oklo

Last week, we had a one-day seminar on planets and planet formation at UC Santa Cruz that brought together researchers from both UCSC and NASA Ames. One of the talks was by Jonathan Fortney, who is currently a post-doctoral researcher in the Planetary Systems Branch of the Space Science Division at NASA Ames.

Fortney and his NASA Ames collaborator Mark Marley have a state-of-the-art radiative transfer code which can compute the emergent (and reflected) spectrum from a hot Jupiter. (See this recent post.) They’ve recently applied their code to compute how the intrinsic radiation from the flow pattern on the surface of the planet would look if you could resolve it with a pair of night-vision goggles. Jonathan writes:

Here’s an MPEG of the full 360 orbit of HD 209458b, in 36 10-degree increments. This is as seen from Earth. It’s the Cooper & Showman (2006) dynamical simulation, run through our radiative transfer solver.

Red is 5 microns, green is 3.3 microns, and blue is 2.2 microns. The Cooper and Showman model predicts a day side that is very similar to a blackbody, leading to a whitish appearance. On the night side, which is fairly cool, strong methane absorption knocks out the blue and green, leaving only red. I have artificially pumped up the red on the night side so that you can actually see it on the monitor. If you don’t, it’s a dark red which is hard to see compared to black–the night side has little flux compared to bright (hot) day.

Here’s a sequence of frames from the movie:

The animation draws on calculations described by Fortney, J. J., et al., 2006, “The Influence of Atmospheric Dynamics on the Infrared Spectra and Light Curves of Hot Jupiters”, which has been submitted to the Astrophysical Journal.

This is a big step forward for the “computational imaging” of extrasolar planets, and I’m really excited about the future directions that Fortney is planning to take these calculations. For starters, it will be very interesting to see the movie with the reflected light component added in. It will also be cool to place the point of view above a particular spot on the planet and animate the time-dependant flow pattern (the above movie rotates a single snapshot model of the planet, but it does not show the actual time evolution that is computed in Cooper and Showman’s hydrodynamical simulations). Animations of the time-dependant flow will start to bring exoplanets into the territory covered by the cloud-pattern movies that the Voyager and Cassini probes radioed to Earth as they flew past Jupiter. Finally, by using John Moore’s integrating sphere to produce the actual visible colors corresponding to individual computed spectra, it will also be possible to produce true visible light (rather than night-vision-goggle infrared) animations of the simulated surface of HD 209458 b and other hot Jupiters. (In particular, HD 80606!)

Regular oklo readers all know that HD 209458 b is a lot bigger than it’s supposed to be.

In a previous post, we saw that the theoretical models that provide reasonable matches for the other 9 transiting planets predict that HD 209458 b’s radius should be slightly larger than Jupiter’s radius. The observations, on the other hand, make it clear that the planet is actually has a diameter about 1.35 times larger than Jupiter. HD 209458 b is by far the best-studied exoplanet, so it’s of more than passing interest to understand why it’s so large.

There’s general agreement that HD 209458 b must be privately tapping an unusual source of internal heat. Somehow, a lot of extra energy is being generated in the planetary interior. The surplus heat allows the planet to maintain an expanded outer envelope, and hence endows the planet with a larger overall size. The big question is: what is the anomalous extra heat source? A few years ago, I was enthusiastic about the idea that there might be a second companion planet that is gravitationally perturbing HD 209458 b, forcing it to maintain a slightly eccentric orbit. This eccentricity would be continually damped as a result of tidal interactions with the parent star, which would generate a sufficient amount of interior heating. Such a state of affairs is analogous to the heating of the inner Jovian satellites. The heat generated by tidal friction lends Io its off-the-hook volcanism, and maintains Europa as the Astrobiology poster world.

Unfortunately, however, the perturbing companion model no longer seems to be a viable explanation of HD 209458 b’s large size. That is, if you use the systemic console to fit to the HD 209458 b radial velocities, you’ll find that there is very little latitude for inserting significant extra planets. Try it and see, and upload your fits to the systemic back-end.

Josh Winn (MIT) and Matthew Holman (Harvard-Smithsonian CfA) have written a paper that presents an interesting hypothesis for resolving the HD 209458 b radius dilemma. Winn and Holman propose that the planet is caught in a so-called Cassini state, which is a resonance between spin precession and orbital precession. In a future post, I’ll give a heuristic discussion of the dynamics of how this situation can arise, and how the Cassini states work, but in short, if HD 209458b is trapped in the “Cassini state 2”, then its spin axis will lie almost in the orbital plane. Like all hot Jupiters, the planet will spin once per orbit, but it will literally be lying on its side as it orbits the parent star. A synchronous planet in Cassini state 2 will experience a large amount of tidal heating, even in the complete absence of any other planets in the system.

