Hot, helpful Jupiters may spawn Poseidon planets

When astronomers started discovering so-called “hot Jupiters” in extrasolar systems, many thought the jig was up for life-bearing worlds there. Many of the 280+ star systems so far discovered have either hot Jupiters or hot Neptunes… gas giants or ice giants that have migrated since planetary formation into the interiors of their systems. It’s likely that random orbital interactions just after planet formation cause outer giants, in some cases, to spiral inward toward their stars and take up permanent residence at a close orbit… sometimes so close the gas giants almost graze their sun’s surface.

Considering how the inward migration of such giants would likely wreak havoc among inner terrestrial worlds, it was assumed generally among planetary scientists that such systems would be barren of life. As an object the size of Jupiter or Saturn moves inward, spiraling in response to some original perturbation in the outer system, it would tend to dislodge small, inner planets and send them hurtling into the void… or into their sun. Unless the Jupiter-in-motion carried its own icy moons, which could melt to form oceans if their giant settled into a stable orbit in its star’s habitable zone (HZ), life wouldn’t have much of a future in systems with a hot Jupiter.

Or maybe it would. Planetquest recently posted an article about the work of Sean Raymond, at the University of Colorado, who models the formation of planetary systems containing hot Jupiters. Using computer models that simulate the accretion of planets during solar nebula collapse, Raymond has spent some time exploring what happens when gas giants form in a system and then careen inward to stable orbits before their solar system matures. His work casts doubt on the prevailing wisdom that hot-Jupiter systems are by definition devoid of life.

According to Raymond’s work, when hot Jupiters swing inwards toward their star, some of the orbital disturbance resulting from their big move causes icy and rocky asteroid material to swarm inward, too. Pulled in thrall to the wandering giant, gigatons of ice and rock swish closer to their star and begin to congeal into a “second generation” of inner, rocky planets. With the rogue giant safely ensconced in a close orbit, its gravity is not longer a threat to planet formation further out, allowing Earths and Super-Earths to accrue within the HZ. According to Raymond’s calculations, such next-gen planets would be likely to have many times Earth’s complement of water and volatiles… meaning they would be ocean worlds. Raymond’s work is still in progress, and I look forward to seeing the published results of his work. If his calculations hold up to scrutiny, there may be far more potential for life out there than previously assumed… and Earths may not be so rare after all.

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~ by Planetologist on April 7, 2009.

12 Responses to “Hot, helpful Jupiters may spawn Poseidon planets”

  1. Could storms or waves cause mixing of the surface and deeper waters? I’d imagine you’d get some pretty vicious weather systems on a waterworld.

    • Tropospheric storms would stir the lower atmosphere, that’s true. Depending on pressure and density values, it might be possible to have sufficient wind force to drag surface waters around, and that might cause some vertical mixing. Although a very deep ocean, deep as in 100s of kilometers deep, might not undergo much vertical mixing no matter how powerful the surface winds, if the ocean were strongly stratified, thermally. I can’t give a prediction either way, without first doing some fairly intense calculations…. which might make an interesting project, actually. 🙂

      • Another interesting project might be modeling the early formation of such a planet. I’m speaking specifically of the period when the surface is well above the triple point of water, so that there wouldn’t be an actual meniscus, instead the density would simply increase as the depth did. It seems likely to me that there would be liquid precipitation higher in the atmosphere, driving a meteorology similar to Earth’s (or perhaps Saturn’s). However, with no meniscus, the strophic effects (geo- and cyclo-) would presumably have continued right down to the surface.

        Another interesting point is rotation. One of the papers you linked to for me on another thread refered to Why Venus has No Moon (by Alemi, Alex; Stevenson, D.), whose abstract said, among other things, “The accepted explanation for Earth’s moon is a giant impact with an impactor on the order of one Mars mass. Given current theories of solar system formation, it is unlikely that Venus would have avoided such a large collision.” If this is so, it seems likely that at least a portion of the water worlds under discussion would also have a moon.

        This suggests two things: based on Earth’s history, the initial rotation would have been very fast, and the moon would have created very strong tides. Along with much greater heat from early radioactive decay, the heat generated from those tides would have been added to whatever stellar radiation energy reached the surface to drive the meteorology. The faster rotation would have created a much more powerful geostrophic (planetostrophic?) effect, helping to isolate the cold tops of atmospheric upwellings from areas of downwelling. The latter could well have been quite warm (especially in the early phases), helping to cool the atmosphere.

        As I understand it, wind in very dense atmospheres will tend to be comparatively slow (relative to ours) but apply much greater force.

        All of these together could have contributed to erosion and mobility of lithic elements into the atmosphere, available for later solution.

        At some point during the cooling, the earliest oceans would have developed, at which point only part of the surface would have been inundated. The rest would have been subject to erosion aided by solid elements carried by the atmosphere. Again, much of the results of this erosion might have been available for later solution after the planet had cooled to the point that it had world-ocean and an atmosphere like Earth’s.

