Q: What’s Jupiter’s Q?

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With the flood of detail from extrasolar planets, one can forget that our knowledge of the worlds in our own solar system is literally centuries ahead of what we know about planets orbiting other stars. For example, careful naked-eye observations can be used to derive better orbital models for Venus et al. than we currently possess for any exoplanet (assuming, of course, that one owns a good watch and eyesight sufficient to resolve the disk of Venus when it transits the Sun). One of the best ways to learn about what’s out there is to learn as much as we can about what’s right here.

In this vein, an important paper came out in Nature last summer, in which Lainey et al describe a direct and unprecedentedly accurate measurement of the present value of Jupiter’s tidal quality factor, Q. The tidal quality factor encapsulates the ability of an object to dissipate disturbances raised by tidal gravity. The lower the Q, the more capable is the body at damping out the perturbations generated by tidal forcing. Q can depend quite sensitively on the frequency at which perturbations occur, and with a few notable exceptions (for example, the Earth and the Moon), it is notoriously tricky to determine. Previous estimates for Jupiter’s Q ranged from Q~60,000 to over a million. By extension, Q values for Jupiter-mass extrasolar planets are often assumed to lie in this range.

In order to directly measure the Jovian Q, Lainey et al. adopted a procedure that’s conceptually very similar to what goes on inside the systemic console. They first collected measurements of the positions of the galilean satellites that were obtained from 1891 all the way through 2007. They then constructed an orbital N-body model that includes the full gravitational forces acting on Jupiter and the galilean satellites, and which incorporates the non-axisymmetric gravitational pulls exerted by the tidal bulges of Jupiter and Io. The fitted parameters — that is, the initial conditions and undetermined constants — for their model are the osculating orbital elements of the moons, and the values of Q/k2 for Jupiter and Io. (The Love number, k2, is a measure of the degree of central concentration of a body, and has a value of k2~0.37 for Jupiter. For more, see these posts, one, two, from last summer).
Lainey et al. varied the parameters and repeatedly carried out new integrations until the the agreement between where the integrated orbital model said the moons should be located and where they were actually observed was optimized. For this type of direct integrations, goodness-of-fit is highly sensitive to the amount of tidal dissipation in Io and in Jupiter — the larger the dissipation, the larger the effect on the orbit. As a consequence, when a best-fit orbital model is attained, one has direct estimates for the Q‘s of both Jupiter and Io.

And the result? The integrations suggest that the current value of Jupiter’s Q is of order 30,000. This suggests that Jupiter is much more dissipative than has been assumed, and is indeed quite comparable to Neptune or Uranus in terms of its ability to damp out tidal disturbances. The measured Q is low enough, in fact, to suggest that Jupiter currently lies in a state where the tidal forcing by Io is leading to a historically large rate of dissipation. Over the past several billion years, as the orbital frequencies of Io, Europa and Ganymede evolved through a range of values, Jupiter’s Q was on average likely quite a bit higher than it is now.

Jupiter’s low Q hints that the transiting Neptune-mass planet Gliese 436b is even more mysterious than previously though. Gliese 436b has a significantly eccentric orbit whose non-circular figure can only be understood if (1) there’s a suitably influential perturber in the system, or (2) there was a relatively recent disaster, or (3) if the planetary Q has somehow stayed anomalously high through billions of years of orbital evolution. No matter which one of these possibilities turns out to be correct, it’ll be a very interesting story.

the last first look

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As is usually the case, there’s been little or no shortage of interesting developments in the field of extrasolar planets. The biggest recent news has been the announcement at the Barcelona conference of a definitive mass for the ultra-short period transiting planet CoRoT-7b. It weighs in at a mere 4.8 Earth Masses (copy of the Queloz et al. preprint here).

Recall that CoRoT-7b caused quite a stir earlier this year with its weird properties. The planet’s year is a fleeting twenty hours and twenty nine minutes, and it induces a tiny transit depth of 0.03%. Unfortunately, the parent star presents a less-than-ideal target for high-precision radial velocity work. It has spots that come and go, and its stellar activity produces frustratingly noisy Doppler measurements. As a result, at the time of CoRoT-7b’s initial announcement, there was no definitive measurement of the planet’s mass.

That’s changed, however, with an unprecedentedly all-out deployment of the HARPS spectrograph. From the Queloz et al. preprint:

A total of 106 measurements between 30 and 60 minute exposure time each were obtained over 4 months, and with sometimes 3 measurements being taken on the same night.

