After more than a decade of telescopic monitoring, astronomers have added two newfound worlds to a nearby planetary system already known to harbor four other planets, and one of the new discoveries looks to be the kind of place where life might be able to take hold.

"Since the beginning of this hunt we've tried to find planets at about the size of the Earth with temperatures so that water can exist," said one of the researchers, Steven Vogt of the University of California, Santa Cruz, in a Webcast press briefing on September 29. "This is the first exoplanet that really has the right conditions for water to exist in liquid form on its surface." Vogt and his colleagues are set to publish their findings in a future issue of The Astrophysical Journal.

By monitoring a small, nearby star for 11 years with one of the 10-meter Keck telescopes in Hawaii and combining the data with 4.3 years of similar observations published by another team, Vogt and his co-authors found two orbiting planets, with respective masses of at least 3.1 times and seven times the mass of Earth. Both qualify as quite small in the field of known exoplanets, in which most of the hundreds of worlds that have been discovered are giants larger than Jupiter. The planetary system, which encircles the red dwarf star Gliese 581 only 20 light-years away, now ranks among the largest known. (In August it was announced that another planetary system boasts at least five, and possibly seven, worlds.)

Of the four previously known planets orbiting the diminutive star, two bracket what astrobiologists call the habitable zone, or the "Goldilocks zone"—the region of space surrounding a star that is neither too hot nor too cold for liquid water and just possibly life. The smaller of the two new worlds, Gliese 581g, orbits right between those two planets, placing it more squarely in the star's habitable zone. Nevertheless, Earthlings would not mistake Gliese 581g for their home planet—in addition to its so-called super-Earth dimensions, it orbits a star far smaller and dimmer than the sun, and its average surface temperatures would vary dramatically, from well below freezing on its night side to scorching hot on the day side.

But somewhere between those temperature extremes, which Vogt estimated might range from –35 to 70 degrees Celsius, would exist stable climatic bands, which Vogt called "eco-longitudes," within which liquid water might persist. Because the planet is probably tidally locked, showing only one hemisphere to its star just as the moon does to Earth, the temperate band between permanent daylight and permanent night might afford life a toehold. "There is a continuum of temperatures in between that are stable," Vogt said. "You just have to move around on the surface."

The actual surface temperatures of Gliese 581g depend on a number of factors that are currently unknown—such as the planet's reflectivity and the strength of any greenhouse effect it might have. "We can't say anything for sure about its atmosphere or about water," study co-author Paul Butler of the Carnegie Institution of Washington said during the Webcast. But what is known about the planet qualifies it as at least potentially habitable. "Its mass would be sufficient to hold a nice, strong atmosphere like Earth," Butler said, "and there would be places on the surface that would be sufficient for water."

The radial-velocity, or "wobble," technique used to identify the new planets relies on tracking Doppler shifts in the host star's light as orbiting planets tug on the star, drawing it nearer to and then farther from Earth. The radial-velocity method has been an extraordinarily productive technique since it was used to identify the first exoplanet orbiting a sunlike star in 1995, but it yields a partial portrait of an exoplanet—revealing only lower bounds for planetary masses, for instance, which can produce ambiguities about whether a planet is rocky, like Earth, or gaseous, like Jupiter, or whether it is an even larger object such as a brown dwarf. In the case of Gliese 581g, it appears that the stability of the planetary system would be compromised if the planet were much more than 4.3 times as massive as Earth.

To get a better picture of the newfound world, astronomers would need a complementary observation, such as watching a partial eclipse (known as a transit) as the planet passes in front of its star, or making a precision measurement of the star's side-to-side motion in the sky. With a planetary transit, researchers can even identify constituents of a planet's atmosphere that might indicate the presence of biological organisms there. But Gliese 581g does not appear properly aligned to transit its star from Earth's vantage point. And instruments for astrometry, which measure stellar positions on the sky, are not yet up to the task. "We're hopeful that continuing advances in astrometry will lead to a confirmation of this discovery and lead to a more precise mass estimate for this planet," Butler said.

Even so, the new paper presents "a marvelously intriguing result," says Geoff Marcy, a University of California, Berkeley, astronomy professor who has collaborated with Vogt and Butler on numerous planet discoveries in the past but was not involved in the new study. Gliese 581g, Marcy says, "is certainly extraordinary for its low mass and for being in the habitable zone."

"It's a very exciting step forward," says David Charbonneau, an astronomer at the Harvard–Smithsonian Center for Astrophysics, who adds that the group collected some "really beautiful data" over the years. Teasing out the subtle signature of small planets in radial-velocity data takes a wealth of observations, especially when the signal is dominated by larger planets in the system, and others are sure to investigate whether the signature of Gliese 581g is real. "There are competing groups that will do their best to see if they can confirm the signal or not," Charbonneau says. "They're probably running those analyses right now, because they learned about this five minutes ago."

Even if the planet proves out, the question of whether Gliese 581g actually hosts any biological activity will remain open. "Any discussion of life at this point is of course speculative," Butler cautioned. "That being said, on Earth, anywhere you find liquid water you find life in abundance."

31 Comments

1. 1. Davy 12:40 PM 9/30/10

   This is fascinating and exciting, I agree. I am however slightly concerned that plantets like this one are being characterised as "extraordinary for its low mass and for being in the habitable zone." Clearly, as observational techniques improve, we will begin to "see" many such, and ever smaller, planets.
   
   To me the most extraordinary thing about this is its nearness to the Earth.

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2. 2. ennui 02:07 PM 9/30/10

   Please reserve my seat aboard the well-thought-out Heavy Lifter.
   I am looking forward to an extended trip of about 2000 years. I will put a $5 account in the Bank of America. The accumulated sum will pay for the whole trip.
   

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3. 3. buddhacosmos 02:28 PM 9/30/10

   i wonder if scientists could use starlight backing the planet to determine atmospheric composition. maybe this is the technology they say is not yet available.

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4. 4. jtdwyer 02:48 PM 9/30/10

   Great! Now, lets send a few billion of our favorite people there to 'explore'. That might allow the rest of us to survive on the remainder of the Earth's resources...

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5. 5. maxsmart 03:09 PM 9/30/10

   so there we are, we could send a lewis and clark expedition to see where the goodies are located and then mount a commercial venture to convert any barbarians that happen to be there by getting them to work for us at cheap labor... but if it has location location location perhaps we could move there right away instead of waiting until earth has been completely ruined first...

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6. 6. Spiff 03:18 PM 9/30/10

   It is truly a scientific marvel that we can determine that humans can live on a distant planet 20 light years away but we can't design/invent an automobile that can go much further then "around the block" without refueling...Sorta like dangling a new carrot in front of an old mule!
   Spiff

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7. 7. Tucker M 03:31 PM 9/30/10

   buddhacosmos,
   
   As many stars as there are in the cosmos, they're not infinite; direct lineups of unrelated stars (as viewed from Earth) are actually pretty rare. I suspect the chance of having a precise exoplanet/distant-star lineup is similarly tiny, and probably much smaller than the chance that it transits its own sun (which would support the more commonly-used technique referred to in the article). So whether or not a distant-star transit would work, it seems unlikely we'd get so lucky.

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8. 8. dilettante 04:12 PM 9/30/10

   What are the estimated ages of Gliese 581 and Gliese 581g? Is the age of Gliese 581g consistent with the planet having a molten iron core and therefore possibly a shielding magnetic field?

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9. 9. jmonea 04:40 PM 9/30/10

   i think this is so awsome maybe we could live there one day

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10. 10. Steez 05:36 PM 9/30/10

    Dibs. That said, for the sake of knowledge I am amazed anything like this is possible and cannot fathom what we will discover in the next century. I see this type of exploration as mere art though. I don't mean to depreciate it; it is truly admirable. On the other hand useless at the moment.

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11. 11. Offlogic 05:43 PM 9/30/10

    The paper I read described GL 581d as potentially habitable, not GL 581g.
    Just in case you were planning to head that way, it would save having to re-park.

