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Gliese 667C: Three Habitable Zone Planets

Gliese 667C keeps getting more interesting. In the past we’ve looked at studies of this star in a triple system just 22 light years away, work that had identified three planets around the star. As one of these was in the habitable zone, this small red dwarf (about a third of the Sun’s mass) quickly engaged the interest of those thinking in terms of astrobiology. Now we get news that GJ 667C may actually host up to seven planets, with three evidently in the habitable zone.

I would say Mikko Tuomi (University of Hertfordshire, UK) is guilty of a bit of understatement. He’s quoted in this ESO news release thusly:

“We knew that the star had three planets from previous studies, so we wanted to see whether there were any more. By adding some new observations and revisiting existing data we were able to confirm these three and confidently reveal several more. Finding three low-mass planets in the star’s habitable zone is very exciting!”

Exciting indeed — we’ve never found three super-Earths within the same star’s habitable zone, in this case a realm closer to the parent star than the planet Mercury in our system. The work drew on data from the UVES spectrograph on ESO’s Very Large Telescope (Chile), as well as the Carnegie Planet Finder Spectrograph at the Magellan II site in Chile, the HIRES spectrograph on the 10-meter Keck instrument on Mauna Kea, and previous data from the HARPS (High Accuracy Radial velocity Planet Searcher) on the ESO 3.6-meter instrument in Chile.

What we wind up with after a thorough analysis of the radial velocity data for GJ 667C are five signals described by ESO as ‘very confident,’ with a sixth signal that is tentative and a seventh that is more tentative still. From the paper:

— – Up to seven periodic signals are detected in the Doppler measurements of GJ 667C data, being the last (seventh) signal very close to our detection threshold.
— The significance of the signals is not affected by correlations with activity indices and we could not identify any strong wavelength dependence with any of them.
— The first six signals are strongly present in subsamples of the data. Only the seventh signal is unconfirmed using half of the data only. Our analysis indicates that any of the six stronger signals would had been robustly spotted with half the available data if each had been orbiting alone around the host star.

A Densely Packed Habitable Zone

The habitable zone here is found to lie between 0.095–0.126 AU and 0.241–0.251 AU. Two planets exist on the star-side of the habitable zone, three within it, and the last two further out in the system. The assumption here, echoed by ESO, is that all five of the inner planets including the three in the habitable zone are tidally locked, with one side in permanent sunlight and the other in darkness. The skies above one of the habitable zone planets could present an interesting view indeed, as the ESO artist’s impression below conveys.


I want to look more closely at the author’s conclusions on the three habitable zone planets, starting with planet c, which is closer to the inner edge of the HZ than the Earth is in our system. Global climate here would depend upon the properties of the atmosphere:

If the atmosphere is thin, then the heat absorbed at the sub-stellar point cannot be easily transported to the dark side or the poles. The surface temperature would be a strong function of the zenith angle of the host star GJ 667C. For thicker atmospheres, heat redistribution becomes more significant. With a rotation period of ∼ 28 days, the planet is likely to have Hadley cells that extend to the poles (at least if Titan, with a similar rotation period, is a guide), and hence jet streams and deserts would be unlikely. The location of land masses is also important. Should land be concentrated near the sub-stellar point, then silicate weathering is more effective, and cools the planet by drawing down CO2 (Edson et al. 2012)….

The authors describe planet f as ‘a prime candidate for habitability’:

It likely absorbs less energy than the Earth, and hence habitability requires more greenhouse gases, like CO2 or CH4. Therefore a habitable version of this planet has to have a thicker atmosphere than the Earth, and we can assume a relatively uniform surface temperature. Another possibility is an “eyeball” world in which the planet is synchronously rotating and ice-covered except for open ocean at the sub-stellar point (Pierrehumbert 2011).

And finally, about planet e, which receives:

…only a third the radiation the Earth does, and lies close to the maximum greenhouse limit. We therefore expect a habitable version of this planet to have > 2 bars of CO2. The planet might not be tidally locked, and may have an obliquity that evolves significantly due to perturbations from other planets. From this perspective planet e might be the most Earth-like, experiencing a day-night cycle and seasons.


Image: This diagram shows the system of planets around the star Gliese 667C. A record-breaking three planets in this system are super-Earths lying in the zone around the star where liquid water could exist, making them possible candidates for the presence of life. This is the first system found with a fully packed habitable zone. The relative approximate sizes of the planets and the parent star are shown to scale, but not their relative separations. Credit: ESO.

