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One of the several planets within the Gliese 581 star system, called Gliese 581d, ranks among the most potentially habitable alien worlds on a new scale.
Astronomers have come up with a livability index for alien planets and moons, and the winners are ... Titan in our own solar system, and the Gliese 581 planets in the extrasolar league.
Rating systems for Earthlike and habitable planets may not make much difference now, but the developers of the Earth Similarity Index and the Planet Habitability Index say they could be crucial in the years ahead.
"With a new generation of telescopes and missions on the way, the discovery of many more exoplanets can be expected," they write in a paper to be published in the December issue of the journal Astrobiology. "That, in turn, will drive the need for a classification scheme for assigning astrobiological potential for exoplanets based on estimates derived from quantitative data of their probability for supporting life."
If such a scheme could truly reflect whether or not a given planetary environment is habitable, that could drive the priorities for exploration in our own solar system, as well as high-resolution observations of extrasolar systems.
Habitability indexes have been in the works for at least the past couple of years. Traditionally, astrobiologists have focused on three conditions that appear essential for life on Earth: organic compounds, the presence of liquid water, and an energy source such as the sun or undersea volcanoes. But in the search for alien Earths, those conditions aren't easily determined, and they may even be irrelevant.
The newly proposed indexes take a two-track approach to the classification challenge.
"The first question is whether Earthlike conditions can be found on other worlds, since we know empirically that those conditions could harbor life," Dirk Schulze-Makuch, an astrobiologist at Washington State University who is one of the study authors, said in a news release. "The second question is whether conditions exist on exoplanets that suggest the possibility of other forms of life, whether known to us or not."
The Earth Similarity Index looks at the size, density and orbital distance of a planet or moon, as well as the size and temperature of its parent star, and compares those parameters with Earth's. Earth has the maximum global ESI of 1. Mars has a 0.70 rating, and Mercury is the next on the list with 0.60. For what it's worth, the dwarf planet Pluto and Neptune's moon Triton register a measly 0.075 and 0.074, respectively. And Enceladus, the icy Saturnian moon that is thought to harbor a subsurface ocean and perhaps life, is right down there with them at 0.094.
Looking beyond the solar system, the researchers worked up ESI values for a variety of extrasolar planets. The top finishers were Gliese 581g (whose existence is in dispute) with 0.89, and Gliese 581d with 0.74.
But that's just the first part of the job: The researchers' Planet Habitability Index looks at a different set of factors: Does the planet have a rocky or frozen surface? Is there an atmosphere, and how thick is it? How about a magnetic field? How much energy is available, either through tidal flexing or from the parent star? Could there be organics present, and is a liquid solvent available for chemical interactions?
By those measures, Earth has a relative PHI of 0.96, which is nearly as close as you can get to the maximum of 1. Based on what's known about the rest of the solar system, the runner-up is not Mars, as you might expect, but the Saturnian moon Titan (0.64 vs. 0.59 for Mars). The Jovian moon Europa is next on the list (0.47), but Enceladus (0.35) ranks lower than Venus, Jupiter and Saturn (0.37).
The authors stress that expectations based on earthly life may not apply to extraterrestrial environments.
"Habitability in a wider sense is not necessarily restricted to water as a solvent or to a planet circling a star,” they write. "For example, the hydrocarbon lakes on Titan could host a different form of life. Analog studies in hydrocarbon environments on Earth, in fact, clearly indicate that these environments are habitable in principle. Orphan planets wandering free of any central star could likewise conceivably feature conditions suitable for some form of life."
So how does the Gliese 581 system's PHI look? Gliese 581g's value was estimated at 0.45, 581d registered 0.43, and 581c came in at 0.41. By that scale, the chances of finding life in a red-dwarf system 20.5 light-years away (or sustaining life if we ever get there) are about as good as they are for Europa. OK, but not great.
It's important to keep a couple of things in mind about this research: First of all, there's a fair amount of speculation about the various factors and their relative value for habitability. Further observations may shift the values for those factors, as well as the mathematical formula into which they're fed.
Perhaps more importantly, the numbers game can't take the place of actual observation and exploration. The ESI and PHI may well turn out to be thought experiments like the Drake Equation, which takes your assumptions about a variety of cosmic factors (How many planets like Earth come into existence every year? How likely is it that intelligent civilizations arise on alien Earths? How long do they last?) and turns them into a number. At least that's the message from David Morrison, director of the Carl Sagan Center for the Study of Life in the Universe, headquartered at the SETI Institute in Mountain View, Calif.
