≡ Menu

Is Kepler 452b a Rocky Planet or Not?

Where is the dividing line between a large, rocky planet and a ‘mini-Neptune?’ It’s a critical issue, because life is at least possible on one, unlikely on the other. But while we’re getting better at figuring out planetary habitable zones, the question of how large a planet can be and remain ‘terrestrial’ is still unresolved. As Andrew LePage explains below, our view of potentially habitable planets like Kepler-452b depends upon how we analyze this matter — clearly, just being in or near the habitable zone isn’t enough. A prolific essayist with over 100 articles in venues like Scientific American and Sky & Telescope, LePage writes the excellent Drew ex Machina site, where his scrutiny of recent exoplanet finds is intense. The work seems a natural fit given his day job at Visidyne, Inc. near Boston, where he specializes in the processing and analysis of remote sensing data.

by Andrew LePage

Andrew_LePage_2014_small

A couple of weeks ago, the media was filled with reports about the discovery of Kepler 452b. While NASA’s Kepler mission had found a number of potentially habitable planets earlier, all of these previous discoveries orbited dim K and M-dwarf stars which are very different than our Sun and present a number of still unresolved issues affecting habitability (see A Review of the Best Habitable Planet Candidates in Centauri Dreams for a full review of earlier finds). What made this new Kepler find unique was that Kepler 452b was a nearly-Earth-sized planet orbiting inside the HZ of a Sun-like star – the first of potentially many more such exoplanets to come from the continuing analysis of Kepler’s data set. But being a bit of a skeptic when it comes to often overhyped media reports about the potential habitability of any newly discovered exoplanet, I wanted to dig deeper into this claim.

Is It in the Habitable Zone?

According to the discovery paper submitted for publication in The Astronomical Journal with Jon Jenkins (NASA Ames Research Center) as the lead author, Kepler 452 is a G2 type star like the Sun with a surface temperature of 5757±85 K, a mass of 1.04±0.05 times that of the Sun and a radius of 1.11 +0.15/-0.09 times the Sun’s. Based on these data, it can be calculated that Kepler 452 has a luminosity about 20% greater than that of the Sun making this it a slightly heavier and brighter version of the Sun. Comparison of the known properties of this star with standard models of stellar evolution yields an age of 6±2 billion years or about 1½ billion years older than the Sun and its system of planets. Compared to the stars earlier announced with potentially habitable exoplanets, Kepler 452 was certainly quite Sun-like.

While a full assessment of the habitability of any exoplanet would require very detailed information about all of its properties, obtaining such information is simply beyond the reach of our current technology. At this early stage in our search for other Earth-like worlds, the best we can do is compare what properties we can derive to our current expectations of the range of properties for habitable worlds to determine if a new find is potentially habitable. One of those important set of properties is the orbit of an exoplanet. According to Jenkins et al., Kepler 452b is in a 384.84-day orbit with an average orbital radius of Kepler 452b is 1.046 +0.019/-0.015 AU. This orbital radius is far outside that where a planet would become tidally locked and be affected by severe stellar flare activity – two unresolved issues that call into question the potential habitability of worlds tightly orbiting much dimmer stars like those found to date.

This orbital radius combined with the stellar properties yields an effective stellar flux for Kepler 452b that is 1.10 +0.29/-0.22 times that the Earth receives from the Sun. This effective stellar flux places Kepler 452b just inside the conservative HZ of a Sun-like star as defined by the runaway greenhouse limit. Given the current uncertainties in the properties of Kepler 452b and the star it orbits, Jenkins et al. calculate that there is only a 28.0% probability that Kepler 452b actually orbits inside of the conservatively defined HZ but there is a 96.8% chance that it orbits inside a more optimistic definition of the HZ corresponding to early conditions on Venus. However, it appears that Jenkins et al. used a definition of the HZ limits for an Earth mass planet. If Kepler 452b has a mass closer to five times that of the Earth (or 5 ME), which is likely to be the case, the effective stellar flux for the inner edge of the HZ increases from 1.10 to 1.18 times that of the Earth raising the chances that Kepler 452b orbits inside of the HZ to probably better than even odds. And since Kepler 452, like all stars, would have been dimmer in its youth, Kepler 452b would have been even more comfortably inside the HZ for billions of years. Considering all these facts combined with the limitations of current models in defining the true inner boundary of the HZ, this is close enough even for this skeptic to consider Kepler 452b as potentially habitable at least in terms of its orbit and effective stellar flux.

Is It a Rocky Planet?

The other important planetary property we can measure using current detection techniques is the size of a planet. Unfortunately, it is here where many past discoveries have run into trouble. It has been suspected for some time now that somewhere between the size of the Earth (or 1 RE) and Neptune with a radius of 4 RE, planets transition from being predominantly rocky with some chance of being habitable like the Earth to being rich in volatiles such as water, hydrogen and helium becoming mini-Neptunes with little chance of being habitable in the conventional sense. Based on recent analyses of Kepler data on the radius of exoplanets smaller than Neptune combined with independently derived masses from radial velocity measurements and other techniques, we now know that this transition from predominantly rocky worlds to predominantly volatile-rich worlds occurs somewhere around 1½ to 2 RE although the precise value and nature of this transition is uncertain due to the small number of planets with measured radii and precisely determined masses in this size range as well as the measurement uncertainties of those values (see The Transition from Rocky to Non-Rocky Planets in Centauri Dreams).