Now in my notoriously biased opinion, such observational enthusiasm is perhaps best reserved for stars such as Alpha Cen B, but a fair argument can be made that the massive investment of time did pay off. Remarkably, the radial velocity data set shows that there are two short-period planets in the CoRoT-7 system. The outer companion, which doesn’t transit, has a period of 3.7 days and at least eight Earth masses. Most dramatically, by combining the mass and radius measurements of CoRoT-7b, one arrives at a density of 5.5 grams per cubic centimeter, essentially identical to that of the Earth, suggesting that the planet is largely composed of refractory materials. (I hesitate to apply the term “rocky” to the CoRoT-7c landscape for the same reason that I’d refrain from describing the Amazon Delta as “icy”.)

In a very real sense, the HARPS campaign on CoRoT-7b has given us our last first look at a fundamentally new category of planet — that is, a world lying in the factor-of-fourteen mass gap spanned by Earth and Uranus. And, from exo-political point of view, the stakes surrounding this discovery were very high. The first density measurement of a planet in this category could just as easily have been made by teams combining high-precision Doppler measurements with either (1) Warm Spitzer, (2) ground-based photometry, (3) Kepler, (4) MOST, (5) HST, or (6) CoRoT. So I can imagine that there was a certain impetus underlying the scheduling of that huge block of HARPS time.

The discovery could, however, still be waiting to be made. Despite all the effort with HARPs, there remains a hefty 70% error on the density determination. This means that there’s a ~16% chance that CoRoT-7b is actually less dense than Neptune.

I’ll go out on a limb: CoRoT-7b’s density will turn out to be anomalously high. More than 90% of “super Earths” will turn out to be “sub-Neptunes” as far as their density is concerned.

campaign mode

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Full-resolution Poster-sized .pdf of the above.

The next HD 80606 transit is coming up this week. While the sky position of the star will be much more favorable during the coming January event, observers across the US have an opportunity to get photometric measurements of the ingress early Thursday morning.

The transit begins just after 11 AM UT on Sept. 24, and will unfold over the next 12 hours, meaning that observers in Japan and East Asia will be able to catch the egress.

Josh Winn of MIT is organizing a repeat of the successful June campaign (detailed in this post). If you’re a capable photometric observer, and if you’re interested in participating in the campaign, definitely get in touch with him.

intellects vast and cool and unsympathetic

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I was jazzed to learn that the recent spate of saucer-themed Google “doodles” and coded messages were a tip of the hat to H. G. Wells. From the Google blog:

Now, we’re finally acknowledging the reason for the doodles with an official nod to Herbert George, who would be 143 years old today.

Inspiration for innovation in technology and design can come from lots of places; we wanted to celebrate H.G. Wells as an author who encouraged fantastical thinking about what is possible, on this planet and beyond. And maybe have some fun while we were doing it.

I’ve always thought that it would be almost impossible to improve upon the first paragraph of War of The Worlds:

No one would have believed in the last years of the nineteenth century that this world was being watched keenly and closely by intelligences greater than man’s and yet as mortal as his own; that as men busied themselves about their various concerns they were scrutinised and studied, perhaps almost as narrowly as a man with a microscope might scrutinise the transient creatures that swarm and multiply in a drop of water. With infinite complacency men went to and fro over this globe about their little affairs, serene in their assurance of their empire over matter. It is possible that the infusoria under the microscope do the same. No one gave a thought to the older worlds of space as sources of human danger, or thought of them only to dismiss the idea of life upon them as impossible or improbable. It is curious to recall some of the mental habits of those departed days. At most terrestrial men fancied there might be other men upon Mars, perhaps inferior to themselves and ready to welcome a missionary enterprise. Yet across the gulf of space, minds that are to our minds as ours are to those of the beasts that perish, intellects vast and cool and unsympathetic, regarded this earth with envious eyes, and slowly and surely drew their plans against us. And early in the twentieth century came the great disillusionment.

Rings on the TV

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Image: JPL/Cassini

Just a heads-up for those of you who haven’t yet firmed up your television viewing schedules for tomorrow night.

I’ll be appearing in a episode devoted to astrophysical disks (that is, rings) that’s set to air Tuesday night on the History Channel’s Universe series. Time is 9PM/8C. (Not sure when it goes down on the West Coast, “check your local listings”.)

The show delves into the ubiquity of disk-like structures in astrophysics, covering the range of scales from the band of our geosynchronous satellites to the rings of the Jovian planets all the way up to quasars and disk galaxies.

The swarm of satellites and space debris, including the ring of geosynchronous satellites (Source).