Nature of the Super-Earths

As to the composition of the super-Earths around GJ 667C, the authors note the ‘packed configuration’ of the system, with all planets inside 0.5 AU, and go on to say:

…the planets either formed at larger orbital distances and migrated in (e.g. Lin et al. 1996), or additional dust and ice flowed inward during the protoplanetary disk phase and accumulated into the planets Hansen & Murray (2012, 2013). The large masses disfavor the first scenario, and we therefore assume that the planets formed from material that condensed beyond the snow-line and are volatile rich. If not gaseous, these planets contain substantial water content, which is a primary requirement for life (and negates the dry-world HZ discussed above). In conclusion, these planets could be terrestrial-like with significant water content and hence are potentially habitable.

Is GJ 667C the first among many M-dwarf systems containing several potentially habitable worlds each? The authors speculate that this is the case, and if that is so, then we may ultimately learn that life is more common on worlds around these small red dwarfs than around any other class of star. It’s a notion worth thinking about, given that M-dwarfs make up perhaps as much as 80 percent of the stellar population.

It’s worth mentioning that this study includes a reanalysis of earlier data that underlines the growing power of our archived observations to inform new work. Thus Guillem Anglada-Escudé (University of Göttingen), who worked with Tuomi on this project: “These new results highlight how valuable it can be to re-analyse data in this way and combine results from different teams on different telescopes.” The new tools of digital storage and analysis mean that we are gathering data at a clip far beyond what we can exhaustively analyze, meaning that surprises may await in numerous datasets when weighed against hints from new studies.

The paper is Anglada-Escudé et al., “A dynamically-packed planetary system around GJ 667C with three super-Earths in its habitable zone”, accepted for publication in Astronomy & Astrophysics.


Comments on this entry are closed.

  • Harry R Ray June 25, 2013, 9:40

    This is NOT the first “detecion” of 3 planets in the habitable zone of this star, although it WILL go down as the first ACCURATE one! Phil Gregory used a Baysian analisys to find the extra planets, AND, got their MASSES right. His problem was with their PRECICE ORBITS( especially the REDICULARLY CLOSE PROXIMATION of c to9 d), which led to a quick verdict of an extremely unstable system! I assume Gregory has re-analysed his original data. I am curious to see whether the two data sets NOW reconcile with each other!

  • Tulse June 25, 2013, 11:33

    Given that the host star is an M-dwarf, does that make it easier to do spectral studies of planetary atmospheres?

  • Ronald June 25, 2013, 12:04

    Is the paper freely available yet (Arxiv or so)?
    What are the masses of c, e and f?

  • Rob Flores June 25, 2013, 12:19

    This system is fairly close, hopefully we find better figures for these
    planet’s Density and Radius.
    I have read that the latest speculation on rocky super earths is that
    their crust maybe too thick for magma to penetrate to the surface.
    If this is true it would put hole through these planets habitabilty, since
    it would mean an surface/atmosphere devoid of important elements
    (at least from an organic chemistry point of view) . There about 40 elements
    critical to life on this planet, could complex life do without 25 of them?

    I find it hard to believe there is no surface effect due to internal heating
    if it has a thick atmosphere it can’t exactly radiate heat into space.
    Some models of Venus suggest that w/0 techtonics that world simply heats
    up the crust and in the thinner portions (every hundreds millions years) Decompose and melt on large scale. So instead of volcanoes
    you would have lava oceans that resurface venus infrequently.

    Anyone know if its harder to spot planets in the habitable zone of red dwarf
    star or the habitable zone of G type star. There is a lot noise from red dwarfs generating false positives , but at 1 AU from G type star we get far fewer candidates because of the distance from the stellar body.

  • Ronald June 25, 2013, 12:28

    Ok, according to Philip C. Gregory, Additional Keplerian Signals in the HARPS data for Gliese 667C from a Bayesian Re-analysis, the masses (*sin i) of c, e, f are resp. 4.8, 2.4 and 5.4 Me.
    Since these are minimum masses, I would suggest that c and f are at least mini-Neptunes or even regular Neptunes, rather than terrestrial planets. Which leaves e as the most terrestrial planet. But e is at the very outer edge of the HZ.

  • Michael June 25, 2013, 12:55

    My worry about tidally locked worlds is the potential lack of a magnetic field, normally it requires a convecting core to produce one.

