs just on the verge of being announced. Time will tell.

And as always, it’s interesting to spend time with the correlation diagram tool over at exoplanet.eu.

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It’s sometimes a little weird to realize that my daily schedule is dictated by the orbits of alien planets. HD 80606b went through periastron passage at 07:00 UT last Tuesday, with the Spitzer Space Telescope’s rattlesnake’s eye vision trained intently upon it. Over the past few days, it’s been hurtling away from the star, gradually reducing its velocity as it climbs up the gravitational potential well of the star.

At 07:45 UT on Monday morning, HD 80606b is scheduled to go through inferior conjunction. In the 1.6% a-priori geometric chance that the orbital plane of the planet is in near-perfect alignment with the line of sight to the solar system, then it will be possible to observe the planet in transit. The 1.6% transit probability is fairly high for a planet with a period of 111 days, but much lower than the 15% probability that a secondary eclipse can be observed. If the planet is undergoing secondary eclipse, then we’ll know as soon as the Spitzer data comes in.

Back in early 2005, Transitsearch.org coordinated a campaign to check for transits of HD 80606b. At that time, there were fewer radial velocities available, and so the transit window was less well constrained. A number of observers got data, and there was no sign of transit, but the coverage was not good enough to rule out a transit. I’m thus encouraging observers to monitor HD 80606 during the next 48 hours on the off chance that it can be observed in transit. Given the small chance involved, it seems appropriate to refer to the transit probability in terms of basis points. As in, “In ’05, we got about 40 basis points. That means there’s still 120 basis points out there to collect.”

HD 80606 is a visual binary. The companion, HD 80607, provides a good comparison star in telescopes with a large enough aperture under good seeing conditions. For most observers, however, the light from the two stars is combined. A transit by HD 80606b is expected to have a depth of order 1.4%, and (if it’s a central transit) will last about 16 hours. It’s a long-shot for sure, but worthwhile and fun nonetheless.

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As I write this, it’s JD 2454425.219 (17:16 UT, Nov. 20 2007). HD 80606 b whipped through periastron a little more than 10 hours ago, and the Spitzer Space telescope is literally just finishing its 31-hour observation of the event. Next comes the downlink of the data to Earth on the Deep Space Network, and then the analysis. Definitely exciting!

The Spitzer Space Telescope is scheduled to run out of cryogen in early 2009. When the telescope heats up, we’ll lose our best platform for mid-infrared observations of hot extrasolar planets, and so there was a palpable urgency last week as everyone prepared their proposals to meet the submission deadline for Spitzer’s last general observing cycle. During the next few years, there is going to be intense development of detailed 3D radiation-hydrodynamical models for simulating the time-dependent surface flows on extrasolar planets. These models will need contact points with hard data. It’s thus vital to bank as wide a variety of observations of as wide a variety of actual planets under as wide variety of different conditions as possible. A number of fascinating exoplanet observing proposals were submitted last week by a variety of highly competent teams. I’m urging that they all be accepted!

Most of the exoplanet observations that have been done with Spitzer have focused on tidally locked transiting planets on circular orbits. HD 189733b, HD 209458b, TrES-1 and HD 149026b are the flagship examples of this class. In the past year, however, eccentric transiting planets have started turning up. Gliese 436b (e=0.15) was the first, followed by HAT-P-2b (e=0.5), and HD 17156b (e=0.67).

Drake Deming, Jonathan Langton and I decided that the most interesting proposal that we could make would be for Gliese 436 b. This is the Neptune-mass, Neptune-sized planet transiting a nearby red dwarf star. Here’s the to-scale diagram of the 2.644-day orbit:

After Gliese 436b was discovered to transit last spring, it triggered a Joe Harrington’s standing Target of Opportunity program. Both a primary and a secondary transit were observed (see this post) which confirmed the startlingly high eccentricity, and which allowed an estimate of the planet’s temperature (or, more precisely, the 8-micron brightness temperature). This turned out to be 712±36 K, which is significantly higher than the ~650 K baseline prediction.

