Archive for March, 2008

1:1 eccentric

March 30th, 2008 2 comments

Image Source.

The range of planetary orbits that are observed in the wild is quite a bit more varied than the staid e < 0.20 near-ellipses in our own solar system. For regular oklo readers, the mere mention of Gl 876, 55 Cancri, or HD 80606, is enough to bring to mind exotic worlds on exotic orbits.

Non-conventional configurations involving trojan planets have been getting some attention recently from the cognescenti. Even hipper, however, is a configuration that I’ll call the 1:1 eccentric resonance. Two planets initially have orbits with the same semi-major axis, but with very different eccentricities. Conjunctions initially occur close to the moment of apoastron and periastron for the eccentric member of the pair.

Here’s a movie (624 kB Mpeg) of two Jupiter-mass planets participating in this dynamical configuration.

At first glance, the system doesn’t look like it’ll last very long. Remarkably, however, it’s completely stable. Over the course of a 400-year cycle, the two planets trade their angular momentum deficit back and forth like a hot potato and manage to orbit endlessly without anyone getting hurt.

Here’s an animation (1 MB Mpeg) which shows a full secular cycle. The red and the blue dots show the planet positions during the two orbit crossings per orbit made by one of the planets. It’s utterly bizarre.

These animations were made several years ago by UCSC grad student Greg Novak (who’ll be getting his PhD this coming summer with a thesis on numerical simulations of galaxy formation and evolution). As soon as we can get the time, Greg and I are planning to finish up a long-dormant paper that explores the 1:1 eccentric resonance in detail. In short, these configurations might be more than just a curiosity. When planetary systems having three or more planets go unstable, two of the survivors can sometimes find themselves caught in the 1:1 eccentric resonance. The radial velocity signature of the resulting configuration is eminently detectable if the planets can be observed over a significant number of orbital periods.

Categories: detection Tags:

one seven one five six redux

March 22nd, 2008 3 comments

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Stefano, and Eugenio and I have been completely immersed in several time-critical projects during the past few months, and as a result, the frequency of posts here on has not been as high as I would like. We’re starting to see our way clear, however, and very shortly, there’ll be a number of significant developments to report. Also in the cards is a major new release of the console, and a refocus on the research being carried out on the systemic backend. In any case, sincere thanks to all the backend participants for their patience.

Oklo regulars will recall all the excitement last fall surrounding the discovery of transits by HD 17156b. The transit was first observed on September 10th by a cadre of small telescope observers, and was then confirmed 21.21 days later on October 1.

Jonathan Irwin at Harvard CfA has led the effort to analyze and publish the October 1 observations of the transit. The work recently cleared the peer-review process, and was posted on the web a few days ago. (Here’s a link to the paper on astro-ph.)

The night of October 1 was plagued by atrociously aphotometric conditions across the North American continent, and most of the observers who tried to catch the transit were clouded out. Southern California, however, had reasonably clear skies, and three confirming time series came from the Golden State. The Mount Laguna observations were taken from SDSU’s Observatory in the mountains east of San Diego, the Las Cumbres observations were made from the parking lot of the LCOGT headquarters in Santa Barbara, and participant Don Davis got his photometry from his backyard in suburban Los Angeles.

The aggregate of data from the October 1 transit allowed us to refine the orbital properties of the planet, and additional confirming observations in a paper by Gillon (of ‘436 fame) et al have given a much better characterization of the orbit.

Because of the high orbital eccentricity, the planet should have very interesting weather dynamics on its surface. Jonathan Langton’s model predicts that the planet’s 8-micron flux should peak strongly during the day or so following periastron passage as the heated hemisphere of the planet turns toward Earth.

By measuring the rise and subsequent decay of the planet’s infrared emission, it’ll be possible to get both a measure of the effective radiative time constant in the atmosphere as well as direct information regarding the planet’s rotation rate. Bryce Croll is leading a team that successfully obtained time on the Spitzer telescope to make the observations.

In another interesting development, a paper by Short et al. appeared on astro-ph last week which proposes the existence of a second planet in the HD 17156 system. The Short et al. planet has an Msin(i) of 0.06 Jupiter masses and an orbital period of 111.3 days. It’s quite similar to the slightly more eccentric (and hence dynamically unstable) version of the HD 17156 system proposed by Andy on the Systemic Backend last December, which was based on the radial velocities and transit timing then available:

The existence of a second planet in the HD 17156 system would be extremely interesting! The immediate question, however, is, how likely is it that the second planet is actually there?

To make an independent investigation, it’s straightforward to use the downloadable systemic console to fit to the available published data on HD 17156. I encourage you to fire up a console and follow along. Now that the Irwin et al. paper is on the web, we have the following transit ephemerides:

These can be added to the HD17156.tds transit timing file in the datafiles directory. The file should be edited to look like this:

When the HD17156v2TD system is opened on the console, it shows both the radial velocity and the transit timing data.

