About That Habitable Planet…

This NASA image represents an artist's conception of what the first four planets around Gliese 581 were thought to look like in 2010.  (click for larger version)
This NASA image represents an artist’s conception of what the first four planets around Gliese 581 were thought to look like in 2010. (click for larger version)

Gliese 581 is a star that exists about 20 light years away from earth. It became important in the astronomy community back in 2005, when a planet (dubbed Gliese 581b) was detected orbiting it.1 Two years later, two more planets (dubbed Gliese 581c and Gliese 581d) were found there.2 Two years after that, a fourth planet (Gliese 581e) was found.3 All of that was interesting, but it didn’t catch much attention outside of the astronomy community.

Then something amazing happened. A year later, Dr. Steven Vogt and his colleagues found two more planets (Gliese 581f and Gliese 581g) orbiting the star. The amazing thing is that one of those planets (Gliese 581g) was found in the habitable zone of the star! What does that mean? If a planet is too close to its star, it will get very hot. If it is too far from its star, it will remain cold. Life as we know it requires a fairly narrow range of temperatures to exist, so if a planet is to host life, it can’t be too close or too far from its star. It must be in a position that is “just right,” and we call that position the habitable zone of the star. Gliese 581g was in that zone, and even more amazing, it had a mass that was only 3.1 times as much as earth’s mass!4

What did that mean? It meant that Vogt and his colleagues had found the first planet that might be enough like earth to support life. Since it was only 3 times as massive as earth, it was expected to be rocky (like earth), and since it was in the habitable zone, it could have the right temperature to support life. Of course, there are lots of other requirements for a planet to be able to support life as we know it, but that was ignored in all the excitement. NASA released an image (shown above) of what the four inner planets orbiting the Gliese 581 might look like. Notice that the most prominent planet is Gliese 581g, and notice how similar it looks to earth. Dr. Vogt said:

Personally, given the ubiquity and propensity of life to flourish wherever it can, I would say, my own personal feeling is that the chances of life on this planet are 100 percent.

I blogged about this planet back in 2010, noting that it might not even exist. Well, it turns out that the latest, in-depth study of Gliese 581 confirms that Gliese 581g, along with Gliese 581d and Gliese 581f, don’t exist.

Paul Robertson and his colleagues have spent a lot of time trying to analyze all the data from Gliese 581 and use it to determine exactly what planets are orbiting the star. One thing their analysis takes into account is the activity of the star, which obviously affects the data we receive from it. They say that when this factor is taken into account, only three planets remain in the system: Gliese 581b, Gliese 581c, and Gliese 581e. The other planets, including the one that generated so much excitement, don’t exist.5 The title of their paper says it all:

Stellar activity masquerading as planets in the habitable zone of the M dwarf Gliese 581

Why am I writing about this? There are two reasons. First, I am sure the news that this planet doesn’t exist won’t get nearly as much attention as its supposed discovery. I am hoping that in some small way, this blog post will help correct that problem.

The second reason is to point out how overblown the reaction to the supposed discovery was in the first place. Even if Gliese 581g did exist, the fact that it is in the habitable zone of its star doesn’t tell us much at all about the planet. In addition, the analysis that was done couldn’t even determine whether or not the planet was rocky. It could have been a gas planet, which can’t support life as we know it. Nevertheless, in their zeal to promote the idea that there is nothing special about our planet, NASA drew an absurd diagram of the supposed planet, and Dr. Vogt (who is clearly a good scientist) went off the deep end, claiming 100% certainty that there was life on this nonexistent planet.

Whether or not you want to believe it, earth clearly is a special planet. It has all sorts of characteristics that make it an oasis for life. Given what we know about planets so far, it is clearly special. Is is possible another planet like earth might be found one day? Most certainly. Is it possible that this planet might support life? Of course. However, the scientific way to approach this issue is to make conclusions based on the data we have. That’s not what was done in the case of Gliese 581g, and unfortunately, I suspect the science media (and many in the astronomy community) will learn nothing from this fiasco.


