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Jupiter’s Protective Role Questioned

How likely are we to find other planets in the universe that are as habitable as Earth? One key to the puzzle has long been thought to be the presence of Jupiter in our own Solar System. In fact, the presence of the giant planet has become a player in the so-called ‘rare Earth’ argument that sees Jupiter as just one factor that makes our Solar System unique. Put a gas giant in the proper position in any solar system and, so the argument goes, dangerous objects from the outer system will be deflected, protecting the inner planets and allowing life to flourish. The issue gets a hard look from Jonathan Horner (University of New South Wales) and Barrie Jones (The Open University, UK) in a paper delivered in Canberra in September of 2011.

Jupiter as protector has a certain appeal. Voyager, Galileo and other probes have shown us a massive planet that is otherwise cold and forbidding, but a world with enough mass to have huge effects on other objects in the Solar System. Horner and Jones perform a series of dynamical studies to see just how potent this effect is, noting that when the question was first studied, long-period comets were the objects most thought of in terms of Earth-crossing orbits. Jupiter’s effect on these seemed clear — a significant fraction of them would be ejected from the system entirely by its influence, keeping life-threatening impacts to a much smaller number.

But our picture of the Solar System has changed dramatically in the years since, and we now believe that long-period comets are only a small part of the total picture. The ‘impact flux,’ those objects hitting the Earth, also includes near-Earth asteroids and short-period comets. Near-Earth asteroids come from the inner Solar System as well as the main belt, and it may be that some are the remains of short-period comets. Given the numbers of NEAs we’re finding, some researchers suggest that they may constitute as much as 75 percent of potential impactors.

Image: NASA’s Cassini spacecraft took this true color mosaic of Jupiter while on its way to Saturn. The smallest visible features are approximately 60 kilometers (37 miles) across. Although Cassini’s camera can see more colors than humans can, Jupiter’s colors in this new view appear very close to the way the human eye would see them. Credit: NASA.

Short-period comets are likewise a danger, their orbital periods short enough that we can observe their return, with many of them having periods of around five or six years. With origins in the Centaurs (Jupiter family comets) as well as the Edgeworth/Kuiper Belt, the Jovian and Neptunian Trojans and perhaps the inner Oort Cloud, these objects are also thought to constitute a major part of the impact threat. The authors’ dynamical simulations tell an interesting tale about the role of all three impact scenarios. Long-period comets are indeed deflected by Jupiter in its present orbit, but the paper argues that they constitute only about 5 percent of the total threat. It turns out that the interactions of the other two populations present a more complicated picture:

In each case, the impact rate from such objects is markedly lower for planetary systems that include a massive Jupiter (such as our own) than for those that have a Saturn-mass (or slightly smaller) planet at the same location. However, for masses lower than ~0.15 times that of our Jupiter, the impact flux experienced by an Earth-like planet falls dramatically in both cases, such that the impact rate were no Jupiter present (or only a very low-mass planet occupied Jupiter’s orbit) would actually be lower than that for the scenarios involving our Jupiter. As such, it seems that Jupiter can easily be at least as much, if not more, of a foe than it is a friend.

So much for the protective Jupiter motif. What happens if we make Jupiter’s orbit more eccentric than it is now? The impact flux from these simulations turns out to be greater, though ‘not punishingly so,’ to use the authors’ words. Orbital eccentricity seems to be of secondary importance compared to mass in determining the impact flux in the host system. What does turn up — and this is with increased orbital inclination rather than eccentricity of the orbit — is a greater than 50 percent depletion of the asteroid belt on relatively short time-scales (107 years) for all but the least massive ‘Jupiter’ tested (0.01 MJ). Systems like this would, after that time, contain a much depleted asteroid belt, posing a correspondingly lesser threat to the inner system.

The finding is clearly stated: “The simple notion that giant planets are required to ensure a sufficiently benign impact regime for potentially habitable worlds to be truly habitable is clearly therefore not valid.” What level of impact flux is best suited for the development of life is a separate question. Here the authors have to punt, noting that planets like the Earth should form inside the so-called ‘snow line,’ where the only water present would be found trapped in hydrated silicates. These would be dry worlds that need an external source for their oceans.