Kepler_452b_Earth_comparison

Image: This artist’s concept compares Earth (left) to Kepler-452b, which is about 60 percent larger in diameter. Credit: NASA/JPL-Caltech/T. Pyle

While many earlier claims of finding potentially habitable planets have run afoul of this transition and turned out to be much more likely to be mini-Neptunes than rocky terrestrial planets, in recent months astronomers have started making some effort to address this issue in discovery papers of potentially habitable planets including Jenkins et al.. Based on the analysis of the Kepler photometric data and the properties of the star, Jenkins et al. report that Kepler 452b has a radius of 1.63 +0.23/-0.20 RE which is close to the transition value. Based on their calculations, Jenkins et al. claimed that there was a better than 50% chance that Kepler 452b is a rocky planet. But how did they arrive at this figure?

Jenkins et al. used two different distributions of probable radius values for Kepler 452b and compared them to two different published mass-radius relationships for sub-Neptune sized planets to calculate the odds that their find has a density consistent with a predominantly rocky composition. The radius value distributions were derived from the measured 1.63 +0.23/-0.20 RE radius of Kepler 452 combined with two different models used to determine the host star’s properties. The first model, SPC (Spectral Parameter Classification), determines the star’s parameters by comparing its spectrum to a collection of synthetically generated stellar spectra to find the best fit. The second model, called SpecMath, is considered more conservative and compares the star’s spectrum to a collection of 800 well-studied stellar spectra to derive the star’s properties.

To calculate the probability that Kepler 452b is a rocky planet based on those radius distributions, Jenkins et al. used two different mass-radius relationships. The first was formulated by graduate student Lauren Weiss and famed exoplanet hunter Geoff Marcy (University of California – Berkeley) which was published in March 2014. Weiss and Marcy fitted radius and mass data for 65 exoplanets to come up with a deterministic mass-radius function where a particular radius value corresponds to a single mass value. While simple, this model admittedly does not reflect the fact that exoplanets with a particular radius value can actually have a range of possible mass values reflecting a variety of bulk compositions.

The second mass-radius relationship used by Jenkins et al. was derived by Angie Wolfgang (University of California – Santa Cruz), Leslie A. Rogers (California Institute of Technology), and Eric B. Ford (Pennsylvania State University) and was submitted for publication in April of 2015. They evaluated data for 90 exoplanets using a hierarchal Bayesian technique which allowed them not only to derive the parameters for a best fit of the available data, but also to quantify the uncertainty in those parameters as well as the distribution of actual planetary mass values. Using their approach, they derived a probabilistic mass-radius relationship where the most likely mass and the distribution of likely values are determined that better reflects the uncertainties in the data and the fact that exoplanets with a particular radius value can have a range of actual masses (for a detailed discussion of this work, see A Mass-Radius Relationship for ‘Sub-Neptunes’ in Centauri Dreams).

Using the radius distributions for Kepler 452b derived from SPC and SpecMath, Jenkins et al. found that the mass-radius relationship created by Weiss and Marcy yielded 64% and 40% probabilities, respectively, that their new find has a bulk density consistent with models of rocky planets. When employing the mass-radius relationship of Wolfgang et al., they found a 49% and 62% probability, respectively, that Kepler 452b is a rocky planet. The average of these results is the origin of the quoted greater than 50% odds that the new find is a rocky planet.

Is It Really a Rocky Planet?

While this is a clever solution to a difficult problem, there are problems with this approach. First of all, while the work of Weiss and Marcy was an excellent first attempt to derive the mass-radius relationship using the newly available Kepler data set, the relationship derived by Wolfgang et al. is superior since it uses more data of higher quality that is analyzed in a mathematically more rigorous way. While Jenkins et al. recognize this and prefer the higher probabilities calculated using Wolfgang et al., they used the parameters of the mass-radius relationship derived using all 90 planets with radii up to 4 RE in the original analysis. Based on earlier work by Leslie Rogers, it was recognized that the transition from being predominantly rocky to predominantly volatile-rich takes place at radius values no greater than 1.6 RE (for a full discussion of this work, see Habitable Planet Reality Check: Terrestrial Planet Size Limit on my web site, Drew Ex Machina).

When Wolfgang et al. analyzed just the subset of exoplanets with radii less than 1.6 RE, they derived different parameters for the mass-radius relationship for these smaller planets. For a planet with a radius of 1.6 RE, for example, the most probable mass when using parameters derived from fitting all planets with radii less than 4 RE, as used by Jenkins et al., comes out to about 5 ME. If the parameters derived from just smaller planets with radii less than or equal to 1.6 RE are used, a smaller probable mass value of 4 ME is found. As a result, the probabilities derived by Jenkins et al. are biased towards higher mass outcomes with corresponding higher probabilities of finding Kepler 452b to be a rocky planet.

A better approach for determining the probability that Kepler 452b is a rocky planet would be to compare its properties directly to the population of exoplanets with known radii and accurately determined masses. Unfortunately, Rogers’ paper does not include a simple function that others can use to calculate such a probability since this was outside the scope of her work. Despite this shortcoming, the title of her paper published in March 2015 in The Astrophysical Journal really says it all: “Most 1.6 Earth-Radius Planets are not Rocky”. In other words, Kepler 452b with a radius of 1.63 RE is most likely not a rocky planet but is a mini-Neptune instead, contrary to the claims by Jenkins et al..