I like the Winn-Holman hypothesis because it’s potentially testable. If the planet is in Cassini state 2, then the pattern of illumination on the surface, and hence the time-dependant global infrared signature, will be very different than if it is locked into the standard upright configuration. In the standard scenario, a hot Jupiter has a fixed substellar point on its equator that does not wander significantly as the planet executes its orbit. One hemisphere of the planet is in perpetual day, while the other hemisphere experiences an endless night. Hydrodynamic calculations by James Cho and his collaborators (link), and by Adam Showman and his students (link), suggest that hot Jupiters should have a single strong equatorial jet that advects heat from the hot dayside to the cool night side. The oklo splash image has been adapted from Cho’s calculations, and shows this jet in action (see this post for more discussion).

I’ve been advising UCSC Physics graduate student Jonathan Langton, who has recently begun a study of what the flow pattern on a hot Jupiter should look like if the planet is caught in Cassini state 2. If the planet’s rotation axis lies in the orbital plane, and if the planet spins on its axis once per orbit, then the play of light and shadow across the planetary orb has a pattern that is totally unlike our seasons here on Earth. At the north and south poles, of a spin-synchronous Cassini-state-2 planet, the parent star rises, passes directly overhead, and then sets once per orbit. At one special spot on the equator, on the other hand, the star is always visible, and additionally passes directly overhead once per orbit. At the opposite spot on the equator (which we’ll call the anti-stellar point), the star never fully rises, but rather peeks half of its diameter above opposite horizons once per orbit.

Jonathan has made two short .avi format animations that help to illustrate the situation. In the first animation, we hover above the point on the equator that receives maximum illumination. In the second animation, we hover above the point on the equator that receives the least illumination. The mythology on such a world would likely be pretty interesting.

When a spin-synchronous planet is illuminated in this bizarre manner, the flow pattern on its surface should be very different than the flow pattern that would occur if the planet is in the standard upright configuration. Jonathan has finished a preliminary set of simulations using the so-called shallow water approximation which indicate that this is indeed the case. (The shallow water approximation is a 2-dimensional method for simulating atmospheric dynamics on the surface of the planet under the assumption that the depth of the fluid is much smaller than the horizontal scales of interest. Use of this approximation doesn’t require us to assume that the red-hot Jupiter is actually covered with water!)

Here are two of Jonathan’s .avi format animations that show the (still very) preliminary results. The first animation [11 MB, modem users watch out!] shows the evolution of the temperature distribution (on the anti-stellar hemisphere) for a planet in Cassini state 2. Here’s a snapshot at a particular moment in time:

The second animation [12 MB] shows the distribution of vorticity across the planet surface. The vorticity at a particular spot in a fluid flow can be thought of as the ability of the flow to cause a tiny imaginary paddle-wheel to spin. Here’s a snapshot from the animation. The high-vorticity orange structure is a giant fiery hurricane-like storm on the surface of the planet:

I think readers sometimes wonder why oklo.org posts about extrasolar planets tend to be illustrated with seemingly random photographs from my house, my yard, and my neighborhood that bear (at best) a distant relation to the topic at hand. I’m liable to get written up on charges of pseudo-artistic pretentiousness for attempting to run the Jones Soda of Astronomy.

We’re taking this approach for several reasons. The oklo blog is designed to recruit users for the systemic collaboration, and and I’m paying the ISP out-of-pocket. An idiosyncratic format makes it easier to keep the posts flowing in the midst of the chronically never caught up academic routine of teaching, research, qualifying exams, topic defenses, homework grading, proposal writing, committee meetings, undergraduate theses, graduate advising, editing, visiting speaker hosting, etc.

Another reason is to drive home my belief that the real interest, the real fascination with extrasolar planets will ultimately lie in their tiny ephemeral details. It’s one thing to gain an abstract, theoretical understanding of the growth of planetary cores through the accretion of small bodies — it’s quite another to see a one-kilometer bolide succumb to the fiery tendrils of fluid instability as it slams into the toxic atmospheric murk of an unsettled four Earth-mass world. It’s one thing to know Neptune’s orbital eccentricity to five significant figures, it’s quite another to swoop in close to see the flow of feathered cirrus outline the turbulent core of the great dark spot.

I can’t get over the fact that I can photograph the subtly intricate details of a habitable planet available in my own backyard, and later that afternoon have them transmitted digitally across the globe. Just think, if we had an autonomous lander with a 5-megapixel camera engaged in a small-scale survey of a hectare-sized region of an Earth-mass terrestrial planet in the habitable zone of any G2V star other than the Sun, then this site would be getting a lot more than 400 visitors per day.

Finally, in resorting to the use of photographs of familiar objects to illustrate unfamiliar things, I want to underscore the urgent need for scientifically correct visualizations of extrasolar planets.

New planet-related discoveries are the subject of numerous NASA and NSF press releases and press conferences, and because these dicoveries generally report information obtained by indirect observational techniques, there’s a need for illustrations to accompany the releases “to capture the public interest”.