If the bulk of Earth’s water was delivered from comets from the outer system, then the role of Jupiter may have been significant. But the paper notes that the hydration of the Earth probably occurred during the migration of the outer planets, when there would have been destabilization and redistribution of the Solar System’s population of small bodies as well. This paper picks up in the post-migration era and is not designed to study the hydration question.

As to life itself, the following passage is interesting:

Once Earth-like planets have been hydrated, the role of impacts will clearly shift from having import in the delivery of volatiles to otherwise dry worlds to directly affecting the course of the development of life. Since the development of life on our planet, a significant number of ‘mass extinctions’ have occurred, in which the great majority of organisms have been extinguished. Although many of these are currently believed to [have] been caused by factors other than impacts, at least a few are thought to have been at least partially the result of collisions between the Earth and small bodies. At first glance, it seems reasonable to assume that the most promising conditions for life to develop, once a host planet has received sufficient hydration, would be those featuring the lowest impact rate (i.e., those with the least massive giant planets, or no giant planets at all, or very massive giant planets). However, it could equally be argued that at least some impact flux is necessary in order to trigger occasional mass extinctions — without the mass extinction that wiped out the dinosaurs, for example, it is debatable whether we would currently be here, debating the importance of such extinctions!

We’re left with a complex picture regarding the role of giant planets, one with clear implications for exoplanet studies. Each system needs to be analyzed carefully in terms of the complicated impact scenario, and it is certainly not enough to base everything on the existence of a gas giant in a particular orbit. Given the number of interesting targets we’re likely to find with Kepler and CoRoT and later searches of more nearby stars, prioritizing systems for astrobiological investigation is going to be important. The configuration of gas giants will be one factor in making these decisions, but Horner and Jones remind us that the variables of the impact flux are wide enough to allow life-bearing planets to exist in a wide variety of solar systems.

The paper is Horner and Jones, “Quantifying Jupiter’s influence on the Earth’s impact flux: Implications for planetary habitability” (preprint). Thanks to Dave Moore and Antonio Tavani for the reference.


Comments on this entry are closed.

  • Ole Burde February 20, 2012, 16:07

    To put it short , we dont have the slightest idea if any kind of planetsize combinations might increase the probablity of finding life . The only reason to deal with it , is because it CAN be done with the existing data .
    Chemical evidence is a very different story . Once life has established itself on a planet , there is every reason to expect it to produce chemical conditions which could not be explained otherwise , and these chemical changes also has longrange influence on the devlopment of a planets general condions.. how would earth lok now at the age of 4 bill without life ? Perhabs much more like Venus , with a CO2 dominated atmosphere and a temperature high enough to speed up the loss of hydrogen to space ?
    It might be , that earth is much colder than it should have been without life .
    The James Webb telecope is sadly not build specificly to do this job , even if it has some spectroscopic capabilities . The biggest of all astronomical questions seems to have had a very low priority , when the gigantic fund-gopling project was put together . On the other hand , luck has often been better than planning .

  • andy February 20, 2012, 16:14

    Yet another “Rare Earth factor” that does not hold up under close scrutiny.

    That point about extremely high impact rates resulting in depletion of the asteroid belt is a good one. Taking this in the other direction, it could be that low impact rates may also be associated with highly-populated debris belts. For example, the news of the substantial debris disc around the old star Tau Ceti has been interpreted as indicating that any planets there are being pummeled by large numbers of impactors. On the other hand it may simply indicate that very little material is being lost from the belt over time (perhaps due to a lack of perturbing giant planets), so the impact rates may not be so extreme as may be expected.