  • Paul Gilster June 25, 2013, 13:39

    Ronald writes:

    Is the paper freely available yet (Arxiv or so)?
    What are the masses of c, e and f?

    Not up on arXiv when last I looked, but probably soon. For now, you can find the paper here:


  • FrankH June 25, 2013, 14:21

    From the paper: “A dynamically-packed planetary system around GJ 667C with three
    super-Earths in its habitable zone”

    The MINIMUM masses (the real masses are certain to be greater) are:
    c : 3.86
    f : 1.94
    e : 2.68

    Sexing up a PR release by mentioning “habitable” planets is pretty annoying.
    This quote from Greg Laughlin should be tattooed on the foreheads of anyone who puts “habitable planet” in a paper title.

    “I think we currently have substantially less understanding of the extrasolar planets than is generally assumed. Thousands of planets are known, but there is no strong evidence that any of them bear a particular resemblance to the planets within our own solar system. There’s always a tendency, perfectly encapsulated by the discipline of astrobiology, with its habitable zones and its preoccupation with water — to make wild extrapolations into the complete unknown.”

  • Stan Erickson June 25, 2013, 14:38

    The word you use frequently, ‘habitable’, is confusing to readers. There are two meanings and they have different implications for mankind. One is ‘Could we live there if we managed to get there?’, and that involves one calculation, including the radiation level on the surface mentioned in another comment. The other meaning is ‘Could life form there?’, which is answered by a different calculation. Since recent results on abiogenesis indicate a high level of UV intensity is needed for at least one of the necessary chemical transformations, it may well be that the answer is ‘Very unlikely’ for red dwarfs, whose spectra does not contain as much of it as brighter stars.

  • andy June 25, 2013, 14:41

    So, the narrow width of the red dwarf habitable zone doesn’t look like such a problem now, does it?

  • Paul Gilster June 25, 2013, 15:02

    Stan Erickson writes:

    The word you use frequently, ‘habitable’, is confusing to readers. There are two meanings and they have different implications for mankind. One is ‘Could we live there if we managed to get there?’, and that involves one calculation, including the radiation level on the surface mentioned in another comment. The other meaning is ‘Could life form there?’, which is answered by a different calculation.

    Stan, the actual term I use throughout the piece is ‘habitable zone’ rather than habitable, the HZ generally being taken to allow liquid water on the surface, and I would read no more into it than that. In other words, we know almost nothing about these planets other than a basic read on minimum mass and their evident position in the HZ (as defined above) for this star.

  • andy June 25, 2013, 15:39

    The MINIMUM masses (the real masses are certain to be greater)

    They are unlikely to be substantially larger: low inclinations (which correspond to higher true masses) are a priori less likely on geometrical grounds. The area of the Earth between 5°N and 5°S is larger than that between 80°S and 90°S, despite both ranges covering 10° of latitude. According to the stability analysis, the minimum inclination of the system is around 30° for an integration time of 1 Myr, corresponding to a factor of two increase in mass.

  • CatharSeamus June 25, 2013, 15:45

    A consideration that i haven’t seen mentioned: we usually think that these orbiting objects are single planetoids. But what if what looks like a single object is actually two smaller objects, like an earth-moon system, but with a moon taking a higher fraction of the mass. In this case, not only the masses of the planets is smaller: but they might be actually tidally locked to each other, rather than to their star

  • coolstar June 25, 2013, 16:15

    Extremes on both ends: Laughlin’s quote isn’t really very helpful (it basically states the obvious, we haven’t found an earth analog yet) and putting “habitable” in every press release is equally unhelpful in the other direction. As Paul said, saying “habitable zone” is pretty much the best we can do for now. For these particular planets, I have grave doubts that any are truly habitable, (2 BARS of CO2?) but the they do meet the definition of being in a loosely defined HZ. I’d also be skeptical about being too stringent about the requirements for life starting as we know very little about how that began here. This star was certainly chromospherically active when it was younger, for example, and the surfaces of the planets could be enriched in what anyone happens to think (today) is essential for life by impacts.

  • coolstar June 25, 2013, 16:17

    @Andy, I suppose it depends on how one defines “substantially”. If I were twice as tall, or twice as massive…..