To create a visual analogy for Saturn’s rings, we visited a Pizza My Heart in Santa Cruz where they still hand-throw the pizza dough. I lecture about how the elastic forces in the spinning dough play a role similar to a the gravity of the central planet in providing inward centripetal acceleration. All the while, they’re throwing the dough in the background.

Throwing pizza dough to emulate an astrophysical disk.

Later, they got dramatic close-up footage of the spinning disks. There were were several moments when the spinning dough was severed azimuthally, causing the outer edge of the dough to go flying off at a tangent, narrowly missing camera and crew. I ad libbed that this is similar to what would happen with the ring particles if Saturn’s gravity could somehow be cut off.

Tune in to see whether it all bakes up as a credible piece of science popularization…

Ups and Downs and Ups And

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Visitors to oklo.org may have noticed that the site was down for most of Sunday. I’d been neglecting to update my WordPress installation, which lead to a problem with the database, and a huge load spike for the server. Everything seems stable now, and I’m now flossin’ 2.8.4 inch rims.

In the relatively near future, I will be modernizing some aspects of the look and feel of the site, which will make it more discussion-friendly, and more smoothly slotted into the hum of the outside world. No need to worry, though. We’ll continue to roll ad-free.

I’ve updated the second systemic console tutorial which guides the user through the remarkable Upsilon Andromedae radial velocity data set. The back-end database is getting closer to its relaunch, and the systemic console (version 1.0.97) is freely available for download here.

Read on to work through the tutorial.

Continue reading →

Hot enough for ya?

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A recent article in Nature reports that WASP-18b has emerged victorious in the ongoing exoplanetary limbo competition.

WASP-18b is also a strong contender in the least-habitable-planet-yet-detected competition. It has a mass roughly ten times Jupiter’s and skims 2.6 stellar radii above the surface of the parent star. The orbital period is a mere 22 hours 36 minutes. A year in less than a day.

To the offhand glance, even the simple presence of the planet seems puzzling. It’s so close to its parent star that tidal orbital decay should haul it in for destruction on a timescale that’s alarmingly short in comparison to the ~1 billion year age of the parent star. Either WASP-18b has been found on the very cusp of its dénouement (which seems unlikely) or tidal dissipation in the parent star is much lower than in a star like the Sun.

Darin Ragozzine pointed me to to a recent article by Barker and Ogilvie that indicates that WASP-18 may indeed be very poor at dissipating tidal energy. It’s an F-type star, somewhat more massive than the sun, with a negligible convective envelope, and no good recourse to turning tidal waves into heat. It’s like a bell that can ring and ring without making a sound. According to Barker and Ogilvie, similarly inviscid F-type parent stars are also responsible for the survival of WASP-12 and OGLE-TR-56b. Their prediction for WASP-18b would be that changes in the orbital period will not be observable, even with the excellent precision that will be obtained by timing the orbit over periods of a decade or more.

Darin also pointed out something else that’s pretty cool. As is also the case with HD 209458b and HD 189733b, the transit of WASP-18b is readily visible in the archived photometry from the Hipparcos mission. Indeed, the planet has been sitting in open view on the web for well over a decade, assuming, of course, that one knew exactly where to look. To see it with 20-20 hindsight, use the folding applet provided at the Hipparcos web site. Enter the Hipparcos catalog number (7562) for the parent star, and fold the 130 published photometric measurements at the 0.94145299 day orbital period. Can you see the transit?

On worlds like WASP-18b, surface temperatures are well in excess of 2000 K. Under such conditions, the ionization fraction is high enough that the planetary magnetic field can affect the weather.

On Earth, where air is composed of neutral atoms and molecules, the wind blows right through magnetic field lines. By contrast, on WASP-18b, the ionization fraction is high enough that the winds will have a tendency to drag the planetary magnetic field lines along. This stretches the field lines, and like rubber bands, they offer a restoring force. Whereas ordinary exoplanetary weather can be described using the equations of hydrodynamics, on an ultra-hot Jupiter, the richer behavior of magnetohydrodynamics comes into play. As a consequence, I have little intuitive sense of what’s going on at the sub-stellar point of WASP-18b, but I’ve got little doubt that it’s interesting and complicated.

Lobbying for Alpha Cen

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Philippe Thebault sent me a link to an article on the Alpha Centauri planet search published earlier this month in the Frankfurter Allgemeine Zeitung. The text is in German, but the Google translator does a passable job of getting the gist across.