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12. 12. edromar 05:46 PM 9/30/10

    (It took our guardians (aka "angels" and EBEs (Extraterrestrial Biological Entities) 20,000 years to get here for each visit until they colonized ua 40,000 years ago. But we will have super-enhanced visual graphics from their planet within a few years.

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13. 13. vendicar9 06:03 PM 9/30/10

    This planet is almost certainly lifeless of course, and for a simple reason. It's axial rotation is tidally locked to it's orbital rotation such that one side always faces the parent star.
    
    As a result...
    
    1: the surface of the planet facing the star is desert.
    2: The surface of the planet facing away from the star is a full ice cap.
    3: Any Water Vapour that enters it's atmosphere will condense on the opposite side of the planet.
    4: The planet has no internal dynamo with which to generate a magnetic field, and hence has had most of it's atmosphere blown away by it's parent star, and it's surface irradiated and sterilized by ultraviolet light.
    
    Tra La.
    
    
    

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14. 14. jtdwyer in reply to vendicar9 06:13 PM 9/30/10

    I still say let's send a few billion adventurers there to explore. We can even pack a lunch, then send them on their way. Based on these commentators, there'd be plenty of volunteers...

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15. 15. vendicar9 in reply to jmonea 06:19 PM 9/30/10

    "live there some day" - Yukyukyuk
    
    20 light years distant.
    
    You ain't gonna live long enough to do that.
    
    
    

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16. 16. Wayne Williamson 06:56 PM 9/30/10

    dilettante...good question on the age...wikipedia gives an estimate of 7 to 11 billion years old...the rest of the article is pretty good and up to date to...
    

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17. 17. hotblack 07:13 PM 9/30/10

    Or maybe there happens to be a wide enough band of warm water between the hot and cold sides that life started there, and adapted as extremophiles (relative to us) into other areas.
    
    The possibilities are endless.
    
    There could be one super-intelligent 3bn year old algea bloom growing on the side of an iceberg for all we know.
    
    You know we don't find life there we're gonna seed it, for the sake of preservation of life in general. Perhaps in a couple billion years complex life forms will evolve there too, and devise theories about where they came from, any traces of the event long gone.

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18. 18. edromar 08:32 PM 9/30/10

    Since the guardians in the lights upon the mountains (aka "angels" and EBEs (Extraterrestrial Biological Entities) arrived after the last of two 20,000 year trips to colonize earth, they have fostered our rac/es,cultures,and religions who will soon enable us to photograph their homeland and clarify their limittions on out destructiveness whilr tyhey demonstrate the futility of resisting their orders to leave nuclear and worst weapons behind. Seephotos of the UFOs on my face page.

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19. 19. jtdwyer 08:51 PM 9/30/10

    Looking for an Earth-like planet for intelligent life to flourish likely requires not only the right size and location for liquid water to form but a liquid iron core and sufficient rotation for the necessary protective magnetic field, as vendicar9 pointed out, and perhaps the stabilizing influence of a large inert moon that may be the product only of the oblique collision of two nearly identical planets to produce an extra large planetary iron core.
    
    To my knowledge the magnetic field and large moon may undetectable even when James Webb Space Telescope becomes operable. The search for habitable planets is likely a fools errand given that humanity may not survive the century on the overpopulated, mismanaged garden of Eden that Earth provided us.
    
    If we did find another New World to infect and misuse, how many of the nearly 10 billion poor souls left on Earth would benefit? I empathize with the attraction of such escapist fare, but wouldn't humanity be better served by improving life on Earth?

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20. 20. r2vettes 10:42 PM 9/30/10

    Gliese 581 has gotten a lot of attention the past few years. Maybe because it's the best speciman we have to study. Reply: how do we know the planets don't have internal dynamics and a magnetic field? I don't think our observations have taken us far enough to decide that. There's a billion other galaxies with a billion or so stars of their own. Einstein's cosmic speed limit will keep our feet off the turf if there is any.

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21. 21. ScarletSeraph in reply to Spiff 02:52 AM 10/1/10

    Haha, mathematics is grossly ahead of technology, by about 60 years in most areas.
    