      • Well, it’s actually very likely that Venus was hit by something big, at some point. Venus has a retrograde rotation, compared with the other planets in our system… meaning it had a gravitational close encounter with something massive enough to reverse its direction of spin. A Mars-sized object might have done that, but frankly I’m not up on the peer-reviewed geophysics literature addressing this question. In any event, although it’s very likely Venus was smashed a few times in its early history, our Moon is the result of a particular impact event where the angle and velocity of impact produced a ring-system of material that could aggregate into a single massive object in a stable orbit around Earth. If the conditions were different, we’d have ended up with no Moon, or a smaller one, or an unstable orbit that later smashed into Earth, etc. Venus would have had to undergo almost an identical collision, with all the variables lining up exactly, to get our result. The odds of two such identical events happening to neighboring planets are very low.

        I need to correct one or two things you said about tides… specifically, a moon’s rotational spin has no effect on tides. Our Moon causes tides on Earth because of its gravity, which is unaffected by the Moon’s rotation speed. Our Moon was once closer to Earth, just after it formed, and back then we probably had slightly higher tides than now, but not catastrophically higher…. probably at most around twice the tidal bulge height Earth experiences from the Moon now, more or less. Tides at that scale wouldn’t create measurable amounts of heat in Earth’s oceans, crust or mantle.

        In regard to a giant water planet, wind wouldn’t contribute to erosion in any way if there isn’t any land. Yes, during ocean formation there’d be all sorts of things going on, but to support a biosphere over geologic time there would need to be a continuing, stable and reliable mechanism for delivering lithophile elements from the crust/mantle to the ocean surface.

      • Sorry for the late reply, I’m still not used to how this software places replies, and only looked right under my other post.

        [… O]ur Moon is the result of a particular impact event where the angle and velocity of impact produced a ring-system of material that could aggregate into a single massive object in a stable orbit around Earth. If the conditions were different, we’d have ended up with no Moon, or a smaller one, or an unstable orbit that later smashed into Earth, etc. Venus would have had to undergo almost an identical collision, with all the variables lining up exactly, to get our result. The odds of two such identical events happening to neighboring planets are very low.

        I haven’t actually read the Alemi and Stevenson paper, but the abstract says (among other things):

        Simulations suggest that most large collisions create a disk from which a moon forms. Moreover, the natural outcome is one where the sense of orbital motion and planetary spin are the same, leading to outward tidal evolution. Despite the smaller sphere of influence of Venus relative to Earth, and the larger solar tidal influence, only very large moons or very dissipative tides allow such a moon to escape.

        Absent a perusal of the peer-reviewed literature, I would assume no consensus otherwise (ie that the Earth’s condition was unique or close to it). If I had access to the literature (other than abstracts, when I can find the papers in the first place) I would do it myself. As it is, I don’t see how we can estimate the probability that an arbitrary water world might have undergone a moon-creating impact.

        A more general question comes to mind: what are the chances that some sort of “fly-by” by another star or large interstellar body messed with the orbits (of any arbitrary planetary system)? Have the probabilities and outcomes of this sort of event even been modeled? As an explanation for a specific anomaly, it can’t be ruled out no matter how improbable (even if there are more plausible alternatives), but when it comes to star systems in general, I wonder how often we can expect this sort of thing to happen.

        [A] moon’s rotational spin has no effect on tides. Our Moon causes tides on Earth because of its gravity, which is unaffected by the Moon’s rotation speed. Our Moon was once closer to Earth, just after it formed, and back then we probably had slightly higher tides than now, but not catastrophically higher…. probably at most around twice the tidal bulge height Earth experiences from the Moon now, more or less.

        I was actually talking about the Earth’s rotation, which would have been faster that early. After all, it is the Earth’s angular momentum and (part of the) rotational energy that drive the Moon’s orbital increase. IIRC estimates of the Earth’s early day are around 10 hours. As for the tidal bulge, AFAIK it would depend on the plasticity of the mantle (and perhaps core), which might have been higher with the greater heat flow through the surface which implies higher mantle temperature (especially immediately after the Great Collision, when the surface may have been molten for a while).

        Tides at that scale wouldn’t create measurable amounts of heat in Earth’s oceans, crust or mantle.

        I wonder if this has been modeled (with respect to the mantle, I agree regarding the crust and ocean), or is everybody just going on intuition? (Please don’t go to any trouble to find ref’s if it has, just knowing would be enough for me.)

      • Well, it’s actually pretty likely that some form of gravitational disturbance engendered the solar system’s formation to begin with. A passing star or other form of gravitational wake would have been responsible for destabilizing the local interstellar medium (ISM), causing local rotational collapse which resulted in our Solar System in probably a number of other sibling systems around the same time. One option is a supernova nearby, and there’s actually some evidence for that model: the presence of Mg-26 in meteoric material requires that Al-26 had to be there initially… Mg-26 in the ISM can plausibly only come from radioactive decay of Al-26, which has a very short half-life (~716,000 years). For any Al-26 to have been present when our Solar System formed, a nearby supernova would have needed to go off and seed our primordial nebula.

        As far as the Moon goes, long odds are just a natural outcome of having to get so many variables to fit. I couldn’t tell you what precisely the odds would be of another Moon-forming event at Venus, Mars or Mercury (the only other terrestrial worlds we have), without some pretty intense calculations, but qualitatively there are a wide range of possible outcomes when any two bodies interact. Only a few of those outcomes will result in a stable large moon.

