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Extraterrestrial Life Common in the Universe --Wishful Thinking?



When speaking about the probability of extraterrestrial life, Carl Sagan, always said that extraordinary claims required extraordinary evidence. Recent discoveries of planets similar to Earth in size and proximity to the planets' respective suns have sparked scientific and public excitement about the possibility of also finding Earth-like life on those worlds.

"Fossil evidence suggests that life began very early in Earth's history and that has led people to determine that life might be quite common in the universe because it happened so quickly here, but the knowledge about life on Earth simply doesn't reveal much about the actual probability of life on other planets," says Princeton astrophysical sciences professor Edwin Turner.

But Princeton University researchers have found that the expectation that life — from bacteria to sentient beings — has or will develop on other planets as on Earth might be based more on optimism than scientific evidence.

Turner and David Spiegel, a former Princeton postdoctoral researcher, analyzed what is known about the likelihood of life on other planets in an effort to separate the facts from the mere expectation that life exists outside of Earth. The researchers used a Bayesian analysis — which weighs how much of a scientific conclusion stems from actual data and how much comes from the prior assumptions of the scientist — to determine the probability of extraterrestrial life once the influence of these presumptions is minimized.

Turner and Spiegel, who is now at the Institute for Advanced Study, reported in the Proceedings of the National Academy of Sciences that the idea that life has or could arise in an Earth-like environment has only a small amount of supporting evidence, most of it extrapolated from what is known about abiogenesis, or the emergence of life, on early Earth. Instead, their analysis showed that the expectations of life cropping up on exoplanets — those found outside Earth's solar system — are largely based on the assumption that it would or will happen under the same conditions that allowed life to flourish on this planet.

In fact, the researchers conclude, the current knowledge about life on other planets suggests that it's very possible that Earth is a cosmic aberration where life took shape unusually fast. If so, then the chances of the average terrestrial planet hosting life would be low.

"Information about that probability comes largely from the assumptions scientists have going in, and some of the most optimistic conclusions have been based almost entirely on those assumptions," Turner said.

Turner and Spiegel used Bayes' theorem to assign a sliding mathematical weight to the prior assumption that life exists on other planets. The "value" of that assumption was used to determine the probability of abiogenesis, in this case defined as the average number of times that life arises every billion years on an Earth-like planet. Turner and Spiegel found that as the influence of the assumption increased, the perceived likelihood of life existing also rose, even as the basic scientific data remained the same.

"If scientists start out assuming that the chances of life existing on another planet as it does on Earth are large, then their results will be presented in a way that supports that likelihood," Turner said. "Our work is not a judgment, but an analysis of existing data that suggests the debate about the existence of life on other planets is framed largely by the prior assumptions of the participants."

Joshua Winn, an associate professor of physics at the Massachusetts Institute of Technology, said that Turner and Spiegel cast convincing doubt on a prominent basis for expecting extraterrestrial life. Winn, who focuses his research on the properties of exoplanets, is familiar with the research but had no role in it.

"There is a commonly heard argument that life must be common or else it would not have arisen so quickly after the surface of the Earth cooled," Winn said. "This argument seems persuasive on its face, but Spiegel and Turner have shown it doesn't stand up to a rigorous statistical examination — with a sample of only one life-bearing planet, one cannot even get a ballpark estimate of the abundance of life in the universe."

"I also have thought that the relatively early emergence of life on Earth gave reasons to be optimistic about the search for life elsewhere," Winn said. "Now I'm not so sure, though I think scientists should still search for life on other planets to the extent we can."

Promising planetary finds Deep-space satellites and telescope projects have recently identified various planets that resemble Earth in their size and composition, and are within their star's habitable zone, the optimal distance for having liquid water.

Of particular excitement have been the discoveries of NASA's Kepler Space Telescope, a satellite built to find Earth-like planets around other stars. In December 2011, NASA announced the first observation of Kepler-22b, a planet 600 light years from Earth and the first found within the habitable zone of a Sun-like star.

Weeks later, NASA reported Keplers-20e and -20f, the first Earth-sized planets found orbiting a Sun-like star. In April 2012, NASA astronomers predicted that the success of Kepler could mean that an "alien Earth" could be found by 2014 — and on it could dwell similar life.While these observations tend to stoke the expectation of finding Earth-like life, they do not actually provide evidence that it does or does not exist, Spiegel explained.

Instead, these planets have our knowledge of life on Earth projected onto them, he said.Yet, when what is known about life on Earth is taken away, there is no accurate sense of how probable abiogenesis is on any given planet, Spiegel said. It was this "prior ignorance," or lack of expectations, that he and Turner wanted to account for in their analysis, he said.

"When we use a mathematical prior that truly represents prior ignorance, the data of early life on Earth becomes ambiguous," Spiegel said."Our analysis suggests that abiogenesis could be a rather rapid and probable process for other worlds, but it also cannot rule out at high confidence that abiogenesis is a rare, improbable event," Spiegel said.

"We really have no idea, even to within orders of magnitude, how probable abiogenesis is, and we show that no evidence exists to substantially change that."Considering the sourceSpiegel and Turner also propose that once this planet's history is considered, the emergence of life on Earth might be so distinct that it is a poor barometer of how it occurred elsewhere, regardless of the likelihood that such life exists.

In a philosophical turn, they suggest that because humans are the ones wondering about the emergence of life, it is possible that we must be on a planet where life began early in order to reach a point so soon after the planet's formation 4.5 billion years ago where we could wonder about it.