1. X. Bonfils, T. Forveille, X. Delfosse, S. Udry, M. Mayor, C. Perrier, F. Bouchy, F. Pepe, D. Queloz, and J. L. Bertaux, “The HARPS search for southern extra-solar planets VI. A Neptune-mass planet around the nearby M dwarf Gl 581, Astronomy and Astrophysics Letters 443L15-L18, doi:10.1051/0004-6361:200500193, 2005
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2. S. Udry, X. Bonfils, X. Delfosse, T. Forveille, M. Mayor, C. Perrier, F. Bouchy, C. Lovis, F. Pepe, D. Queloz, and J. L. Bertaux, “The HARPS search for southern extra-solar planets XI. Super-Earths (5 and 8 M) in a 3-planet system, Astronomy and Astrophysics Letters 469L43-L47, doi:10.1051/0004-6361:20077612, 2007
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3. M. Mayor, X. Bonfils, T. Forveille, X. Delfosse, S. Udry, C. Perrier, J. L. Bertaux, H. Beust, F. Bouchy, C. Lovis, F. Pepe, D. Queloz, and N.C. Santos, “The HARPS search for southern extra-solar planets XVIII. An Earth-mass planet in the GJ 581 planetary system, Astronomy and Astrophysics 507487-494, doi:10.1051/0004-6361/200912172, 2009
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3. M. Mayor, X. Bonfils, T. Forveille, X. Delfosse, S. Udry, C. Perrier, J. L. Bertaux, H. Beust, F. Bouchy, C. Lovis, F. Pepe, D. Queloz, and N.C. Santos, “The HARPS search for southern extra-solar planets XVIII. An Earth-mass planet in the GJ 581 planetary system, Astronomy and Astrophysics 507487-494, doi:10.1051/0004-6361/200912172, 2009
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4. Vogt, S.S., Butler, R.P., Rivera, E.J., Haghighipour, N., Henry, G.W. & Williamson, M.H., “The Lick-Carnegie Exoplanet Survey: A 3.1 M Earth Planet in the Habitable Zone of the Nearby M3V Star Gliese 581,” The Astrophysical Journal 723:954-965, doi:10.1088/0004-637X/723/1/954, 2010
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5. Paul Robertson, Suvrath Mahadevan, Michael Endl, and Arpita Roy, “Stellar activity masquerading as planets in the habitable zone of the M dwarf Gliese 581,” Science doi:10.1126/science.1253253, 2014
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14 thoughts on “About That Habitable Planet…”

  1. Very interesting. I am quite curious if there are legitimate planets out in space that could comfortably support life (I admit I doubt it, though). If I may pose the question, Dr. Wile, what exactly was the process these astronomers used that led them to their discovery that this ‘Gliese 581g’ planet was an illusion?

    1. Zorcey, as far as we know, there aren’t any other planets in space that could comfortably support life. Most of the planets discovered are not rocky, way outside of the habitable zone, etc., etc. Of course, there are probably lots of planets out there we haven’t detected, so it’s possible that one day, a life-supporting planet could be found.

      In answer to your question, most planets outside our solar system are not directly observed. Instead, a planet makes its star wobble a bit, and if you analyze those wobbles carefully enough, you can deduce what planets orbit a star, their positions in the system, and their periods of orbit. The problem is that a star’s natural variability can make it look like it is wobbling. What this new group did was use independent markers of the star’s activity to disentangle the variability from the wobbling. They showed that when they did that, there was only enough wobbling to indicate the three planets: b, c, and e.

  2. Hi Dr. Jay,

    It looks like they may have jumped the gun with their excitement and desire to be first with a big discovery (assuming that they made the mistake rather than the folks behind the new paper… it can take time to sort this stuff out). These guys are astronomers who have spent most of their lives trying to figure out a small part of the bigger question, so they are understandably excited by a discovery like this.

    There are other planets known now in the so-called habitable zone, like Kepler-186f. But it looks like habitable zones might be bigger than previously thought as both Europa and Enceladus appear to have habitable environments and Ganymede and Titan might have habitable environments. All of this, along with the discovery that exoplanets are quite common, is real progress on the capital-L Life question.

    But just because we have found a habitable environment doesn’t mean it is inhabited 🙂 This is why many people want to send probes to Europa and Enceladus so badly… both have water oceans where Earth bacteria *could* live, so the burning question is anything there. And even if there are bacteria in these environments, it could have come from Earth or the same origin as Earth life because we know asteroid impacts can move material between the planets. And even if we find life in these places *and* show that it arose independently from Earth life, it might be much less complex than Earth life. Life might not be rare, but eucaryotic life may be… there is so much unknown, it’s big scientific news whenever someone figures out even a small part of it.