  • Adam June 25, 2013, 16:30

    I echo the complaint about “habitable” being overused in these kinds of studies and associated press. What people need to be mindful of is that “Earth-like” is a much narrower specification than “habitable” – an Earth-like planet exists in a very narrow insolation range, from about 110% to 90% current insolation levels, while habitable planets exist (arguably) between 400% to 1%, depending on assumptions made. Most habitable planets aren’t Earth-like – AFAIK none have been found in that narrower range yet.

  • andy June 25, 2013, 17:11

    @coolstar: the factor of 2 is an upper limit, the true value is more likely to be closer to 1. For a randomly-oriented system with inclination greater than 30 degrees, the expectation value for 1/sin(i) is about 1.2 (assuming I still remember how to integrate over the surface of a sphere), which is comparable to the uncertainties on the masses.

    Regardless, it looks like these planets are in the <10 Earth mass regime, so they are not Neptunes (mini-Neptunes is another matter), let alone gas giants.

  • torque_xtr June 25, 2013, 18:22

    I guess it’s a very long way ahead to define the second meaning of habitability, “can we fly there and live”. The “habitable” for humans is very likely much different from “capable of producing and hosting a (native) biosphere” and the first isn’t a subset of the second.

    An oceanworld 10 times the Earth’s mass with the Neptune-like composition, but an abiogenic oxygen-rich atmosphere, pruduced by massive photodissociation of H2O early in it’s history, would be a decent place to build floating colonies, walk without spacesuits, surf on the waves and ski on the polar ice, but not capable to produce any native life, no heavy elements in the ocean, separated fron the core by thousands of kms of high-pressure ices, and no organics in the ocean, oxidized in the same event which removed any H2 and saturated the atmosphere with O2. The temperate terminators of hot and dessicated superearths may again be habitable for humans, if the escape velocity prevents the escape of oxygen – even the probability that there are places on Alpha Centauri Bb where humans can walk without protective suits, is somewhat different from zero!
    The question of biosphere compatibility is something entirely unknown and possibly very complicated. How will organisms interact if the another biosphere is based on something completely different than DNA, but still made of carbon, hydrogen, oxygen and nitrogen? Earth’s biosphere has organisms which can break down almost any organic matter and build it’s own biomolecules from the simplest level ingredients – their analogs, if capable of consuming DNA-based biomass, would be a terrible hazard (no easy way to become immune!) And our own omnivorous microbes would too be a devastating plague to all the other biosphere. If the biochemistry resembles our own, then the possibility of pathogenic interactions is higher, but the higher the resemblance, the less fatal the interactions themselves and the easier it would be to somehow engineer the immunity. But the (more effective than the closed cycle) extraction of nutrients from native biosphere requires the highest level of compatibility, which could be a very rare occurence. But that’s conservative vision, there could be unforeseen constructive interactions as well. And all this is pure speculation, since we don’t really know, how different to our own or how similar the other biochemistries can be in principle.

    And possibly we won’t know for a while – the trade of surface material between Solar System solid bodies means that there can be a common Solar System biosphere and any differences between terrestrial and (if there’s any) martian life don’t reflect all possible biochemistry diversities. To get something really alien, we should go to other stars :-)

  • Sedjak June 25, 2013, 21:39

    More solid evidence that some stars have earth-like nurseries of planets. Since its just plain hard to find smaller habitable planets, the Drake equation factors of f(p)*n(e), estimated to be around .5 now, may be an extreme lower bound. Gliese 667C may not even be in the upper part of the range for n(e), but the norm, at least for M-dwarfs. Thus, future studies and telescopes may hopefully nail down f(p)*n(e) to be closer to 2 or even 3, especially if one includes exomoons.

  • Stan Erickson June 26, 2013, 0:26

    Thanks for the clarification of how you use the term ‘habitable zone’. I don’t see the link between the English word ‘habitable’ and a zone defined by a temperature range where water can exist. Perhaps the press could help the public become better informed if you and others used the term ‘water zone’, which has no implications about the possibility of life there, either originating there or migrating there, and is actually what you mean to say. Granted, calling things super-Earths or Earth-like (@Adam) may get the public more excited and supportive of research, but it doesn’t seem proper to me.

  • Doug M. June 26, 2013, 9:33

    “Earth’s biosphere has organisms which can break down almost any organic matter and build it’s own biomolecules from the simplest level ingredients”

    Pretty much every organism on Earth requires a specific suite of elements — the famous CHON, but also S, P, K, Fe, Mg, Ca, and so forth. These are non-negotiable; there’s no microbe, no matter how extreme, that can be built without them.

