What makes the Earth habitable?

[Ann]

I grew up thinking that really the only thing that determined the habitability of a planet was whether it was inside the habitabe zone of its solar system or not.

When I was a kid in the sixties, my parents had a coffee table Reader's Digest style book about the natural world. I read that book a lot, and I definitely thought that it was serious. My favorite chapter was one that predicted that people in the year 2000 would visit Mars in the same way that people in the sixties visited Hawaii. There was an illustration of a little girl of perhaps three playing in a charter spaceship taking her and her family and a lot of other people to Mars to see the sights there. The author of the text reminded us that visits to Mars would require many more precautions than the usual malaria and tourist flu shots, and in all probability visitors to Mars would have to remove their wisdom teeth and appendices before embarking on their trip, just in case. Getting to see Mars would be a lot of fun, however, and while there would be no little green men there to greet the visitors (due to the atmosphere, which was too thin to sustain such complex life forms such as little green men), the Earthlings would at least be able to admire the Martian vegetation, which was undoubtedly the cause of the seasonal variation of the coloring of the Martian landscape which could be seen from the Earth. The Martian vegetation was expected to be particularly verdant and flourishing on the banks of the Martian canals.

Just a few years ago I came across an English translation of a Soviet book written around 1960, when people in the Soviet Union were still bursting with pride after their Sputnik achivement in 1957. The book took pains to explain many aspects of space flight and more basic concepts such as gravity, but it also speculated on what future cosmonauts might see when they reached other planets. On Mars the cosmonauts would be greeted by the intelligent inhabitants there. The sophisticated Martians had already made their presence known to us Earthlings through their planet-encompassing system of canals, which could be seen from the Earth. Or so this book said, anyway.

The book also predicted that Soviet cosmonauts would visit Venus. Much less was known about Venus than about Mars, the book explained, because the surface of Venus was perpetually hidden by thick clouds. However, since Venus was situated inside our solar system's habitable zone, Venus was expected to be much like the Earth, but warmer. They book hypothesized that Venus was covered by huge swamps where exotic animals may be roaming.

I vividly remember an article in my local newpaper from about 1970, where the science-dabbling reporter was shocked and horrified at the latest news from NASA, which revealed the surface of Venus to be hot enough to melt lead. It was not supposed to be like that, the reporter wailed. The surface of Venus was supposed to have a temperature of about 80-90 degrees Celsius, and there were supposed to be steaming hot ponds of water there were heat-loving bacteira would thrive. How, asked the reporter, can Venus be inside the habitable zone and still not be the least bit habitable?

In another folder in this forum, there has been a thread suggesting that the Kepler mission has found "hundreds of Earth-like planets". While the thread in question clears up the confusion between Earth-like and Earth-size planets, the thread still remains, in my opinion, very optimistic about the number of habitable (and, we may suppose, therefore also inhabited) worlds in our galaxy:

There may very well be hundreds of millions of potentially habitable rocky planets in the Milky Way, but solid estimates are still a ways off.

Hundreds of millions of habitable, or at least potentially habitable, rocky planets like the Earth in the Milky Way? Well, if the most optimistic interpreters of the Drake equation are right when they suggests that there may be a hundred million technological civilisations in our galaxy, then there had better be several million planets whose inhabitants have not yet become advanced enough to, like the Martians, build a planet-spanning system of canals.

Oh. There are no intelligent Martians on Mars and no planet-encompassing system of canals. I forgot.

This is my point. I was made to believe, when I was a kid, that being inside a theoretical habitable zone was all it took for a rocky planet to produce fairly complex life forms. When I read today's news about extra-solar planets, I feel I'm being told that these planets are expected to be habitable and probably inhabited as long as they are made of rock and orbit inside their star's habitable zone. I definitely think that the general public will get this impression of the news they are exposed to.

So I'll ask you this question. What makes the Earth habitable and so abundantly inhabited? Is it just that the Earth is made of rock and orbits at such a comfortable distance from the Sun? Venus is also made of rock, and it is almost exactly the same size as the Earth, but it orbits too close to the Sun. What if we could put Venus in Earth's orbit? Would Venus become habitable? Would it suddenly have oceans? Would it start sprouting all kinds of life?

Maybe Venus has had such a runaway greenhouse effect that it can't be made habitable even if it could be put in an Earth-like orbit. What if Venus could have been put in an Earth-like orbit when the planet was young, however? Would that have made Venus evolve into a kind of Earth? Would it have had an Earth-like atmosphere and Earth-like oceans, if Fate had just been kind enough to put in an orbit exactly one A.U. from the Sun?

Personally I really doubt that Venus could be "fixed" that easily. And I definitely believe that many more Goldilock factors were required before the Earth could start teeming with life.

I just think that the search for life-friendly planets in the Milky Way is too simplistic. Surely there is more to the life-friendliness of the Earth than the rocky composition of the planet and the size of its orbit around the Sun?

Ann

[Henning Makholm]

I grew up thinking that really the only thing that determined the habitability of a planet was whether it was inside the habitabe zone of its solar system or not.

By that criterion, the Moon should have been habitable.

