More Evidence that Gliese 581 Has Planets in the Habitable Zone

The discovery of Gliese 581 was one of the most exciting moments in extrasolar planetary researcher. Astronomers found an Earth-massed planet orbiting within the habitable zone of a distant star. This would mean that liquid water could be on its surface – and maybe life. Now there’s even more evidence that Gliese 581 is living up to the speculation. Astronomers have published two independent studies this week, claiming that there are least 2 Earthlike planets orbiting the star within the habitability zone.

The first team, led by Franck Selsis, computed the properties of planetary atmospheres at various distances from the star. As we’ve seen with Venus, Earth and Mars in our own Solar System, your distance from the star matters a great deal. Get too close, and the water is vaporized and blown out into space. Get too far away and your carbon dioxide can’t trap in enough heat to keep the planet warm. You want to be just right.

Selsis and his team calculated that the inner boundary of this habitable zone around Gliese 581 should be somewhere between 0.7 and 0.9 astronomical units (an AU is the distance from the Earth to the Sun). And the outer zone should be between 1.7 and 2.4 AU. At least one planet orbiting Gliese 581 falls within this range.

The second team used a different technique to calculate habitability. They studied a narrower region where Earth-like photosynthesis is possible. For the super-Earths thought to be orbiting Gliese 581, they calculated the sources of atmospheric CO2 (volcanos and ridges) and then the potential sinks through weathering. If a planet’s too old, if might not be active any more, and wouldn’t release enough CO2 to keep the planet warm.

Once again, the age of the planets, and therefore the amount of carbon dioxide, is within this region of habitability.

Thanks to this new research, the planets orbiting Gliese 581 are primary targets for future planet hunting observatories, such as ESA’s Darwin and NASA’s Terrestrial Planet Finder. These observatories should be able to directly measure the atmospheres of these planets, and determine if they harbour life.

A third paper on the topic has recently been accepted for publication in the journal Astronomy and Astrophysics. In this, another team of researchers have studied the long term orbits of planets going around Gliese 581. Here you want stability, without highly eccentric orbits that might cause extreme warm and glacial eras. Once again, the planets around Gliese 581 are surprisingly stable.

Things are looking really hopeful. Now we just need someone to uncancel the Terrestrial Planet Finder.

Original Source: Astronomy and Astrophysics

18 Replies to “More Evidence that Gliese 581 Has Planets in the Habitable Zone”

  1. I don’t think there is any way right now for us to know if planets in Gliese 581 have moons, and I doubt we will find that information until we have a telescope like the Terrestrial Planet Finder to see them.

  2. Harry’s question is very pertinent. The only life sustaining planet we know of has a sizeable moon. I’m guessing if we can’t yet take a decent look at a planet, we certainly won’t be able to detect any moons any time soon. I remain very hopeful about Gliese 581 however… the latest news is fantastic.

  3. With all the advances in astronomy, and I have read as much about all the new things we have learned in the past decades as I can absorb [but sadly I’m too dense to understand the serious amount of mathematics and physics required to truly appreciate all the beauty of it] I don’t really read a lot about advances in space transportation that would allow us one day to seriously entertain the notion of trying to visit other stars.

    Really, is there any astronomer out there who would not sell his or her Ph.D. to hop onto a ship that was actually able to make the journey?
    Obviously I am aware of the big daddy of limitations with regards to space travel, but there is this deep nagging doubt in me that is convinced that we just don’t understand enough physics yet to build us a system that will actually take us there [in a time frame that is reasonable within the limits of the human experience].

    All this playing around with chemical propellants is heartwarmingly charming and it does work, but for the serious work of interstellar travel, we’re really going to have to find something out of this world ;).

    Other than this juvenile dreaming it is my deep conviction that everybody on the planet should be offered a basic course in astronomy just so they could look past the horizon of their own personal limitations and understand how amazing the universe really is.

  4. M type stars are not anything to get excited about. They are abundant and well worth study when it comes to discovering planets and understanding our universe, but the probability of life sustaining atmosphere is very low. Gl 581b and c are not in the habitable zone and wouldn’t be good candidates for further research (although we always want to know about everything). The accuracy of measurements to determine if Gl 581d is within the habitable zone (HZ) are questionable and the probable elliptical orbit would likely take it out of the HZ at times. Radiation from m type stars is intense and would be expected to destroy any atmosphere on a planet in the HZ. Just as the same face of the moon faces the earth, the same face of Gl 581d probably faces its parent star constantly. The dark side probably couldn’t sustain itself for the time that it’s atmospheric currents take to pass back into warmth. In other words, the atmosphere would fall as ice on the dark side and life couldn’t be sustained. Determining the actual size of this planet is very difficult to do with our current abilities, just figuring out that the planet exists is a miracle. This research wasn’t about determining the parameters of the HZ or what conditions effect it, but that is a subject that greatly interests me personally thus I digress. The mass of a planetary body determines the parameters for liquid water and the availability of liquid water; I regard to be a key to any habitability. Temperature and pressure will determine the availability of liquid water. Mars didn’t have the mass to sustain it even though the evidence is quite clear that it had liquid water at one time. The maximum size of a planet in terms of having a rocky surface is assumed to be about ten times the mass of earth. Although in realistic terms, we probably aren’t interested in searching for life on any planet that is anywhere near that size. The size of a planet is very important to us because life as we understand it would have a hard time existing under conditions where our weight were significantly heavier. Can you imagine weighing 2000 pounds and trying to get on your feet? The work of Dr. Bloh and the Potsdam Institute is extraordinary, but I have no optimism of life in any form on Gl 581d (nor do I think that they do).