    Well, so: there’s no reason to think that alien life would use the same suite. That list? is of stuff that’s pretty abundant on Earth’s surface, Earth’s oceans, and Earth’s atmosphere. It’s not hard to imagine an alien biology that made no use of (say) magnesium, but did require some other elements — barium, say, or vanadium or strontium or nickel — that Earth biology mostly ignores*. We’d be immune to attack by such life; it would choke on our lethal magnesium, while starving for lack of strontium (or whatever).

    We’re pretty sure that most exoplanets are going to have noticeably different elemental abundances, BTW — especially in the upper crust and the oceans (if any). Earth’s mix is pretty clearly contingent.

    *mostly ignores, but not completely. There are biological systems that use all of those elements. Proof-of-concept; if they were more abundant, or other stuff less available, there’s no reason to think life couldn’t evolve to use them more widely.

    Doug M.

  • Robert Feyerharm June 26, 2013, 11:23

    Found this recent article published by researchers in the Monthly Notices of the Royal Astronomical Society (http://mnras.oxfordjournals.org/content/430/2/1247.full.pdf+html). The article examines the estimated masses and radii of seven super-Earths and calculates theoretical models of their atmospheres.

    The authors conclude that the seven super-Earths in their dataset have likely retained an extended hydrogen atmosphere from their formation. Applying this evidence to the three proposed Gliese 667C planets in the habitable zone (and given the true masses will be larger than the minimum mass estimates), I’d wager they’re not rocky terrestrial planets as we had hoped but rather mini gas giants, or a weird hybrid of terrestrial and gas giant planet.

  • Rob Flores June 26, 2013, 11:33

    Let me clear, I don’t think we will find Organic chemistry using the same
    elements that we have here. But it will use other elements that are readily available. I can imagine a whole smorgasboard of new compounds that
    in an alien world with higher or lower partial pressures and temperatures will turn out to be analouges of earth proteins.
    We may even find these alternate chemistry on Titan or the Jovian

    Well atleast Kepler functioned long enough to give us an idea of
    how common these under 2 Earths sized worlds are. 12.8%, Despite
    my discarding these as mostly not colonizable, many in the HZ have the potential for some life.

  • kzb June 26, 2013, 12:59

    I like the word “biocompatible”. “Habitable” should have been reserved for planets where humans could live unprotected by high technology, on at least some of the surface.

  • ljk June 26, 2013, 13:11

    New lease of life for hobbled planet-hunter Kepler

    15:37 18 June 2013 by Jacob Aron

    Reports of the death of our principal planet hunter have been greatly exaggerated. The prolific Kepler space telescope may instead be entering early retirement, spending its golden years seeking out planets with a gravitational magnifying glass.

    Since its launch in 2009, NASA’s Kepler mission has discovered 132 exoplanets and thousands of other possible worlds, making it one of the most celebrated exoplanet missions.

    To catch sight of far-off worlds, Kepler must stare at stars with an unwavering eye, looking for tiny dips in starlight when a planet transits, or crosses in front of, its host star. To do this, the craft needs at least three orientation-controlling reaction wheels to stabilise its vision. Two of its four wheels have now failed.

    Shaky eyesight doesn’t have to mean curtains for Kepler, says Keith Horne of the University of St Andrews, UK. He and Andrew Gould at Ohio State University in Columbus suggest that the hobbled telescope can use its gear to take up microlensing, an alternative way to spot planets.

    Full article here:


    To paraphrase Monty Python, Kepler’s not quite dead yet! Plus there is a lot of data the satellite has already collected that needs to be processed and analyzed, so more exoplanet candidates await.

  • Alex Tolley June 26, 2013, 14:53

    @Rob Flores
    Let me clear, I don’t think we will find Organic chemistry using the same
    elements that we have here. But it will use other elements that are readily available. I can imagine a whole smorgasboard of new compounds that
    in an alien world with higher or lower partial pressures and temperatures will turn out to be analouges of earth proteins.

    So what might actually work in lieu of carbon? Or are you talking about the more minor elements only?

  • FrankH June 26, 2013, 15:26

    @andy –
    Without transits or direct observation, these planets are just points with a range of mass and nothing more.