This is my point. I was made to believe, when I was a kid, that being inside a theoretical habitable zone was all it took for a rocky planet to produce fairly complex life forms. When I read today's news about extra-solar planets, I feel I'm being told that these planets are expected to be habitable and probably inhabited as long as they are made of rock and orbit inside their star's habitable zone. I definitely think that the general public will get this impression of the news they are exposed to.

So how would you propose the news be framed?

Until recently it was genuinely an open question whether Earth-Mars-Venus-Moon like planets at all are a common phenomenon. It might have been just a freak once-in-a-galaxy accident of the particular circumstances of the cloud of matter that became the Solar system that caused such planets to form and settle into nice circular orbits. Finding that planets of this kind are indeed common would remove a significant amount of uncertainty from the Drake formula, even though there is still uncertainty enough to go around.

Star type, planet size, position, atmosphere, water, tides, life, plate tectonics, ores, technology -- it is very easy to come up with necessary conditions for a recognizably Earth-like technological civilization arising elsewhere, if not otherwise then by raising the bar for what you consider "recognizably Earth-like". But nobody at all knows any sufficient criteria. As long as we know only one example of a stellar system that did in fact produce a technological civilization, there is no telling how much of the responsibility for that boils down to pure accidents that could not have been foretold before they happened.

I just think that the search for life-friendly planets in the Milky Way is too simplistic. Surely there is more to the life-friendliness of the Earth than the rocky composition of the planet and the size of its orbit around the Sun?

I'm confused as to what you think should be done differently. Do you argue that it is "too simplistic" to do anything that won't resolve your entire favorite question in one fell swoop? We'd never have gotten anywhere without incremental advances.

Was is "too simplistic" to invent the wheel before internal combustion was invented? Surely there is more to a car than having it roll?

[makc (guest mode)]

I bet it's life itself that transformed the planet out of giant rock surrounded with gas into organic paradise. Those little, microscopic green fellows who feed on sunlight.

[orin stepanek]

Maybe Venus is still cooling? http://www.science-frontiers.com/sf014/sf014p03.htm

[Chris Peterson]

So I'll ask you this question. What makes the Earth habitable and so abundantly inhabited?

That is two questions, and very different ones.

Your choice of the word "habitable" may be a little off, too, since as applied to humans now it means something very different than when applied to life in general. But my answer to what you need for a planet to support life is this: a suitable liquid solvent, and a source of energy. In the case of the Earth, the solvent is water and there are multiple energy sources- the Sun, internal heat, and chemical energy. I think any planet with these two things will generally be habitable, assuming it doesn't exist in some sort of extreme state, like massive gravity or gamma ray bombardment. Of course, the idea of habitability might be extended if you consider more exotic definitions of "life".

The much more complicated question has to do with whether life will actually develop on a habitable planet, and whether it will thrive as it has on Earth. That would seem to be a question that can't be answered much beyond pure speculation. It seems quite possible that a planet with a simpler environment- say, no tides, no tectonics- might produce a much more uniform sort of life. I'm imagining a planet covered with algae, for instance, where there is very little in the way of evolutionary pressure to force diversity. That pretty much describes life on Earth for a billion years or more. Not that you wouldn't get change, but it could be very slow.

[neufer]

The much more complicated question has to do with whether life will actually develop on a habitable planet, and whether it will thrive as it has on Earth. That would seem to be a question that can't be answered much beyond pure speculation. It seems quite possible that a planet with a simpler environment- say, no tides, no tectonics- might produce a much more uniform sort of life. I'm imagining a planet covered with algae, for instance, where there is very little in the way of evolutionary pressure to force diversity. That pretty much describes life on Earth for a billion years or more. Not that you wouldn't get change, but it could be very slow.

Satellite, (from Latin Stelles, -itis, an attendant.)
Stromatolite, (from Greek στρώμα, strōma, mattress, stratum, and λιθος, lithos, rock)

Stromatolites at Lake Thetis, Western Australia

---

<<Stromatolites are layered accretionary structures formed in shallow water by the trapping, binding and cementation of sedimentary grains by biofilms of microorganisms, especially cyanobacteria (commonly known as blue-green algae).
Stromatolites are a major constituent of the fossil record for about the first 3.5 billion years of life on earth, with their abundance peaking about 1.25 billion years ago. They subsequently declined in abundance and diversity, which by the start of the Cambrian had fallen to 20% of their peak. The most widely-supported explanation is that stromatolite builders fell victims to grazing creatures (the Cambrian substrate revolution), implying that sufficiently complex organisms were common over 1 billion years ago. The connection between grazer and stromatolite abundance is well documented in the younger Ordovician evolutionary radiation; stromatolite abundance also increased after the end-Ordovician and end-Permian extinctions decimated marine animals, falling back to earlier levels as marine animals recovered.

[img3="Pre-Cambrian stromatolites (commonly referred to as "Mary Ellen jasper") on display at the North American Museum of Ancient Life. The fossil was originally excavated in the Iron Range of Minnesota."]http://upload.wikimedia.org/wikipedia/c ... olites.JPG[/img3]

While older, Archean fossil remains are presumed to be colonies of single-celled blue-green bacteria, younger (that is, Proterozoic) fossils may be primordial forms of the eukaryote chlorophytes (that is, green algae). One genus of stromatolite very common in the geologic record is Collenia. The earliest stromatolite of confirmed microbial origin dates to 2.724 billion years ago. A recent discovery provides strong evidence of microbial stromatolites extending as far back as 3,450 million years ago.

While prokaryotic cyanobacteria themselves reproduce asexually through cell division, they were instrumental in priming the environment for the evolutionary development of more complex eukaryotic organisms. Cyanobacteria are thought to be largely responsible for increasing the amount of oxygen in the primeval earth's atmosphere through their continuing photosynthesis.>>

[Ann]

Chris said:

Your choice of the word "habitable" may be a little off, too, since as applied to humans now it means something very different than when applied to life in general.

Rest assured that I mean life in general.

But my answer to what you need for a planet to support life is this: a suitable liquid solvent, and a source of energy.

I get that. My real question is how likely it is that rocky bodies within a star's habitable zone will have solvents and an energy source.

I think any planet with these two things will generally be habitable.

How common are these two things on rocky planets inside a star's habitable zone? Is it Venus that is an odd planet because it has no solvent at all, or is it the Earth that is remarkable because it has what it takes to support life?

I wonder how much the Copernican principle has confused our thinking about the universe. This is what Wikipedia says about the Copernican principle:

In physical cosmology, the Copernican principle, named after Nicolaus Copernicus, states that the Earth is not in a central, specially favored position.[1] More recently, the principle has been generalized to the relativistic concept that humans are not privileged observers of the universe.[2] In this sense, it is equivalent to the mediocrity principle, with important implications for the philosophy of science.

And this is what Wikipedia says about the mediocrity principle:

The assumptions of mediocrity principle is the notion in philosophy of science[citation needed] that there is nothing special about humans or the Earth. It is a Copernican principle, used either as a heuristic about Earth's position or as a philosophical statement about the place of humanity.

My impression is that the media as well as certain scientists are using the mediocrity principle to argue that alien life and extraterrestrial civilizations must be extremely common. If there is nothing special about humans or the Earth, then we ourselves and our planet must instead be typical of the universe. Precisely because we are nothing special, we must be the norm. From that it follows that most other planets which are rocky and located inside their star's habitable zone must be teeming with life, just like the Earth. It also follows that simple life forms will naturally evolve into more complex life forms and eventually into intelligent life and technological civilizations.

I strongly question this reasoning. In my opinion, our own solar system refutes the idea that planets and moons are easily predictable. It was not easily predictable that Venus would turn out to be the hottest planet of our solar system, even though it is not the planet closest to the Sun:



No one had predicted that Jovian moon Io would prove to be the most geologically active body in our solar system:



No one had predicted that tiny moon Enceladus would shower its environment with sprays of water ice:



Before astronomers were able to really observe Titan, no one would have guessed that Titan would have a nitrogen atmosphere thicker than the Earth's or that it would have methane and ethane lakes and a methane/ethane precipitation cycle:



And before astronomers were able to really observe Iapetus, no one would have predicted that the moon had a light/dark dichotomy and a huge long ridge that gives Iapetus a walnut-like shape:



Planets and moons are surprising. They have characteristics that we have often been unable to predict. Why, then, should we assume that life is so predictable and so all-but-inevitable on rocky bodies inside a star's habitable zone?

Why should we assume that we and our planet are the norm of the universe?

Ann

[owlice]

Why should we assume that we and our planet are the norm of the universe?

Ann

WE don't assume that. Your interpretation is not everyone else's; it certainly isn't mine.

[Henning Makholm]

I get that. My real question is how likely it is that rocky bodies within a star's habitable zone will have solvents and an energy source.

Sometimes you just have to make do with the answer that nobody knows. This is one of those times.

My impression is that ... certain scientists are using the mediocrity principle to argue that alien life and extraterrestrial civilizations must be extremely common.

[citation needed].

[Chris Peterson]

My real question is how likely it is that rocky bodies within a star's habitable zone will have solvents and an energy source.

We have absolutely no way of knowing. I think that the idea of a "habitable zone" is overrated. There is much more to having liquid water than simply being a certain distance from a star. It's likely that liquid water (or possibly other suitable liquids) exist in the Solar System well outside what is termed the habitable zone, and it is clear that planets within the habitable zone can be quite inhospitable. Venus has a runaway greenhouse, Mars lacks the mass to sustain a magnetic field, so it has lost most of its atmosphere. Earth is on a knife-edge: it seems quite possible that it could be driven into a runaway greenhouse situation similar to Venus. There are just too many variables, and not enough examples.

[neufer]

Why should we assume that we and our planet are the norm of the universe?

• • ___ Memorable quotes for "Norm" (1999)
  Mr. Denby: The men's bathroom is disgusting. Tell her, Norm.
  
  Norm Henderson: There's a bathroom in this building? I've been using the donut shop across the street.
  
  Laurie: You go all the way across the street just to use the bathroom in the donut shop?
  
  Norm: There's a bathroom in the donut shop?

[Ann]

Why should we assume that we and our planet are the norm of the universe?

Ann

WE don't assume that. Your interpretation is not everyone else's; it certainly isn't mine.

You are right, Owlice. It isn't everybody's assumption, I know that. I certainly didn't mean to imply that you, for example, believe in it.

But I think this assumption is made often enough even in popular astronomy magazines, and certainly in other media. Perhaps a year ago or so I read an article in Astronomy Magazine which claimed that life probably arises on almost every planet that is potentially habitable.

I think the dominant thinking about life on other planets is simplistic. While I don't doubt that you, Owlice, and many other people here realize that planets and their habitability aren't easily predictable, I also think that too many people, even scientists, really are that naïve in their thinking. I am the owner of a book by Ken Croswell, Planet Quest from 1997, where a professional astronomer is quoted saying that it should be possible to tell the difference between massive planets and brown dwarfs by the shape of their orbits, because planets will move in circular orbits and brown dwarfs in elliptical ones. Well, it turned out that elliptical orbits are extremely common among extrasolar planets. The astronomer in Ken Croswell's book assumed that planets follow circular orbits because that is what most planets do in our solar system, including the Earth, and the Earth is assumed to be the norm.

For a time I read science fiction, but I never came across the idea that a planet may follow an elliptical orbit. The concept of elliptical orbits was too weird and improbable for sci-fi writers to toy with it. And yet, now that hundreds of extrasolar planets have been discovered, almost the only things we know about them are their masses and orbits, and we can see that most of them have orbits that are totally different than anything we find in our solar system. Almost the only thing we know about these planets flies in the face of one of the most basic assumption about planets made by even most educated people, namely that planets follow circular orbits! Why, then, should we assume that all the things we can't know anything about will turn out to be comfortably predictable and just like things are here on Earth?

I know. I said "we" again. I talked about "us" making assumptions. Okay, I don't mean to imply that many people here are making such assumptions. Nevertheless, I think these things should be pointed out. We here at Starship Asterisk should say that astronomers don't know what those extrasolar planets are like. We don't know that they are the least bit "Earth-like" just because they are rocky and orbit inside their star's habitable zone. Yes, maybe some of them will indeed turn out to be Earth-like in the sense that they will indeed host life. But we can't know how many of them them will do that, and we can't assume that very many at all will do it.

I find the expression "Earth-like planet" extremely irritating. If Kepler has indeed found hundreds (or a hundred) "Earth-like" planets, does that mean that it has found hundreds of planets with surface-dwelling alien life forms? Certainly not. In my opinion it is more likely that each and every one of those planets are either lifeless or, at best, have bacterial life forms or life forms existing deep in the interior of the planet, than that even one of them has a rich and complex and easily accessible biosphere comparable to the Earth's.

Ann

[Ann]

My real question is how likely it is that rocky bodies within a star's habitable zone will have solvents and an energy source.

We have absolutely no way of knowing. I think that the idea of a "habitable zone" is overrated. There is much more to having liquid water than simply being a certain distance from a star. It's likely that liquid water (or possibly other suitable liquids) exist in the Solar System well outside what is termed the habitable zone, and it is clear that planets within the habitable zone can be quite inhospitable. Venus has a runaway greenhouse, Mars lacks the mass to sustain a magnetic field, so it has lost most of its atmosphere. Earth is on a knife-edge: it seems quite possible that it could be driven into a runaway greenhouse situation similar to Venus. There are just too many variables, and not enough examples.

Good points, Chris. However, as to what you said about the Earth being posed on a knife-edge when it comes to habitability: at least the Earth has managed to remain on that knife-edge for more than three billion years and have life forms thriving on it during all those years. That is a good balancing act, if you ask me. As to what you said about liquid water existing well outside what is termed the habitable zone in our solar system, we haven't found evidence that life exists in the liquids in or on planets or moons outside the habitable zone of our solar system. Admittedly there may be life in the interiors of moons like Europa, Ganymede or Enceladus, but this is pure speculation, and we have no evidence suggesting that this may be the case. The idea that life may exist in the incredibly cold lakes of Titan seems even more far-fetched.

Ann

[bystander]

I think the dominant thinking about life on other planets is simplistic. While I don't doubt that you, Owlice, and many other people here realize that planets and their habitability aren't easily predictable, I also think that too many people, even scientists, really are that naïve in their thinking. I am the owner of a book by Ken Croswell, Planet Quest from 1997, where a professional astronomer is quoted saying that it should be possible to tell the difference between massive planets and brown dwarfs by the shape of their orbits, because planets will move in circular orbits and brown dwarfs in elliptical ones. Well, it turned out that elliptical orbits are extremely common among extrasolar planets. The astronomer in Ken Croswell's book assumed that planets follow circular orbits because that is what most planets do in our solar system, including the Earth, and the Earth is assumed to be the norm.

Am I missing something here? Or am I under a fallacious understanding that all planets in our solar system have elliptical orbits with the Sun at one focus? Doesn't Kepler's Laws of Planetary Motion require this?

[Chris Peterson]

Good points, Chris. However, as to what you said about the Earth being posed on a knife-edge when it comes to habitability: at least the Earth has managed to remain on that knife-edge for more than three billion years and have life forms thriving on it during all those years. That is a good balancing act, if you ask me.

