The question of how and where life arose on Earth is a massively complex puzzle. The puzzle pieces themselves--the physical evidence of what really happened--have long since vanished. The best biologists can do is to try reconstructing what the pieces might have looked like, and how they might have fitted together. Every breakthrough in origin-of-life studies to date has been an important but very small step toward a convincing explanation of how it really happened. It may be that life is inevitable, given the right conditions, as Sagan thought. It may equally be that life is terribly, terribly unlikely to happen, even under the best of circumstances. The fact that life on Earth survives in so many harsh environments, moreover, doesn't prove that life arises easily. It proves only that that life can adapt like crazy after it arises.
If you're a pessimist, therefore, you might conclude that the search for extraterrestrial life might well prove to be fruitless. If you need further ammunition to bolster your pessimism, you might take a look at the book Rare Earth, published by paleontologist Peter Ward and astronomer Don Brownlee in 2000. The authors advance a series of arguments to suggest that while life might well be common in the Milky Way, the sort of advanced life we'd really love to find is very rare. Each argument by itself sounds pretty convincing; taken together, they appear at first to be devastating.
Take Jupiter, for example. If our biggest planet had spiraled in toward the Sun to become a hot Jupiter, it would probably have disrupted Earth's orbit. But if we had no Jupiter at all, that could be a problem as well. The reason, argue Ward and Brownlee, is that Jupiter shields the Earth from comet impacts. Comets originate from the outer solar system, and most of them stay there. When one does fall in toward the Sun, however, it's almost always flung away by Jupiter before it can get anywhere near Earth. The astronomer George Wetherill showed decades ago that if Jupiter didn't exist, we would get about ten thousand times more comets smashing into Earth than we do--not a good thing for the emergence and evolution of anything more advanced than bacteria.
Ward and Brownlee also point out that our Moon is much bigger in relation to Earth than any planet-moon pair in the solar system. It's so massive that its gravity helps stabilize the tilt of the Earth. Mars, whose moons are tiny, wobbles something like a spinning top that's close to falling over. Without the Moon, our planet would do the same, making the seasons highly unstable and making it hard for plants and animals to adapt.
And then there's plate tectonics, which recycles the Earth's crust back into the interior over hundreds of millions of years. That process also recycles carbon dioxide after it binds chemically to surface rocks, ensuring that the atmosphere doesn't undergo a runaway greenhouse effect, turning our planet into a hothouse like Venus. Of all the rocky bodies in the solar system, only Earth has plate tectonics, so it's probably rare in the universe. And then there's Earth's magnetic field, which protects us against energetic particles streaming in from the Sun or from deep space. And then... well, suffice it to say that Rare Earth makes a sobering read.
It does, that is, until you talk to Jim Kasting. "A lot of people read [Rare Earth] and believed it," he told me during a conversation at a Vietnamese restaurant in Seattle. "I think they sold a lot of copies because it was the anti–Carl Sagan. It appealed to people who didn't want to believe this whole line of stuff that Carl had been selling."
One by one, Kasting addressed the arguments in Rare Earth and made it clear that he wasn't impressed. For example, he said, it's true that if you eliminated the Moon, Earth's tilt would wobble chaotically. But if Earth were spinning faster--if the day were twelve hours long rather than twenty-four--the chaos would go away. "So you have to ask," said Kasting, "How fast would the Earth be spinning if you didn't have the Moon? And that's complicated." In short, Ward and Brownlee raise a plausible argument, but hardly a definitive one.
It's also true, continued Kasting, that Jupiter protects Earth from comet impacts. But it actually raises the odds we'll be struck by asteroids. That's because the asteroid belt is just Sunward of Jupiter, so it's relatively easy for the giant planet to nudge a mountain-size chunk of rock into an Earth-crossing orbit. "It appears," Kasting writes in his 2010 book How to Find a Habitable Planet, where he devotes a full chapter to presenting counterarguments to Rare Earth, "that having a Jupiter-sized planet... is a mixed blessing."
As for plate tectonics, he said, Venus is the only other planet in our solar system besides Earth big enough to have them in the first place (a planet smaller than Venus would have cooled off by now, so it wouldn't have the semi-molten rock that allows continents to slide around). But Venus lacks the water it would need to lubricate the motion of crustal plates, which could be why, despite its adequate size, it doesn't have plate tectonics. Out of two planets that might have plate tectonics, one of them does, and Kasting sees no reason at all to assume that Venus is somehow typical of exoplanets while Earth isn't.
