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Galileo’s discovery of the four major Jovian satellites — his Medicean Stars — revealed that Jupiter is accompanied by a planetary system in miniature. In his Dialogue on the Two Chief World Systems, Galileo drew on the obvious analogy between Jupiter and its moons on the one hand and the Sun and the planets on the other as evidence in favor of the Copernican worldview.

The pattern is that when an orbit is larger, the revolution is completed in a longer period of time; and when smaller, in a shorter period. Thus Saturn, which traces a greater circle than any other planet, completes it in thirty years; Mars in two; the moon goes through its much smaller orbit in just a month; and in regard to the Medicean stars, we see no less sensibly that the one nearest Jupiter completes its revolution in a very short time (namely about forty-two hours), the next one in three and one-half days, the third one in seven days, and the most remote one in sixteen. This very harmonious pattern is not changed in the least as long as the motion of twenty four hours is attributed to the terrestrial globe (rotating on itself).

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Two decades ago, prior to the discovery of the extrasolar planets, the Galilean satellites, along with the regular satellite systems of Saturn and Uranus, constituted one of the strongest hints that extrasolar planets should exist. In each case, the total fractional mass and relative orbital scale of the satellites is quite similar, implying that a robust and generic formation process was at work. There’s a factor-of-twenty difference between the masses of Jupiter and Uranus, but the fractional mass caught up in their satellites differs by only a factor of two. The Jovian satellites add up to 0.021% of Jupiter’s mass, whereas the Uranian moons amount to 0.011% of Uranus’ mass. Similarly, the Saturnian satellite system (which is completely dominated by Titan) has a total mass amounting to 0.025% of Saturn. In all three cases, the orbital distance of the outermost large satellite is between 20 and 60 planetary radii.

Robin Canup and Bill Ward of Boulder’s Southwest Research Institute have developed a compelling formation model that naturally accounts for the similarities between the giant planet satellite systems (see here and here). In their picture, regular satellites build up from solid particles that flow into the circumplanetary disks from the surrounding solar nebula. Once a nascent moon reaches a non-trivial size, it decouples from the inward spiral of gas, and is able to rapidly accrete large quantities of solid particles. Ultimately, a satellite’s ability to grow to very large size is shuttered by Type I migration, whose timescale decreases in inverse proportion to the satellite mass. In the Canup-Ward picture, a succession of Jovian satellites form and are accreted onto the central planet when their mass exceeds ~0.02% of the planetary mass.

The flow pattern in the outer region of a protoplanet’s Roche lobe that regulates the flow of gas into the circumplanetary disk is quite complicated. Here’s an image adapted from the hydrodynamical simulations of Steve Lubow and his collaborators (paper here) that shows the streamlines in the vicinity of the forming planet’s Roche lobe:

The gravity of the Sun produces a tidal barrier which meters the flow of gas into the protoplanetary disk, and Canup and Ward compare the Jovian satellites to the buildup of mineral deposits on the interior of a pipe through which a great deal of water has flowed.

Squeezing out regular oklo posts is a bit of a challenge. I want to keep the posting schedule fairly regular in order to keep the readership up, but at the same time, its sometimes hard to keep coming up with post-worthy topics. In trolling for ideas, I often go to the Extrasolar Planets Encyclopaedia and comb through the tables, looking for patterns or analogies. A bit more than a year ago, I noticed that Gl 876 d, with its 1.92-day orbit and its 0.007% mass ratio is reminiscent of a Jovian satellite. Could it have arisen from a direct analog of the Canup-Ward formation process?