    We now have the technical know-how to send probes to Europa and Enceladus and look and see if we can find life there. We also could build space telescopes that could take measurements of exoplanet atmospheres and look for atmospheric chemistries that *could* be caused by alien life.

    If we keep looking, we’ll hopefully eventually get enough evidence to say that we aren’t alone or that we are… either result is terribly exciting… and maybe a little terrifying too. Maybe there at lots of alien civilizations out there and we may not want to advertise our existence with all of our radio waves… or maybe we are all there is and we should do a better job of looking after each other because human life is even more precious.

    -Nathan (IASMH ’93)

    1. Thanks for your comment, Nathan. I agree that Kepler 186f is in the habitable zone of its star, but it is very near the edge. It gets about 30% of the illumination that the earth gets, so to be warm enough to be inhabitable, there has to be a very precise mix of atmospheric gases and albedo, along with many other properties. Of course, we don’t even know its density, so we have no idea whether or not it even has the physical property of being able to support life, not to mention the thermal properties, magnetic properties, atmospheric properties etc., etc.

      I think you are really being a bit too optimistic to think think that “Europa and Enceladus appear to have habitable environments.” It’s true that Europa has an ocean, however, but it probably doesn’t have enough energy to support life. As a recent article in Astrobiology makes clear:

      Possible sources of energy for life on Europa have been identified, but considerable uncertainty exists as to whether, or at what rate, such energy is made available in the ocean.

      Enceladus has the same problems. Of course, things are even worse for Ganymede and Titan. In addition, water and energy are only two of many factors necessary for life. Given the fact that we have no idea about the energy or the other necessary factors, claiming that these moons “appear to have habitable environments” or “might have habitable environments” is going far, far beyond the data that we currently have. This is, of course, exactly what NASA and Dr. Vogt did with Gliese 581g.

      Now I do agree that we should gather as much data as possible, and sending probes is one way to do that. I also agree that a habitable environment doesn’t mean a place is inhabited. That, of course, means we need to exercise even more caution when making claims about life on other planets (or moons)!

  3. “Ubiquity and propensity of life to flourish wherever it can”? Surely it’s a little hasty, even for an evolutionist, to call life in the universe ubiquitous! Never mind the lack of habitable planets- even if life can come about by spontaneous generation, how would we know how frequently or infrequently it does so…? We only have one instance of it (allegedly) happening to base our predictions on.

  4. @Keith… looking for life in potentially habitable environments tests all sort of theories about the origin of life on Earth, and whatever we find in looking very likely will change a lot of what biologists think they know today. It’s the big honkin’ unknown, and that’s what I find so exciting. …one aside, we could possibly find life on other planets with a common origin to Earth life. Some models suggest that life could spread between stars over long timescales. But with only one data point (Earth) those are all just speculation.

    @Jay I know the authors of that paper 🙂 I suspect they would agree that Europa currently ‘appears’ to be habitable. But this is an assessment based on a limited data set and a lot of modeling that may not hold up when we get more data.

    There is very strong evidence for both the Europan ocean and for two possible energy sources (tidal energy and radiation interaction with the surface). What is unclear, but predicted by models, is if the ocean is in contact with a source of nutrients and with the energy source. I wouldn’t say that they have ‘no idea’ about energy or other factors. I think they have a pretty good idea, but they are wary to jump to conclusions because every new mission uncovers totally unexpected things. But, based on what we know now, Europa is most likely habitable.

    However, for Enceladus we have much stronger evidence from the Cassini data. Currently the only models that match the Cassini data predict that Enceladus has a water sea with an energy source and contact with rock for a nutrient source. Of course, some of the Cassini data (like the salt detection) could be in error, or maybe there is another yet-to-be discovered model that fits the data better… but its still very strong evidence of habitability. But the environment may have only been habitable for a few million years and this might not be long enough for life to arise… but maybe the seeds of life were already there and it just ‘woke up’ Who knows… but getting more information about what’s there would let us know what’s going on 🙂 And Ganymede’s ocean is too deep to know much about… it might be trapped between layers of ice without energy or nutrients… not enough is known to tell yet. Titan is weird because is may be able to support life with a different chemical basis from Earth life on the surface, and Earth-like bacteria in it’s internal ocean… again, too little is known to say if it is habitable or not.