The hotter-than-expected temperature measurement could arise from a number of different effects (or combinations of effects). By measuring the secondary eclipse, you strobe one hemisphere of the planet. If there are significant temperature variations across the surface of the planet, then a high reading might arise from chancing on the hotter side of the planet. Alternately, the effective temperature implied by measuring the energy coming out at 8-microns could be seriously skewed if the spectrum of the planet has deep absorption or emission bands at the 8-micron wavelength. Another possibility is that we’re observing tidal heating in action. Gliese 436b is being worked pretty hard in its eccentric orbit, and it should be generating quite a bit of interior luminosity as a result. If its structure is similar to Neptune, then a 712K temperature is completely understandable.

Io, of course, is subject to a similar situation. Here’s a K-band infrared photo of Io in transit in front of Jupiter:

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Gliese 436b is in pseudo-synchronous rotation, and spins on its axis every ~2.3 days. The eccentricity of the orbit leads to an 83% variation in the amount of light received from the star over a 1.3 day timescale. This leads to a complicated flow pattern on the surface.

Here’s what Jonathan Langton’s model predicts for the appearance of the hemisphere facing Earth at five successive secondary eclipses:

Globally, the hydrodynamical model produces a statistically steady-state flow pattern that is dominated by a persistent eastward equatorial jet with a zonally averaged speed of ~150 meters per second. This eastward flow in the planet’s frame produces a light curve in the lab frame that has a ~3 day periodicity. This period is significantly longer than both the planet’s orbital period and the planet’s spin period. Our Spitzer proposal is to observe a sequence of 8 secondary transits in hopes of confirming both the amplitude and the periodicity of this light curve.

It’s certainly the case that our current hydrodynamical model is not the definitive explanation of what these planets are doing. I won’t be at all surprised if the flux variation from eclipse to eclipse is more complicated than what we predict. I’m highly convinced, however, that the model is good enough to indicate that the situation on Gliese 436b will be interesting, dynamic, and complex. The actual variation in the real observations will provide an interesting and non-trivial constraint that a definitive model of the planet will need to satisfy. The observations, if approved, will thus be of great use to everyone in the business of constructing GCMs for short period planets.

Stay tuned…

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As soon as the new data sets for 55 Cancri from the Keck and Lick Observatories were made public last week, they were added to the downloadable systemic console and to the systemic backend. The newly released radial velocities can be combined with existing published data from both ELODIE and HET.

Just as we’d hoped, the systemic backend users got right down to brass tacks. As anyone who has gone up against 55 Cnc knows, it is the Gangkhar Puensum of radial velocity data sets. There are four telescopes, hundreds of velocities, a nearly twenty year baseline, and a 2.8 day inner periodicity. Keplerian models, furthermore, can’t provide fully definitive fits to the data. Planet-planet gravitational perturbations need to be taken into account to fully resolve the system.

Eugenio has specified a number of different incarnations of the data set. It’s generally thought that fits to partial data sets will be useful for building up to a final definitive fit. Here’s a snapshot of the current situation on the backend:

The “55cancriup_4datasets” aggregate contains all of the published data for all four telescopes. This is therefore the dataset that is most in need of being fully understood. The best fit so far has been provided by Mike Hall, who submitted on Nov. 9th. After I wrote to congratulate him, he replied,

Thanks Greg, […] It actually slipped into place very easily. About 13-30 minutes of adding planets and polishing with simple Keplerian, then 25 iterations overnight with Hermite 4th Order.

The problem is that it seemed like I was getting sucked into a very deep chi^2 minimum, so getting alternative fits may be tricky!

Here’s a detail from his fit which illustrates the degree of difference between the Keplerian and the full dynamical model:

and here’s a thumbnail of the inner configuration of the system. It’s basically a self-consistent version of the best 5-Keplerian fit.