Other astronomers trying to calculate the odds that their finds are rocky planets or not have derived probabilities in different ways. Guillermo Torres (Harvard-Smithsonian Center for Astrophysics) on January 6, 2015 announced the discovery of eight habitable zone planets using Kepler data where they quantified the probabilities that their finds were rocky (see Habitable Planet Reality Check: 8 New Habitable Zone Planets on my web site, Drew Ex Machina). Although somewhat different from the method used by Rogers, the approach used by Torres et al. to calculate the probability that a planet with a particular radius is rocky gives qualitatively similar results. Using their model, the chances that Kepler 452b is rocky is about 45%. This is closer to the low-end 40% figure derived by Jenkins et al. than the often quoted “greater than 50%” figure.

Unfortunately, the chance that Kepler 452b is a terrestrial planet might not be as good as even 40%. Recent work by Rebekah I. Dawson, Eugene Chiang and Eve J. Lee (University of California – Berkeley) recently submitted for publication in Monthly Notices of the Royal Astronomical Society strongly suggests that planets with masses greater than about 2 ME (which would have a radius of about 1.2 RE, assuming an Earth-like bulk composition) which orbit stars with a high metallicity are more likely to be mini-Neptunes. This is because stars with higher metallicities tend to have more solid material available to form planetary embryos more quickly making it more likely for them to acquire some gas directly from the protoplanetary disk before it dissipates. Only 1% or 2% of a planet’s total mass in hydrogen and helium is sufficient to puff up its observed radius and make it a mini-Neptune. Stars with lower metallicity values tend to form planetary embryos more slowly and they might not reach the required 2 ME mass threshold fast enough to begin to acquire any more than trace amounts of gas before it has already dissipated from the protoplanetary disk. With a iron-to-hydrogen ratio about 60% higher than the Sun, Kepler 452 has a slightly higher metallicity than the Sun increasing the odds somewhat that Kepler 452b is a mini-Neptune. Taken together with Rogers work, this strongly suggests that the odds that Kepler 452b is a rocky planet are less than 50% not greater as is being claimed.

Conclusion

To be perfectly honest, quibbling over a couple of tens of percent probability one way or the other about the nature of Kepler 452b is most likely not all that important considering the uncertainties in its properties as well as the still substantial uncertainties in the mass-radius relationships available at this time. In the end, we will have to wait for a more definitive derivation of the mass-radius relationship and a more quantitative description of the nature of the transition from rocky planet to mini-Neptune to settle this question more accurately. Despite the outstanding issue of the nature of Kepler 452b, it still has very real prospects of being potentially habitable. But even if it proves not to be, future studies of its properties will provide scientists with vital information on the limits of planetary habitability.

While some might be disappointed by this less rosy assessment, it should be remembered that scientists are still actively analyzing the Kepler data set and performing follow up observations. There are already several potentially habitable Earth-size planet candidates found orbiting Sun-like stars that are being actively studied by members of the Kepler science team and their colleagues. It is only a matter of time before the discovery of true “Earth twins” is announced.

The preprint of the Kepler 452b discovery paper by Jenkins et al., “Discovery and Validation of Kepler-452b: A 1.6-RE Super Earth Exoplanet in the Habitable Zone of a G2 Star”, can be found here.

tzf_img_post

Comments on this entry are closed.

  • Brett August 7, 2015, 11:44

    There are already several potentially habitable Earth-size planet candidates found orbiting Sun-like stars that are being actively studied by members of the Kepler science team and their colleagues.

    I remember that was in the chart you provided for the 12 new planet-candidates – one of them looked a lot more promising than Kepler-452b. It’s strange how the media didn’t report much on the other candidates when Kepler-452b showed up.

    It’s sounding like the bigger “Super-Earths” would need some pretty unusual events to be rocky planets, like impacts with very massive impactors that conveniently blow off their hydrogen envelopes and so forth.

  • spaceman August 7, 2015, 12:31

    So, would it be safe to say that the majority of habitable, rocky planets are less than or equal to 1.2 earth-radii? A lot of focus is on the transition from mini-neptune type planets to rocky terrestrial type planets. What about once we are already solidly in the rocky planet regime below 1.2 earth-radii? What is the lower limit in terms of the radius of a rocky planet below which the planet would likely no longer be habitable? I remember reading somewhere that planets smaller than 0.80 the diameter of earth are unlikely to be habitable…

  • ljk August 7, 2015, 12:44

    http://arxiv.org/abs/1508.01202

    On The History and Future of Cosmic Planet Formation

    Peter Behroozi (STScI), Molly Peeples (STScI)

    (Submitted on 5 Aug 2015)

    We combine constraints on galaxy formation histories with planet formation models, yielding the Earth-like and giant planet formation histories of the Milky Way and the Universe as a whole. In the Hubble Volume (10^13 Mpc^3), we expect there to be ~10^20 Earth-like and ~10^20 giant planets; our own galaxy is expected to host ~10^9 and ~10^10 Earth-like and giant planets, respectively. Proposed metallicity thresholds for planet formation do not significantly affect these numbers.

    However, the metallicity dependence for giant planets results in later typical formation times and larger host galaxies than for Earth-like planets. The Solar System formed at the median age for existing giant planets in the Milky Way, and consistent with past estimates, formed after 80% of Earth-like planets.

    However, if existing gas within virialised dark matter haloes continues to collapse and form stars and planets, the Universe will form over 10 times more planets than currently exist. We show that this would imply at least a 92% chance that we are not the only civilisation the Universe will ever have, independent of arguments involving the Drake Equation.