The bottom line, he said, is that "there are a lot of things that we don't know, so we make conjectures. Ultimately, if we can do TPF and follow up with post-TPF missions, we'll figure out what happens, and where." "I'm an optimist," he admitted. "I agree with Carl Sagan. I think there's probably life all over the place, and there are probably other intelligent beings. I'm just not as good at speculating as he was."
There's another reason you might lean in the direction of optimism. The concept of the habitable zone applies if you're assuming life is confined to the surface of a planet. If you discard that assumption and consider places where conditions are favorable beneath the surface, you've suddenly got a lot more places to look. In our own solar system, Earth has the only habitable surface, but planetary scientists think the Martian subsurface might be habitable as well. In November 2011, NASA launched its biggest, most capable rover toward Mars, where the six-wheeled, SUV-size Curiosity will, among other things, drill into the Martian soil to look for organic chemicals (but not, on this mission, for life itself).
The right conditions for life could also exist on even more exotic worlds. Astronomers have known for years that Jupiter's moon Europa and Saturn's moon Enceladus both have subsurface water. The energy to keep the former from freezing solid right down to the core comes from tidal squeezing, as it orbits through the powerful gravitational field of Jupiter; Enceladus's heat source is a mystery. More recently, theorists have suggested that even Pluto might harbor liquid water, one hundred miles or so beneath its icy surface--the heat in this case coming from the decay of radioactive potassium. As for complex carbon molecules, they're abundant in the bodies of both comets and asteroids, which have been crashing into the moons and the outer planets for billions of years.
Yet another plausible reason for optimism arises from the fact that the universe is under no obligation to follow the "life as we know it" rule. Carbon is abundant in the Milky Way and combines easily with other atoms to form the elaborate organic molecules that underlie all of terrestrial biology. Water is abundant as well, and acts as a versatile solvent. So it's not absurd to think that carbon-based life might be universal, and is exactly what astrobiologists should be looking for.
Excerpted with permission from Mirror Earth: The Search for Our Planet's Twin (Walker & Company) out today. Michael D. Lemonick is a senior writer at Climate Central, Inc. and a former staff writer for Time. Mirror Earth is his fifth book. Buy it here.
So what this basically boils down to is that astro-biologists, and advocates of abiogenisis are pulling un-testable hypotheticals out of their butts to keep the faith alive.
What a grouch! What this really boils down to is that the jury is still out on whether or not life exists elsewhere and how common or uncommon it might be, and as technology advances to enable the search for extraterrestrial life there are competing theories on the best place to look first. Or we could just not bother to search for life anywhere because "God did it". Either God or a robot monster from outer space.
@Bagpipes100 I agree, they are trying to keep he faith alive although I have to disagree that their theories are pulled out of their butts. Each point is reasonable and logical even if they are mostly un-testable.
@HBillyRufus even if 'God did it,' a robot did it, or a space monster spawned life here, why would we stop looking? Besides, it would be pretty cool if we found the life spawning space monster itself. Unless it turns on us in rage and completely annihilates us. On second thought, maybe we should stop looking...
@strider99k2 Yeah just like in Prometheus. (SPOILER ALERT) Well travel to find our maker and then they'll try to kill us.
The thing is we can never say life doesn't exist elsewhere unless we thoroughly search every planet in the universe. So have fun with that.
The odds of finding a planet just like Earth may be low but given that we have seen many planets significantly larger than Jupiter and that these large planets tend to have very large moons - earth sized and maybe larger. Moons are much more abundant than planets and are the most likely places where life will be found IMO. I would estimate the odds of finding an Earth Sized moon are much higher than an earth sized planet.
I think that depending on your point of few, either side can be the pessimist or the optimist. It may be optimistic to think that life is very rare in the universe as, if a civilization more advanced than ours came into contact with us, most likely bad things would happen. We could easily be enslaved and/or eaten, or wiped from the universe. You may think that they would be friendly but why should they be. After all, using the most intelligent form of life we know as a reference, they would most likely take advantage of us. We kill and eat almost all forms of life below us, and yet we do not see anything wrong with this. Why is socially ok to eat pigs but not to eat other humans? Probably because they are not as intelligent as us. So why should beasts smarter than us feel that it is not ok to eat anything of lower intelligence than them?
Just a thought...
So you like gaps arguments?
'we haven't looked at every planet so there still might be one that shows what we hope'.
'given enough time life might generate from non life, even though it hasn't happened in the time we've been watching for it'
'science hasn't been able to verify by observation one species turning into another but some day it could'
There is no question as to whether other carbon based life exists on other planets but how man millions of planets they live on.
The questions remaining are things like:
How many varieties of atmospheres are these beings able to live in?
Do they need one that is close to the chemical make up of ours?
How much different can it be?
We know that it isn't required that other "Earths" have exactly the same gravity. So ...
How much different would we be if our planet had 1.5 times our gavity?
How about 2 times our current gravity?
How about 0.8 times, etc.
How much does/would gravity have an effect on our legs and torso and arms?
If gravity was three times what we have, would ape/human like beings ever end up standing upright?
Would people/animals end up with more legs because of higher gravity on bigger earths? Would dogs or horses have six legs to counteract the increased gravity?
Of course other earths won't just have more gravity. Maybe an even amount would have less.
But how small could be the smallest earth like planet be? The smaller it is doesn't just mean less surface space but it would also reduce the gravity that beings are faced with on that planet. How would plants be different. With lower gravity they could be many times taller.
The same is true in the opposite way. What is the largest an earth like planet could be? The bigger the planet the more space but also the more gravity would pull down on beings but also plants. How short and fat would red cedar like trees be?
This is my main obsession, if you haven't noticed. How would increased or decreased gravity effect life on earth planets that are bigger or smaller than ours? How would everything be different because of it?
Before any exoplanets where found, you had a similar debate. Perhaps our solar system was different in some way and there were no other planets. Somewhat plausible but false.
Now we have evidence to show there are more planets than stars in the Universe. We are fairly certain that instead of 8 planets of several types, there are at least a trillion trillions planets. Think of the variations!
Who in their right mind can believe that with these almost infinite variations, only Earth's "perfect" setup allows for life?
Again that is only somewhat plausible. It is far more plausible that we will find so many variations of star-planet formations that eventually we will see the Earth as a wonky version that just made the cut.
I would say for those who believe in intelligent design of our universe, the obvious answer is yes. Life is everywhere with evolution as a tool or automation process to fill the universe with life on a grand scale with minimal intervention.
If you believe the universe and life is a coincidence then the outlook for alien life is more bleak though there are a great number of stars and planets in the universe.
I tend to agree with the intelligent design theory. I have a blog post here about science/religion and alien life. www.jdraniel.blog.com
"There is no question as to whether other carbon based life exists on other planets but how man millions of planets they live on."
I mean, besides bacteria and stuff which we may have inadvertently launched onto mars (or other body in our solar system) on one of our probes or rovers. Where on earth are you coming up with this assertion?
E.T. Phone you or something?
Until they have any reason to believe that life can spontaneously generate from non life here on earth, I think hypothesizing as to what other conditions it can do so could be put off until later.
I know naturalism just assumes that abiogenisis can occur....but come on...where's you'r evidence? That's what 'science' is about right?
It's not absurd to think anything, evidently. There may be silicon-based life, or any other element x-based life. It's interesting that what we think is normal here would be universal everywhere, since it could be the opposite. Element x-based life may be more nomal elsewhere with us being the EXCEPTION. We have no reason to think carbon-based life is universal, quite the opposite, since we haven't detected carbon-based life on any of the other planets, in fact I recently heard of an arsenic based life here on Earth.
Regarding the idea of an Earth "twin", given millions of planets it stands to reason 1 or 2 may be earth-like, but the odds that we'd find it basically zero because there's only so much we can tell from a telescope and it's beyond the scale of human experience to send a probe out to relay information back to us, like if it took 1,000 years for a probe to get to it, who could keep track of that? Space is basically too big for us to know if there's another Earth out there, and I'm talking about the NEAREST neighboring solar systems.
Carbon-based life is most likely, since carbon readily forms complex bonds and is also nearly ubiquitous in the interstellar clouds we've been able to analyse. This doesn't make silicon-based life impossible, but it's reasonable to assume that organic, carbon-based life would out-compete most other varieties. Carbon & liquid water is good stuff!
(Incidentally, the arsenic-based life mentioned above was supposedly a carbon-based bacteria that had replaced Potassium in it's makeup with arsenic. The research was sloppy, and has since been disproven by other scientists.)
In any case, even other carbon-based life is unlikely to be similar enough to humans to ever want us as food. And unless they have some instant-anywhere space drive, it would never be practical to make war over resources. No, they don't want our water, they don't want our flesh. They might want to trade knowledge though, if we have anything they might lack. (Bootleg copies of GTA: LasVegas to swap for Betelgeusean VR recordings?) You'd have to have a good reason for travelling hundreds of light years.