  • Martin J Sallberg February 20, 2012, 16:39

    Impacts, while they do throw up dust that block the sun, can also stimulate hydrothermal activity. That means they create and destroy life opportunities at the same time. That may place them in the extinction category where adaptability is more important than minimalism. After all, it is absurd that minimalism-favoring disasters should have been crucial to our evolution, and there is fossil evidence that dinosaurs did not die out until about 300000 years after the K/T impact. The K/T impact was probably only one of many contributors. Is it possible that the mass of Earth has increased due to impacts delivering matter, and that large fliers in past geological eras was possible because gravity was lower? Far from all impacts need to leave a iridium signature, because many asteroids have a differentiated layer structure like planets and many more is almost certainly fragments of such. Maybe the Siberian traps actually mitigated the Perm/Trias extinction by providing a hydrothermal ecosystem basis, rather than causing it. Who is to say that the chaotic part of the Solar System`s history is over?

  • NS February 20, 2012, 18:34

    I don’t recall if it was posted here, but I once saw a comment from a chemist saying that if we do find some form of exotic life, we’ll probably be slapping our heads saying “Why didn’t we think of that?”. It may be generally true that questions like “What sort of planets can support life?” are too complicated to be addressed by anything except direct observation.

  • Abelard Lindsey February 21, 2012, 1:04

    The only “Rare Earth” factor that still holds up is plate tectonics. I think Brownlee and Ward are spot on about plate tectonics. The only question is how common are they? There is evidence that Mars had plate tectonics early in its history. If so, this means that having the big Moon making impact is not necessary for plate tectonics. However, the evidence for early Mars tectonics is not 100% conclusive.

  • Ronald February 21, 2012, 6:45

    @andy: what you say about Tau Ceti and its massive debris disc is indeed very interesting, also possibly as an indicator of the presence or absence of giant planets.
    Though recent (Kepler, HARPS) results are less conclusive about a small planet versus metallicity relationship, there seems to be a very clear relationship between the occurrence of giant planets and metallicity. Tau Ceti has very low metallicity (about 1/3 of solar). It is likely to lack giant planets, which in turn may have resulted in its present massive debris belt (lots of remaining material not swept up and/or ‘failed planet’ material like our asteroid belt).
    Whether it has small planets or not, and, if present, whether these are subject to heavy bombardment remains to be seen.

  • Ronald February 21, 2012, 6:58

    @Martin J Sallberg: “Is it possible that the mass of Earth has increased due to impacts delivering matter, and that large fliers in past geological eras was possible because gravity was lower?”
    Seems very unlikely, because the annual influx of meteorites (est. 30,000 tonnes per year) is very low in comparison with the earth mass (6 * 10^21 metric tonnes). Even if it was very significantly higher in the early days of our solar system (such as during Late Heavy Bombardment), the prime time of the dinosaurs and flying reptiles was so recent relatively, that meteorites are very unlikely to have contributed anything significant to the earth mass since then: say, in the last 100 million years, 10^8 * 3 * 10^4 = 3 * 10^12 tonnes = 0.00000005 % of the earth mass, totally insignificant.

  • Martin J Sallberg February 21, 2012, 13:54

    @Abelard Lindsey: An ecosystem without plate tectonics is capable of being sustained long-term provided that plants can extract CO2 from minerals in which it is bound. There is ideas about terraforming Mars by introducing plants genetically engineered for that, and similar ecosystems may have developed on exoplanets. As for speciation, what you lose in allopatric speciation you gain in individual variation, and sympatric speciation does not need isolation at all. One note on the former: individual variation dramatically reduces the risk of species extinction during environmental change. And even if plate tectonics was necessary (which it is not) it can easily result from impacts stimulating geological activity.

    @Ronald: You are (most likely falsely) assuming that the impact rate is invariably the same as it has been during impact-recording history. It is possible that lots and lots of heavy bombardments have happened in geological history. One possible mechanism is the shattering of a large, layer-differentiated asteroid into a shower of fragments. Layer-differentiated asteroids do not leave iridium signatures. Consider the existence of ring systems that most likely formed from shattered moons. Also, it has now been discovered that the Moon was geologically active as recently as 50 million years ago. The signs of heavy bombardment on the Moon may just be a tiny fraction of the impacts that actually happened.