Yes, "knife edge" is the wrong analogy. Our climate is metastable- at any time, we spend most of our time in stable regions that we periodically get knocked out of (and into others). While that sort of pattern probably exists on other planets, it is very complex on Earth, partly because of life. Without the feedback mechanisms created by a biosphere, most planets probably end up with conditions that are way down the slope of stability on one side or the other- very much like Venus and Mars.

I do think that most habitable planets have life, but I think that many planets identified as habitable are, in fact, not. We don't have much to use for that assessment when it comes to extrasolar planets, after all.

[Ann]

I think the dominant thinking about life on other planets is simplistic. While I don't doubt that you, Owlice, and many other people here realize that planets and their habitability aren't easily predictable, I also think that too many people, even scientists, really are that naïve in their thinking. I am the owner of a book by Ken Croswell, Planet Quest from 1997, where a professional astronomer is quoted saying that it should be possible to tell the difference between massive planets and brown dwarfs by the shape of their orbits, because planets will move in circular orbits and brown dwarfs in elliptical ones. Well, it turned out that elliptical orbits are extremely common among extrasolar planets. The astronomer in Ken Croswell's book assumed that planets follow circular orbits because that is what most planets do in our solar system, including the Earth, and the Earth is assumed to be the norm.

Am I missing something here? Or am I under a fallacious understanding that all planets in our solar system have elliptical orbits with the Sun at one focus? Doesn't Kepler's Laws of Planetary Motion require this?

Sorry, Bystander. Yes, I know that all planetary orbits are elliptical, because even the (incredibly unlikely) perfectly circular planetary orbit can be said to be elliptical, since the circle is really just an extreme case of an ellipse.

You probably realize that when I said "circular orbits", I referred to orbits like the Earth's, which are of course elliptical but with a very small degree of eccentricity.

Ann

[neufer]

I find the expression "Earth-like planet" extremely irritating. If Kepler has indeed found hundreds (or a hundred) "Earth-like" planets, does that mean that it has found hundreds of planets with surface-dwelling alien life forms? Certainly not. In my opinion it is more likely that each and every one of those planets are either lifeless or, at best, have bacterial life forms or life forms existing deep in the interior of the planet, than that even one of them has a rich and complex and easily accessible biosphere comparable to the Earth's.

So we are the only "Earth-like planet" with creatures sophisticated enough to draw a picture of a yellow sun?

No mind...those snotty little Crayola kids are just cattle for the true ruling creatures on earth: prokaryotes

_Microbial Population Explosion_ By David Tenenbaum (1998)

<<Spare a tear for the poor microbiologist, eager to pin down the total number of bacteria and other prokaryotes on Earth. (The prokaryotes comprise two of the broad kingdoms of life -- bacteria and archaea. The third kingdom -- eukarya -- contains all organisms with cellular nuclei, including fungi, plants and animals.) Prokaryotes lack a cellular nucleus, yet they can live from the upper atmosphere to the ocean floor, from the human gut to rocks a mile deep. And there are lots of prokaryotes. Even though each one weighs less than a less than a quadrillionth of a gram when toweled dry, overall they weigh about as much as all the plants in the world -- roughly a gigaton.

So how many prokaryotes share the planet with us? According to William Whitman, a microbiologist at University of Georgia, the number is 5 x 10³⁰. This is a big number by any standard. If you had that many pennies, Whitman and colleagues David Coleman and William Wiebe calculated, they would make a stack a trillion light years long.

Just where are all these prokaryotes? As we indicated, some bacteria live in the human gut -- a total of 3.9 x 10²³ among all six billion of us. (Before you down another tetracycline as your part in the global crusade against bacteria, remember that most intestinal bacteria are helpful.) Furthermore, the vast majority of prokaryotes live under land or the sea floor, not in us. In fact, 92 to 94 percent of all prokaryotes live underground lives of quiet desperation, hidden in the cracks and pores of rock and sediment, lacking sunlight, fresh air, even cable TV.

Unable to count bacteria individually, Whitman and colleagues opted for a sampling technique. They divvied the world into representative habitats, like forests, deserts, freshwaters, and shallow and deep ocean waters. Then they scoured the science literature looking for studies on the density of bacteria in each habitat. From there it was simple multiplication -- size of habitat in milliliters times number of prokaryotes per milliliter equals total number of bacteria in that habitat.

The math showed that the top eight meters of soil carry 26 x 10²⁸ prokaryotes, and all aquatic habitats carried 12 x 10²⁸. But the real jackpot lies underground. More than 8 meters below the land surface, they found between 25 and 250 x 10²⁸ prokaryotes. And beneath the ocean floor live a staggering 355 x 10²⁸ organisms without nuclei.

Even to doubters, the research demonstrates that all discussions of life, and its effects on Earth, had best take into account the hidden biosphere. Bacteria, after all, make some of the oxygen and virtually all of the nitrogen in our air. They decay pollutants and dissolve rocks. And like redwoods and mahogany trees, bacteria also store carbon. That has bearing on the study of global warming, since removing carbon from the air slows the rise of carbon dioxide that is causing the planetary cook-out.