    Calling them “super-Earths” is again a biased nomenclature. Planet f, at it’s minimum of 2 Earth masses… maybe. The rest (and just about every other “super-Earth”) should be called mini-Neptunes because all can probably retain Helium and many can retain Hydrogen. Mini-Uranus would be more accurate – but this field is already full of jokers. Maybe mini-Urectums?

    There are so many, many unknowns with these worlds – they’re orbiting a low metallicity red dwarf, which may compound the issue of volatiles (like water) available on the planets. We also don’t know how the low metallicity will impact the planet’s bulk composition- maybe they’re carbon planets like 55 Cancri e

  • Rob Flores June 26, 2013, 16:03

    Actually I think it will be mostly carbon based life we find. Just a different
    brew of proteins based on local conditions.

    But if pressed I would say that oxygen coupled with metallic ores
    would be a possible path to life in a hot enviroment. As long as
    Hydrogen is absent, it would be viable. This would be a hot world
    something where a carbon base molecules would be too fragile.
    Since were are finding quite a few terrestrials in the hot zone it’s worth
    exploring the idea. and maybe it’s the one surprise surface of Venus would
    have for us.

    The other side of the coin is where reactions organic chemistry is way too slow on a cold world such as the Jovian mOons. There there is the posibility of sulfur or ammonia based chemistry.

    Just on basis of the large number of planets that are terrestrial but not
    earth like, I have to believe we will find more than just a trivial amount
    of life not based on carbon and H2O.

  • Dave Weeden June 26, 2013, 16:16

    If there were aliens on Gliese 667 c, f, or e, and they had by some strange coincidence astronomical tech similar to ours, wouldn’t they have calculated the mass of the 3rd planet in our system as 1.17 earths? Aren’t we measuring the planet/moon systems at each data point, in other words?

    The earth-moon system is almost a ‘double planet’ and Pluto-Charon are even closer to each other in mass (unless there’s been a new development there that I’ve missed). Now, Robert Feyerharm is very likely right about “super Earths” being “mini gas giants”, but what are the chances that one of the “super Earths” we’ve discovered is, in fact, two rocky “slightly-super Earths”? (Or a “slightly-super Earth” and a couple of large bodies in Trojan orbits? — Though this seems more fanciful and almost certainly unstable.)

    On a different note, it may be worth saying that the moon is slap-bang in the “habitable zone” but quite dead. Don’t get too excited yet.

  • andy June 26, 2013, 16:51

    @FrankH: and most of the Kepler transiting small planets are just a radius and nothing more.

    We have a better idea of the orbital properties of these objects than for a transiting planet like Kepler-22b, where we basically have the orbital period and that’s about it (plus some constraints on the eccentricity from transit duration, actual value is highly dependent on the unknown argument of pericentre).

    And as for the directly-imaged planets, so much depends on highly-uncertain models of planetary evolution that the fundamental properties mass and radius are really just “guesstimates”.

  • Doug M. June 26, 2013, 16:57

    “wouldn’t they have calculated the mass of the 3rd planet in our system as 1.17 earths?”

    The Moon has about 1/80th the mass of the Earth. So, more like 1.01 earth’s. And if their error bars are that small, they can probably suss out the existence of the Moon by other means.

    Doug M.

  • Doug M. June 26, 2013, 17:12

    On Earth, every photosynthesis reaction uses manganese and copper. (There are several different sorts of photosynthesis on Earth, but they’re all homologous.)

    Chemically speaking, does it have to be those two? Probably not — one suspects there are other transition metals that could do just as well. On an exoplanet with a different distribution of elemental abundances, an independent evolution of photosynthesis would probably choose some other metals entirely — even if it was otherwise proteins-in-water burning yellow dwarf sunlight at STP.

    Or: on Earth, magnesium is a vital trace element, used by every single cell of life. Lithium, on the other hand, plays little or no biological role, despite being chemically very similar to magnesium. (They’re in different columns of the periodic table, but they have a “diagonal relationship” that causes them to participate in many reactions in a similar way.) The reason seems to be that Earth is relatively lithium-poor. But there’s reason to think that Earth is oddish in this regard, and that terrestrial exoplanets may have a lot more lithium. In which case it might well replace magnesium, or play some other biological role.

    And so on, and so forth.

    Doug M.

  • andy June 26, 2013, 18:20

    “Habitable” should have been reserved for planets where humans could live unprotected by high technology, on at least some of the surface.

    This definition needless to say excludes the Earth for much of its history.