  • Joy February 21, 2012, 15:01

    “andy February 20, 2012 at 16:14
    Yet another “Rare Earth factor” that does not hold up under close scrutiny.”


  • andy February 21, 2012, 15:25

    @Ronald: one of the predictions of current models of planet formation is that systems with significant debris discs are probably good places to search for terrestrial planets: the giant planet scattering events that would tend to disrupt the terrestrial planet region would also be efficient at clearing the system of debris.

    The following two papers are worth a look:

    Raymond et al. (2011) “Debris disks as signposts of terrestrial planet formation

    Raymond et al. (2012) “Debris disks as signposts of terrestrial planet formation. II Dependence of exoplanet architectures on giant planet and disk properties

  • Ole Burde February 21, 2012, 16:35

    “It may be generally true that questions like “What sort of planets can support life?” are too complicated to be addressed by anything except direct observation.”
    Perhabs , but how would you make any “direct observation” possible if you dont have any idea about what you are looking for ? the only thing that COULD be observed directly is an understandable difference between how a planet looks after eartlike life has shaped its chemistry , and how it would have been without life . Only if no such planets can be found , does it make any sense to start consentrating on exotic alternative lifechemistries

  • andy February 21, 2012, 18:24

    Regarding plate tectonics, early Mars is an interesting case. It seems on Earth that having liquid water around helps the subduction process, and it is subduction which seems to be the problematic part: there are numerous examples of rifting on other planets. Unfortunately getting hold of another terrestrial planet with liquid water to check how easily such planets end up in the plate tectonics regime is fairly tricky. Studying early Mars is probably the best we’ll be able to do for quite a while. Venus seems to have had massive amounts of resurfacing and is in any case extremely inaccessible due to hostile surface conditions, and I don’t see anyone exploring the ocean floors of the outer solar system ice moons any time soon.

  • NS February 22, 2012, 1:59

    Sorry for the belated reply, Ole Burde. I actually posted something here a while ago like what you said, suggesting that we identify a baseline of lifeless planets and use that to look for any spectral anomalies in other planets that might indicate life. It was pointed out to me that (among other things) we won’t have any way of establishing the baseline or knowing what spectra indicate life until we’ve already visited quite a number of both lifeless and life-bearing worlds.

  • Martyn Fogg February 22, 2012, 4:14

    Horner and Jones’ conclusion that planetary systems lacking any giant planets at all might be even safer than our own with respect to HZ based worlds is particularly noteworthy. Is because, whilst almost all stars probably host planets (apart from some binaries where dynamical stability would be impossible), giant planets themselves appear to be relatively rare. In most cases, giant planet cores cannot grow fast enough to undergo rapid gas accretion from the protoplanetary disk before it evaporates. As far the need for Jupiter to divert icy outer planetesimals inward to supply water to inner planets… There are other ways of doing this. Protoplanetary migration seem to have played a major role in many of the systems we observe, although it is poorly understood. This brings water into inner systems having already accreted onto a planetary embryo at a greater radial distance from the central star.

  • Ole Burde February 22, 2012, 11:45

    “we won’t have any way of establishing the baseline or knowing what spectra indicate life until we’ve already visited quite a number of both lifeless and life-bearing worlds.”
    That remains to be seen. We might not have any emediate 100% sure way of knowing in general what spectra would indicate life , but if the data gets good enough many kinds of specifik conklutions can be reached . The simplest example is if a planet will look very close to what we would expect earth to look from the same distance : as far as we know a high concentration of oxygen cannot exist on a rocky world under earthlike conditions without being continously produced by fotosyntesis . The chemical activity of fotosyntesis is in direct contradiction with the laws of Entrophy , which in any other connection states that a a system will eventually reach its lowest energy lewel . The chemical results of fotosyntesis is the fingerprint of life , the simlest and most obvious clue .
    It would be great if earthshine could be examined spectroscopicly from a serious distance , from outside the solar wind boundary layer ..