Mike’s fit has a reduced chi-square of 7.72. This would require a Gaussian stellar jitter of 6.53 m/s in order to drop the reduced chi-square to unity. Yet 55 Cancri is an old, inherently quiet star, and so I think it’s possible, even likely, that there is still a considerable improvement to be had. It’s just not clear how to make the breakthrough happen.

This situation is thus what we’ve been hoping for all along with the systemic collaboration: A world-famous star, a high-quality highly complex published data set, a tough unsolved computational problem, and the promise of a fascinating dynamical insight if the problem can be solved.

I’ll end with two comments posted by the frontline crew (Eric Diaz, Mike Hall, Petej, and Chris Thiessen) that I found quite striking. These are part of a very interesting discussion that’s going on right now inside the backend.

When something is this difficult to solve using the ordinary approaches, I start to look to improbable and difficult solutions. In the case of 55C, my hunch is that it’s a system where the integration is necessary, but not sufficient to build a correct solution. I think that the parameter space of solutions is so chaotic that the L-M minimization doesn’t explore it well, or that the inclination of the system is significant enough to skew the planet-to-planet interactions in the console, or both. Trojans or horseshoe orbits would fit these conditions. Perhaps other resonant or eccentric orbits would as well.

I think the high chi square results and flat periodograms after fitting the known planets also point to a 1:1 resonant solution or significant inclination. I just don’t think there’s enough K left to fit another significant planet unless it’s highly interactive with the others.

I’m going to keep working on this system in the hopes that we can find a solution (and because it’s really, really fun), but I suspect that a satisfactory answer won’t be found without a systematic search of the parameter space including inclination.

— Chris

“Nature is not stranger than we imagine but stranger than we can imagine.” Or words to that effect, I can’t remember who said that but in all probability this system shall have more questions answered about it (or not as is often the case!) by direct imaging e.g. such as by the Terrestrial Planet Finder (TPF) mission to show what is really happening (if it is ever launched). The 55 Cancri system is listed as 63 on the top TPF 100 target stars.

In the meantime, we struggle on… I don’t think I can add anything else to what Eric and everyone else has said…

— Petej

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Here’s a development that systemic regulars will find interesting! In a press release today, came announcement of the detection of a fifth planet in the 55 Cancri system (paper here). The new planet has an Msin(i) of 0.144 Jupiter masses, a 260-day orbital period and a low eccentricity. The detection is based on a really amazing set of additions to the Lick and Keck radial velocities:

For background on the 55 Cancri system, check out this oklo.org post from December 2005.

The outer four planets in the 55 Cancri system all have fairly low eccentricities in the new five-planet model. This leads to a diminished importance for planet-planet interactions, but nevertheless, the system does require a fully integrated fit. Deviations between the Keplerian and integrated models arise primarily from the orbital precessions of planets b, c, and e that occur during the long time frame spanned by the radial velocity observations.

Eugenio has added the velocities onto a fully updated version of the downloadable systemic console. The new version of the console adds a wide variety of new features (including dynamical transit timing) that were formerly available only on the unstable distribution. Check it out, and see the latest news on the console change log and the backend discussion forum. Over the next month, we’ll be talking in detail about the new features on the updated console.

Very shortly, a new entries corresponding to the updated 55 Cancri data sets will be added to the “Real Stars” catalog on the systemic backend. I’ll then upload my baseline integrated 5-planet fit to the joint Keck-Lick data set. I’m almost certain that with some computational work, this baseline model can be improved. Such a task is not for the squeamish, however. Obtaining self-consistent 6-body models to the 55 Cancri data set is a formidable computational task for the console. There are 29 parameters to vary (if the Lick, Keck, ELODIE and HET radial velocity data sets are all included). The inner planet orbits every 2.79 days, and the data spans nearly two decades. Fortunately, Hermite integration is now available on the console. Hermite integration speeds things up by roughly a factor of ten in comparison to Runge Kutta integration.