    Comments: MNRAS accepted

    Subjects: Astrophysics of Galaxies (astro-ph.GA); Earth and Planetary Astrophysics (astro-ph.EP)

    Cite as: arXiv:1508.01202 [astro-ph.GA]
    (or arXiv:1508.01202v1 [astro-ph.GA] for this version)

    Submission history

    From: Peter Behroozi [view email]

    [v1] Wed, 5 Aug 2015 20:05:50 GMT (1260kb,D)

    http://arxiv.org/pdf/1508.01202v1.pdf

  • ljk August 7, 2015, 14:11
  • Michael August 7, 2015, 14:58

    I am seeing this world as a super hot Venus type world with a thick >5000 bar Hydrogen, Helium, Nitrogen, Methane, Carbon monoxide/dioxide, steam bathed world.

  • Michael Spencer August 8, 2015, 6:50

    One wonders how the (very) young earth would have been seen before the removal of the mass of the moon.

  • Andrew LePage August 8, 2015, 9:41

    @Michael Spencer August 8, 2015 at 6:50

    “One wonders how the (very) young earth would have been seen before the removal of the mass of the moon.”

    I probably would have been about 10% SMALLER in mass (or around 3% smaller in radius) since, according to the impact theory of the origin of the Moon, the Moon was created from some of the debris of the impact of the proto-Earth and a Mars-size protoplanet (most of the papers I recall reading on the topic have the majority of the debris from both bodies either falling back to Earth or forming the Moon with comparatively little escaping). While the Moon is large compared to the Earth when looking at other planetary moons, it is still only 1% of the mass of the Earth.

  • Hop David August 8, 2015, 13:04

    “Where is the dividing line between a large, rocky planet and a ‘mini-Neptune?’ It’s a critical issue, because life is at least possible on one, unlikely on the other. ”

    There may be life within gas giants.

    It’s possible a gas giant can have a layer of atmosphere with temperature and pressure comparable to earth’s surface.

    I know of two science fiction stories looking at life in such layers. One by Arthur C. Clarke and another by Robert Forward:

    https://en.wikipedia.org/wiki/A_Meeting_with_Medusa
    https://en.wikipedia.org/wiki/Saturn_Rukh

  • Geoff August 8, 2015, 19:22

    Could Kepler 452b really hold on to H/He over billions of years? With surface gravity of 2g and Earthlike temperatures, the escape rate for molecular hydrogen should be about the same as for He escaping from Earth.

  • Rob Henry August 8, 2015, 21:10

    @Hop David, I agree that life is possible suspended high in the atmosphere but if it was photosynthetically based, it would probably be above layers that vigorously mix with wind swept particulates from the surface. That would mean the system is limited by the infall of meteoric dust for its supply of transition metals used in metabolism. As such I suspect it would be a couple of orders of magnitude less powerful than Earth in its energy capture.

  • Enzo August 9, 2015, 4:33

    Hi Andrew,

    Can you explain the following statement ?
    “If Kepler 452b has a mass closer to five times that of the Earth (or 5 ME), which is likely to be the case, the effective stellar flux for the inner edge of the HZ increases from 1.10 to 1.18 times that of the Earth ”

    Why a larger mass for the planet moves the HZ inwards ?

  • andy August 9, 2015, 8:20

    A science fiction novel, no matter how well written or how famous the author, does not constitute evidence that something is possible.

    The idea of life on gas giants has several severe problems, notably the strong vertical turbulence that would fling any organisms between extremes of temperature and pressure. Even if you could find a gas giant without such turbulence, there is still the fact that in a hydrogen/helium atmosphere, the possibilities for lifting gases are very limited (plus lighter-than-atmosphere balloons do not work well at small sizes), the difficulty of concentrating the necessary chemicals within the atmosphere.

  • Andrew LePage August 9, 2015, 9:41

    @Enzo August 9, 2015 at 4:33

    “Why a larger mass for the planet moves the HZ inwards ?”

    Because a more massive planet has a compressed atmospheric column compared to a less massive planet slightly lessening the impact of a water vapor-driven greenhouse effect (with water vapor being the primary greenhouse gas near the inner limits of the HZ). More details and caveats can be found in:

    Ravi Kumar Kopparapu et al., “Habitable zones around main-sequence stars: dependence on planetary mass”, The Astrophysical Journal Letters, Vol. 787, No. 2, Article ID. L29, June 1, 2014

  • Brett Bellmore August 9, 2015, 10:34

    I find these planets being discovered academically interesting, but I’m actually more interested in the discovery of asteroids and comets around nearby stars. Let’s face it, if we do go interstellar, the trips are likely to be only to the few nearest stars, we’re not going to up and visit a star 200 light years away. And the journeys won’t begin until we’ve been colonizing smaller bodies in the Solar system for a while.

    People living in asteroid habitats, and colonizing comets, aren’t likely to switch to living on a planet when they get to another star system. Ok, maybe if it’s really suitable for terraforming; Be nice to create a fall-back ecosystem in case your smaller ones crash.

    But, for interstellar colonization, the real question is whether a star has suitable smaller bodies around it. We don’t need a rocky planet, we need asteroids and comets.