        I’m not sure about whether the Hadean Earth would have been spinning fast enough to cause any appreciable interior heating, but I doubt it, frankly. I’m going off of intuition on that one, but also there’s been a lot of work on heat sources inside the primitive Earth, and that potential source never comes up in the conversation. That question may have been dealt with at some point, or not. Incidentally, Al-26 probably conferred quite a bit of heat to the accreting Earth (and Mars, Venus, Mercury, etc.) just after formation. Al-26 is intensely radioactive, and generates a lot of heat as it falls apart (as does any highly radioactive isotope, actually). In any event, there’s an active literature on tidal heating of planetary bodies… mostly inspired by the modern discoveries of tidal-heat generated liquid water inside (at least) Europa, Titan and Enceladus.

      • Yet another mistake: In my previous post I said “when the surface is well above the triple point of water“. What I meant was “when the surface is well above the Critical point of water“.

        I really apologize for the careless errors. (I’m running under time constraints and keep trying to fit too much in.)

  2. This is really great stuff.

  3. Much of this has already been published, e.g. here, here, here, which I found through a quick search on adsabs. One question would be what the mineral concentrations in the oceans of such waterworlds would be like without continental runoff.

    • Well, that’s the big question, although the problem wouldn’t be with marine salt concentrations. Earth’s salinity mostly comes from basaltic oceanic crust, and halogen emissions (e.g. HCl, HBr, HI) from volcanoes. Fresh basalt contributes alkali and alkaline-earth elements to ocean water from weathering and exchange reactions with sea water, while marine sulfate mostly comes from volcanic SO2 emissions. Continental weathering contributes a bit, but most of the “salt” in the oceans comes from volcanoes and weathering ocean crust.

      The real issue would be with nutrient cycling, specifically delivering phosphorus (P), potassium (K) and metallic micronutrients (Ni, Cr, Cu, Mo, etc.) to the marine photic zone. Phosphorus especially is a major issue, because it’s a primary limiting nutrient in most ecosystems today. Continental weathering sends a lot of P to the oceans in river runoff. Upwelling zones along the continental margin help to recycle P from deep water to the surface… and sustain most of the ocean’s primary productivity in the process. Without a source of P to the photic zone the marine biosphere would wind down, eventually dying back to the only available sources of P, like seafloor hydrothermal vents and hotspots on the ocean floor.

      On a wet super-Earth or an oceanic Neptune-sized world, there would have to be some means of recycling deep water to the surface on an ongoing basis, or else any sun-based biospheres would be stillborn. The problem is that a solar-heated ocean without any continents to bounce currents off of would probably become thermally stratified… warm water on top, cold water below… with no convection. Thermohaline top-bottom currents might work, if the salinity, gravity and ocean depth variables converged properly. But otherwise, in a deep ocean without continents there aren’t many ways of getting all the essential nutrients of life into the near-surface environment. Ironically, the biggest, wettest worlds are probably also the worst suited for big biospheres.

      Now, if photic-zone life on such worlds could build root systems that reach down through 50 – 100 km of clear water, to the rocky seafloor below…

      • The real issue would be with nutrient cycling, specifically delivering phosphorus (P), potassium (K) and metallic micronutrients (Ni, Cr, Cu, Mo, etc.) to the marine photic zone. Phosphorus especially is a major issue, because it’s a primary limiting nutrient in most ecosystems today.

        How sure can we be that life that originated in another world would need these elements?

        As for the micronutrients, it’s my impression that their inclusion in specific enzyme systems is almost accidental; IIRC cobalt is required for the standard process of nitrogen fixation, but there’s an “older” process that does not require it. Perhaps if cobalt weren’t available, another process that didn’t require it would have arisen that was as efficient as the standard process. A similar thing may be true of the other micronutrients.

        As for potassium, the differential diffusion with sodium across the cell wall is essential in a lot of processes on Earth, but perhaps if it weren’t available other processes would have evolved that didn’t need it.

        Phosphorus is the most critical element, especially its place in Earth life. However, it may not be necessary for the informational process (nucleic acids with an amino acid backbone have been proposed). In fact, I would guess that the greatest value of the phosphodiester bond (as well as linking diphosphate bonds as in CoA and NAD) is the ease with which they can be broken when necessary. Since the amide group (peptide) bond is also easily broken, it may be that life could get by using it everywhere and thus not need phosphorus.

      • It’s certainly possible that alien life might use a different suite of essential nutrients than conventional life…. a classical example would be life that uses arsenic (As) instead of P. Also, particular metallic micronutrients that are used for some purposes on Earth might easily be used for other purposes elsewhere, as metal cofactors in completely different protein structures. But the problem with these “Poseidon” planets would be more basic – getting any lithophile elements into the photic zone. Elements that are sparingly soluble in water, and which are mostly found in silicate and oxide minerals, are not going to have an easy time getting into the bulk water column on a world without landmasses. Volcanism on the sea bed would help, but in a stratified water column lithophile element circulation would still be limited, probably.

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