A Potentially Habitable Earth-like World

by Paul Gilster on April 24, 2007

This is a big one, and it happens several years earlier than I had expected. A planet of about five times Earth mass, one whose radius is only 1.5 times that of our own world. Moreover, a planet that’s smack in the middle of its star’s habitable zone, with a mean temperature estimated at between 0 and 40 degrees Celsius. The models in question say that this is a rocky world, and its temperatures tell us that oceans could exist there. The first detection of a planet where carbon-based life could conceivably exist makes this one a find for the history books.

Planetary system around Gliese 581

The star is Gliese 581, already known to be home to a planet of Neptune mass and a possible third world about eight times as massive as Earth. It’s an M-class red dwarf, far smaller and cooler than the Sun. The new planet, the smallest found up to this point, orbits it in 13 days. Gliese 581, it should be noted, is comparatively close to our own Solar System, about 20.5 light years away in the constellation Libra. Radial velocity methods seem to have been made to order for this small star and the planets that circle it.

Image: Artist’s impression of the planetary system around the red dwarf Gliese 581. Using the instrument HARPS on the ESO 3.6-m telescope, astronomers have uncovered 3 planets, all of relative low-mass: 5, 8 and 15 Earth masses. The five Earth-mass planet makes a full orbit around the star in 13 days, the other two in 5 and 84 days. (c) ESO.

Xavier Delfosse (Grenoble University), a member of the discovery team, has this to say about the significance of the new world:

“Liquid water is critical to life as we know it. Because of its temperature and relative proximity, this planet will most probably be a very important target of the future space missions dedicated to the search for extra-terrestrial life. On the treasure map of the Universe, one would be tempted to mark this planet with an X.”

Yes, and Gliese 581, with clear evidence of a second ‘super-Earth’ in an 84-day orbit (well outside the star’s habitable zone) and its Neptune-class world (5.4 day orbit) as well, is one of the most interesting planetary systems analyzed to date. We’ve looked many a time in these pages at what a terrestrial world around a red dwarf might be like on the surface. Surely tidally locked to its parent, with that dim red sun eternally fixed in the same place in the sky. Given a thick enough atmosphere, heat transfer could occur that keeps the dark side warm enough to prevent its gases from freezing out. We’re left with the possibility of a temperate region on the day side that could, for all we know, support life.

Gl 581

Image: Gl 581. Just a dot at screen center, but perhaps home to the first habitable world ever detected. Credit: Sloan Digital Sky Survey (via systemic).

The new planet may not be anything like this, and it will take more work — and surely space-based instrumentation — to learn what its true characteristics are. But ponder a planet where infrared predominates rather than visible light, and periodic flare activity acts as an evolutionary stimulus. The consider the long lifetimes — more than a 100 times the Sun’s paltry ten billion years — that M-dwarfs have to let evolution work its wonders. There are arguments to be made for and against this scenario, but if it’s remotely true, then the number of habitable planets in our galaxy may be far higher than we’ve previously believed.

Physicist and science writer Douglas Blane recently interviewed astrobiologist Giovanna Tinetti on the subject of hunting for life-markers on exoplanets. Part of their discussion involved M-dwarfs, with Tinetti recalling a talk at Caltech by John Raven (Royal Observatory, Edinburgh), who described his work with Ray Wolstencroft on M-dwarf habitability. What kind of photosynthesis might happen on a world lit by the longer wavelengths of such a star? From the interview:

They provided a scheme for photosynthesis that uses three photons instead of two, as vegetation does on Earth. They showed that you can still have photosynthesis with a cooler star. So that started me thinking about what would happen to the red edge [photosynthesis shows high reflectance at the far red of the optical spectrum, a useful signal of the presence of vegetation]. I showed that it would be shifted, so you would need to look for a slightly different signature.

The next question of course was whether planets of such a star would have the same atmospheric characteristics as Earth. Now a scientist called Joshi had already provided a 3-D model for a terrestrial planet in the habitable zone of an M-star. He’d shown that you probably need more greenhouse gases to warm up the area not illuminated.

This is because for such a cool star the planet has to be very close. So it could be tidally locked, with one face always illuminated and the other always dark. That meant you needed a circulation of the atmosphere and a particular composition. We put that model together with my calculations on the shifted red edge, and discovered that the strength of the edge feature on an M-star terrestrial planet can exceed that on Earth, given the right conditions.