In the Gl 876 system, the middle planet c would have metered the gas flow into Gl 876’s inner circumstellar disk. A considerable amount of the inward flowing gas in the nascent Gl 876 system would have accreted onto planet c, but there was likely a stream (or streams, given the additional presence of planet b) that bypassed the planets and flowed onto the inner disk. The low density of steadily flowing gas in the inner disk would have allowed planet d to feed on the incoming solid material while staving off demise via Type I migration. The formation of d through this process would have occurred entirely within the Gl 876 snowline, and so in this picture, planet d is composed largely of iron and silicates. Figure 10 from the Lubow et al. paper gives a nice sense of how gas and small solid particles would have slipped by planet c on their way in:

Willy Kley and his collaborators have done hydrodynamical simulations which model the interaction between that the outer two Gl 876 planets and the parent gas disk. The flow pattern in the vicinity of the planets is more complicated than in the single-planet case, and streams of gas (and small particles) are able to flow into the disk region interior to the 30-day orbit of planet c. It’s not unreasonable to imagine that the combined presence of planets b and c mediated an inner circumstellar disk around GJ 876 that was reminiscent of the circumplanetary disk around a Jovian planet. Here’s an example figure from the Kley et al. paper showing the hydrodynamical flow in the vicinity of planets b and c:

It thus seems plausible that GJ 876 d could indeed owe its origin to the same process that produced the Jovian satellites. The planet d that shows up in the radial velocity data might be the largest survivor among a number of similar iron-silicate planets that formed in the gas-starved inner disk and were then lost to the star via type I migration. In keeping with the analogy to Jovian satellites, this scenario would hint at additional, somewhat smaller iron-silicate planets circling Gl 876 in orbits with periods in the 4-12 day range. Looks like more RVs are in order!

A manufacturing scheme akin to the giant planet satellite formation process is, however, not the only way to produce Gl 876 d. Doug Lin and his collaborators, for example, have suggested that GJ 876 d formed from pre-existing icy planetesimals that were herded inward during the resonant migration of the massive outer planets b and c. In their picture, Gl 876 d is made largely from water, and would thus have a larger physical radius than if it was built primarily from silicates via a Jovian satellite-like formation process. Mandell et al. outline a related mechanism by which d could have formed via resonant shepherding.

It’s a shame that d doesn’t transit.

Are there any other inner planets that might be candidates for formation via the Canup-Ward mechanism? Plausible clues would consist of a short orbital period, a ~0.01% mass ratio, and a massive outer planet in a ~10-50 day orbit. 55 Cancri e just might fit that bill…

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Mike Valdez pointed me to an interesting paper by Pasquini et al. that was posted to astro-ph today. The authors examined the frequency with which Jovian-mass planets are detected around giant stars and dwarf (that is, ordinary main sequence) stars as a function of the metallicity of the host star. Their main result is summed up in this redrawn figure:

The red histogram shows the well-known result that detectable Jovian-mass planets are preferentially found around metal-rich stars. The blue histogram shows a result that seems surprising at first glance. It indicates that for giant stars, the metallicity effect essentially goes away. The distribution in the blue histogram is not much different from the overall distribution of stellar metallicities in our local galactic neighborhood.

Pasquini et al. give several possible explanations for their result. Their favored interpretation is that the planet-metallicity correlation is due not to high intrinsic metallicity, but rather to stellar pollution. The idea is that after a planet-bearing star forms, its thin convective envelope is enriched by the accretion of heavy elements. The planet-bearing stars that have metal-rich spectra are in actuality ordinary stars sheathed in enriched envelopes. As polluted stars evolve off the main sequence, their convective envelopes grow deeper, and the apparent metallicity enhancements largely disappear.

As an inveterate adherent of the core-accretion hypothesis for the bulk of giant planet formation, my knee-jerk reaction is to be unhappy with a pollution interpretation. Disks and (by extension) stars that are metal-rich are more capable of building planetary cores while there’s still gas remaining in the protoplanetary disk. The planet-metallicity connection is thus a natural consequence of the core accretion hypothesis.

Pasquini et al. point out that the giant stars in their sample are systematically more massive than the main-sequence stars for which the planet-metallicity connection has been established. This leads them to speculate:

Since the fraction of planet-hosting giants is basically independent of metallicity, it is feasible that intermediate mass stars favor a planet formation mechanism, such as gravitational instability, which is independent of metallicity. One could speculate that such a mechanism is more efficient in more massive stars, which (likely) have more massive disks.