Here's what Morrison told me in an email:
"Very interesting. Discussing such conceptual indexes is a good way to organize our thinking about worlds that may be suitable for life. But it doesn’t actually add value, in my opinion. For the Earth Similarity Index, we already have thought that liquid water, and a solid surface, and enough gravity to hold on to a substantial atmosphere, are important indications of habitability. Hence the interest in Earth-size planets within the habitable zone (meaning surface liquid water is possible). To go further, as by considering the composition of the atmosphere, we are quickly into the effort to identify life by its chemical signatures, not just habitability. The broader habitability index in also interesting, but we just don’t know how to define habitability. And if Titan is an example, we may never have the data on exoplanets that could distinguish the hydrocarbon liquid lakes that we see on Titan.
"Bottom line: This (like the Drake Equation) is a good teaching tool. It helps is to organize our thoughts. But I doubt it will be very useful as a research tool, because we know so little about what properties truly define habitability. Without a much better idea of what alien life is like, we don't know how to define habitability. And probably nature is much more creative than we can imagine."
What do you think? Where would you target the search for extraterrestrial life, and what criteria would you use to prioritize the targets? Feel free to weigh in with your comments below.
More about the search for alien life:
- A new equation for life
- Case builds for habitable alien planet
- Super-Earth on the edge of habitability
- More about astrobiology from Cosmic Log
In addition to Schulze-Makuch, the authors of "A Two-Tiered Approach to Assessing the Habitability of Exoplanets" include Abel Mendez, Alberto G. Fairen, Philip von Paris, Carol Turse, Grayson Boyer, Alfonso F. Davila, Marina Resendes de Sousa Antonio, David Catling and Louis N. Irwin.
Connect with the Cosmic Log community by "liking" the log's Facebook page, following @b0yle on Twitter or following the Cosmic Log Google+ page. You can also check out "The Case for Pluto," my book about the controversial dwarf planet and the search for new worlds.
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I don't really think we have enough information to make an informed judgment on the habitability as of yet. We are sending the Curiosity rover to Mars to determine that planet's habitability. Without similar first hand knowledge of other places in the cosmos I think this kind of index is a little premature. But that's just one guy's opinion. If it helps our exploration of space then I'm all for it.
weeeellllll.....maybe if you look at it as "theoretical" livability list maybe it has some value. But I agree, until we can get there or send a probe there in a reasonable amount of time it's a fairly pointless exercise.
Let's just keep our fingers crossed that those little neutrinos really are as fast as some physicists think they are and maybe FTL speed will be a possibility in another 100 years. I don't know about you but I can't wait...no, really...I'm 61 years old and can't wait. But some of you youngsters might be amazed.
In the meantime, I'm anxious to see what the new Mars rover digs up. I'm still betting on a past civilization buried beneath the sands of Mars. John Carter, Dejah Thoris, Tars Tarkas, they're all there just waiting to be found. Well, at least in my dreams.
I don't know, mob; I think we can at least make estimates of habitability based on current knowledge as long as we keep it to habitability for critters like ourselves and don't make too many assumptions about what ET may be like, and as long as there's a very big asterisk next to the statement acknowledging how much we don't know.
I'm with you, though, if it increases interest in the subject and furthers space exploration, it's a good thing.
Skip unfortunately FTL travel will only be beneficial for use in our own solar system or systems very close to ours, when you take relativity into account any long term travel would have to be by another means otherwise you could run into a scenario where we travel 1000 light years and find out that our descendants already developed technology that was better so they beat us to our destination.
Skip, Gosh those names brought back memories!!! Ah, good ol' S.F.!! as for the topic, I'll have to agree with Mob'as usual...
Think of it this way. The Wright brothers first flew in 1903. 100 years later we have had manned missions to the moon, robotic missions to mars and beyond. We fly to every corner of the globe. The first telegraph was sent in 1866. Now we communicate instantly all over the planet. That took a little longer. But the way technology advances, we will probably have a totally different mode of transportation and communication in the next 50 years. C'mon transporters, warp drive, and subspace communication!
I can't wait for the day that human survivability on the Mars is no longer a distant hope. Mars is a hostile place but I beleive humans will be able to live somewhat comfortable with the right supplies and will be able to thrive with the right technology to be self sufficient. Once that occurs then the universe is our sandbox. There are enough people with the urge to explore that they will devote their life to see what is out there and beyond first hand. Perhaps one day...