There have been hints of the 260-day planet for a number of years now because it presents a clear peak in the residuals periodogram. After the 2004 announcement of planet “e” in its short-period 2.8 day orbit, Jack Wisdom of MIT circulated a paper that argued against the existence of planet “e”, and simultaneously argued that there was evidence for a 260-day planet in the data available at that time. More recently, a number of very nice fully self consistent fits to the available data have been submitted to the backend (by, e.g., users thiessen, EricFDiaz, and flanker). Their fits all contain both the 2.8 day and the 260-day planets, and happily, are fully consistent with the new system configuration based on the updated velocities. Congratulations, guys!

Interestingly, the best available self-consistent fits to the system indicate that planets b and c do not have any of the 3:1 resonant arguments in libration. It will be interesting to see whether this continues to be the case as the new fits roll into the systemic backend.

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The Spitzer telescope recently observed HAT-P-2b (data not yet analyzed) and the Nov. 19-20th encounter with HD 80606b is coming right up. No better time, then, to go out on a limb with our predictions of what will be seen. Our latest paper (Langton & Laughlin 2007) has been accepted by the Astrophysical Journal, and will be posted to astro-ph shortly. In the meantime, here’s a .pdf file containing the full paper. We’re happy with the way it came out, and we’re working hard to push the models to the next level.

From the conclusion:

A short-period Jovian planet on an eccentric orbit likely presents one of the Galaxy’s most thrilling sights. One can imagine, for example, how HD 86060 b appears during the interval surrounding its hair-rising encounter with its parent star. The blast of periastron heating drives global shock waves that reverberate several times around the globe. From Earth’s line of sight, the hours and days following periastron are characterized by a gradually dimming crescent of reflected starlight, accompanied by planet-wide vortical storms that fade like swirling embers as the planet recedes from the star. It’s remarkable that we now have the ability to watch this scene (albeit at one-pixel and two-frequency resolution) from a vantage several hundred light years away.

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This has not been the best month to get swamped with work and as a result essentially ignore my extrasolar planet weblog. The discoveries have been coming thick and fast, and many of them have some very interesting ramifications. So I’m going to make an effort to get back to a regular schedule of posts.

Staying up-to-the-minute on extrasolar planets can involve quite a bit of work. Fortunately, for the past year, Mike Valdez has been combing astro-ph each day as soon as the new mailings are released. He applies a strict standard of applicability to select the papers relevant to extrasolar planets, and reports the most interesting ones here on the systemic backend. I recommend this service to everyone.

HAT P-5b, WASP-3, OGLE-TR182b, WASP-4, HAT P-6b, WASP-5. Man. It seems like the transit detection rate is ramping up significantly. In all probability, the bottleneck is now the pace of RV confirmation with 8-meter class telescopes, rather than any shortage of transits themselves. It’ll be very interesting to see what the correlation diagrams and the planet catalog looks like one year from now.

Earlier this month, Ruth Murray-Clay visited UCSC, and gave an interesting talk about work that she’s been doing with Eugene Chiang on a model for the winds that flow off of hot Jupiters. Back in 2003, the Hubble Space Telescope was used to observe the HD 209458 b transit in the ultraviolet region of the spectrum surrounding the Lyman-alpha line. It turns out that the HD 209458 b transit has a depth of order 15% in Lyman alpha, indicating that a comet-like wind of hydrogen is flowing off the planet. Here’s a cartoon view:

The Murray-Clay and Chiang model assumes a steady-state flow, which allows them to adopt a time-independent treatment of the equations of hydrodynamics. It would be interesting to relax the time-independence and extend the analysis to the recently detected transits of HD 17156b. Because HD 17156b has such an eccentric orbit, any comet-like wind that it produces should be time-variable in nature. It should thus be possible to make some interesting predictions that can be tested when the community eventually regains the capability of observing transits in the ultraviolet.