It’s quick work to dial in a one planet fit to the RV and transit timing data. I get a system with the following fit statistics:

The required jitter of 2.12 m/s indicates that a one planet fit to the data should still be perfectly adequate, since the star (which is fairly hot and massive) has an expected stellar jitter of order 3 m/s. Nevertheless, the residuals periodogram does show a distinct peak at ~110 days:

Using the 110 day frequency as a starting point, one finds that ~0.1 Mjup planets do indeed lower the chi-square. I’ve uploaded an example two planet fit to the systemic backend that harbors a second planet in a 113 day orbit and a mass of 0.13 Jupiter masses. Its periastron is aligned with that of planet b, and the RMS has dropped down to 3.08 m/s (for a self-consistent, integrated fit). The implied stellar jitter is a bargain-basement 0.59 m/s, which is almost certainly too good to be true.

When I do an F-test between my one and two planet fits, the false alarm probability for planet ccomes in at 38%. It’s thus fairly likely that the second planet is spurious, but nevertheless, it certainly could be there, and it’ll be very interesting to keep tabs on both the transit timing data and the future radial velocity observations of this very interesting system…

Categories: detection Tags:


March 11th, 2008 1 comment

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Over the past two days, I got the opportunity to fly to Hawaii to give two talks for the Keck Observatory’s Evening With Astronomers series. The talks focused on extrasolar planets (here’s a link to the slides in Quicktime format, ~40MB , along with the audio files of (1) a planetary system in a 2:1 resonance, (2) an unstable planetary system, and (3) another unstable system). Both talks were on Kona coast of the Big Island, where, behind the palm trees, Mauna Kea looms up 13,796 feet in the hazy volcanic distance.

The landscape here resembles nothing so much as a habitable, terraformed Mars. Hardened ropes of lava run down to the water’s edge:

In the pre-dawn light this morning, the air was totally silent, and it was easy to imagine that I was actually on Mars, before the water was gone, when a Northern hemispheric ocean lapped up against the lava of the lowermost slopes of Elysium Mons:

Image Source.

In the last few years the Martian landscape has become much more familiar, as the Spirit and Opportunity rovers crawl across the surface and radio home their photographs:

At Kona, looking out toward the lava fields, the view is positively Martian, with the most immediate difference being a sky that is a hazy blue-white rather than a hazy salmon-white. Here, the Ala Loa trail recedes into the jagged distance of what could easily be Mars:

On Mars, however, one generally has a fairly reasonable sense of what the 360-degree panorama will look like even if only part of the horizon is in view. On Earth, the situation can be quite different. Here’s the view that one gets simply by turning and looking in the opposite direction down the Ala Loa trail:

(On a marginally related note, our Alpha Centauri ApJ paper is starting to pick up some news coverage. Here’s a link to a story by National Geographic News.)

Categories: worlds Tags:

And four point five billion years later…

March 4th, 2008 2 comments

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The last mile of the San Lorenzo river in Santa Cruz is strongly affected by the twice-daily ebb and flow of the tides.

It’s always startling to see the tidal bore, a solitary breaking wave that runs upstream at a ~8 minute per mile pace when the tide is coming in. The San Lorenzo bore is small, usually six to nine inches high, but dramatic nonetheless. In its wake, there’s a turbulent froth of whitewater, whose eddies eventually cascade into viscous dissipation, turning the kinetic energy of organized flow into a slight heating of the water. As the Moon recedes, the Earth spins down, and the bore expends itself in a swirl of eddies.

The energy that powers the bore was all imparted during the Moon-forming impact, in which a Mars-sized object collided with Earth, leaving the planet violently shaken and stirred and spinning crazily through days that were originally just a few hours long. Now, 4.5 billion years later, the bore running up the river is a distant echo of the impact that was large enough to cause Earth to glow with the temperature of a red dwarf star.

From Robin Canup's moon-forming impact simulation

Adapted from: Source.

There’s a nice discussion of tidal bores in the 1899 popular-level book The Tides and Kindred Phenomena in the Solar System, by Sir G. H. Darwin (son of the naturalist). The book in its entirety can be downloaded from The Internet Archive.

The Moon-forming impact, which occurred somewhere between 10 and 100 million years after the collapse of the pre-solar molecular cloud core, essentially marked the end of terrestrial planet formation in our own solar system. From a dynamical standpoint, a system undergoes a lot of evolution during a time scale of 100 million orbits. By contrast, the Milky Way galaxy is only about 40 orbits old, and is still in an effectively pristine, dynamically unrelaxed configuration.

At Darwin’s time, the first photographs of spiral galaxies were appearing, and there’s a remarkably good photo of the Andromedae galaxy on page 339 of the book:

Darwin writes:

There is good reason for believing that the Nebular Hypothesis presents a true statement in outline of the origin of the solar system, and of the planetary subsystems, because photographs of nebulae have been taken recently in which we can almost see the process in action. Figure 40 is a reproduction of a remarkable photograph by Dr. Isaac Roberts of the great nebula in the constellation of Andromeda. In it we may see the lenticular nebula with its central condensation, the annulation of the outer portions, and even the condensations in the rings which will doubtless at some time form planets. This system is built on a colossal scale, compared with which our solar system is utterly insignificant. Other nebulae show the same thing, and although they are less striking we derive from them good grounds for accepting this theory of evolution as substantially true.

In 1899, the extragalactic distance scale hadn’t been established, and so Darwin thought that M31 was a lot closer than it actually is. In dynamical terms, he would have guessed that it’s many thousands of orbits old rather than only a few dozen. Nevertheless, it’s interesting to think about what will happen to an isolated spiral galaxy by the time it’s 10^18 years old…

Categories: worlds Tags: