These days, every exoplanet discovery is still rich with excitement, as astronomers scrutinize each distant world and consider its possible characteristics. But this could get tedious pretty soon, as the number of confirmed exoplanets climbs into the thousands. When that happens, astronomers and especially astrobiologists will have to start sifting planets according to their interestingness. A new paper to be published next month describes a new two-step ranking system to make this process easier. We spoke to astrobiologist Dirk Schulze-Makuch to get some details.
Unlike other astrobiology criteria, this new system doesn't assume that habitability only applies to a rocky place with liquid water. Dirk Schulze-Makuch, first author on the new paper and an astrobiologist at Washington State University, said the team wanted to be as open-minded as possible.
Their proposed Earth Similarity Index looks at the size, density and orbital distance of a planet or moon, as well as its star's size and temperature, and compares these with Earth. The companion Planetary Habitability Index is based on the presence of some type of stable substrate, an energy source, appropriate chemistry, and the potential for holding a liquid solvent, not necessarily water. Both use a scale from 0 to 1.
So what's the most Earthlike habitable place (other than here) in the cosmos? In the burgeoning exoplanet pantheon, it's Gliese 581d, with an index of 0.74. The PHI has some different results, with the best option apparently Titan, with an index of 0.64. Mars rings in at 0.59, incidentally.
These numbers are only as meaningful as the data used to inform them, however, so the researchers stress there's room for updates and improvements.
PopSci talked with Schulze-Makuch to get some more details about the twin indices and how they could help astronomers make intergalactic classifications.
PopSci: Why do you think there are several habitable exoplanets?
Dirk Schulze-Makuch: Right now, the exoplanets we have discovered are still heavily biased toward gas giants, because they are easier to detect. But the longer we research, we are getting closer and closer to Earth-like planets. Most of them are still super-Earths.
One planet I am quite excited about is Gliese 581d, which is about 10 times the mass of Earth. It's in orbit around a red dwarf, so it is a sun that isn't as bright as our sun. It orbits closer to the central star, but if you put it in our solar system metrics, it is in an orbit basically like Mars. Mars is pretty small compared to Earth, but if Mars, in our solar system, was 10 times the mass of Earth, and still had a magnetic field and a thick atmosphere, it would have liquid water oceans. And in that way, it would be a habitable planet.
The point of the scheme that we came up with is really to prioritize and categorize, of the many exoplanets that are already discovered and will be discovered. We have now more than 700 confirmed exoplanets, and about 1,500 unconfirmed. And the count will be in the several thousands very soon. Since we only have limited resources, especially in these dark budget times, we really have to think about where we put our resources. What planets are interesting for life, and what planets do we want to study?
Within our solar system, you only have about 15 planetary bodies that are interesting from an astrobiology perspective, really. But we are finding so many exoplanets, we really have to pool our resources. Some of those are actually relatively close by. The Gliese 581 system is 20 light years away. It's far away, but that's not that far, in the overall perspective, so this is something that really should interest us.
So how does this new classification scheme work? Why does Titan look better than Mars?
We propose two indices. One is an Earth Similarity Index, that is based on size, and how similar that planet is to Earth. The second is a Planetary Habitability Index. There, we take a different approach, to be very open-minded. There could be planets that are not in the traditional habitable zone, meaning Earth-like orbit where water could be in its liquid form. In this index, [Saturn's moon Titan] comes out above Mars.
The reason is because Titan has a relatively thick atmosphere, composed of nitrogen and methane. It's actually very close to the early Earth. Titan has hydrocarbon lakes, so you have your liquid solvent, but that solvent is not water. Hydrocarbons as a solvent may be a possibility. We know carbon works very well for organics, and there are a lot of complex carbon compounds on Titan, so that is something we have to look at.
Another example is Europa — we have a liquid ocean, as [NASA announced last week], under the ice on that moon.
What does habitability actually mean? How do you define something like that, which is all relative to the place you're studying?
There are several parameters. One, for example, is whether a planet has an atmosphere. That it has carbon on it. We will look for nitrogen and phosphorus. Whether it has a magnetic field to protect it from cosmic radiation. Whether it has any liquid solvents on its surface that are stable there.
With that kind of categorization, we purposely are not Earth-centric. We are not saying there has to be liquid water, or it has to be in an orbit like Earth is around the sun. We look for organic compounds, but it's not that it has to be amino acids or anything.
One of our parameters is to have places where organisms could still do photosynthesis. Here, in our solar system, that extends all the way from Mercury to Saturn. Obviously, Earth, Mars and Venus are in a more suitable zone, but it could theoretically be to Saturn. So if you had a brown dwarf that has less light output, the planet has to be closer where you would still get enough light for photosynthesis. We tried to be as open-minded as we can.
As the Kepler Space Telescope (and the Corot Telescope) have made more exoplanet discoveries, we've started to think more carefully about what life might look like if we do ever find it. Seth Shostak at the SETI Institute has even suggested that we look not only for intelligent biological systems, but intelligent machines. Does your new system take any of that into account, planets that could harbor intelligent life or machines?
Our approach is life in general. In a previous version of the paper, we did have a biological complexity index. For certain complex microscopic life, we defined parameters, and said this is a planet that might have more complex life on it. But we thought this was really too speculative. On the microbial or generic level, we have a much better idea, so it would be easier to just make a general habitability index.
For Earth, for some kind of outside observer, it would be very difficult to detect any kind of spacecraft. They are too small. But what you would detect would be ozone and methane, and those compounds are not stable. They have to be constantly reproduced. If they would not be reproduced, you would have carbon dioxide out of those compounds. But we do measure ozone and methane, and that means there is some equilibrium, and there is something produced. This is a generic biosignature for life.
It could work for intelligent life, too, but what if you had something like this planet but 5 million years ago? You could have complex life, but not intelligent by any means.
What are some potential drawbacks of this new classification?
DSM:One thing that we cannot do is study the planetary history of a planet. We always see ti at a specific time. This is especially important with Mars. Current Mars, in its planetary habitability, is below Titan. But ancient mars, 500 million years after its formation, had an ocean on it, as it looks. There was sediment, there were flowing rivers. At that time, it still had a magnetic field, it still had a thicker atmosphere. So ancient Mars would have scaled a lot higher up on that index. But this we cannot know from any exoplanet that we look at right now. Mars is still reasonably high, but it used to be higher in the past, and from this we can infer that there is a good chance that there is still life on Mars. But with exoplanets we cannot do that.
We just want to make sure that no favorable target falls in the cracks. If the people who discovered exoplanets say, 'OK, this one was not in an Earth orbit, so by default this has no chance' — we want to have objectively some kind of parameter, where you might say, 'This is not an Earth-like planet, but this is still a very interesting planet for some exotic life form that could possibly exist on there.'
When do you think we'll find life on other planets?
I think the discovery will be in our solar system first, and I think Mars and Titan are the two places where the chances are the highest. Mars is probably most likely because of its history and its closeness to Earth. We know there are asteroids that can travel between the two planets. If we know there was life on early Earth, and Mars was very conducive to life, there should be life on Mars, we just have to find it and confirm it.
With Titan, it's more of a long shot, but would be even more exciting. If we do find life, it would be exotic; it would be very different from our biochemistry. Titan has these ethane and methane lakes, it's just so different. It would mean probably that there was a separate origin of life. For me, that would be the most exciting thing. On Mars, it would still be very exciting, but you can only say something about early evolution of life; you can't say something about how common life is in the universe and how different it may be.
With exoplanets, we may find some biosignatures. But at these great distances, it will be very difficult to confirm. If you do astrobiology you really have to get up close and personal. I can't see how that would be done, unless it was an intelligent civilization that was sending us signals or something like that. But you can say, 'That is a planet that is very conducive to life.' Whether there is life or not will be incredibly difficult to prove.
Earth has such extreme variety of life in extreme variety of locations.
We could look right at a habitable planet and think no life, but in fact be completely wrong since we did not see in some corner of that planet where everything was just right for some kind of life to exist.
I think we be more successful to find a planet that has an abundant about of life, since we are observing so extremely far away. But hey, I am just brain storming off the top of my head.
I want to wish everyone HAPPY THANKS GIVING!!!!!!
I am just so Qurious!
"Earth has such extreme variety of life in extreme variety of locations.
We could look right at a habitable planet and think no life, but in fact be completely wrong"
that is completely correct.
Life could very well take on a form we have not encountered on earth.
but they could be rated on HUMAN or mammal sustainability since we really are not concerned with seeding a planet with monera, then again... that may exactly what we want to do.
The mistake is to believe that most aliens are very much like us. Actually, though alien species are similar to human species, many of them are far more evolved than we are, a few are less evolved while even fewer are exactly at the same level as us. If we only look for planets that are suitable for people like us, we are restricting ourselves too much and making the task especially difficult. Those aliens who are more evolved than us can easily survive in environments which are impossible for us to live in. See ufocoverup.org for more info
Consider how long human civilization has been around. 10 to 30 years or longer and today with our modern technology we still surprise to find a new life form on our own planet.
So my point is about Mars. We see it as a whole and I see sand and no life. It may actually take a 1000 years searching across that planet to find life in some tiny corner.
So as scientist if we will wish to find life in our lifetime across the vast cosmos, I think we do better to look for a planet that has life in a gigantic way. Just consider earth as you see pictures of it from space afar. All the water, atmosphere, various colors, it just screams to have life.
If we spend our time looking for bacteria life on Mars, our search very well go on beyond our life time.
I am just suggesting we try to find life in a more obvious way as we search the cosmos!
I am just so Qurious!
ha ha, my writing skills, lol
Please insert 10 to 30 THOUSAND years above or longer, thank you.
I am just so Qurious!
"ha ha, my writing skills, lol"
um yeah... you spelled curious wrong too.
That is a Q thing. I did that on purpose.
But hey, if it gives you a smile, that is fine.
I am just so Qurious!
Aside from the point that the system is based on Earth, where would Earth land on that scale?
Why don't we put a hold on cataclysmic technology (aka nukes, big ones) until we colonize at least 4 or 5 planets plus.
That would be funny if aliens discover where Earth is based on remnants of a computer that specified the ideal human habitat. They pull up their galactic map and look for what fits the par.
@xalar "cataclysmic" technology is going to help us develop the technology that we're lacking for space travel. I think you'll be surprised what happens in the next 20 - 30 years
@Infinion right, nuclear reactors; the big batteries can easily be wired for self destruct(aka makeshift nuke). XD
(get it, stellar <3)
@xalar nope, nuclear reactors and nuclear bombs are two COMPLETELY different things
Are we alone in our solar system?
Life began in earth's oceans over 3.7 billion years ago it didn't start on land until 700 million years ago. Our outer solar system is where we found evidence of water ice, water geysers and organic chemistry on ice covered water worlds, all the necessary marker for life as we know it. Europa ice covered ocean, and the water geysers of Enceladus are just a beginning of more discoveries to come. As been suggested Titan is now being considered as yet another one of those ice covered water worlds that exist in our outer solar system, the list goes on and on which may also soon include a couple of planet size moons of Jupiter's, Ganymede and Callisto.
We are looking for life on a habitable worlds similar to Earths near a star which is a mistake. We are now finding out that a ice covered water-world can exist anywhere in space. Life exist at the bottom of our oceans where there is less light reaching this area than there is light from the sun in deep space far beyond Pluto. We now know where there is a large gravitational field such as exist at our gas giants that give these worlds enough heat through the friction of the constant tidal tugging over billions of years there could be life under a thick layer of ice with the right rich organic compounds that exist in a much greater amounts in the frigid parts of our solar system.
Recently we are finding out that warm interior worlds heated by internal radiation alone, even small one's like "Asteroid Lutetia May Have Heart of Hot Melted Metal," exist everywhere in our universe no matter how close to their local star.
Aliens visiting our planet may be from one of our outer solar systems ice covered water worlds. The aliens may not be looking for a nice warm spot to land but are looking for a nice comfortable spot that exist only at the bottom abyss of our oceans such as in the movie the "Abyss." The flying alien spaceships are only seen coming and going then classified as a UFO's, but our surface dwelling world is too toxic to spend much time because they are, fish out of water -- it's like humans spending time outside their shell in the vacuum of space orbiting the earth.
See reclassifying the term Habitable zone here:
I believe to have an astrobiologist look for new life on Earth defeats the meaning of his title. Astrobiologist. He's trained to look for life on these exoplanets and thats what he should do. Biologist look at like on Earth. Not astrobiologists.
And I'm not understanding what they mean when they bring up intelligent machines. They never really address it in this article. I'm assuming by the context it is in, they are talking about non-biological signs of life... An image of a robot just pops into my head every time i think of intelligent machines, but that may be just my own ignorance.
Here's a thought: they talk about "super-earths" with a mass 10x that of our own planet. At what point would gravity become a factor in the development of life?
Depends on your weight, if your a strong able body person around 80kg or less then it probably be ok to handle 2.0x the amount of G's. In fact it probably have some health benefits.
I'm not sure if a 10x larger planet automatically equals 10x the gravity, it would need 10x the mass at its core generating that gravity well, and the fact people will be 10x further out from the central point might mean that the gravity is reduced significantly.
I don't think a Super-Earth would always means its not able to support human life. Think we will haft to leave that question until we send out the first 0.5-0.8 x Speed of light craft out to have a look. Atm we are still fixated on using the slow chemical propulsion techs, so really our society is not ready to answer anything more then a whole bunch of maybes and similar speculation.
Even if we found a nice populated alien earth (somehow) out there we would not be about to do shit about it, Earth human society is facing a (major) financial crisis along with everything else, so its going be something for a future society completely different to today's to consider achieving.
Dreaming is nice, but our world is built on WAY too many assumptions and not enough science, in my opinion!
Were are not alone in the Solar System!
I am here now, in our Solar System, typing this! ;)
Science sees no further than what it can sense.
Religion sees beyond the senses.
There's actually a lot of considerations regarding weight. Weight = mass * gravity. However, gravity is going to change based on your mass, the mass of the planet, and your distance from it.
If you take a planet and simply multiply its mass by 10, not increasing its size, standing on the surface your weight would multiply by 10 exactly. But if you also increased its size by 10, your weight on its surface would actually divide by 10. If you took a planet with the same mass as Earth, but it was 10 times smaller, you would weigh 100 times more. The same holds true closer to Earth's core - if you mined down 90% of the way to the core of the earth, you would weigh 100 times more than normal (well, technically not -- cause now you have so much mass directly above you!).
However, that's far too simplistic. In reality, you may not be standing directly on the surface. Aquatic life may be somewhere closer to the centre of the planet. And as you see, distance from the centre has a more dramatic affect on the weight of a creature.
But that also brings into consideration other forces like buouancy. If life begins on such a planet in water, the force of gravity may not be nearly as relevant so long as the organism can float (although oceanic pressure would also raise dramatically the further down you go, possibly providing other issues).
As for why we're searching for planets we think have the likelihood of harbouring life, I think the article addresses it. We don't have the resources to search every planet we find (in our galaxy alone it's estimated there may be as many as over a million rocky planets orbiting stars).
While it's true life may -- even likely does -- exist far different from our carbon-based, land-loving (but water originating) selves, we don't know what those life forms will look like or the conditions they would thrive in, if it's even possible such life would ever naturally get its kick-start.
We do, however, know matter of fact 1 planet that has life, and the exact characteristics they require and that they can indeed begin naturally. So we search for that form of life - the one life form we know without a doubt exists and can exist elsewhere.
Later, as technology advances and we learn more, we'll expand our horizons. For now -- baby steps.
Alien life is more advanced than us? And you know this because you are part of a secret military organization that uses ancient Egyptian stargates to travel the universe and form alliances with alien civilizations. No? Ah, it was a shot in the dark.
Until we find the Asari I refuse to believe in aliens lol
Science is based on observable fact right?
We have observed life on how many other planets?
We have what observed evidence to suggest that we should reject the null hypothesis there is no other life out there?
And some of you all think the 'religious' are non-scientific.
For similar emphasis, like religion for spiritual people, alien life enthusiasts see the possibility as a positive force of hope, thus putting faith in the likelihood alien life does exist.
For people without the presence of 'god' in there life (whatever god might be based on your religious practice; monotheistic and polytheistic), there is the perception of being master of their universe. There is also the lack of hope and a swift channel into a river of despair. They chose not to believe in anything greater than themselves, so when things outside of their control take their joy away they have nothing to turn to of great enough magnitude to set them back on the right path.
The same could be said about anything someone tries to put their faith into. Faith in general is a positive force. It's a prime motivator. The people who speak ill of the religious (ignoring the fact that religion is culturally based spiritual practice unique to a specific culture, thus being universally contradictory) find more faith in the possiblity that life exist elsewhere beyond the this planet than the possibility of a powerful being or powerful beings that lord over us from some ethereal world we can not perceive.
The fact of the matter is, people put there faith in different things. Because each of these things are different, people will always disagree with the source of spiritual enlightenment and contentment. In the end it doesn't really matter where you place your faith. As long as it motivates you to do good, be better, and advance the race for the greater good, it's all good.
Only a little over half of science is directly based on observable fact -- although for a theory to be confirmed, yes, it does indeed require observational data. However, science itself is often conducted purely in the realm of the hypothetical, only later to be observably proven or disproven.
Hawkings radiation is one such phenomenon believed to be true in theory, but there's no observable evidence yet for it. That said, if and when we ever do have observable evidence of Hawkings Radiation, Dr. Hawking will get his Nobel prize for it.
As for proof of life out there -- you're absolutely right. And no scientist that I know of would say life definitely exists, despite there being no proof (or at least, no scientist would stake their reputation on such a claim).
You'll find individuals out there such as some of the folks on the forums here claiming there has to be aliens/ufos/whatever -- but they aren't scientists.
Having said that, there's still a very, very high probability life does exist somewhere. There's actually a number of studies done to determine the actual probability of life existing out there, and that probability increases with every new planet we find.
The vast majority of life out there -- if any exists at all besides us -- is likely to be far less advanced, but there may be some, perhaps small, chance they're even more advanced -- though it's incredibly unlikely they're anywhere near our level of technology, either way.
I should point out I'm an atheist, but I'm also an extremely conservative believer (hopeful?) regarding life existing out there. It's certainly not something I'd put my faith into, and it doesn't make me sleep any better (or worse).
Though, your point that we all put our faith into something is certainly true - that's a human condition, I would say. For me, I have faith that all things have a material cause - some potentially understandable precursor.
And this is something that gives me hope in life.