Three Potentially Habitable Planets Found Orbiting Gliese 667C

A closer look at the previously-studied nearby star Gliese 667C has revealed a treasure trove of planets – at least six – with three super-Earths in the habitable zone around the star. Gliese 667C is part of a triple star system (Gliese 667) and is just over one third of the mass of our Sun. Now that we know there are multiple planets in the so-called Goldilocks zone – a region where liquid water could exist — Gliese 667C might be the best candidate for harboring habitable exo-worlds.

“We knew that the star had three planets from previous studies, so we wanted to see whether there were any more,” said Mikko Tuomi from the University of Hertfordshire in the UK, one of the astronomers who led the new study of Gliese 667C. “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!”

Artist’s conception of the seven planets possibly found orbiting Gliese 667C. Three of them (c, f and e) orbit within the habitable zone of the star. Image is courtesy of Rene Heller/ Carnegie Institution for Science.
Artist’s conception of the seven planets possibly found orbiting Gliese 667C. Three of them (c, f and e) orbit within the habitable zone of the star. Image is courtesy of Rene Heller/ Carnegie Institution for Science.

Tuomi, along with Guillem Anglada-Escudé of the University of Göttingen, Germany looked at existing radial velocity data from the HARPS spectrograph at ESO’s 3.6-metre telescope in Chile. The team said they are extremely confident on the data on the first five planets, while the sixth is tentative, and a potential seventh planet even more tentative.

The team writes in their 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.

If all seven planets are confirmed, the system would consist of three habitable-zone super-Earths, two hot planets further in, and two cooler planets further out.

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
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

But the team said the three in the habitable zone are confirmed to be super-Earths. These are planets more massive than Earth, but less massive than planets like Uranus or Neptune. This is the first time that three such planets have been spotted orbiting in this zone in the same system.

“The number of potentially habitable planets in our galaxy is much greater if we can expect to find several of them around each low-mass star,” said co-author Rory Barnes from the University of Washington, “instead of looking at ten stars to look for a single potentially habitable planet, we now know we can look at just one star and find several of them.”

Gliese 667 (a.k.a GJ 667) is 22 light-years away from Earth in the constellation of Scorpius.
The planets in the habitable zone and those closer to the star are expected to always have the same side facing the star, so that their day and year will be the same lengths, with one side in perpetual sunshine and the other always night.

The researchers say that the ‘f’ planet is “a prime candidate for habitability.”

“It likely absorbs less energy than the Earth, and hence habitability requires more greenhouse gases, like CO2 or CH4,” the team wrote in their paper. “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.”

The other stars in the triple system would provide a unique sunset: the two other suns would look like a pair of very bright stars visible in the daytime and at night they would provide as much illumination as the full Moon.

Are there more planets to be found in this abundant system? Perhaps, but not in the habitable zone. The team said the new planets completely fill up the habitable zone of Gliese 667C, as there are no more stable orbits in which a planet could exist at the right distance to it.

An artist’s impression of the orbits of the planets in the Gliese 667C system:

Read the team’s paper.

Sources: ESO, Carnegie , Planetary Habitability Laboratory

36 Replies to “Three Potentially Habitable Planets Found Orbiting Gliese 667C”

  1. If the exo-habitable planets always face the same way to their host sun, how can they support life? Just curious.

  2. Based on the info from wikipedia, the planet with the highest potential to have liquid water, f, is massive – 5.68 times the mass of the Earth. I don’t know how that would translate into gravity, but I know that would not want to get out of bed on that planet!

    The star it orbits is much, much, much, much cooler and smaller than our Sun which, according to the wikipedia page, means that most of the radiation from the sun would be in the infrared range. Therefore, plants as we know them would not survive there. If there is life there (and it is a BIG if, given how little we know about what conditions life can survive in on other planets and how little we know about this planet), it would probably look quite different from life on Earth. This star is also a flare star – I don’t know what that would mean for a closely orbiting planet, but it doesn’t sound good to me. The planet is probably tidally locked, too, making it a significant challenge for life.

    I think it is a little to early to call these “potentially habitable planets”, given that we don’t know much about these worlds yet.

    1. Agreed better to call these 3 Goldilocks planets. Btw all the co-called habitable exoplanets are just Golidilocks.

    2. Potentially habitable means that as far as we know they could or couldn’t be habitable. So I think the article title is pretty accurate.

      A massive planet with 5.68 time the mass of the Earth can have exactly the same gravity on surface that we have on Earth. F = m / d2. Gravity on surface not only depends on mass, but it depends more on the radius of the planet, which depends of its density.

      Anyway aquatic life will have no problem with 5x times Earth gravity.