  • Rob Henry August 9, 2015, 16:27

    @Andy, bacterial sized life has a diameter sufficiently low to prevent bulk flow of air around it. That is why fine particles reaching the stratosphere are said to become suspended – but they haven’t. They will slowly diffuse to lower levels, taking a couple of years or so to fall out – contrast this to their doubling time which can be as low as 15 min. A few bacteria can be raised by electromagnet forces – or even the energy generated by seeding clouds, and so the process begins again. The true problem is the transition to larger life.

    Let us start your gas bladder process by pumping up a vacuole such that a micron sized bacteria bloats to 3 microns but with the same mass, and what do we find? This is not sufficient to float it, but it is sufficient to allow bulk flow, and the particle falls out like a stone in just a few hours. I hope this helps.

  • ijv August 9, 2015, 22:24

    What would a “transition planet” with about 1% hydrogen in its atmosphere and a radius in the range 1.2-1.6Re look like? Water world, super Venus, something totally different? With higher metallicity, would there a higher proportion of radioactives in the core potentially driving even more vigorous plate tectonics than on Earth?

    I assume that it would have a solid surface of some kind, rather than just an increasing density of gas with a gradual transition to a liquid state, is this correct?

    Also, given the estimated age of the Kepler 452 system and higher solar luminosity how well could such a planet hold onto its hydrogen over 6billion years or so?

  • kzb August 10, 2015, 7:35

    Notice that NO macroscopic Earth organism has evolved atmospheric buoyancy as a lifestyle, despite it being (on the face of it) an attractive option.

    I suppose you could argue that such organisms would be too vulnerable to predation on Earth, something that would not happen when the predators are similarly slow-moving?

    But there are also the recently-discovered cloud bacteria, which even affect our weather and climate. It seems to me that similar organisms could exist on gas giants or mini-Neptunes. If they can exist here, why not a mini-Neptune in a similar climatic zone to Earth?

  • xcalibur August 10, 2015, 8:04

    This is probably a very layman question, but bear with me: could spectroscopy be used to directly analyze the atmosphere of an exoplanet, and provide evidence as to whether it’s rocky or gaseous?

  • andy August 10, 2015, 13:29

    Sigmund Nastrazzurro has a series on lighter than air “balloon organisms” on the (unfortunately now in infrequent-update mode) Furahan Biology and Allied Matters blog. Here are the links to part I (not titled as such), II, III, IV and V. Conclusion is that gas giants are a really bad bet for these things and they don’t really seem particularly feasible in general, unless the balloon size is very large which then raises the question of how you evolve that in the first place.

    I still am sceptical about the possibilities for abiogenesis in a gas giant environment, and with a long fall into the infernal depths rather than a watery surface, I don’t see panspermia as being a particularly good option either. I suspect life on gas giants would be extremely rare.

  • Alex Tolley August 10, 2015, 14:30

    @kzb – that we don’t see floating macro-organisms on Earth, doesn’t mean that they may not have evolved elsewhere. The gas balloon would have to contain a light gas like hydrogen or methane that could be split from water, perhaps using a photosynthetic pigment. Whether this makes sense on a planet with a denser atmosphere allowing a smaller gas envelope I don’t know. Clearly organisms have evolved that can generate light gases, so it is theoretically possible, and most fishes and some molluscs do have gas bladders to be buoyant in water. Perhaps on planets with very dense atmospheres, buoyancy does evolve.

  • Rob Henry August 10, 2015, 19:39

    Okay Alex let’s do it and see how far we can go. We need a dense gas and a lifting one. Producing a lifting gas in an oxidising atmosphere is prohibitively expensive so we need a reducing one. N2 dominated Titan has the right characteristics so lets start here and just dial up gravity ten fold, but leave it 10AU out for ease calculation. That gives us a density of about 10kg per cubic metre at ground level where about 2/3rds light is already absorbed by smog. To lift to this level would take the generation of about 200 moles of H2 for each kg.
    CH4 + 2H20 –> CO2 + 4H2
    that adds to about 6,000kJ/kg or about the same energy density as hydrated starch as stored in a living body in an oxidising atmosphere.

    On a mini Neptune the atmosphere will extend much deeper than our hypothetical ‘ground level’ – also its warmer so more smog unless we dial down the CH4 content. Of cause, we need the same number of moles of H2 for lifting a kg no matter how deep we go (within sensible limits), but we will have less photosynthetic potential to do so. We could get cheaper by collecting and pumping traces of H2 directly across a membrane, but I’m unsure that is practical.

    Perhaps this is all wrong, and air NOT water is life’s medium here – in which case the cell matrix is an aerogel, and lifting involves trapping more H2 in the internal latices.

  • Rob Henry August 10, 2015, 21:46

    Andy, Sigmund Nastrazzurro work on ‘ballonts’ is interesting, but he neglects ways in which air and water are fundamentally different. In his scheme there are flying things and floating things and things in between, but he misses all others. Air can hold a much higher electrostatic charge than water, which gives another suspension mechanism that micron sized organisms can use. Yet another happens if we have a super-saturated gas with a very high latent heat such as water vapor. Quorum sensing organisms can create a permanent updraft with a vertical component of at least 1m/s (using Earth’s example) by nucleating rain drops through dimethyl sulphide release. This would create permanent weather features such as Saturn’s hexagon, or the great red spot, in which rather large fluffy or gliding organisms could evolve.

    PS. earlier I made a factor of three error due to reliance on memory, so we only need 2000 kJ/kg – for balloon animals that should be no problem at all.