  • andy February 22, 2012, 15:05

    Out of interest, how many of the mass extinctions are thought to have been caused by asteroid impacts? The only one I’m aware of conclusive evidence for a large impact was the K-Pg extinction, and even then you had large scale volcanism in India going on at the time (the Deccan Traps), a mechanism which has been implicated in the far larger P-T extinction (the Siberian Traps).

  • Rob Henry February 22, 2012, 17:20

    Ole Burde, what about oxygen produced by photodisassociation of water and then hydrogen loss through the exosphere. Surely a safer signature is a combination of high O2 and N2 in the presence of water vapour or oceans. I realise though that N2 is hard to detect.

  • Ronald February 23, 2012, 11:22

    @andy: yes, the first paper I already knew, the second one is also fascinating, and even more so.

    Two quotes:

    “We find that systems with equal-mass giant planets undergo the most violent instabilities, and that these destroy both terrestrial planets and the outer planetesimal disks that produce debris disks. In contrast, systems with low-mass giant planets efficiently produce both terrestrial planets and debris disks”
    “Our main result is a prediction that debris disks should be anti-correlated with systems containing eccentric giant planets and correlated with the presence of terrestrial planets. Solar-type stars with bright cold debris disks and no giant planets are excellent candidates to search for Earth-like planets. In contrast, systems without debris disks and with eccentric giant planets are probably not good candidates for terrestrial planets.”

    So, giant planets, and in particular very large and/or eccentric ones tend to both suck up debris disks and terrestrial planets alike.

    And since the occurrence of giant planets is positively correlated with (high) metallicity, very high metallicity systems might appear to be poor in terrestrial planets and vice versa, terrestrial planets may be correlated with intermediate metallicity.

  • ljk February 23, 2012, 15:19


    Geological cycle causes biodiversity booms and busts every 60 million years, research suggests

    Posted On: February 22, 2012 – 4:00pm

    A mysterious cycle of booms and busts in marine biodiversity over the past 500 million years could be tied to a periodic uplifting of the world’s continents, scientists report in the March issue of The Journal of Geology.

    The researchers discovered periodic increases in the amount of the isotope strontium-87 found in marine fossils. The timing of these increases corresponds to previously discovered low points in marine biodiversity that occur in the fossil record roughly every 60 million years.

    Adrian Melott, a Professor of Physics and Astronomy at the University of Kansas and the study’s lead author, thinks these periodic extinctions and the increased amounts Sr-87 are linked.

    “Strontium-87 is produced by radioactive decay of another element, rubidium, which is common in igneous rocks in continental crust,” Melott said. “So, when a lot of this type of rock erodes, a lot more Sr-87 is dumped into the ocean, and its fraction rises compared with another strontium isotope, Sr-86.”

    An uplifting of the continents, Melott explains, is the most likely explanation for this type of massive erosion event.

    “Continental uplift increases erosion in several ways,” he said. “First, it pushes the continental basement rocks containing rubidium up to where they are exposed to erosive forces. Uplift also creates highlands and mountains where glaciers and freeze-thaw cycles erode rock. The steep slopes cause faster water flow in streams and sheet-wash from rains, which strips off the soil and exposes bedrock. Uplift also elevates the deeper-seated igneous rocks where the Sr-87 is sequestered, permitting it to be exposed, eroded, and put into the ocean.”

    The massive continental uplift suggested by the strontium data would also reduce sea depth along the continental shelf where most sea animals live.

    That loss of habitat due to shallow water, Melott and collaborators say, could be the reason for the periodic mass extinctions and periodic decline in diversity found in the marine fossil record.

    “What we’re seeing could be evidence of a ‘pulse of the earth’ phenomenon,” Melott said. “There are some theoretical works which suggest that convection of mantle plumes, rather like a lava lamp, should be coordinated in periodic waves.”

    The result of this convection deep inside the earth could be a rhythmic throbbing—almost like a cartoon thumb smacked with a hammer—that pushes the continents up and down.

    Melott’s data suggest that such pulses likely affected the North American continent. The same phenomenon may have affected other continents as well, but more research would be needed to show that, he says.