The data also help explain the astonishing level of the diversity of prokaryotes. It is this diversity which allows them to prosper in virtually every habitat, from ice to boiling water, from deep in the Earth to high in the atmosphere. Prokaryotes come in varieties that can cause human and crop diseases, or supply medicines and new molecules for industry. These abilities derive directly from the DNA that has directed the growth and replication of prokaryotes during 3.8 billion years of existence (on a planet that's roughly 4.5 billion years old). Each habitat offers new chemical challenges that bacteria, through their diversity, have learned to meet. Indeed, prokaryotic diversity may be immeasurable: Scandinavian researchers found at least 4,000 species of bacteria growing in a single gram of soil.>>

[orin stepanek]

Chris said:

Your choice of the word "habitable" may be a little off, too, since as applied to humans now it means something very different than when applied to life in general.

Rest assured that I mean life in general.

But my answer to what you need for a planet to support life is this: a suitable liquid solvent, and a source of energy.

I get that. My real question is how likely it is that rocky bodies within a star's habitable zone will have solvents and an energy source.

I think any planet with these two things will generally be habitable.

How common are these two things on rocky planets inside a star's habitable zone? Is it Venus that is an odd planet because it has no solvent at all, or is it the Earth that is remarkable because it has what it takes to support life?

I wonder how much the Copernican principle has confused our thinking about the universe. This is what Wikipedia says about the Copernican principle:

In physical cosmology, the Copernican principle, named after Nicolaus Copernicus, states that the Earth is not in a central, specially favored position.[1] More recently, the principle has been generalized to the relativistic concept that humans are not privileged observers of the universe.[2] In this sense, it is equivalent to the mediocrity principle, with important implications for the philosophy of science.

And this is what Wikipedia says about the mediocrity principle:

The assumptions of mediocrity principle is the notion in philosophy of science[citation needed] that there is nothing special about humans or the Earth. It is a Copernican principle, used either as a heuristic about Earth's position or as a philosophical statement about the place of humanity.

My impression is that the media as well as certain scientists are using the mediocrity principle to argue that alien life and extraterrestrial civilizations must be extremely common. If there is nothing special about humans or the Earth, then we ourselves and our planet must instead be typical of the universe. Precisely because we are nothing special, we must be the norm. From that it follows that most other planets which are rocky and located inside their star's habitable zone must be teeming with life, just like the Earth. It also follows that simple life forms will naturally evolve into more complex life forms and eventually into intelligent life and technological civilizations.

I strongly question this reasoning. In my opinion, our own solar system refutes the idea that planets and moons are easily predictable. It was not easily predictable that Venus would turn out to be the hottest planet of our solar system, even though it is not the planet closest to the Sun:



No one had predicted that Jovian moon Io would prove to be the most geologically active body in our solar system:



No one had predicted that tiny moon Enceladus would shower its environment with sprays of water ice:



Before astronomers were able to really observe Titan, no one would have guessed that Titan would have a nitrogen atmosphere thicker than the Earth's or that it would have methane and ethane lakes and a methane/ethane precipitation cycle:



And before astronomers were able to really observe Iapetus, no one would have predicted that the moon had a light/dark dichotomy and a huge long ridge that gives Iapetus a walnut-like shape:



Planets and moons are surprising. They have characteristics that we have often been unable to predict. Why, then, should we assume that life is so predictable and so all-but-inevitable on rocky bodies inside a star's habitable zone?

Why should we assume that we and our planet are the norm of the universe?

Ann

Wow! I'll bet that when we get better information on the exoplanets; we'll find even more surprises than we found in our solar system! And I,m sure that a lot of these suns have a lot more planets than we now can find. I believe that there are so many stars out there that there has to be civilizations out there; and most stars must have a so called habitable zone. If not; we are truly alone in the universe.

[rstevenson]

... For a time I read science fiction, but I never came across the idea that a planet may follow an elliptical orbit. The concept of elliptical orbits was too weird and improbable for sci-fi writers to toy with it. ...

We all know the caution to "never say never.' But it's especially true of science fiction, a field whose authors have been extremely imaginative, and, lest we forget, a field that includes among its authorial ranks many scientists.

You gave up reading too soon. I've read science fiction for over 50 years and I can certainly recall stories involving odd orbits. Larry Niven in his Known Space series describes a planet named Jinx orbiting so close to its primary that it is pulled into an egg shape, with the narrow end of the planet sticking out of its atmosphere. And if you want a complicated (likely impossible) planetary system, try the one in Asimov's Nightfall, where the protagonist's planet orbits its primary, which is part of a system including 5 other stars. I also vaguely recall a story - perhaps a series - about a civilization that is cyclical due to the extremely elliptical orbit of its planet. And another about a planet that every once in a long while switches from orbiting one of a binary pair of suns to orbiting the other. And another about two Earth-sized moons of a gas giant, co-orbiting much like Pluto and Charon do the sun, which regularly occlude each other's view of the giant, and each of which developed a totally unique set of life forms. Someone with a better memory than I could no doubt think of many other examples.

As for your initial question, and the title of this thread, I can only reply, "Who knows?" A data set containing only one point makes a poor chart.

Rob

[Henning Makholm]

I also vaguely recall a story - perhaps a series - about a civilization that is cyclical due to the extremely elliptical orbit of its planet.

The most famed implementation of that concept is Brian Aldiss' Helliconia series.