Of course, we can only speculate about the new Gliese 581 planet because thus far all we really have a read on is its orbital period, distance from the star and minimum mass. Even so, what a find, one that will surely spur yet more interest in the red dwarf category that accounts for up to 80 percent of the stars in our galaxy. Kudos to the discovery team and the amazing HARPS (High Accuracy Radial Velocity for Planetary Searcher) spectrograph, located on ESO’s 3.6-meter telescope at La Silla (Chile). HARPS detected velocity variations in this star between two to three meters per second — we’re talking about the speed of a brisk walk!

Planet hunter extraordinaire Michel Mayor (Geneva Observatory) has this to say about HARPS:

“HARPS is a unique planet hunting machine. Given the incredible precision of HARPS, we have focused our effort on low-mass planets. And we can say without doubt that HARPS has been very successful: out of the 13 known planets with a mass below 20 Earth masses, 11 were discovered with HARPS!”

All true, and you have to like where this is going. Mayor again:

“And we are confident that, given the results obtained so far, Earth-mass planets around red dwarfs are within reach.”

Oh for the dedicated (and lengthy) observing run sufficient to let HARPS do its number on Centauri B! And if you’re wondering about previous super-Earths, Gliese 876 does indeed have a planet with a minimum mass in this same range — 5.89 Earth masses — but its orbit (completed every two days) takes it too close to its star for liquid water to exist. Likewise, the icy world OGLE-05-390L weighs in at 5.7 Earth masses but is much more distant from its primary and out of the habitable zone.

The upcoming paper in Astronomy and Astrophysics is Udry et al., “The HARPS search for southern extra-solar planets : XI. An habitable super-Earth (5 MEarth) in a 3-planet system.” I’ll point you to the abstract as soon as it becomes available. Thanks to Darnell Clayton for additional information on this story.

Update: Here’s a link to the paper on this work; thanks to Malcolm Ramsay for the address.

andy May 4, 2007 at 6:10

So what’s the consensus here: Is Gliese 581 c clearly out of the habitable zone, or can we use a phrase like ‘on the edge of the habitable zone?’

I think there’s a pretty good case for “not in the habitable zone at all”.

First off, we can work out the flux at the location of Gliese 581 c. This is proportional to L/d^2, so for L=0.013*solar luminosity and d=0.073 AU, the flux is 2.44 times that at Earth. This is greater flux than at Venus (L=1*solar, d=0.723 AU, flux=1.91), which is not a habitable planet by a long way, and has in fact undergone runaway greenhouse.

Secondly, estimates of the habitable zone of the Sun put it between 0.95 and 1.65 AU. Scaling these distances to the zone around Gliese 581 where a planet would receive the same flux gives 0.11 to 0.19 AU, outside the orbit of Gliese 581 c, whose apastron is around 0.085 AU. In addition, calculating the habitable zone in this way probably puts it too close to the star, see below.

Thirdly, a paper on the detectability and evolution of hot ocean planets suggests that ocean planets (planets with ice envelopes constituting a large fraction of the planetary mass) interior to 0.84 AU have supercritical water envelopes rather than liquid oceans. Scaling this distance to Gliese 581′s luminosity gives 0.096 AU, still outside the apastron of planet c. It is thus unlikely that if Gliese 581 is an ocean planet that it has a liquid ocean. Note also that the surface temperature of Venus is above the critical point of water.

Fourth, the planet is very likely tidally locked to the star. If the planet somehow manages to have a non-greenhouse atmosphere with enough pressure to support liquid water (unlikely – the atmospheres of Venus and Mars are mainly carbon dioxide, and water vapour is itself a powerful greenhouse gas), it would probably not be able to transport enough heat to the dark side of the planet to prevent runaway atmospheric freezeout, or at least freezeout of water. This would not leave a habitable planet. If Gliese 581 c is a terrestrial planet (as opposed to an ocean planet), it is likely either in the runaway greenhouse state, or frozen out.

Fifth, water absorbs more radiation in the infrared than the visible, so the planet would absorb a greater fraction of radiation intercepted than Venus, even with a Venus-like reflective cloud layer. This has the effect of shifting the habitable zone around an M-dwarf star outward with respect to where it would be if we just scale distances to equivalent values of the solar constant (which is how I got the estimates for equivalent distances above).

Sixth, while the effective temperature range of the planet reported in the media is in the range that liquid water is possible under atmospheric pressure, effective temperature doesn’t really say much about the planet, except that there may be a region in the atmosphere where the temperature is roughly that value. Both Earth and Venus have sub-zero effective temperatures, around -20 degrees C, and both planets are significantly warmer than this. Effective temperature is only a reasonable estimate for the surface temperature on airless worlds. The only habitable temperature region on Gliese 581 c is probably in the atmosphere.

It might be possible to make a case for the third planet, Gliese 581 d, being able to support liquid water (though it might be too cold for that), but I think Gliese 581 c is only habitable in the eyes of the hype machine.

andy May 4, 2007 at 6:12

Hmmm… I seem to have mangled a close tag there… can we have a “preview post” button please?

Administrator May 4, 2007 at 12:43

Andy, you make a strong case here. For those following the Gl 581 c story, here’s how the discovery paper by the Geneva team discusses habitability: “For an M3 dwarf, this separation puts the planet within the habitable zone around the star. From the 0.013 L stellar luminosity (Bonfils et al. 2005b), we compute an equilibrium temperature for the planet of −3◦ C (for a Venus-like albedo of 0.64) to +40◦ C (for an Earth-like albedo of 0.35). With a planetary radius of ∼1.5 R⊕ (Valencia et al. 2006) and a temperature that would be +20 C for a 0.5 albedo, Gl 581c is probably the most Earth-like of all known exoplanets.” There are, obviously, many dissenters. Any other comments from readers?

A ‘preview post’ button would be good. I’ll see if I can find a WordPress plugin that handles such.

valeria May 5, 2007 at 11:33

I have always had an interest in space stuff, the moment I saw the news about gliese581 gave me an immence hope of life visit my blog,you’ll like it!!

jesper May 6, 2007 at 6:53

how long time does it take to go to gliese 581 by the tecnolegy at the time…

Administrator May 6, 2007 at 7:35

Well, the numbers are only approximate, but with travel times like these, it doesn’t much matter. It would take a spacecraft traveling at Voyager-style speeds well over 70,000 years to get to Alpha Centauri. Since Gliese 581 c is roughly five times the distance, that would mean it would take a similar probe more than 350,000 years to get there. Clearly, new methods are needed!

george scaglione May 6, 2007 at 11:25

paul yes i agree 1000% !! this site itself is one of the ways in which we are seeking those “new methods”!! we have to learn to fly alot faster or manipulate space time in some way! e.g. – warp drive or traversable worm holes! hope everybody is having a good week end and i look forward to hearing some new ideas soon and discussing them with everybody.maybe even authoring one or two.thank you very much your friend george

Adam May 6, 2007 at 17:10

Hi Jesper

Paul’s just going off the current state of the art, but the distance to Gl581 will be different by the time a vehicle arrived if it took that long. Both the Sun and Gl581 are moving around the Galaxy at slightly different speeds – the difference between the two is how fast they’re approaching or moving apart.

But a space-probe could be built to go a lot faster if we used a big enough booster to launch enough propellant into a Jupiter approaching orbit that then dropped the probe in close to the Sun. Then a brief engine burn at high-thrust could add a HUGE speed to the probe by borrowing a bit of gravitational energy from the Sun. Speeds of about 1 light-year per 5,000 years (about 60 km/s) might be reached in that fashion. I’m not sure where Gl581 would be in about 100,000 years so the trip might be a bit quicker.

If we didn’t use chemical rockets there’s two different approaches for “extreme” speed that we could do with technology we have now, but have yet to refine – nuclear-pulse rockets and solar sails. Nuclear-pulse rockets can get to quite high speeds, enough to travel a light-year in 50 years so Gl581 would be about 1,000 years away. Very large solar sails that are launched to open up very close to the Sun can get up to speeds of about 1 lightyear per 200 years, so Gl581 is about 4,000 years away. Both those travel-times could be survived by space-probes – there are components of electronics for example that have failure times of over 10,000 years, so we could make an ultra-reliable probe even now.

Sending people is much, much harder.

Adam

burak ateş May 12, 2007 at 21:32

this is amazing and ı cant believe dreams come true

Hiro May 14, 2007 at 15:07

One of the major problems about large solar sails is space-rocks. They are perfect targets for space dust which travels around 20Km/s. If we can develop fusion propulsion around 2050-2100, then we might reach Gl581 in the next generation. The second idea is antimatter propulsion. In order to have enough amount of antimatter for the trip, we need to build an antimatter factory and I don’t think we can do that in the next 100 years.

nurul saadah May 15, 2007 at 8:15

i like the gliese 581 c…i love it….
it’s all about our future tomorrow..n tomorrow…n tomorrow..