      We don’t know if they are tidally locked, they can have moons that have anchored their rotation. Besides, tidally locked planets can have large inhabitable zones (Eyeball Earth Planets).

      We need more information to know if they sustain life. But as far as we now know, they “could” be habitable.

      1. Nice and informative comment.

        What do you think of the idea of a manned search for life on Europa? If the presence of liquid water is a condition for habitability, then Europa makes a good target since its liquid water is powered by Jupiter?

      2. The “surface” gravity (gravity at the cloud tops, since there is no surface) of Saturn is only 0.84 that of Earth, as an example of this effect. For Jupiter in comparison it is 2.5g (remember that Jupiter is 318 times the mass of earth).

        Calculations suggest a 5-10e mass super-earth, if made of rock like the earth, will have a surface gravity about 3g. (So solid planets, of normal matter forming from normal planet formation processes, with 5g, 10g or more surface gravity are actually not possible, or at least appear to be impossible given our limited understanding of the processes at this point. Paradoxically, the highest “surface” g planets have to be made of gas. Planets substantially more massive than Jupiter made of gas have diameters similar to Jupiter’s, due to compression of their gas, so a 10J super jovian planet would have gravity about 25g at its “cloud” tops.)

        A 5-10e mass water world, made mostly of water and other volatiles, could actually have a surface gravity very similar to 1g.

    3. No, habitability is assessed from a perturbation analysis of what we know works (which is why the HEC uses an Earth Similarity Index). You are arguing about smaller effects that may or may not influence habitability potential. What we are interested of so far is potential habitability, and these measures gives us that.

      Tidal lock is _not_ “a significant challenge for life”, how could it be? As they say, a thicker atmosphere would redistribute heat nicely and make such a planet conventionally habitable. If not, there will be areas that are less habitable, but it shouldn’t affect abiogenesis for example.

      Surface gravity estimate, assume Earth density for simplicity:

      M ~ D*r^3 so Fsurface ~ M/r^2 ~ r ~ M^1/3.

      HEC gives the expected mass for Gliese 667C f as 3.1, so it has ~ 3.1^1/3 or ~ 1.5 our surface gravity. It also expects the radius as 1.5, giving the same answer within this estimate.

      If the mass had been ~ 6, the surface gravity had been ~ 6^1/3 or ~ 1.8 ours. This is all well within what cells (100’s or 1000’s of atmospheres pressures or g’s equivalent forces) and even complex multicellular life can handle.

      1. A planet that is tidally locked could have an atmosphere that distributes heat. However, this requires a Goldilocks criterion for atmospheres. If it is too thick the planet is more like Venus and too thin then thermal conditions may not be stable on the annular region where stellar irradiance is near parallel to the planet.

        Red dwarf stars also exhibit a lot of flaring. We are not knowledgeable enough to know whether M class stars go through phases of flaring and other phases of quietude with little flaring. Gliese 667 might just be a star in a quiet phase.

        If I had to guess I would say these planets are likely to be Veunsian.


    4. Your question regarding the impact of mass and solar activity on habitability is a good one and not often addressed. The habitability index is a simple calculation of whether or not liquid water could exist on the surface of a planet. Recently, habitability zones have been adjusted for the amount of infrared output for stars. Although this is a nice starting point, you may be realizing that there are many more variables that are essential for a planet to have in order to be habitable. As we cannot observe these planets yet, most of these are unknown, so we simply have no idea if these planets really are habitable. I will post a link to an article that demonstrates this:

      I will go over one of the major categories that matters to give you an idea of the complexities involved. Fortunately, in the near future we will be getting data on this from more advanced telescopes in the pipeline.

      The atmosphere of a planet has a large impact on whether or not the planet is habitable. The larger the mass of the planet, the more atmosphere it should retain. In fact once you get to 10x the mass of the Earth, a planet will retain hydrogen and helium as it develops which transforms it into a Neptune like planet. The thicker the atmosphere, the more CO2 it should retain and therefore the warmer it should be. As the temperature rises it becomes easier for lighter gasses to escape the atmosphere. The reason is that a gas at a given temperature has a range of velocities, some of which will reach escape velocity and as the temperature rises, an increasing percentage reaches escape velocity. This is where solar activity comes into play. Solar winds significantly enhance the loss of water from atmospheres via hydrogen leakage. Examples in our solar system are Venus and Mars. However, a strong magnetic field greatly minimizes this effect and a planet with one will experience a greatly reduced rate of water loss. Losing water is a bad thing since it acts as a heat sink and will tend to lower surface temperatures. If a planet is a water world and endowed with a large amount of water from onset, it will mitigate the temperature and allow habitability closer to a star. It is of course possible to have too much CO2 in the atmosphere resulting in uncomfortably high temperatures. Once the boiling point of water is exceeded you can get a runaway effect where Venus like conditions are created given enough time and enough water loss into space.