[Ann]

Personally I believe that the Earth is extremely rare. I'm not saying that we are alone in our galaxy, and certainly not that we are alone in the universe. A statement like that can only ever be philosophical, because how could we ever, ever hope to prove it? By contrast, the statement that our galaxy is teeming with life and that there are millions of civilizations in the MIlky Way should be much easier to prove. It should be enough that we find clear evidence of life on one extrasolar planet relatively soon. We have every reason to believe that we have found only an extremely minute fraction of all the planets in the Milky Way, and even when we have found hundreds of times more planets than today and hundreds of Earth-sized planets, we will still only have found extremely, extremely few of the planets that are out there. So if we find life on even one of those few planets - for example, if we find an atmosphere that is very strikingly "out of non-biological equilibrium" - then we have every reason to believe that there is a lot of life not only on that planet but on many other planets, too. So when I'm saying that I think that there is very little life in the Milky Way, I take the risk of being proved wrong quite soon!

I think that the Earth is rare for a number of reasons. I think we have been lucky in many ways:

1) The Sun. We orbit an untypical star of our galaxy, a G2V star, which is in the top 5 percentile of bright and massive stars in our galaxy. Why do we find ourselves orbiting such an untypical star, instead of one of the dime-a-dozen M-type red dwarfs? There are two possible answers. One is that life is so extremely common in our galaxy that even the untypical stars like the Sun have planets with life on them (and then it follows that there must be billions of red dwarfs with living planets, too). The other answer is that few stars are really suitable for life, and that our Sun happens to be one of them.

If so, our Sun might be suitable for a number of reasons. Unlike the tiny red dwarfs, it gives off enough heat that a planet can orbit far away from the star not to have a locked rotation around it - that is, the planet doesn't have to have the same side facing the Sun all the time, so that it doesn't have to have one side with perpetual, scorching day and one side with perpetual, ice-cold night - and still it can receive enough warmth from the star to have liquid water on its surface.

Second, our Sun may have a mild and benign magnetic field. Ous Sun rotates very slowly, probably remarkably slowly as stars go, and it is not at all given to having huge, devastating outbursts. Even so, its magnetic field is strong enough to ward off many harmful cosmic particles and keep them out of our solar system.

Also our Sun has a comparatively high metallicity, which may make it more likely for planets like the Earth to come into existence in the first place.

Finally our star is single, which may well create a better and more stable environment for planets than most double stars would.

2) The Earth. The Earth is the biggest of the terrestrial planets. It has the highest iron content, and it was probably born with the largest amount of radioactive elements. Therefore, the Earth had a greater supply of internal heat than the other terrestrial planets. When the Earth was young and the Sun was young, too, the Sun was probably fainter and less energetic than it is today, because studies of stellar evolution strongly suggest that stars grow ever brighter and more energetic as they age. Perhaps because of its internal heat the Earth could be warm enough very early on to allow life to come into existence there. Also, because the Earth's internal heat production has likely grown weaker over the years while the Sun has grown brighter, the net balance of the heat that the surface of the Earth has received from the inside and from the outside over billions of years has remained remarkably constant.

The Earth is unique in having long-lasting oceans on its surface. Mars may have had a large ocean in its youth, but if so it dried out more than two billion years ago.

There are two mysteries about the Earth's oceans. One is the mystery of where they came from in the first place. The second is how they could last so long.

A terrestrial planet with long-lasting liquid water on its surface can't have been born with that water. If a planet is close enough to its sun for the sun to keep water liquid on the planet's surface, then the planet formed in a position of the accretion disk where water was a gas, and that gas could not be incorporated in large amounts as the planet formed. Further out the accretion disk was cold enough for water to be a solid, and therefore water ice makes up much of the bulk of the moons of Jupiter and Saturn, and it makes up much of the bulk of large cold planets Uranus and Neptune. But the water ice that we find in these parts of the solar system will not be a liquid on the surfaces of these bodies.

No, the Earth must almost certainly have received its water when it had already been born, when huge numbers of water-rich comets bombarded the inner solar system. Venus, the Earth and Mars all received a good helping of water from these comets. Mars may have had a large ocean for a few million years in its youth, but Mars has nevertheless been all but bone-dry for most of its history. If Venus has ever had an ocean, we have failed to find evidence of it.

The Earth recieved not only water from these comets but nitrogen, too. We have a nitrogen atmosphere. Venus and Mars received nitrogen, too. On Venus, that nitrogen may have created nasty compounds in the planet's atmosphere. Where it went on Mars I don't know. However, Mars and Venus both have CO2 atmospheres. On the Earth, most of the CO2 was incorporated into our rocks, leaving our atmosphere mostly free of it.

So the Earth was born without water, but then it received water along with nitrogen and CO2 from comets. Venus and Mars probably recieved similar helpings, but while the Earth made long-lasting water oceans and a nitrogen atmosphere, Venus and Mars made CO2 atmospheres and no really long-lasting bodies of water.

The Earth remains a dynamic world with precipitation cycles and plate tectonics, which help renew and recycle liquids and substances that living things need to survive.

On the Earth, green algae and later green plants came into existence, which fed themselves on sunlight, CO2 and water and created O2 as a rest product. Thereby they cleared the atmosphere of too much CO2 while at the same time they created the O2 that larger organisms need to survive on dry land.