:)

m1 May 18, 2007 at 16:12

even a atm. of pressure of only 13 percent that of earth will sustain non freezin nightside so andy is wrong

andy May 18, 2007 at 18:28

From what I have read, 13% prevents collapse of a carbon dioxide atmosphere (for an atmosphere composed entirely of carbon dioxide!) – it does not prevent collapse of the hydrosphere, for which more atmosphere is needed. Plus, the effect of receiving more radiation in the infrared increases the mixing ratios of methane and N2O, further enhancing the greenhouse effect.

The issue is that the planet is receiving a solar flux 2.4 times that of Earth – this is going to cause significant evaporation of water from the planet’s oceans, and water vapour is a greenhouse gas. Probably atmospheric collapse wouldn’t happen (it is difficult to see how a 5 Earth-mass planet would end up with almost no atmosphere anyway), in which case you might be able to argue for habitable conditions on the dark side, but you wouldn’t have photosynthesis!

Besides, the Gliese 436 detection strongly suggests that hot Neptunes around M dwarfs formed further out in the ice-rich regions and migrated inwards. Since Gliese 581 c is outside the orbit of such a hot Neptune, it is likely to have also formed in ice-rich regions and thus the “ocean planet” scenario applies, and that would give a supercritical “hot ice” world, though maybe water clouds could exist high in its atmosphere.

Shonte May 21, 2007 at 21:01

Is it true that warp factor 1 is in reach, but the engine that could reach such speeds deteriorates itself?

Administrator May 22, 2007 at 8:30

Shonte, I’m afraid we’re nowhere near warp-style speeds. But research continues, and we all hope that one day it will happen. Until then, we’re still looking at solutions that are closer to our current technological development, like solar sails and beamed magsails. No one knows when — or if — a warp-style breakthrough will happen, but one way or another, we’ll get to the stars. Some ways just take longer — much longer — than others.

george scaglione May 22, 2007 at 9:12

paul,thank you i surely hope that warp drive will come into use some day.however i guess and it is just a guess that the first starship’s mission will be propelled by something much different.but it is really good to know that people are giving it all alot of thought.we have already begun the first step.but,come to think of it? what did shonte mean? about some kind of warp drive being “close”? my guess respectfully would be that he was just mistaken about something he had read or the like.anyhow thank you very much your friend george

MirrorStephen May 22, 2007 at 9:22

Shonte – say it with me – Star Trek and Star Wars are NOT REAL.

Now repeat and go read a real science book.

ljk September 10, 2007 at 0:37

The Survival Rate of Ejected Terrestrial Planets with Moons

Authors: J. H. Debes, S. Sigurdsson

(Submitted on 6 Sep 2007)

Abstract: During planet formation, a gas giant will interact with smaller protoplanets that stray within its sphere of gravitational influence. We investigate the outcome of interactions between gas giants and terrestrial-sized protoplanets with lunar-sized companions. An interaction between a giant planet and a protoplanet binary may have one of several consequences, including the delivery of volatiles to the inner system, the capture of retrograde moons by the giant planet, and the ejection of one or both of the protoplanets.

We show that an interesting fraction of terrestrial-sized planets with lunar sized companions will likely be ejected into interstellar space with the companion bound to the planet. The companion provides an additional source of heating for the planet from tidal dissipation of orbital and spin angular momentum. This heat flux typically is larger than the current radiogenic heating of the Earth for up to the first few hundred million years of evolution. In combination with an atmosphere of sufficient thickness and composition, the heating can provide the conditions necesary for liquid water to persist on the surface of the terrestrial mass planet, making it a potential site for life. We also determine the possibility for directly detecting such systems through all-sky infrared surveys or microlensing surveys. Microlensing surveys in particular will directly measure the frequency of this phenomenon.