    Such a planet would presumably need to have an existing biosphere to supply the oxygen* and therefore may well have the potential for unpleasant effects arising from the interaction between humans and the native organisms, and you may as well say there is only one habitable planet in the universe.

    * other methods for getting substantial free oxygen into the atmosphere are likely to be pretty bad news for humans trying to live there as well, e.g. photodissociation of water during a runaway greenhouse transition.

  • Dave Weeden June 27, 2013, 3:48

    Doug M, you’re right of course. I was going by gravity on the surface. I had no idea that the moon was *that* small.

  • kzb June 27, 2013, 7:55

    Andy, I think it’d fair enough that Earth would’ve been excluded for most of its history. Previous to the last few hundred million years it would’ve been classed as biocompatible not habitable. I thought it was more or less a given that a habitable planet (in my sense) would need an extant biosphere to provide oxygen in the atmosphere. To say that there is necessarily unpleasant effects from interacting with the native organisms, I don’t think this is known yet. We’re unlikely to be able to eat anything, and I personally wonder about allergic reactions, but beyond that I don’t know.

    On the topic of elemental abundances, I think some people are overestimating the impact of this. Organisms are not short of trace metals here becasue they are rare in the crust. They can be short of them because of bioavailability, but that is a completely different thing. We get algae growing in distilled water at my workplace. Microorganisms can concentrate trace metals by amazing orders of magnitude. To say that organisms could be short of magnesium (for example) <> it is say half the crustal abundance of Earth I think is not true.

  • andy June 27, 2013, 17:26

    It’s also unclear what value the concept of “habitable for humans” is when discussing extrasolar planets anyway. “Canned monkey to the stars” seems like rather more of a flight of space cadet fantasy than an actual, genuinely realistic prospect.

    If human technological civilisation does eventually get to the stage of travelling between the stars, it is likely that significant biological engineering would be required, assuming we send biological rather than machine travellers. It would be rather silly in this case to send something that has the same tolerance as ourselves (who are adapted to life on Earth) rather than adapting the travellers to better survive the destination.

  • Rob Henry June 27, 2013, 18:39

    Stan Erickson Wrote
    “Since recent results on abiogenesis indicate a high level of UV intensity is needed for at least one of the necessary chemical transformations, it may well be that the answer [to where life is] is ‘Very unlikely’ for red dwarfs, whose spectra does not contain as much of it as brighter stars.”

    If the first part is true, then there is a problem in that UV light is even better at destroying complex chemistry than creating it. Thus, if we need it, it is better in short bursts or flares, so the second part of that comment should read
    “it may well be that the answer is ‘Very likely’ for red dwarfs, who dose this out in flares, but very unlikely for brighter stars whose spectra contain it without relent.”

  • amphiox June 27, 2013, 19:54

    It is interesting to me that the relative positions of these three planets within their stars habitable zones seems roughly analogous to the relative positions within Sol’s habitable zone of Venus, Earth, and Mars.

  • amphiox June 27, 2013, 19:57

    I personally do not see the logic of restricting the term “habitable” to environments wherein humans can survive.

    We are, after all, not (yet!) looking for these planets to find places that humans can colonize.

    We are looking for these planets to find other worlds where life might exist.

    Our definitions and classifications should, ideally, fit the purpose for which we are actually studying the subject.

  • Doug M. June 28, 2013, 4:24

    “Organisms are not short of trace metals here becasue they are rare in the crust.”

    Never said they were. There are elements that are very abundant in the crust but that are not biologically significant (aluminum) and vice versa (zinc).

    Bioavailability is a complicated topic. Some ions don’t get used because they don’t dissolve well in water at STP. Others dissolve, but aren’t reactive in a useful way. Aluminum is probably the best example of this — it exchanges ligands very slowly, making it of limited use in biological reactions. Life seems to have turned its nose up at aluminum, opting instead for rarer but more chemically flexible ions like Fe (III) and Mg (II).

    Barium and strontium are much more common than sulfur, chlorine, copper or zinc. But strontium is used rarely and barium not at all, while all those others are biologically important. Why? Well, in this case it’s probably because life already had a couple of abundant alkaline earth metals — calcium and magnesium — to play with. And so forth.