  • Alex Tolley August 10, 2015, 22:12

    @ Rob Henry, I don’t see why one needs to increase the gravity to get a denser atmosphere, as Titan and Venus attest.

    Ultimately some sort of external energy will need to power the ecosystem, solar seems likely, but there may be other options.

    Creating lifting molecules chemically may be unnecessary – just pumping out heavier gases in a mixture may be sufficient, e.g retaining CO in a CO + CO2 atmosphere, or H2 in a CH4 + H2 atmosphere, or CH4 in a mixture of low carbon number organic compounds.

    On Earth, fish use buoyancy to allow rest while maintaining position in the water column, or at least above the bottom. For an atmospheric buoyant organism, I would speculate that similar factors must apply, so that dynamic lift alone has lower fitness.

    The idea that such organisms are vulnerable seems to me to be proven otherwise with Earthly organisms like jellyfish – slow moving but protected with poisonous tentacles and especially dangerous in shoals/swarms. Such organisms might either be autotrophs or feed by sweeping up small organisms in the atmosphere, or both.

  • Michael August 11, 2015, 0:54

    @Enzo August 9, 2015 at 4:33

    “Why a larger mass for the planet moves the HZ inwards ?”

    @Andrew LePage August 9, 2015 at 9:41

    ‘Because a more massive planet has a compressed atmospheric column compared to a less massive planet slightly lessening the impact of a water vapor-driven greenhouse effect (with water vapor being the primary greenhouse gas near the inner limits of the HZ)…’

    I would have thought a more massive planet would have a larger light intercept, this coupled with a denser atmosphere would drive a more powerful greenhouse effect.

  • Andrew LePage August 11, 2015, 12:24

    @Michael August 11, 2015 at 0:54

    “I would have thought a more massive planet would have a larger light intercept, this coupled with a denser atmosphere would drive a more powerful greenhouse effect.”

    No. A larger planet not only has a larger “light intercept” (whatever exactly that it is), it also has a larger surface area to heat so the two effects precisely cancel out. Also, since there has yet to be a clear connection made between a planet’s volatile content and its mass, there is no evidence to support your claim that a larger planet necessarily has a denser atmosphere. Once again, I can only suggest reading Ravi Kumar Kopparapu et al., “Habitable zones around main-sequence stars: dependence on planetary mass”, The Astrophysical Journal Letters, Vol. 787, No. 2, Article ID. L29, June 1, 2014

  • andy August 11, 2015, 13:35

    @Rob Henry: even if various mechanisms might work for micro-organisms, you still have the problem of abiogenesis: the prebiotic systems would need to be able to use these mechanisms sufficiently well to avoid being destroyed before producing more complex living systems. A gas giant would tend to lack the kind of convenient surface chemistry and chemical concentration mechanisms that a terrestrial world could offer. Even if you believe in panspermia, a gas giant is a far less promising target for incoming life-bearing meteoroids than a terrestrial. It’s not just a matter of what would work, it’s also a question of whether the intermediate steps to arrive at a working solution are feasible.

  • Andrew LePage August 11, 2015, 14:31

    @xcalibur August 10, 2015 at 8:04

    “This is probably a very layman question, but bear with me: could spectroscopy be used to directly analyze the atmosphere of an exoplanet, and provide evidence as to whether it’s rocky or gaseous?”

    While spectroscopy could help to determine the composition of an exoplanet’s atmosphere (an ability we are only just now developing), it will only provide an indirect hint about whether a planet is primarily rocky or a volatile-rich mini-Neptune. A better indicator would be a planet’s bulk density but that requires knowing a planet’s radius and mass (the latter of which is currently difficult to measure for potentially habitable Earth-size planets orbiting Sun-like stars using currently available technology).

  • Hop David August 12, 2015, 11:39

    Andy writes
    “A science fiction novel, no matter how well written or how famous the author, does not constitute evidence that something is possible.”

    Agreed. However that silly notion seems to be your invention, it didn’t come from me. I mentioned the Clarke and Forward stories as a point of interest, not offering them as evidence.

    “The idea of life on gas giants has several severe problems, notably the strong vertical turbulence that would fling any organisms between extremes of temperature and pressure. Even if you could find a gas giant without such turbulence, there is still the fact that in a hydrogen/helium atmosphere, the possibilities for lifting gases are very limited (plus lighter-than-atmosphere balloons do not work well at small sizes), the difficulty of concentrating the necessary chemicals within the atmosphere.”

    What are you basing this on? Planetary formation models? Even well crafted sims are often discredited when probes return actual data. What do we actually know about possible mini-Neptunes?

    I’ll look at a few of your arguments.

    1) Vertical turbulance would subject life to extremes of temperature and pressure and kill it.

    What do we know about convection cells on Mini-Neptunes? Looking at our own planet there are life forms that manage to stay within limited strata of the atmosphere and oceans.

    2) Organisms wouldn’t be buoyant in a hydrogen helium atmosphere.

    You assume the hospitable strata is hydrogen and helium. Yes, proto planetary disks are predominantly hydrogen. We could expect most Jupiter and Saturn sized gas giants to be mostly light gases in their upper atmosphere.

    Is the same true of all mini-Neptunes? I could imagine a scenario where a small gas giant wanders inward increasing it’s temperature. Jean’s escape mechanism would boil off hydrogen and helium leaving increased concentrations of O2, N2, H2O and CO2.