I don’t completely agree with this interpretation either, but I do think that the correct explanation is tied into a systematic difference in stellar mass between the giant sample and the dwarf sample. While it’s somewhat difficult to get accurate masses for giants, its reasonable to assume that the average mass of the giants in the above histogram is ~2 solar masses. If we assume that protostellar disks scale in mass with the mass of the parent star, then the average disk around a 2 solar mass star had roughly twice the surface density of solids than the average disk around a solar mass star. This is equivalent to a 0.3 dex increase in metallicity in a disk around a solar mass star, neatly explaining the magnitude of the offset between the red and the blue histograms.

The paucity of planets around high-metallicity giants probably stems in part from small number statistics and from the fact that there are very few super-metal-rich giants in the survey. Note that the histograms plot the distributions in metallicity for planet-bearing stars, and not the fraction of planet-bearing stars in a complete sample as a function of metallicity Although a detailed Monte-Carlo experiment is definitely in order, I think that Pasquini et al.’s result will end up being fully in line with the expectations of the core-accretion theory.

This argument would have had a lot more weight if I’d done a detailed Monte-Carlo analysis in advance, rather than monday-morning-armchair-quarterbacking (that is, blogging) with a smug postdiction. I think, however, that the core-accretion theory indicates that these general trends will all continue to hold true:

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Last weekend, I participated in the “Future of Intelligence in the Cosmos” workshop at NASA Ames. In an age of ultra-specialized conferences, the focus for this one bucked the trend by pulling back for the really big picture:

The Future of Intelligence in the Cosmos” is an interdisciplinary two-day workshop that seeks to elucidate potential scenarios for the evolution of intelligent civilizations in our galaxy and thus, perhaps, to find a resolution for this seeming paradox. The probability that intelligent civilizations exist has been succinctly stated by the Drake Equation. While the first few terms in the equation, such as the number of stars in the Milky Way Galaxy, the fraction of stars that have planets, and the number of planets in the habitable zone, are becoming better known, the last three terms that depict the fraction of planets that evolve intelligent life, the fraction that communicate, and the fraction of the lifetime of the Milky Way Galaxy over which they communicate, are not well known. It is these last three terms in the Drake Equation that are the focus of the workshop.

In most venues, extrasolar planets veer toward the esoteric. At this workshop, however, the galactic planetary census was perhaps the most nuts-and-bolts topic on the agenda. We know that planet formation is common in the galaxy, and its increasingly clear that the “great silence” isn’t stemming from a lack of Earth-mass worlds.

Here’s a link to a .pdf document containing the slides from my talk.

In an upcoming post, I’ll try to pull together a synopsis of what emerged from the conference. Perhaps the most startling moment for me came in Paul Davies‘ talk, when he described the extent to which the simulation argument has been developed.

When I was in graduate school, Frank Drake was a faculty member in our Department. I noticed right away that the license plate on his car read “neqlsl”. I always read this as “n equals one”, until I finally asked him which term was responsible for thwarting all the alien civilizations.

“It’s not N equals one,” he said, “it’s N equals L”.

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When I give a public lecture, I often start by trying to impart a sense of the extraordinarily rarefied character of the local galactic neighborhood. The known catalog of planet-bearing stars is akin to 200 small grains of sand dusting a volume more than 1000 kilometers on a side. It seems amazing to me that we’re able to see the stars at all with the naked eye. Even Sirius appears twelve billion times fainter than the Sun.

At the moment, the Alpha-Proxima trio is the closest group of stars to the Sun, and they are currently drawing closer still. In 27,000 years, they will pass at a minimum distance of 2.75 light years. Already, the Alpha-Proxima system is beginning to have an effect on the Oort Cloud, and as a result of the encounter, roughly half a million comets will be delivered into Earth-crossing orbits over the next several million years. This will generate something like a 10% increase in the arrival of new comets above the long-term average.