    All of this is work in the past 10 years or so… and just for habitability. Looking for life is a whole ‘nuther big step. My main motivation is curiosity… I just want to know more about what’s there 🙂

    1. Nathan, I would like to know more about what’s there as well. However, we don’t get closer to knowing that by overstating things based on limited data, and that’s what you are doing. Europa is not “most likely habitable.” In order for it to be habitable, a host of factors have to be there, and we don’t know if they are. Yes, there are some energy sources, but we have no idea whether or not they couple to the oceans enough to support life. We have no idea about nutrients. We have no idea about pH. We have no idea about all sorts of factors. This is true for Enceladus as well. It is good that you are curious. However, like NASA and Vogt, you are going way beyond the evidence.

  5. For Enceladus, we’ve known for a few years from Cassini mass spectrometer data that the water reservoir is in contact with either a rocky core or chunks of rock mixed in with its icy mantle. Recent modeling by Francis Nimmo was able to predict observed variations of plume activity by modeling a heating mechanism caused by tidal interactions with Saturn and a deep system of cracks in Enceladus’ crust. We also know from Cassini data that there appears to be an environment that could support known methanogens from Earth (i.e. it is habitable for life already known to us). Here’s an article written after Nimmo’s paper came out with some quotes from people like Chris McKay: http://www.dailygalaxy.com/my_weblog/2013/04/saturns-enceladus-emerging-as-the-most-habitable-spot-beyond-earth-in-solar-system.html

    Now, could I challenge these results? Sure. The best challenges raised so far question the way the Cassini mass spectrometer works and its results… but those challenges appear to have been put to rest and also are much weaker than the other challenges to Enceladus habitability that have already been debunked (like models that predicted the plumes were formed by sublimation of hot ice rather than from a water reservoir). Enceladus habitability is pretty widely accepted right now… Sure that may change. But my statement is a pretty accurate assessment of what we know right now.

    There is less known about Europa, and its habitability isn’t as well established as Enceladus. But it has also been studied for a lot longer and there are a lot better models of its internal structure and of its tidal interactions with Jupiter. We know it is primarily silicate rock and that rock is most likely in contact with the ocean (i.e. nutrients). Tidal flexing of that rock is what heats the ocean. This strongly suggests some sort of volcanic activity that could provide the energy input needed for life. Another possible energy input is from radiation interaction with the surface and the transport of that material to the subsurface… This idea has support from lab simulations of the surface, ground based observations, and Galileo observations. All of this makes me say ‘probably habitable’. Would I like confirmation from a Europa mission, sure. But I think that most of the evidence we have right now suggest that Europa could support known Earth life if we put it there.

    There is also evidence that there may be habitable regions on Mars, btw. (not life, just a few places where Earth bacteria could survive)

    I don’t think I’m jumping the gun here. I think I have an informed opinion based upon evidence. Claiming that there definitely is (or is not) life on other planets is a much, much bigger leap. It’s also pretty clear that the idea of the habitable zone ending at Mars orbit is pretty outdated.

    It’s really not fair to say that I am “going well beyond the evidence.” I am providing an educated opinion based upon the evidence we have so far. You posted something about the area I work in and I just tried to be positive and share some information related to the topic that might be interesting. I wasn’t trying to argue anything about worldview. I was just trying to convey why I find all of this stuff exciting and interesting.

    Personally I think complex life like plants and animals may be limited to Earth… this is speculation on my part based upon 3 things: evidence that the genesis of eucaryotic life may have required something very improbable to arise, the Fermi paradox, and the improbable sequence of events that led to the formation of the Moon… but like I said, this is speculation on my part like guessing who’s going to win a football game… we’ll have to wait and see.

    1. I agree that we will have to wait and see, Nathan. However, I still think you are going way beyond the evidence. The article you link is full of a lot of hopeful statements, but short on supporting data. About the only evidence it gives is the following:

      It has liquid water, organic carbon, nitrogen [in the form of ammonia], and an energy source…there is no other environment in the Solar System where we can make all those claims.

      That may very well be true, but that doesn’t indicate in any way that it is habitable. Too much ammonia, for example, would destroy any hope for life, and too little would mean there isn’t enough nitrogen to support life. If the energy source is too sporadic, it wouldn’t support life. In addition, while the article hopes that a food source exists (such as hydrogen and carbon dioxide or acetylene), it provides no evidence that any food source exists in the quantities needed to support life. Of course, it makes no mention of the many other factors necessary to support life (some sort of carbon cycle, some sort of nitrogen cycle, a pH that is conducive to the form of metabolism used, etc., etc., etc.) Thus, your statement, “it is habitable for life already known to us” is clearly false. We have no idea whether or not it is habitable for anything, much less life that we know about. It seems to me you are doing the same thing NASA and Dr. Vogt did – speaking way, way, way beyond the available evidence.