      On the other end of the spectrum is sequestration. This causes atmosphere loss due to it freezing out of the atmosphere and has a larger impact in planets at the cold end of the habitability zone. Mars is a good example of this phenomenon.

      Theoretically, planets further from their star should pick up a larger percentage of water due to closer proximity to the snow line and greater frequency of comet impacts. However if the planet acquires too much water that freezes, it should increase it’s albedo and lower surface temperatures by reflecting more sunlight.

      Another important element of a planet’s atmosphere is its composition which is determined by the nebula from which it formed. Nitrogen composes of 78 percent of Earth’s atmosphere, but the amount present has more to do with its endowment at birth than any other factor.

      To summarize with regards to atmosphere, if your planet is on the cold end of the habitable zone, you want it to be more massive so that it retains a thicker coat of CO2 and thereby has a warmer surface temperature. The opposite should apply for planets at the warmer end of the habitable zone. A strong magnetic field is very helpful no matter where the planet is located, but particularly around small variable stars. We have not had to opportunity to study super-Earths, but he prevailing hypothesis is that they are more likely to have a more diversified interior with a liquid phase that generates a strong magnetic field. They are also more likely to have plate tectonics that recycle elements that get sequestered from weathering and erosion.

      The other concern with a flare star is production of x-rays and gamma-rays which would have sterilizing effects on Earth. Any life around a flare star would have to adapt to such conditions. Simple life forms will likely be able to find niches of shelter, but it is unknown whether complex life can develop at all under such conditions.

  3. startling news: 3 habitable planets would have been seething intelligent life that is quite capable of interplanetary space travel. I hope Finesse mission unravels the mysteries here.

  4. We need to plan and deploy telescopes capable of taking spectra of these relatively nearby planets. Imaging would be even better but far more challenging.

    And just a nit but the sun never sets or rises on these tidally locked worlds.

  5. Given the fact that you are pulling all your information from wikipedia, I would say that you really dont know much of what you are talking about. Yes, they could be tidal locked but that doesnt rule out the possibality that life might have arose there. Just like you stated in your comment, we dont really know how life might arise or live on another planet, doesn’t mean we cant contemplate the fact that it could have. Of couse we need to look for emperical evedience before we confirm something, but thats not the point of this article. The article clearly states in the title “Three Potentially Habitable Planets Found Orbiting Gliese 667C” with the world Potentially, your comment is rendered invalad. Maybe look up your facts in some science journals
    or from a more credited souce next time.

  6. Even a tidally locked planet could have a reasonably tolerable
    atmosphere for life I think. I would hypothesize that there would be
    strong winds caused by the temperature difference between the dark and
    sunlit sides of the planet. Planet would require a thick
    atmosphere/magnetic field to offset the affects of the flares common for
    dwarf stars. From other sources I have read, plants may have very dark
    leaves (almost black) to accommodate the light that is shifted toward
    the infrared that the star emits. These planets could also have a large
    moon that could also be more conducive to life. What if the planets
    moon was tidally locked like our moon, yet because it orbits the planet
    it would have changing day/night cycles over its surface.

    1. Venus is an example of what happens as a thicker atmosphere distributes heat. It is very slowly rotating, yet a human can stand up on its (admittedly windy) surface. IIRC the surface winds are no more than ~ 50 km/h.

  7. We don’t know if they are tidally locked.

    Anyway, tidally locked planets can host vast inhabitable zones. Look for Eyeball Earth Planets.

    1. The habitable region of an Eyeball super-earth could easily encompass a total area as great as Earth’s entire surface area….

  8. The stability of the orbits and the metalicity of Gliese 667 are major factors that need be addressed prior to making any assumptions about the possibility of life in this system. Still, as the number of stars with known planets continues to rise, the odds for finding life somewhere also rises…. Onward!

    1. I don’t think so. I see from the HEC that the star is less than 2 Ga in age, meaning it is likely older than 1 Ga.

      And metallicity of the star is not considered a major factor for habitability. (It can, in cases of much oxygen, affect star lifetime.)

  9. What color would the sky be on a planet orbiting Gliese 667C? (class M1V according to wikipedia)

  10. “Something wonderful is going to happen”, the HEC promised.

    And they delivered. I don’t agree with their news release that “more than one potentially habitable planet per star has been a very rare event”. There has been 1-2 systems, apart from our own, which have had the potential for habitability in their database for a long time now. Of the 12 planets in the HEC, 7 planets or ~ 50 % are now part of many-habitable systems; 3 stars of 8 or ~ 30 % of stars with habitables are such systems.