During the last ten years, these images conveying scientific results have generally been supplied by artist’s impressions. On occasion, some of these have veered toward the bizarre, the lurid, and the just plain wrong. Consider, for example, the above two illustrations of HD 209458 b. Both appeared in fairly recent HST press releases, and both are riddled with profound misconceptions. First look at that painting on the left. At the time when the image was most recently released, it was well-known that satellite orbits around the planet are highly dynamically unstable (e.g. Barnes and O’Brien 2002). The shadowing of the planet only makes sense if the star is much smaller than the planet and is somehow orbiting just above the planetary surface. The number of cloud bands and zones indicate that the planet is rotating with a ~12 hour period (like Jupiter) whereas in reality, the planet must be spin-synchronous with its P=3.5257 day orbit, and should probably have a single prominent equatorial jet. The panel on the right, which attempts to show that the planet is surrounded by an optically thin, yet Lyman-alpha-absorbing hydrogen cloud, is dramatically inconsistent with the laws of perspective, illumination, and radiative transfer. Literally billions of dollars are being spent on extrasolar planets. Surely, at the pinnacle of our dispatches, we can do better than this.

Genuinely realistic visualizations of extrasolar planets that incorporate all known information in a self-consistent way are going to become an oklo.org rallying cry over the coming year. We won’t be flying to Upsilon Andromedae any time soon. Radial velocities, photometric light curves, stellar and planetary models, dynamical integrators, radiative transfer routines, hydrodynamic codes, and Maya are what we have to work with. If it is the destiny of the extrasolar planets to truly inspire, then we must strive to see them as they really are.

I’ve often wondered what the dayside of a Hot Jupiter would look like (with the dark, wraparound shades on, of course).

Jonathan Fortney and his collaborators at NASA Ames Research Center have been making sophisticated calculations to determine the atmospheric structures of giant planets. As a product of this research, they can compute a prediction of what the spectrum of the light coming from a short-period giant planet should be. For example, for HD 209458 b, they get the following distribution of light coming off of the planet:

If we look at the dayside (substellar point) of the planet, the total distribution of light that we would see in this model is given by the solid black curve. This total light is a sum of the light that the planet reflects (the blue line) and the light that is actually generated by the hot planet itself (the red line). As one would expect, the visible-light dayside appearance of the planet is dominated by the reflected light. The nightside image is considerably les bright, and its somewhat battered thermal spectrum is a bit more magenta than the Oklo splash image, (which was generated with a black-body color map).

Molecular absorption due largely to water and methane selectively removes yellow and green light from the reflected optical spectrum. This suggests that the hot Jupiter daysides should have some sort of purplish appearance. But what exactly will the color look like?

John Moores, a graduate student at the University of Arizona, and his advisor, Peter Smith, the PI of the Phoenix Mars Lander have built an optical setup which can generate (in an integrating sphere) the composite color that corresponds to any pre-specified optical spectrum. [Moores’ weblog is here]. Their goal is to obtain a source that simulates the Martian lighting environment. Given the distinctly unearthly cast to the Martian illumination, it’s of interest to see whether long-duration exposure leads to adverse psychological or even physiological effects.

Moore used Fortney’s spectrum as an input to his apparatus to produce the image below. It shows the integrating sphere bathed in the resultant color. There is a small hole cut in the back of the sphere to allow access to a fiber spectrometer (visible as a dark spot to the left of center).

To the best of our knowledge, therefore, the daysides of hot Jupiters are imbued with distinctly trendy distinctly mauve-like hues.

Imagine leaving the front door open on a cold day, and having the inside of your house grow warmer as a result. Curiously, that’s exactly how self-gravitating systems such as stars, nascent giant planets, accretion disks, and globular clusters behave. Drawing energy from any of these systems causes them to heat up. The negative heat capacity of self-gravitating systems is one of the most central concepts in astrophysics.

The dynamics of the Keplerian orbit can be used to understand how this works. Imagine a particle initially on a circular orbit around a central star. The particle slams into a cloud of dust. As a result, the dust and the particle both heat up and radiate energy. The particle decreases its velocity and drops into an eccentric orbit with a smaller semi-major axis.

Here’s the key point: the smaller semi-major axis means that the average squared speed of the particle (averaged over an orbit) has increased. The fact that the particle is slow near apastron is more than compensated by the high speed near periastron. Since the particle’s kinetic temperature is proportional to its speed squared, the temperature of the system goes up. In effect, the reserve of gravitational potential energy gets double billed: once to provide the radiated energy, and a second time to increase the kinetic energy of the particle.

It’s a lot like taking a cash advance on your credit card and using half to pay late bills and the other half to buy a set of 22 inch rims for your Escalade. It’s a little sad to observe Nature operating on such a dissolute and spendthrift principle.