In 1999, Joan Garcia-Sanchez and collaborators filtered the known space motions of nearby stars in order to determine which systems are scheduled to make (or have already made) close encounters with the Solar System. The closest approach that they identified belongs to the currently inconspicuous red dwarf Gliese 710. In 1.4 million years, this half-solar mass star will skim by at a distance of ~1.09 light years, and will appear as bright and as red in the night sky as Betelgeuse. Like most low-mass stars in the galactic disk, Gliese 710 probably has a retinue of terrestrial planets. If the encounter were occurring now, Gliese 710 would likely have both an evocative Arabic name, as well as hundreds to thousands of high-precision radial velocity measurements.

The Gliese 710 encounter will produce a comet shower roughly six times more severe than what Alpha and Proxima will generate. It’s unlikely, however, that the increase in the number of comets will lead to an extinction-level impact. Nevertheless, the impending passage of a red dwarf at a distance of only 70,000 AU has a certain panache.

Given that encounters of the Alpha-Proxima and Gliese 710 variety are occurring on a million-year timescale, what is the most hair-raising encounter that one can one expect on a 4.5-billion year time scale? The mean encounter velocity between stars in the galactic disk is of order 40 kilometers per second, and the density of stars is ~0.1 systems per cubic parsec. Using these numbers, a simple n-sigma-v calculation yields an expected close-approach distance of only 770 AU. An encounter this close would literally thread the orbits of outer solar system bodies such as Sedna.

Imagine waking up to one of two headlines: (1) “Red Dwarf Discovered heading straight toward the Solar System at 400 meters per second!” and (2) “Red Dwarf Discovered heading straight toward the Solar System at 40 kilometers per second!”

Naively, one might expect that headline #2 bears much worse news, but surprisingly, that’s not the case. A red dwarf passing through the outer Solar System at 40 kilometers per second would barely deviate from a straight-line trajectory. Aside from any comets or Kuiper belt objects lying nearly directly in its path, it would barrel past us and produce only a minimal perturbation to the planetary orbits. Headline #2, on the other hand, could potentially be very bad news, as a close encounter with a slowly moving star can be far more damaging. The reason is that the interloping star is in the vicinity for much longer, and has time to build up a far stronger overall perturbation on solar system bodies. When the solar system was forming, the Sun very well could have belonged to an open cluster like the Orion Nebular Cluster. In a cluster environment, a close (several hundred AU) passage of a slowly moving brown dwarf or a low-mass star is a fairly common event, and indeed (as argued here by Morbidelli and Levison) the Sun may well have grabbed Sedna and a few hundred other as-yet undiscovered dwarf planets from an interloping star.

Asteroids that hit the Earth routinely kick clouds of debris into interplanetary space. Large rocks launched in this fashion can harbor hardy bacteria for nearly indefinite periods of time. The outer solar system, then, at any given moment, is often sparsely populated by viable dormant spore-forming bacteria that originated on Earth (see, e.g. here).

Odds are, that once-in-the-solar-lifetime (~770 AU) close encounter involved a red dwarf as the interloping star. A run-of-the-mill red dwarf has 0.3 solar masses and 1% of the solar luminosity. A habitable orbit around such a star lies at a distance of ~0.1 AU, and orbits at a speed of ~50 kilometers per second. This orbital velocity is quite close to the ~40 kilometer per second relative velocity that one would expect for an interloping star at our galactocentric radius. This means that the existence of a habitable terrestrial planet would have given the impinging parent red dwarf a dynamical mechanism for absconding with some of the material that belonged to our own outer solar system. Comets, rocks, and dwarf planets captured in this way would have stuck with the red dwarf, orbiting until they either collided with or were ejected by the red dwarf’s planets.

When that first SETI signal gets picked up, it’s unlikely, but not impossible that it’ll be coming from my trillionth cousin five hundred billion times removed.