    Here nearly 5 planets ended up in the habitable zone. Meaning systems with 4-5 habitables will be seen as the larger stars, which are fewer and are harder to locate small planets around, are properly surveyed.

  11. Funny question, even when taking the implied falsehood out (“habitable”-“how can they support life”). I could as well say:

    “If the exo-habitable planets not always face the same way to their host sun, how can they support life? Just curious.”

    “If the exo-habitable planets always face the same way to their host sun, how can they not support life? Just curious.”

    When people look into tidal lock, the largest difference is a pattern of heat distribution. But it isn’t a severe problem in most cases, or a thicker atmosphere moderates the temperature differentials and winds to less than what we can see on Earth, so IMHO the phenomena just nags the number of habitables and their bioproductivity a bit.

  12. I’m very excited about the possibility of life on Europa :-).

    AFAIK even superior life could be possible (fishes), because there is a mechanism that feeds the ocean with oxygen (solar radiation hits the ice on surface and separates hydrogen and oxygen who sinks into the ocean).

    I can’t wait to see images around the volcanic smokes on Europa’s ocean.

    1. Thank you Marc. Very informative again indeed. I really appreciate your observations. (and others here too)

      I presume the current NASA budge is not sufficient to send people to Europa if the budget is not enough to send us to Mars.
      I really hope that US government would take Nieil Degrasse Tyson’s repeated suggestion to double NASA’s budget.

  13. All this speculation about a habitable planet here & there. Ostensibly so that we humans may migrate & maintain the Human Genome

    Well the situation is a very complex one.

    I suspect that this is already being done by extra terrestrials.

    This is the best way of spreading the linage throughout the Galaxy.

    I suspect that we humans are an adapted species of extra terrestrials.

    It seems the best way to go for colonization

    This is a reasonable conjecture when we consider the long history of anecdotal evidence.

    The vast majority of DNA is being called ‘Junk DNA.’

    It seems that it is a perfect storage medium. DNA just grows and doesn’t throw anything away .

    It activates & deactivates genes .

    We are just beginning to crack the DNA code.

    We have lots of surprises in store

  14. The border between day and night could be perfect. That border is constant when it is tidal locked.

    1. Well we now have a tabletop laser accelerator that uses/produces antiprotons. How long before we have tabletop anti-proton storage?

    2. we would be dead and our children maybe even further as well by the time we get there unless they build a spaceship with light speed built on it we can get there within a certain amount of time…

  15. I do not think we currently even have the technology to send a manned mission to Europa – we do not have life support that will reliably last long enough for such a mission, or propulsion that can move as heavy a spacecraft as would be needed to support the life support for a crew quick enough for the mission to be completed in a reasonable period of time which would also be economical enough to be feasibly built with our current economies. And we may not have the ability to shield the crew reliably from the radiation environment around Jupiter, which is quite harsh, especially around Europa’s orbit (it orbits within one of Jupiter’s radiation bands)

    I think we could plan such a mission on paper, and it would be just within the realm of the feasible, but the cost and risk would be too great to practically build it right now.

    Maybe in another 20 years or so, we could do it.

    1. A manned mission is out of our current reach, indeed, but a robotic probe isn’t. Something like an aquatic Mars Rover will be almost as useful as a manned mission to retrieve information about Europa’s Ocean and its habitability, images, on-site tests, etc. …

      It will be very difficult (reach the inner ocean, send data back, …) but I think it’s feasible with current technology.

      About manned missions, I agree that we need at least 20 years to build fusion-powered rockets, or similar, to reach Europa in a short enough travel time.

      PS: About Jupiter’s radiation, I suppose that any viable habitat on Europa will have to be under the ice sheet, to shield it.

      1. Indeed. Not just the radiation, but also the hard vacuum of space, seeing as Europa has no atmosphere above the ice. 🙂

      2. Thanks for the interesting discussion. I agree that a manned mission to any of Saturn’s or Jupiter’s moons are many decades in the future, however I’d love to see more Huygens type probes dispatched to Titan(again, with improved tech), Enceladus, Europa, Io, etc. These missions are achievable, while manned missions are little more than science fiction at this point. Still, we can dream!

  16. It is more that just budget. The distance means a much longer mission, which means the need for much more fuel for propulsion, the need for life support to last much longer, and the need therefore to keep the spacecraft powered for that much longer, and to have its working parts reliable without breaking down midflight, for that much longer.

    Europa orbits in or near one of Jupiter’s radiation belts, and the radiation within it is much more intense than anything near Mars or Earth. So we would need a means of shielding a crew from that radiation if we want them to have any chance of getting anywhere near Europa.

    I do not think we actually have the technological capacity to do such a mission right now. Mars is just on the edge of what is possible right now. Europa would be an order of magnitude harder.

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