    But exoplanets are likely to have dramatically different abundances. They’ll have different isotopic ratios in the starting nebula. They’ll have different geological histories. (For instance, it’s currently believed that Earth’s crust is deficient in lithophile and chalcophile elements in part because of the Theia impact. An otherwise Earthlike exoplanet that never suffered such an impact would have a very different crustal composition and different salts in its seas.) They’ll be different because of differences in atmospheric composition. (Archaean Earth, three or four billion years ago, had an atmosphere dominated by CO2, resulting in acidic rain that drove very different patterns of weathering, resulting in different ocean salinities and crustal rock compositions.) And they’ll be different because of contingent differences in history — google the “leaky earth” hypothesis, for instance.

    As to microorganisms concentrating trace metals, that only goes so far. There’s a reason that large stretches of open ocean are biological deserts.

    Doug M.

  • Alex Tolley June 28, 2013, 14:31

    Barium and strontium are much more common than sulfur,

    Are you sure? Sulfur is quite common. What abundance table are you using? Chlorine is very common in our oceans, even though dot so common in the universe.

  • amphiox June 28, 2013, 22:22

    “As to microorganisms concentrating trace metals, that only goes so far. There’s a reason that large stretches of open ocean are biological deserts.”

    But even the more desolate of such stretches are not sterile, and support some life.

    This does not seem to necessarily preclude life, but rather limits the fecundity of a habitat.

  • Doug M. June 29, 2013, 7:43

    @Alex, my bad — I misread a line on sulfur. According to WebElements, barium, strontium and sulfur are all around the same proportion by weight in the Earth’s crust — 340 ppm, 360 ppm, and 420 ppm respectively. So sulfur is actually a bit more common than Ba and Sr. Chlorine is about half as common, at 170 ppm.

    Sulfur is much more common in the universe; the Earth’s crust is relatively depleted in it. IIUC, there are a couple of theories as to why. One is the Theia impact, given above. Another is that the reducing atmosphere of early Earth favored metal sulfides (which eventually got subducted and dropped down into the mantle) over metal sulfates (which are lighter and more soluble and so more likely to stick around in the upper crust and ocean).

    Doug M.

  • Doug M. June 29, 2013, 7:45

    Incidentally, this is why some of us were sighing deeply at the “we found arsenic life!” announcement by NASA a couple of years back. Life can be incredibly flexible in terms of adapting to very high (or low) concentrations of different elements. But in terms of actually using different elements in active biochemistry, Earth life tends to be very, very conservative.

    Doug M.

  • kzb July 1, 2013, 8:47

    The limiting factor in open-ocean productivity is said to be iron availability. (You recall the ideas to increase photosynthesis in the oceans by adding ferrous salts, thereby absorbing CO2 ?). Yet in the sediments at the bottom of said oceans there will be copious amounts of iron. The trace amount of iron that is available for photosynthetic organisms in the oceans would hardly be affected if that sediment contained one quarter of its current iron content, or ten times come to that. The limiting factor is the solubility in seawater, not how much iron there is on Earth.

  • coolstar July 1, 2013, 23:22

    @Andy a) you’re the one who mentioned a factor of 2 b) the expectation value of sin(i) is 0.860 IFF i is drawn from an isotropic distribution c) see Ho and Turner (The Astrophysical Journal 739, 2011) for reasons why this might be FAR from being accurate.
    These MINIMUM masses are just too large to be very interesting in terms of habitability.

  • andy July 2, 2013, 13:45


    a) I also mentioned this was the minimum inclination from dynamical stability when I mentioned that factor. I guess that slipped past you.

    b) The expectation value of sin(i) is not really the interesting quantity here. The interesting quantity is how much the true mass is likely to be relative to the radial velocity minimum mass. This is represented by the expectation value of 1/sin(i), and in general

    〈1/sin(i)〉 ≠ 1/〈sin(i)〉

    Furthermore there is the dynamical constraint on the mass, so we are not probing the full range of the mass distribution here. This also excludes the extremely high masses at very low inclinations which biases 〈1/sin(i)〉 to higher masses if used over the full range of inclination. It also allows the approximation that the planetary mass is much smaller than the stellar mass to remain valid over the inclination range, which means the mass function can be inverted in such a way to give the mass constraint as a simple m*sin(i) value rather than the result of inverting a cubic equation (if memory serves correctly).

    Now granted, perhaps there is a strong change in the mass function in the region 2-8 Earth masses (in which these planets appear to be constrained) which would bias things towards the higher end of the mass range, which was why I did make sure to state the whole “assuming random inclination” thing.

    As for whether the mass range precludes habitability or not, I was not arguing that either way.