    Source: University of Chicago Press Journals

  • Ole Burde February 23, 2012, 15:55

    Rob Henry
    You are right , oxygen alone might be misleading . In the solar system there is quite a variety of planets , but only Earth has quantities of free oxygen . This is already an indication in itself , but in order to build up a case for the existense of life on a given planet , a profile of conditions have to identified . Perhabs as you mentioned the presence of N2 would be an critical part of the profile. The incredible sucses of the Kepler telescope sugest that the cheapest and fastest way forward could be to build a telescope specifikly made to this purpose , to concentrate on a limited sample of known exoplanets in the habitable zone ( as soon as kepler gets to finish its work) , and study their atmospheres intensively . In this connection the larger ” super earths” might be the most promissing .
    So why is this not happening ? Probably all good and serious people are waiting for the remaining Kepler results before deciding what the next step should be . Maybe not so smart a strategy , because in the meantime other projects like the James Webb telecope are gobling up all the money …

  • Eniac February 26, 2012, 13:13

    The two best indicators for carbon based photosynthesis would be lack of CO2 and presence of oxygen. Together, I believe they would be as close to incontrovertible evidence as you could get without going there. CO2 is ubiquitous if not fixated by a biosphere, and oxygen quickly reacts with the crust unless continuously replenished by water splitting. Photodissociation is much too weak to produce an effect comparable to that of photosynthesis, and it would not remove the carbon.

    You could imagine silicon based photosynthesis, in which silicon oxide from the crust would be reduced for biomass and free oxygen released. Atmospheres with that going on would have both oxygen and CO2.

  • Ole Burde February 26, 2012, 16:27

    Lots of oxygen and not too much CO2 , agreed . The problem may be in the ablity to detect the relative concentrations and in understanding the general nature of the planet . What we really need to know more about is the degree of detectability of a wide range of parameters which might be linked together to make a”profile” of a planet . An example could be the lack of Carbon in general in a starsystem , due to a combination of its age and position in the galaxy . If this could be corelated to a COMPLETE lack of CO2 in the atmosphere, and if the Planet is relatively smal , then it could be a case of a very old planet which had started out with a great quantity of water , and lost most of the Hydrogen to space over a long period . A big enough quantity of Oxygen would eventually have saturated the environment and produced an oxygen atmosphere. To build up such a profile ,it would help alot to be capable of detecting even an earthlike CO2 concentration .

  • ljk April 4, 2012, 23:48

    We can survive killer asteroids — but it won’t be easy

    Should an extinction-level asteroid begin plummeting toward Earth, it will take more than that Aerosmith song they play at proms to save the planet. Allow astronomer Neil deGrasse Tyson to elaborate on humanity’s options for survival.

    The chances that your tombstone will read “Killed by Asteroid” are about the same as they’d be for “Killed in Airplane Crash.”

    Solar System debris rains down on Earth in vast quantities — more than a hundred tons of it a day. Most of it vaporizes in our atmosphere, leaving stunning trails of light we call shooting stars. More hazardous are the billions, likely trillions, of leftover rocks — comets and asteroids — that wander interplanetary space in search of targets.

    Most asteroids are made of rock. The rest are metal, mostly iron. Some are rubble piles — gravitationally bound collections of bits and pieces. Most live between the orbits of Mars and Jupiter and will never come near Earth.

    But some do. Some will. More than a thousand known asteroids are classed as “potentially hazardous,” based on size and trajectory. Currently, it looks doable to develop an early-warning and defense system that could protect the human species from impactors larger than a kilometer wide. Smaller ones, which reflect much less light and are therefore much harder to detect at great distances, carry enough energy to incinerate entire nations, but they don’t put the human species at risk of extinction.

    Every few decades, on average, house-sized impactors collide with Earth. Typically they explode in the atmosphere, leaving no trace of a crater. Once in about a hundred million years, though, Earth is visited by an impactor capable of annihilating all life-forms bigger than a carry-on suitcase.

    Full article here:


  • ljk September 11, 2012, 9:36