    Further, there is a wide extreme of pressures and temperatures life can thrive in. From bacteria in the upper atmosphere to smoker communities near volcanic vents on the sea floor. The smoker communities show life can exist at hundreds of degrees centrigade as well as hundreds of atmospheres.

    There are bacteria that can remain suspended in the upper atmosphere: http://www.sciencedaily.com/releases/2009/03/090318094642.htm . Since liquid water is less compressible than gas, buoyancy is even less of an issue at 100s of atmospheres.

    Fish have air bladders they use to regulate their buoyancy. What level of complexity is needed to form an air bladder? Perhaps even small bacteria could harbor bubbles. Yes, air bladders don’t help much if the atmosphere is predominantly hydrogen. But once again I question the assumption that hospitable strata within a mini-Neptune must be mostly hydrogen.

  • Michael August 12, 2015, 14:30

    @Andrew LePage August 11, 2015 at 12:24

    ‘No. A larger planet not only has a larger “light intercept” (whatever exactly that it is), it also has a larger surface area to heat so the two effects precisely cancel out.’

    I mean the atmospheres reach into space adds more collection area, it is not a massive amount but it can’t be ignored.

    ‘Also, since there has yet to be a clear connection made between a planet’s volatile content and its mass, there is no evidence to support your claim that a larger planet necessarily has a denser atmosphere.’

    “The above equation suggests that larger planets should have thicker atmospheres.”

    page 4

    http://arxiv.org/pdf/1404.5292v2.pdf

  • andy August 12, 2015, 18:17

    I’m sure that if you apply principles of intelligent design, you can come up with an organism that would work on a gas giant. Especially if you intelligently design the gas giant to be magically free of atmospheric turbulence (and bear in mind that such planets would not have topography to limit the atmospheric motion), or redefine the meaning of the term “gas giant” to include planets which do not possess significant quantities of atmospheric hydrogen. On the other hand, the universe seems to play by the rules of evolution, which forces you to consider intermediate steps to get there from prebiotic chemistry (which is itself harder to achieve on a gas giant which would lack mechanisms to achieve strong and long-lasting chemical potential gradients on small scales in a similar manner to hydrothermal systems at the water/rock interface).

  • Rob Henry August 13, 2015, 4:41

    I find it interesting that pressure differences driven by vertical turbulence have been raised as an issue. True, pressure does result in different chemical potential, but even on Earth some rather complex organisms cycle between the surface and about 2km depth – that is between 0.1-20 MPa, with few ill effects. Of cause, vertical winds promote adiabatic temperature differences with altitude and these can be problematic. IMHO it is safer to stick to that as the problem.

    As a finally thought, what about evidence from Shoemaker-Levy 9 that Jupiter is itself could be infected. Any life in the Jovian clouds would be strictly limited by meteoric infall for its mineral supply, so such an impact should have caused a massive ‘algal bloom’ on its surface. This brings us to a deep mystery – what exactly caused higher levels of light absorbing materials to linger for months around all areas where those fragments impacted?

  • Andrew LePage August 13, 2015, 10:43

    @Michael August 12, 2015 at 14:30

    “I mean the atmospheres reach into space adds more collection area, it is not a massive amount but it can’t be ignored.”

    That effect is taken into account in the modelling I am aware of and its impact is small compared to the total area of the planet (on the order of tenths of a percent or less).

    As far the correlation between a planets mass and it volatile content, Kopparapu clearly states “We should caution that volatile delivery to a planet is stochastic in nature, and may be a weak function of planetary mass. Still, this is the best assumption we can make in the absence of a rigorous theory of how planetary volatile content varies with planet mass. So, please keep this in mind when using the following limits.”

    http://www3.geosc.psu.edu/~ruk15/planets/

  • Leo Serrano August 13, 2015, 22:50

    Is it possible that the mass reading of the planet masks a potential moon?
    how good is the technology to point these sort of differences between a small neptune or a planet having an earth’s moon size or bigger?

  • Hop David August 14, 2015, 17:09

    Andy wrote “…redefine the meaning of the term “gas giant” to include planets which do not possess significant quantities of atmospheric hydrogen.”

    I had used the word “gas giant” to describe a mini-Neptune. My bad. Some are now calling Uranus and Neptune “ice giants”
    https://en.wikipedia.org/wiki/Ice_giant
    Ice giants can be 20% hydrogen with a higher proportion of ammonia, water and other heavier volatile gases.

    A mini Neptune’s smaller gravity well makes it more susceptible to loss of light gases via Jean’s escape. So probably an even higher ratio of CO2, NH3, O2, N2 and H2O than Neptune and Uranus.

    And as mentioned, lower strata can have an even lower proportion of hydrogen and helium.

    Jupiter’s tropopause acts as a barrier keeping water from migrating to the stratosphere. A CHON based organism’s buoyancy would help prevent ascent via vertical turbulence. How many birds are blown into the stratosphere via thunderheads?

    So I reject your notion that intelligent design is needed for strata that can sustain life. And I will reply you’re using lots of handwavium to argue it’s impossible.

  • Michael August 15, 2015, 2:01

    @Andrew LePage

    ‘As far the correlation between a planets mass and it volatile content, Kopparapu clearly states “We should caution that volatile delivery to a planet is stochastic in nature, and may be a weak function of planetary mass. Still, this is the best assumption we can make in the absence of a rigorous theory of how planetary volatile content varies with planet mass. So, please keep this in mind when using the following limits.”

    “We should caution that volatile delivery to a planet is stochastic in nature, and may be a weak function of planetary mass.’