      I am not arguing about any worldview, Nathan. As I have stated previously, I think life on other planets is perfectly consistent with a creationist worldview. What I am arguing about is science, and at this point, science simply doesn’t support your statements about Enceladus, Europa, or any other parts of the solar system being habitable. There are indications that some parts of the solar system are more likely to be habitable than others, but that’s all. Until we have a lot more data, it is simply premature to claim that any place we have studied (other than earth) is “probably” or “likely” or “is” habitable. Making such statements takes us out of the realm of science and into the realm of absurdly wishful thinking.

  6. This is all very interesting! I’ve often wondered what life in other parts of the universe would mean for the fall, sinful man, and redemption story of the Bible. Jesus came to earth to die for “the world.” Suppose there is other life, and that other life may even be human, or intelligible! What does that mean? Could there be other places where God has revealed himself differently? Other dimensions where humans DIDN’T sin?

    1. I think there are a couple of things to consider, Kendall. Life on other planets need not be sentient. There could be all sorts of planets where God created all kinds of cool life forms that don’t have the capacity to sin. When it comes to sentient life, that’s hard to say. Scripture seems to be referencing the universe when it uses the term “creation,” and Romans 8:22, for example, says, “For we know that the whole creation groans and suffers the pains of childbirth together until now.” That seems to indicate that the effects of the Fall were universal. Thus, I don’t see how there could be other sentient life forms that did not experience the Fall. At the same time, of course, we currently see through a glass dimly (1 Corinthians 13:12), so it’s possible that this interpretation is not correct.

  7. I linked to a popular news article, not an academic paper… you shouldn’t hold it to the same standard as a paper. I can’t link to the relevant papers because they are behind a paywall (though you should know that). You state: “What I am arguing about is science, and at this point, science simply doesn’t support your statements about Enceladus, Europa, or any other parts of the solar system being habitable.” But the science does support that conclusion strongly. The best explanation of the observations we have is that Europa and Enceladus have habitable environments. For what its worth I even asked several of the world experts on those moons and on astrobiology yesterday and they agreed with my assessment (including people on that paper you cited). Now I have been working in this field for 15 years, I worked on the mission that discovered the geysers on Enceladus, but you dismiss my expertise in this field with borderline ad hominem statements… You say I’m going way beyond the evidence, but from my point of view it looks like you did a little cursory reading of a few papers and decided to cherry-pick things that fit your thesis and to fill in the gaps with a few rhetorical tricks. I really have no desire to ‘win’ any argument or to even have one for that matter. I was just trying to communicate what the current thinking in the field was. Sure we may get evidence later to disprove it… just like anything else. But using the same sort of reasoning that geologists use to say XX formed this mountain or YY formed this crater, it looks like Europa and Enceladus are probably habitable. No leaps, just saying where the evidence at hand is pointing. There really is no reason to impugn my credibility. Skepticism is an important part of science, but being a jerk about it is optional.

    1. Nathan, I am very sorry that I offended you. I did not mean to give you the impression that I was dismissing your expertise, and I honestly don’t think I made any borderline ad hominem statements. I also don’t think I impugned your credibility. That was not my intent, anyway. I simply disagree with you about the evidence. In my view, there simply is not enough evidence to even come close to concluding that there are other inhabitable places in our Solar system.

      Your knowledge of this subject clearly demonstrates that you are an expert. However, that doesn’t mean your opinion (or those of your colleagues) is correct. As Dr. Vogt shows us, experts can be wrong – dreadfully wrong. After all, he has been in the field longer than you, and he is certainly an expert. Yet he was 100% sure that there is life on a planet that doesn’t even exist!

  8. This is a very interesting exchange, I would only like to comment on one thing that Nathan Strange said:

    “Life might not be rare, but eucaryotic life may be…”

    I would submit that even prokaryotic life is far too complicated to arise naturally, i.e., the spontaneous generation of a living cell. Protein synthesis for example.

    Even prokaryotic life may be exceedling rare.

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