I got my first inkling of the Geneva Planet Search’s Alpha Centauri campaign through Lee Billings’ article in Seed Magazine. (See this post). In the Frankfurter Allgemeine article, Francesco Pepe gives further details — Alpha Cen B is one out of ten stars that are receiving special scrutiny for terrestrial planets at HARPS. They are getting one observation every two weeks, meaning that the star is being hit roughly one out of every two of their planet search nights:

“Allerdings müssen wir uns Harps mit anderen Gruppen teilen”, sagt er. Zudem ist Alpha Centauri B nur einer von zehn Sternen, die sie auf erdähnliche Planeten absuchen wollen. “Aber alle zwei Wochen schauen wir damit auf Alpha Centauri, und das Gerät ist sehr effizient.”

This quote implies that my speculations regarding the Geneva team’s data collection rate on Alpha Cen B were somewhat overheated. Instead of getting 100 ultra-high-precision HARPS velocities per year, it looks like a more realistic estimate of their current rate is 25 velocities per year. Since signal-to-noise increases as the root of the number of observations, this means that the minimum mass threshold for Alpha Cen Bb at any given time is approximately doubled relative to my estimates at the beginning of the Summer. Instead of arriving at 2.5 Earth masses in the habitable zone a bit more than a year from now, they’ll be at roughly 5 Earth masses.

Now nobody likes backseat drivers. As the saying goes, “theorists know the way, but they can’t drive”, and theorists have had a particularly dismal record in predicting nearly everything exoplanetary.

But nevertheless, I’m urging a factor-of-four increase to that data rate on Alpha Cen B. I would advocate two fully p-mode averaged velocities per night, 50 nights per year. I know that because Alpha Cen B is so bright, the duty cycle isn’t great. I know that there are a whole panoply of other interesting systems calling for time. It is indeed a gamble, but from the big-picture point of view, there’s a hugely nonlinear payoff in finding a potentially habitable planet around Alpha Centauri in comparison to any other star.

During the next few months, it’s inevitable that one of the numerous Super-Earths that have been turning up in the radial velocity surveys will be announced to be observable in transit (see, e.g. this post). When that occurs, we’ll effectively have had our last first look at a truly new category of planet — the logarithmic mass interval between Earth an Uranus is currently by far the largest among the 70-odd planets that have accurately determined radii. My own guess is that the emerging population of super-Earths will be better described as a population of sub-Neptunes. That is, the transit depths will indicate compositions that are largely water.

So if 5-Earth mass planets turn out to be primarily water-based rather than rock-based, it’s (in my mind) an argument in favor of cranking up the data rate on Alpha Cen B. There were no structurally substantial quantities of water in the Alpha Cen planet-forming environment. If we’re seeing sub-Neptunes rather than super-Earths in the HD 40307, Gliese 581, et al. systems, then the odds are heightened that any planets orbiting Alpha Cen B are less than 2 Earth masses. There’s no payoff in tuning your Alpha Cen B strategy for sub-Neptunes. Finding truly terrestrial-mass planets will require paying full freight.

In the early nineteenth century, the detection of stellar parallax was a problem fully equivalent in both scientific excitement and prestige to the modern-day detection of the first potentially habitable extrasolar planet. I think it’s worth noting that the prize of discovery of the first stellar parallax went not to the eminently capable (but overly cautious and slow-moving) observer who accumulated data on the best star in the sky, but rather to an observer who focused on a rather obscure star in the constellation Cygnus.

Here’s a link to the article, “Thomas Henderson and Alpha Centauri” by Brian Warner of the University of Cape Town.

Keep hope alive

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At first glance, through a telescope, Venus looks like it just might be habitable. Earth-like mass. Earth-like size. Close to the Sun, yes, but the white clouds reflect most of the incident sunlight.

A lifetime ago, it was perfectly reasonable to imagine that swampy Devonian-era conditions prevail on Venus. In his remarkable book, Venus Revealed, David Grinspoon recounts an expert opinion voiced by the Nobel-prize winning chemist Svante Arrhenius in 1918:

The humidity is probably about six times the average of that on Earth. We must conclude that everything on Venus is dripping wet. The vegetative processes are greatly accelerated by the high temperature, therefore, the lifetime of organisms is probably short.

There’s definite allure to the watery Venus meme. C.S. Lewis does an interesting treatment in Perelandra. I’ve always liked Ray Bradbury’s vision of Venus in The Long Rain:

The rain continued. It was a hard rain, a perpetual rain, a sweating and steaming rain; it was a mizzle, a downpour, a fountain, a whipping at the eyes, an undertow at the ankles; it was a rain to drown all rains and the memory of rains. It came by the pound and the ton, it hacked at the jungle and cut the trees like scissors and shaved the grass and tunneled the soil and molted the bushes. It shrank men’s hands into the hands of wrinkled apes; it rained a solid glassy rain, and it never stopped.