    He states ‘may’, how scientific is that. Are we not assuming a lot already about masses of planets and their densities, the error bars are still way to high to determine if there is a thick atmosphere or even a thick water layer. I will believe in increasing larger planets having increasingly thicker atmospheres and the Earth having an atypical skinny atmosphere until proven otherwise.

  • andy August 15, 2015, 11:25

    @Hop David: As regards your accusations of “handwavium”, I hate to raise the old canard about extraordinary claims and extraordinary evidence, but you’re the one making the claims for life on gas giants. There is currently no evidence for such life. If anything, you are the one handwaving away the issue of getting life to originate in the environments provided on a gas giant where you are fundamentally lacking any kind of analogues to things like black smokers or tidal pools that are candidates for origin of life environments on Earth (instead of solid/liquid interfaces you get generally smooth transitions over scales of many kilometres or more), to focus on whether organisms large enough to use buoyancy (which somehow managed to evolve around the issue that at intermediate scales above microscopic size and below giant skywhale, such mechanisms do not work even in still air).

    I’d love to be proven wrong, but gas giant life strikes me as the kind of phenomenon that is likely to be vanishingly rare, unless someone is going around the universe engineering such lifeforms and deliberately seeding them onto planets.

  • Andrew LePage August 15, 2015, 14:16

    @Michael August 15, 2015 at 2:01

    “I will believe in increasing larger planets having increasingly thicker atmospheres and the Earth having an atypical skinny atmosphere until proven otherwise.”

    Obviously you are free to believe anything you wish no mater what the evidence states but Earth’s “skinny atmosphere”, as you describe it, has nothing directly to do with its mass and everything to do with the carbonate-silicate cycle that has converted most of its original allotment of CO2 into climatically inert carbonate deposits. Without this important cycle that helps to maintain Earth’s long term habitability, it would have an atmosphere filled with an estimated 60 bars of CO2 making it a virtual twin of Venus, albeit a touch cooler owing to its lower effective stellar flux.

  • Michael August 16, 2015, 3:12

    @Andrew LePage August 15, 2015 at 14:16

    ‘Obviously you are free to believe anything you wish no mater what the evidence states’

    There is not much evidence against or for thick atmospheres but just like 30 years ago people thought there were exoplanets out there but there was no evidence, now we just can’t stop finding them. Now to detect the densities and pressure of these worlds it is going to take some doing.

    ‘but Earth’s “skinny atmosphere”, as you describe it, has nothing directly to do with its mass and everything to do with the carbonate-silicate cycle that has converted most of its original allotment of CO2 into climatically inert carbonate deposits.’

    This more likely due to the moon formation event making the atmosphere ‘skinny”, these events are rarer than accretion events.

    ‘Without this important cycle that helps to maintain Earth’s long term habitability, it would have an atmosphere filled with an estimated 60 bars of CO2 making it a virtual twin of Venus, albeit a touch cooler owing to its lower effective stellar flux.’

    I agree with the need for a carbo-silicate cycle without which certain life would be constrained. But if liquid water is present carbon dioxide does not hang around and will naturally combine with certain rocks over time to form carbonates leaving over less volatile gases in the atmosphere such as Nitrogen.

    http://www.innovationconcepts.eu/res/literatuurGPV/45_4_washingtondc_0800_07081.pdf

    Venus has enormous amounts of CO2 due to the temperature but if we drop the temperature and liquid CO2 forms on the surface it will directly form carbonates with the rock without water.

    https://www.ipcc.ch/pdf/special-reports/srccs/srccs_chapter7.pdf

  • Eniac August 19, 2015, 1:10

    It may be worth noting that our best guess about the beginning of life is that it happened under water. To me, that means that neither atmosphere nor rocky surface really matter, what matters is a layer of water. I do not know if “ice giants” become “water giants” when closer to the sun, but my best guess is that they do. There is a fairly wide range of temperature ~250-400K in which biochemistry as we know it is possible, and there is practically no constraint on pressure. So, water giants with a thick hydrogen and helium atmosphere may still be hospitable for life if there is water at the right temperature somewhere inside. Even if you assume you need a rock/water interface, I do not know of any way to exclude that in a mini-Neptune. After all, there is supposedly a lot of rock in there, too, or not?.

    So, my question to the experts is: Can you exclude the existence of liquid water on a mini-Neptune? Or of a suitable rock/water interface? How?

    Why is a dense atmosphere considered hostile to life? I would think the opposite might be more easily argued.

    We tend to interpret “habitable” as being able to stand on a surface, walk around in the thin air, and kick or turn over rocks. As any fish can tell you, this is not really a necessity of life, at all.

  • Michael August 19, 2015, 14:28

    @Eniac August 19, 2015 at 1:10

    So, my question to the experts is: Can you exclude the existence of liquid water on a mini-Neptune? Or of a suitable rock/water interface? How?

    Highly unlikely as water is quite a common compound in the universe, much more common than rock. However if say it is orbiting close to a Red Dwarf which has a long contraction period the hydrogen (water) could have escaped via UV photo breakdown if the planet was not massive enough at that temperature to hold on to it.

    ‘Why is a dense atmosphere considered hostile to life? I would think the opposite might be more easily argued.’

    I would have thought it could be a problem to planet to close to its star and help it if it is far away from it, six and two threes if you ask me. Would a dense atmosphere behave that much different than a water layer? I would have thought not.