  5. Planets not in the HZ are worth exploring because the may have moons. Jupiter and Saturn are not in Sol’s HZ, but both have moons worthy of exploring for life.

  6. How much does an African elephant weigh? Weren’t there dinosaurs that not only got to their feet but also managed to walk and run over the face of earth for millions of years that weighed considerably more than 2000 lbs?

  7. The very ability to discover that another star has a planet is beyond my comprehension. It was only just a few years ago that we were discovering that Pluto had a satellite and we could not even image it. Only by seeing the light variations of Pluto could we know it was there. Now we can image it and even know that there are two others. What may develop in the area of visual research stagger the mind. In 10 op 20 years we may well be able to see this planet and its possible satellites. We need only be patient. But lets do a lot of speculating in the mean time.

  8. Ray,
    Surface gravity, on the planet ten times the mass of earth, is only 2.15 times stronger.
    So you would weigh only 430 lb.

  9. Well, that is, unless you’re an exceptionally heavy person, but I assume that we’re speaking of average male adult weight.

    I think that this is something to be optimistic about, but it should be a very cautious optimism – just because the chemical makeup of the atmosphere, soil, etc. is suitable for life, I’m skeptical about whether or not a theoretically habitable planet would ever develop life, looking at it from a purely scientific standpoint. Though, I assume that a ‘habitable’ atmosphere would likely be one with the right makeup to assume that life had already existed there for some time?

    Regardless of life on any of these planets, though, even life that has taken unimaginable forms to fit such conditions, it would still be valuable just finding a planet that could have the barest chances of life on it.

  10. Just wondering.. to what extent would gravity have an effect on the size, shape and behaviour of possible life on another planet?
    On a planet with light gravity relative to ours, would plants and animals tend to be taller and more spindly? and on heavy grav planets would they be squat and stocky? We have a diversity of forms and behaviour here, but all have evolved under the influence of our planet’s gravity . Would extreme light or heavy grav make life impossible?
    Ray, if you weighed 2000lbs would you even HAVE feet?

  11. ‘”Tony Says:
    December 15th, 2007 at 9:49 am
    How much does an African elephant weigh? Weren’t there dinosaurs that not only got to their feet but also managed to walk and run over the face of earth for millions of years that weighed considerably more than 2000 lbs?”

    The problem is not total weight, but weight per unit volume. It’s the same volume of muscles, and two to three times as much weight. For light reading on problem (pun not intended), try Hal Clement’s “Mission of Gravity”.

  12. A few questions come to mind. Has anyone pointed a radio dish at Gliese 581 to look for radio signals? How far away is it? Assuming that a civilization which has advanced to a similar level as ours could also detect planets in a habitability zone, would it be reasonable to believe they could detect our planet? Any idea how old Gliese 581 is and when the planets formed?

  13. My apologies if I was unclear. I never said that a person would weigh 10 times as much on a planet 10 times the mass of Earth. I was thinking about reasons that we rule out life on large planets even if they were found in the HZ, for example, Gl 581b – if it were in the zone.

    Fraser seemed excited about this story and I am too. But, when I stopped to think about it, I realized that our search for life beyond planet Earth is probably best spent looking at stars more compatible with the Sun.

    We’ve searched the heavens for radio signals and found nothing that seems an indication of advanced life communicating. I’m convinced that it is out there, and probably in large numbers, but the next question is how can we travel fast enough to reach it in a reasonable time?

  14. Ray
    J. Matthews of UBC used the CSA space telescope MOST (Micorvariability and Oscillations of STars) to observe Gliese 581 for a period of six weeks following the April 2007 observation of Gliese 581c. He found that the star (M type) was remarkably stable over the time monitored and thus would provide a stable light/heat source for its planets, as might be expected from an old M type star. (see

    Also, remember that the surface gravity on a planet depends not only on its mass but also on the planets radius (GMm/r^2) so the mass of Gliese being 10 times that of earth must be divided by about 4 (the radius of Gliese 581c being about twice that of earth) yielding a value closer to 500 lbs rather than 2000 lbs.

    The distance to Gliese 581 is about 20.5 ly, relatively close, and within a distance that may make a probe to that system possible in the not too distant future.


  15. I heard of a sun called M8. i think and it was going to be studied (i forget what space program) because it was said to almost be the same as earth and a proabilty that it has life.

  16. i dont know much about life but much like how we adapted to earth conditions and made our way of life couldnt life forms of any sort have the same inpact on any of the planets in the hz of gl 183?

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