Comments: 4 pages, 2 figures, Accepted to ApJL

Subjects: Astrophysics (astro-ph)

Cite as: arXiv:0709.0945v1 [astro-ph]

Submission history

From: John H. Debes [view email]

[v1] Thu, 6 Sep 2007 20:00:13 GMT (45kb)

Epitony May 18, 2009 at 8:29

I have to say, this is exiting…regardless if Gliese 581c is in the habitable zone or not, i find it incredible that we can make such discoveries with current technology. You always see how we would imagine ourselves exploring the vastness of space in warp drives or traversing wormholes somehow, but never the initial discovery! This is it people, this is where our curiosity blossoms, this is where we really shine as Human beings, a constant and adept (maybe not always conscious) tendency for discovery and knowledge, to question, to learn, to dream, to explore. Call me optimistic if you’d like. I for one have always had a fascination for space and the universe….when you stop to think of how small we are compared to whats around us it boggles the mind.
I won’t sit here and pretend i know all the technical words and science behind just how difficult space travel REALLY is but i know that we will some day soar through the stars with much less effort than we do currently…..there is just too much for us to discover for it to never happen. Short of wiping each other out here on earth i see nothing stopping us. I do know this….there are those that argue that there is just not enough resources found upon earth to make that dream reality, not enough metals, not enough fuel, in the literal sense i couldn’t believe that was a problem. I have also read that asteroids near mars contain much more metal and iron than all the metal found on earth….now logically one would see how we could fix that problem if we could manage to mine the asteroid. Mars isn’t exactly a casual walk to the neighborhood park, even i know this. Anything is possible, again, i will not pretend I know anything technical in this case, but i have faith in our ingenuity.

The more i read about this discovery the more fascinated i get. It’s my understanding that planets like Earth give off little radiation, meaning that some other sentient being out there possibly sees us as a very dimly lit “star” or maybe not at all. Would that be accurate? Hypothetically speaking of course. Planets like Earth, rock planets or ocean planets are not as bright as gas planets and suns, right?

What a grand discovery, incredible! So as it is now we do not know what the planet looks like because we lack the right technology, perhaps a few years from having such technology? The HARPS telescope can detect how large a planet is and how far from it’s primary star it is? Still very impressive! There are still lots of calculations to be made i’m sure, even if we had the means to reach something 20.5 light years away there are still trajectory and angle of atmospheric entry assuming the planet has atmosphere like ours or similar…..the planet in question orbits at a very high speed compared to earth. So it’s simple….all we need to do is create a warp drive…..do i have to think of everything? =) You know it’s funny to think that human imagination thinks up things like warp drives and beaming technology, but that is always the first step to actual design and viability…is it not? Maybe one day we will create such things to help us reach further into the stars.

Good day.

Epitony May 18, 2009 at 8:52

MirrorStephen – I find it strange that you say that seeing as at one point human beings thought the world was flat and science was seen as witchcraft. If we can imagine it, it is possible. You lack vision man…you also see an opportunity to put someone down on a forum, which in my opinion lacks class. Ok, so the science of Star Trek isn’t real, doesn’t mean that we won’t create something similar.

dd July 27, 2009 at 1:37

how about tidal lock…. this planet(glies 581) is thidal lock?
and how about the effect..? if we stay on the tidal lock planet?

Frank September 11, 2009 at 17:07

My theory is that it is possibly habitable, but then again it would be almost impossible to get there until technology permits us to. I consider a worm hole theory out of the question because I have read that you can get hrough no problem. Though on the way back you are 1 or 2 million years ahead of time on Earth. I do not know if thats credible but if I were an astronaut, I would not like to find an obliterated Earth or something. So close, but yet so far.

RJ October 14, 2010 at 23:30

We need to prepare to leave NOW! I almost cry because I’m scared my kids will die without a chance… Or their kids, or theirs… And their friends, families… Anything can happen…. Asteroids… Major solar flares… Gamma Ray Burst.. Anything. Some of us need to be somewhere, anywhere, besides Earth if somthing horrible happens… And that is sad… Seeing as I am not even old enough to drive… That should say something….

Samantha January 11, 2011 at 18:05

This didn’t help at freaking all!

Paul Gilster January 11, 2011 at 18:54

Samantha, the ups and downs of the Gl 581 c and d story, and the apparently false detection of e, has been covered again and again in these pages in the past three years. The story has taken a long time to resolve itself, and still leaves many questions open, especially about Gl 581 d. The many posts on Gliese 581 can be found via the search engine here.

Dan September 15, 2011 at 12:20

I can’t see getting started on sending a rocket of some type yet as the times we are talking about to get there are too large. For example if we sent out a rocket which would be half way to the target in 100,000 years technology would be so advanced by that time that we could catch up to that rocket in a couple of days. This is assuming that man is not stupid enough to wipe himself out with this new technology which would make atomic bombs looks like tinker toys. If we take care of the human race it could one day develop into something very important because of our curiosity and abilitys, then these exploration trips with be a piece of cake.

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