I really think that our understanding of habitability would benefit from actually inhabiting something (other than earth). How about Mars? Only after we've colonized a planet will we really know what it takes. What are we waiting for? Why aren't more people supporting this effort? Let's colonize Mars!!
Do you have any idea of what it would take to colonize Mars with say a hundred people. You cannot just wish for a Martian atmosphere and any colonist would have to live in controled huts with everthing they woul d need brought from Earth on a continuing supply of ships. They would have absolutely nothing to send back to Earth except a how long before the next shipment will be sent. The dream of creating an Earth type atmoshere would take a hundred thousand or more years to create a ten foot high belt of breathable air around Mars. It is to cold to grow most of the food stocks indemnic to Earth outside of a hut. Solar radiation would also be a big problem. In time we might be able to tap into Mar's left over water but we would need the equipment to convert this into Oxygen and Hydrogen for fuel. I estimate that it would take two to three hundred years to arrive at a sustainable population of several hundred souls at a cost of several trillions of dollars and Joe Blow won't be one among that number.
N.C., what do you mean "there would be nothing to send back to Earth except a how long before the next shipment will be sent"? There is hematite iron on Mars, which we earthlings can use. There are also probably a good amount of other metals, minerals, and maybe interesting stones that can be used for many things. I would say that probably a colony would be good if miners, smelters, and refiners were the largest part of the population. That way, materials would flow in both directions. Foundries on Mars would help send lots of green house gases into the atmosphere, which would raise the surface temperature which would help in terraforming the planet. And let's face it...if we can find a way to warm a finite area, plants will probably grow there. After all, plants take in carbon dioxide (which seems to be the majority of the Martian atmosphere) and give off oxygen (which is what we need). I'm sure if we all put our thinking caps on, we can work it out.
This is just an inordinately stupid waste of time. You can't define the conditions that result in habitability, and you can't observe those conditions directly if you knew them, and most importantly, it's worthless information even if you could figure it out. Use your brains to cure cancer or something actually useful. Sheesh.
If they used you as an example of life here on Earth, this planet would get an extremely low rating on intelligent life here.
"You can't define the conditions that result in habitability"
Not very well yet. No one pretends to have the last word. Science never does.
"and you can't observe those conditions directly if you knew them,'
So all those telescopes and current/future space probes are for nothing?
"it's worthless information even if you could figure it out"
Mike, it's not worthless because you don't care.
"Use your brains to cure cancer or something actually useful. Sheesh."
I wonder what people will use for an excuse when cancer is cured...?
I think the search for extraterrestrial life should be done on two fronts; human exploration and robotic exploration. Robots are best suited for places that score lower on the planet habitability index while humans should begin with places that score high on the Earth Similarity Index.
But of course my opinions are based on the notion that the Earth harbors lifeforms that are collectively interested enough in their cosmologically origins to commit the resources necessary for discovery and are intelligent enough not to become so embroiled in sociological bickering that they are unable to investigate their surroundings. If there was a scale for these two measures Earth would score kind of low right now.
If we do leave the solar system it will be so far in the future none of this crap is going to matter much. Right now if you asked NASA to get us to the moon they couldn't pull it off.
You can blame cuts in funding for that. Lets face it the moon is not in our reach until China is set to make the leap, then the USA will be shamed to action. Dont believe me look at the cold war.
I postulate that we could get back to the moon in 5 years if we just spent the proper resources. If NASA didn't have to go through a yearly budget axe, It'd be a different story.
"Dont believe me look at the cold war."
Pirate, we perceived the Russians as 'backwards' in the 50's. Thus a 'Sputnik moment.'
We have no such misconceptions about China today. That they might do something highly technical would just not be a surprise. The Cold War is over.
Being Earth-centric and Human-centric, those indexes could potentially be useful for "where's the best place to start a new colony", but much less useful for "where are we most likely to find extraterrestrial life". Life tends to evolve to fit it's environment, so life could be thriving in places quite different than on earth.
As for colonizing other planets, that's not going to happen until the cost of space travel is dramatically reduced, which means using something more efficient and economical than current rocket designs.
Time and money are constructs of man. We just have to decide what is more important, long term survivability on more than one planet, or perceived wealth.
rwalker #7.1, you might as well get used to it we aren't going anywhere, unless Kirk decides to pay us a visit and replenish his crew. We are in a lifeboat, Earth, and it is on a never ending ocean through space. We have to learn to make do with our habitat until nature takes its course as it does with all species and reduces our number to a manageable amount of inhabitants. You may not want to believe it but it did this through wars and smaller skirmishes throughout man's history. This is what kept the population in check. No species can live beyond its means of sustainance. Most animals keep their population in check by limiting the number of offspring they bring to life. Man has no built in physical check to do this and prolificates without regard as to his ability to feed himself. War is the answer, except the moralist and humanitarians have got in the way and have prolonged the onset of war far beyond man'e ability to feed himself. Nature has had to take a hand and is gradually leading us into a big one. I predict that within twenty years we will see a reduction in the world's population by two billion souls through war and its after effects. It will slow us down for a time but a good war will have to do until that next meteor comes along.
It might be disease. Antibiotic resistant bacteria strains are coming along all the time, viruses are mutating faster than ever. With this much population, it won't take much to get a good disease spreading.
I note that these indices are for habitability in the abstract and that even the Earth Similarity Index does not rate planets for habitability by terrestrial life forms. Notably, there does not appear to be a score related to having a gravity in the 1G range, even though we know from the experience of 50+ years in space that terrestrial life does not do well, long-term, in a microgravity environment. Gravity is something we do not know how to manufacture (even in theory), and gravity substitutes (such as constant 1G acceleration or giant centrifuge biospheres) are currently beyond our technological capability. We do not know yet how much gravity is required by terrestrial life forms, so we cannot say how habitable mini-gravity environments such as the Moon and Mars would be. In this solar system, Venus (so hot that it would be necessary to live in artificial underground biospheres) has a gravity roughly comparable to terrestrial gravity, and the upper atmospheres of Uranus and Neptune are also in the 1G range (although it would take the ability to build floating biospheres -- currently beyond our technology -- in order to live there).
Determining habitality is guesswork in the extreme. Some twenty five years ago if you had told our biologist that life could exist and florish four or five miles beneath the sea, he would have told you to apply for a bed at the nearest asylum rest home. Every biologist knew that life had to be solar energized. They stated this as an a priori fact. Then nature reared its ugly head and showed them life living in an environment of hundreds of degrees Farenheit, existing off of bacteria which converted Sulphur into a usable form of life sustaining existence. These are not plants but animal forms of life. Oops, we were wrong and have to rethink our a priori theory. How far wrong are these indices of probable life on other worlds. The universe is so diverse in everything that the only indices which could probably come close to being right are those which start at zero and rate everything else as being a possibility. In other words it is blather guesswork and more suitable for shamans or witchdoctors.
We should send small comet-like probes to them loaded with lower life form's DNA and other biological building blocks to start terraforming. Maybe by the time we can get there they will be along the path to habitability. Hmmm, does anyone out there feel like they've forgotten something, anyone?
Sounds like the "Genesis Experiment" from Star Trek II: Wrath of Khan. Plant the seeds and let it grow.....
Nah, they used proto-matter.
It will be a moot point if humans don't start behaving better on this one.
Morrison of SETI is betraying sheer desperation in light of their ongoing failure in detecting alien life by resorting to semantics and conceptual calistenics in an effort to devalue these habitability indexes by inferring that we really do not know what other other types of life there may be in the universe and how to define habitability. Oh ... we pretty much do know by now. There is for sure life elsewhere but please put a cork on the Hollywood science fiction!
I have to disagree, we don't know for sure there is life elsewhere.
KNLG1,
It's true we don't have proof as yet. But I think what Jose V is referring to is the low odds of not finding life, given there are billions of stars in the universe. And I agree. Life surely exists elsewhere. We have to find out where.
Does this have any implications for where we might direct radio signals in the attempt to communicate with alien life?
Bill, I have a feeling that with enough looking, we might find some kind of markers or zones that life is more likely to occur in - moreso than the goldilocks zone around a star... I could be wrong, but I feel that there may be parts of the universe with galazies a certain age, with a certain amount of stars of a certain type... on and on and on until there is a higher liklihood of life.
Your question is the prime reason that SETI is an exercise in futilty. If we detected a signal, by the time we received it, that body sending it would have moved a distance depending on its speed and direction away from the point of it first message dot or blip and the light years it took for it to travel to Earth. We would probably need upwards of twenty years to decipher the message, and would then be faced with where should we direct a response at a body moving in an unknown direction at an unknown speed. If for instance we received a signal today from an object estimated to be say thirty light years away, we would have to calculate where that body would be eighty years hence. The further problem would be we might think that it was coming from some star thirty light years away but it could be coming from some star a hundred light years away closely aligned with our suspect star. The even greater problem is what society would invest the power requirements to overcome the background radiation of its parent star. If we wanted to send a recognozable signal to Alpha Centauri we would have to use enough power to provide all of the Earth's power output for ten years to send a ten minute series of dots and blips with only the slimest chance that someone would be listening and return a signal some twenty five years hence. This is one of the reasons most governmments stopped funding SETI. One day scientific dreamer man will realize that perhaps the distance between stars is what it is because we are not permitted or allowed to communicate with each other by the universal God being.
Say we do put a colony on some distant rock someday, how long before we flood it with drugs and put up a fence? Food for thought.
Maybe when we vote Obama out of office he could run for office on Mars. But wait there is nothing he could screw up or ruin due to lack of leadership on Mars.
Newt in 2012
Barry could even screw up what's happening on Mars!!
Actually, the question is: Based on what you've just read, where do humans start looking for other homes?
And the answer is: Mars and Titan (with plenty of exploration of other worlds too)
Its time to get started on this project. We're going to need options.
Try Australia or New Zealand!!
Actually, we need to learn to live in space (not the ISS), where people live, work, have families and grow food. Also, Lunar bases. These will help us solve a number of problems before going further out.
If aliens are watching us (doubtful) they are probably wondering why we don't live on our satellite yet.
I believe that the possibilities DO exist that we could go to and live on other worlds. I know I will never see it, being 69, but there's always hope for the youngsters. Besides being an actual Railfan, I'm also a Sci-Fi buff, and I can tell you for sure that a lot of stuff that was poo-pooed when I was a kid are reality today. And in reply to those of you who feel that this is all nonsense and pipe dreams, I reply with a line James T. Kirk never said but should have: "Beam me up. Scotty! There's no intelligent life here!"
I can't help but think even the next 10 or 20 years are going to be just incredible. If I had to guess you will see proof of extraterrestrial life before your days are done.
Pluto is now called a 'dwarf planet'?
Mars and Titan are nice, but for the winter months I prefer Venus
Probability indexes and theoretical indexes aren't worth the paper they are printed on, because we will never, I repeat never get close enough to any other body in the Galaxy to confirm them one way or another. The statistical models are probably more accurate than any other theory, they being that for every billion stars there is probably one thousand that has an Earth like Planet, and of this thousand possibly ten has intelligent life ranging on a level of we being a ten and Neanderthal being a one. Probably the other nine hundred ninety harbor some form of life from viral to crude organisms as found in our fossil history. All stars are essentially created and develop as our star has and has the same complement of elements in its makeup, ergo a good assumption would be that any life forms would be patterned closely to what we find on Earth. If science is correct we could substitute Silicon for Carbon and still have a close resemblance to life as we know it. It will be found that visiting extraterrestrial bodies is an impossibility by man in a space suit or spaceship, and even visiting Pluto is a near impossibility for man. We may visit by unmanned probes, but the logistics pose an insurmountable problem for a manned flight to Pluto. A probe sent to our closest stellar neighbor, Alpha Centauri, would be unable to communicate its findings back to Earth via radio. The power requirements to overcome the background radiation of this star or any of its sisters would be, again, insurmountable. It would take nine thousand years at a minimum for a probe to reach this system. In that time NASA or any other agency would long be out of business and the only receipients of any message, if one came, would be that diehard bunch at SETI if it still existed. For those who think that we can develop some exotic means of propulsion which will put our speed of transit up to close that of light, this is wishful thinking on two counts. First no mass based object can travel at a speed greater than about five hundred thousand miles per hour and remain an intact mass, physically both from a practical and a theoretical basis this is the limit. Secondly, nuclear power is the absolute highest form of energy obtainable and it has limits of endurance and application. Not withstanding recent reports of velocities greater than that of light, they are erroneous from any number of theoretical concerns. While the speed of light depends on the factor of the momentum of time at its point of measurment, it cannot be overcome or exceeded from a relative point of view. An Astronaut fifteen billion miles from Earth might experience light traveling at some relative velocity faster than he did while on Earth but his clocks would still show the same velocity as they did on Earth. We on the other hand back on Earth might find that he was traveling twenty miles faster than our value on Earth called for. Ask NASA about the two probes sent out in 1972, for verification of this.
We have wasted trillions of dollars on space research?? And for what ... what has it brought us that makes those trillions worth what we have found. You think that maybe we could use those trillions for something worth while at this point?? Personally, I'd rather see the trillions spent on cancer research.
And what if the miracle cancer drug has to be made in zero gravity? I guess we will never know if we cut space research.