    Anyway, 20 light years has me completely fascinated. If there is life and it is intelligent, and they start using radio waves we will be able to detect it!

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22. 22. cofu 09:08 AM 10/1/10

    the difference of speeds at capture by the Sun of the Earth - has caused: fast heating of bowels on all weight, as consequence reduction of volume of magma,
    then the break of a firm environment of a planet has followed, hills (an equatorial part) were formed , volume of hills
    (is above the average radius of a planet) allows to define size of reduction of diameter of the Earth, simultaneously with occurrence of hills there was Atlantic ocean - redistribution of continental parts has caused the beginning of processes of III natural phenomenon - these fatal events have taken place on a planet in time (~700 - 800 thousand years); in other Galaxies there are civilizations it confirms the fact of occurrence of the Earth in an orbit of the Sun, is doubtless searches of planets where there is a life it is necessary to continue, but at present processes of the natural phenomenon III such as yet have not terminated it is necessary to find out an epicentre of a thermal stream in bowels near Antarctica and consequently what changes will occur still not on far planets namely on continents in the Southern part of Earth; on a site www.mammoths.50megs.com in more details - chronology of changes during stages II, III, IV
    

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23. 23. BillR in reply to ennui 12:24 PM 10/1/10

    Actually, they would hit you with surcharges for account inactivity and you would owe them 35 trillion dollars on your return. Luckily, inflation will be such that that would be worth only 30 cents in todays money.

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24. 24. BillR in reply to jtdwyer 12:26 PM 10/1/10

    I do not think the human race should be allowed on other planets until they learn to take care of the one they already have.

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25. 25. jtdwyer in reply to BillR 02:41 PM 10/1/10

    You, sir, seem to be a responsible human being - I commend you!

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26. 26. vendicar9 12:08 AM 10/2/10

    "how do we know the planets don't have internal dynamics and a magnetic field?" - whomever
    
    Planetary magnetic fields are generated by a dynamo effect in which differential rotation under the crust creates a drag current that in turn creates a magnetic field which itself creates a new current which then creates a stronger magnetic field, etc, etc. etc..
    
    Where there is no rotation, there is no differential rotation and where the is no differential rotation there is no relative motion and hence no drag current.
    
    You might as well ponder how here on earth, electric power generators can generate power without spinning.
    
    
    

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27. 27. Johell 07:38 AM 10/2/10

    Lets evolve, Walmart shoppers. The transcendence required to gratefully inhabit, and the technology required to travel to Superearth are just beyond our Not-so-super-brains. First we need to eliminate the competition for resources on our planet. That being said, travelling to a Red Dwarf system would probably be only an oasis.

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28. 28. pauldcamp 10:16 AM 10/2/10

    For a tidally locked planet, if the cold side is always below the freezing temperature of water (which is likely in the goldilocks region), it seems that all water in contact with the atmosphere would eventually migrate to the dark, cold side and freeze there, leaving all "eco longitudes" very dry and not capable of holding much, if any, liquid water. Mars provides a simple case of this effect, where the poles hold ice but there is no area with liquid water on the surface, although the low atmospheric pressure makes Mars a less than perfect case study for this effect.

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29. 29. promytius 07:52 AM 10/3/10

    If it's a red dwarf, how did it get to be one? By throwing off most of its energy and collapsing inward, just as our sun will some day. Oh but that isn't very good for organic life anywhere near it, right? So big deal, we're there thousands of years too late to say HI.

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30. 30. vethron in reply to jtdwyer 11:04 PM 10/10/10

    Glad to know you are first to volunteer!

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31. 31. Torchlake 09:43 PM 11/13/10

    This is great information! It's fun to think about "what ifs". What is needed first is to establish ourselves on the moon and MARS!
    The United States desperately needs to maintain pertinence in Space. The starts and stops in our space program..only serves to allow other countries to take the initiative. Today we have to BUY a ride into space from the Russians! How does that happen? Why does that happen? We are well on the way to being an ALSO RAN! Russia,China,Japan, Shameful!

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