Frustratingly, just as the prospect of interplanetary travel was evolving into a concrete engineering problem, Venus’ spoilsport nature was revealed. In the late 1950s, Venus was observed to be glowing brightly in the microwave region of the spectrum (see, e.g. this article). The immediate — and ultimately correct — interpretation is that the microwaves are the long-wavelength tail of blackbody emission from a lead-melting surface, but at that time, the situation was not entirely clear. Even as the first astronauts were orbiting the Earth, one could optimistically chalk up the Venusian microwaves to phenomena in its ionosphere. (See, for example, this 1963 review). The space race, the cold war, the whole twentieth century would have unfolded very differently had Venus been Earth-like beneath its inscrutable clouds.

August 27, 1962 — Launch of an Atlas Agena B with Mariner 2: Destination Venus.

The microwave radiometer on Mariner 2 brought a quick end to fading hopes of a habitable Venus. Here’s the link to the baleful 1964 summary of the mission results. With the equally bleak assessment of Mars courtesy of Mariner 4, genuinely habitable extraterrestrial worlds in the solar system were a no-go. The space race fizzled out. Now we’re looking at retro-futuristic voyages to the Moon in the 2020s and dreaming of Alpha Centauri.

Speaking of which, two recent theoretical papers have come down on the pro-planet side of the ongoing terrestrial-planets-orbiting-Alpha-Centauri debate. In an article that’ll be on astro-ph within the next day or so, Payne, Wyatt and Thebault suggest that outward migration of planetary embryos in the Alpha Cen B protoplanetary disk can provide a mechanism for circumventing the problems associated with habitable planet formation in the binary environment. In the second paper (posted to astro-ph earlier this year) Xie and Zhou argue that a modest inclination between Alpha Cen A’s proptoplanetary disk and Alpha Cen B’s orbit can also tip the balance quite significantly in favor of terrestrial planet accretion around A (and with similar logic applying to planet formation around B).

Last November, in the comments section to the Alpha Cen Bb post, I was asked:

What do you think the odds now are of there being a planet somewhere in the Alpha Centauri system?

I answered:

Hazarding a guess, I’d say 60%. A better answer might be, “High enough to warrant mounting an inexpensive (in comparison to most other planet-search efforts in operation or contemplation) ground-based search.”

I’d like to raise those odds to 68.3%.

Retrograde

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Turn your world upside-down and you’re looking at a very different planet. Antarctica, ringed by the vast exapse of the Southern Ocean, draws all the attention. Viewed from beneath, I think Earth might better resemble the habitable planets that are out there in the local galactic neighborhood, waiting to be found.

Speaking of upside-down planets, last week brought a curious back-to-back development. Three separate papers (one, two, three), posted to astro-ph on two successive days, presented strong Rossiter-McLaughlin-based evidence that both WASP-17b and HAT-P-7b are on severely misaligned, potentially retrograde orbits around their parent stars. Winn et al.’s data for HAT-P-7 are a near-exact inversion of the familiar sawtooth produced by well-behaved hot Jupiters such as HD 209458b or HD 189733b. It would appear that Dr. Kozai exerted a heavy hand during HAT-P-7b’s early days:

The HAT-P-7 system is alarmingly compact. The star is roughly 80% larger than the Sun, and the orbit of the transiting planet is only about four times larger than the star itself. It looks, in fact, when drawn to scale and tilted to the proper inclination, like a schematic cartoon of a transiting system.

Remarkably, HAT-P-7 lies in the Kepler field, and was the subject of a teaser-like “brevia” published in Science a few weeks ago. In the folded Kepler light curve for HAT-P-7b it’s easy to see the phase function of the orbiting planet, along with the primary transit and the secondary eclipse. The well-resolved depth of the secondary eclipse indicates that the spacecraft is performing up to spec and will be able to detect the transits of Earth-sized planets orbiting Sun-sized stars.

Interestingly, a near-perfectly inverted Rossiter-McLaughlin waveform doesn’t necessarily mean that the planetary orbit is retrograde, but rather only that the angle between the planet’s orbital angular momentum vector and the sky-projected spin axis of the star is close to 180 degrees. If the star’s polar axis is pointing nearly in our direction, then the planetary orbit is close to polar. The small vsin(i) for HAT-P-7 provides a piece of evidence that HAT-P-7b’s orbit might in fact be close to polar.