How likely is it that all these lucky circumstances will come together on a planet to make it habitable? I personally believe that it happens very, very rarely, and that our planet really is a rare gem in our galaxy.

Ann

[Ann]

... For a time I read science fiction, but I never came across the idea that a planet may follow an elliptical orbit. The concept of elliptical orbits was too weird and improbable for sci-fi writers to toy with it. ...

We all know the caution to "never say never.' But it's especially true of science fiction, a field whose authors have been extremely imaginative, and, lest we forget, a field that includes among its authorial ranks many scientists.

You gave up reading too soon. I've read science fiction for over 50 years and I can certainly recall stories involving odd orbits. Larry Niven in his Known Space series describes a planet named Jinx orbiting so close to its primary that it is pulled into an egg shape, with the narrow end of the planet sticking out of its atmosphere. And if you want a complicated (likely impossible) planetary system, try the one in Asimov's Nightfall, where the protagonist's planet orbits its primary, which is part of a system including 5 other stars. I also vaguely recall a story - perhaps a series - about a civilization that is cyclical due to the extremely elliptical orbit of its planet. And another about a planet that every once in a long while switches from orbiting one of a binary pair of suns to orbiting the other. And another about two Earth-sized moons of a gas giant, co-orbiting much like Pluto and Charon do the sun, which regularly occlude each other's view of the giant, and each of which developed a totally unique set of life forms. Someone with a better memory than I could no doubt think of many other examples.

As for your initial question, and the title of this thread, I can only reply, "Who knows?" A data set containing only one point makes a poor chart.

Rob

I read Asimov's Nightfall and liked it.

As for Larry Niven, I read his Ringworld. The story is set on a world that is shaped like a ring around the Sun - so the Ringworld isn't orbiting the Sun as such, but the world itself is its own orbit! I was fascinated by the concept, but I was in two minds about the protagonist. He was 200 years old, and I didn't really like his sexual appetite. In the beginning of the book he was together with a girl of twenty, who was therefore a tenth of the man's age. Later, when the protagonist had been separated from his twenty-year-old lover, he met a 3,000-year-old hooker who had devoted millennia to perfecting her craft. She, too, became his lover. Accompanied by her the protagonist roamed the Ringworld. He came to a part where a man was required to "greet" any woman he met by having sex with her, and our hero enthusiastically greeted a lot of women there.

I was a bit turned off by the protagonist's sexual appetite, as I said, but I nevertheless decided to read the follow-up to the first book. In the opening of this second book, the protagonist is hooked up to a lot of wires inserted into his brain. Well, you see, his 3,000-year-old hooker died, and he himself almost died of sexual frustration. So he had the wires inserted into the pleasure center of his brain and spent the following months in a stage of permanent sexual climax.

But after a while he returned to the Ringworld. After some time he met his former lover, the twenty-year-old girl. She had somehow been transformed into a monster. There was a lengthy description of how the hero fought his girlfriend-turned-monster, slowly tearing her limb from limb until she died. Later he found out that the Ringworld had begun wobbling, and he had to stabilize it. But the only way to do that was to fire a deadly jet of some sort into exactly the part of the Ringworld where all those women lived whom he had "greeted" last time he had been there by having sex with them. Nevertheless, a man has to do what a man has to do, and the hero fired the jet into all the women he had exchanged bodily fluids with, killing them all.

I decided then and there that I would never read a book by Larry Niven again!

Ann

[owlice]

the statement that our galaxy is teeming with life and that there are millions of civilizations in the MIlky Way should be much easier to prove.

Who made that statement? Where did it come from? What is your source for that statement?

I'm sorry; I must have missed this. I don't see from where such a statement comes. I've never heard a serious assertion that our galaxy is teeming with life, nor that there are millions of civilizations in the Milky Way. Speculation, yes. Stories that are science fiction, yes. But a flat-out statement that our galaxy IS teeming with life? No. A flat-out statement that there ARE millions of civilizations in the Milky Way? Please let me know what reading material I've been missing by pointing me to a source; thanks!

[rstevenson]

I decided then and there that I would never read a book by Larry Niven again!

I'm pretty sure I could make A Tale of Two Cities or even Little Women sound just as ridiculous if I made as much effort as you've just made to trash Niven's Ringworld. But in making that effort you failed to address the point I was making in response to your slur of all science fiction for its lack of creativity regarding planetary orbits.

Rob

[Chris Peterson]

I'm sorry; I must have missed this. I don't see from where such a statement comes. I've never heard a serious assertion that our galaxy is teeming with life, nor that there are millions of civilizations in the Milky Way. Speculation, yes.

I've heard pretty serious claims that the galaxy might be teeming with life. That's not an unreasonable bit of speculation, looking at our own world in comparison to the sort of planets we are starting to learn exist elsewhere. I've not heard much in the way of serious assertions that the galaxy has millions of civilizations, though.

Personally, I don't see anything to make me think there aren't millions of planets out there very similar to Earth, and I have no reason to think they don't host life. Fortunately, that belief is subject to test, and I expect it won't be long before we start finding extrasolar planets that show evidence of life. Interesting times.