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After several late nights of work, Jonathan Langton and I submitted our new paper that predicts the weather conditions on unevenly heated (read eccentric) short-period planets. We’re hoping that by observing these worlds in the infrared, we’ll be able to learn about the atmospheric dynamics that characterize all of the hot Jupiters.

One of our main results is a head-to-head comparison of the expected 8-micron light curves for the six most promising short-period eccentric planets. HAT-P-2b (in turquoise) comes up the winner in terms of observability, with HD 80606 b (in black) running second, and HD 118203 b (red) in third place:

In a concluding paragraph, we took the liberty to wax slightly-more-than-scientific enthusiastic about home-town favorite HD 80606b:

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 80606 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.

We’ll post the paper online after it makes it through the refereeing process. And stay tuned as we get HAT-P-2b and HD 80606b ready for their multi-frequency screen tests…

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Two evenings ago, Venus and the Moon hung close together in the deep blue twilight. Their alignment, along with the location of the fading sunset glow, gave a suggestion of the sweep of the ecliptic plane.

Zooming in on the pixels of the above photograph, it’s just possible to see that Venus is not a point source. The hint of a half-illuminated world indicates that the planet is now fairly near maximum elongation. In our lifetimes, I think we’ll likely see images of habitable extrasolar terrestrial planets that harbor something like this level of detail.

Venus gets a bad rap because the surface is so unpleasant. The Venera landers were built like submarines, and yet they still managed only an hour or two on the ground before expiring. The coke-bottle lens panoramas of basaltic slabs that they radioed back do little to fire the imagination.

My attitude toward Venus was transformed by David Grinspoon’s Venus Revealed, which I think is probably the best trade book ever written on planetary science. The text is filled with gems of insight. One passage that sticks is:

There is a level in the clouds (about 33 miles up), where the atmospheric pressure is about 70% of the pressure at sea level on Earth, and the temperature is a balmy 107 degrees Fahrenheit. For ballooning at this altitude on Venus, you would need only a thin, acid resistant suit, and oxygen tank and a large supply of cold lemonade. It’s cool enough for liquid water, and small amounts of it exist there (in a strong sulfuric acid solution).

By contrast there’s no place on Mars that could be explored using gear from an Army Surplus store.

Are there other similarly “visitable” environments in the Solar System? Surprisingly, the answer is yes. On Jupiter, at a level where the atmospheric pressure is ~6 times that at sea level, the temperature is chilly, and yet still comfortably above freezing. This level (at which hot tea might be preferable to lemonade) lies in the midst of the Jovian water cloud deck, and is subject to torrential downpours accompanied by lightning and thunder. If one were ballooning at this level, you would see an misty gray expanse, stabbed by lightning discharges, with the rotten-egg smell of hydrogen sulfide seeping in through your uncomfortably heavy scuba-store face mask.

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From all accounts, it looks like Jonathan Langton’s talk at last week’s AAS meeting in Honolulu went quite well. Here’s a link to a gzipped tar file of his Keynote presentation. It weighs in at 10.5 MB, and includes a number of cool animations. The following frames have been grabbed from the 2-orbit animation of HD 185269b:

We’re putting the finishing touches on a paper that we hope to submit this weekend. It shows that there’s a remarkable range in weather patterns and predicted infrared light curves among the short-period planets with non-zero eccentricity. The bottom line is that HAT-P-2b and HD 80606b are the best prospects for Spitzer observations, whereas HD 185269 b seems to produce the most complex and photogenic weather (see the three frames above).

HD 185269 b was discovered by John Johnson during the course of his radial velocity survey of slightly evolved high-mass stars. The orbital eccentricity is a modest yet still significant e=0.3, which leads to a 344% increase in the amount of energy received by the planet between apastron and periastron. This seasonal variation is strong, but not crazy enough to drive the shock waves that show up on HD 80606 b or HAT -P-2b. The combination of Coriolis deflection, periodic heating, eddy formation and Kelvin-Helmholtz instabilities on a global scale lead to a mesmerizing, endlessly evolving flow.