Spiegel and Turner explored how the probability of exoplanetary abiogenesis would change if it turns out that evolution requires, as it did on Earth, roughly 3.5 billion years for life to develop from its most basic form to complex organisms capable of pondering existence. If that were the case, then the 4.5 billion-year-old Earth clearly had a head start. A planet of similar age where life did not begin until several billion years after the planet formed would have only basic life forms at this point.

"Dinosaurs and horseshoe crabs, which were around 200 million years ago, presumably did not consider the probability of abiogenesis. So, we would have to find ourselves on a planet with early abiogenesis to reach this point, irrespective of how probable this process actually is," Spiegel said. "This evolutionary timescale limits our ability to make strong inferences about how probable abiogenesis is."

Turner added, "It could easily be that life came about on Earth one way, but came about on other planets in other ways, if it came about at all. The best way to find out, of course, is to look. But I don't think we'll know by debating the process of how life came about on Earth.

"Again, said Winn of MIT, Spiegel and Turner offer a unique consideration for scientists exploring the possibility of life outside of Earth."I had never thought about the subtlety that we as a species could never have 'found' ourselves on a planet with a late emergence of life if evolution takes a long time to produce sentience, as it probably does," Winn said."With that in mind," he said, "it seems reasonable to say that scientists cannot draw any strong conclusion about life on other planets based on the early emergence of life on Earth."

This research was published Jan. 10 in the Proceedings of the National Academy of Sciences and was supported by grants from NASA, the National Science Foundation and the Keck Fellowship, as well as a World Premier International Research Center Initiative grant from the Japanese Ministry of Education, Culture, Sports, Science and Technology to the University of Tokyo.

The image at the top of the page is the Pinwheel Galaxy, one of the most spectacular spiral galaxies to view from Earth. This three-color composite image was captured by the Isaac Newton Telescope in La Palma, Spain. Known more officially as Messier 101 or NGC 5457, this classic spiral galaxy is 27 million light years from Earth in the Ursa Major constellation, also known as the Big Dipper. Its slight asymmetry is thought to be the result of an encounter with another galaxy in the recent past. This event also left many huge clouds of glowing gas and plasma known as H II regions.

The Daily Galaxy via Princeton University

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In relation to the probability of life evolving elsewhere in the cosmos, surely this will be better evaluated as we examine our own solar system more energetically.

Measured against the time-line of our own existence as a species [much less the 500 million years or more of life of any kind on our own planet!] the period of our exploration is absolutely infinitessimal if spectacular. The morphology of Venus and Mars and other bodies beyond the 'goldilocks region' like Titan, Enceladus or Europa offer obvious examples among many other possibilities for game-changing analyses.

I am not a mathematician but it strikes me that any probability model could justifiably include the view: 'if you find one of something [ie: an earth containing myriad life-forms] the likelihood that there will be another might be reasonably argued especially if an identifiable pattern of pre-conditions [ie: exosolar planetary systems] is repeated, possibly ad-infinitum.

Given some of the unlikely events that occurred to make complex life on Earth possible -- a tide-creating moon, a powerful magnetic field, the right chemistry, and so forth -- it does seem likely that true Earth-like planets are quite rare in the Milky Way.

On the other hand star systems in which to look are certifiably very numerous.

Thus, I'm confident that we'll find several good places to set down colonies. Some may even have already-edible flora and fauna (though I wouldn't lay a bet one way or the other on that).

Extraordinary claims may require extraordinary evidence implicates Humanity's own cognitive dissonance with regards to the universe.

We're getting very close to knowing some fundamental truths about what's out there - and I think we'll be surprised, if not stunned to reveal that life - through evolutionary biology, can take some truly miraculous and bizzare turns.

The biological structures and compositions of creatures out there will be vastly different from us - some forms of life may not even need planets or star systems to evolve from.

Such life could be spread out over vast distances and regions of the universe, limited to one advanced species per 400 billion stars. The reason behind this is the universe's overall hostility to life, and the early demise of civilizations due to natural forces.

Sounds like something from Star Trek? The encounters of Starfleet may not be too far fetched.

world event major product large america muscle action build muscular bodybuilding title compete europe legal emerge popular 2008 relat sleep.

We have already seen robotic probes from an alien world flying around our planet. Aliens have abducted humans for experiments. It just remains for them to come out in the open and identify themselves.

The one thing I do agree with what this article is saying, is that we cannot assume life on other planets to have developed similar to here on Earth.

But.. that hardly reduces the probibility of life IMO. This would only increase it significantly.

The sheer number of galaxies, stars, planets and moons is staggering. IMO the only extraordinary evidence required here is proof that other life DOES NOT exist. But unless we find life, microbiotic or otherwise, in our own solar system, it is unlikey with our current technology that this will be proven any time soon... unless of course intelligent life introduces themselves to us.

When Sagan uttered "extraordinary claims require extraordinary evidence" he wasn't referring to the probability of alien life, but on extraterrestrials visiting earth and supposed UFO encounters people claimed to have experienced.

Sagan was a big believer in the Drake equation, and expected the universe to be teeming with life.

The commonality of life is not 'wishful thinking' but a likely reality. But from microbes and mushrooms to rational, and even technological beings is quite a divide.

Again someone observed that reason may not be terribly efficient or required, as evolution unfolds...

I'm getting sick of reading how peole are postulating the probability of life evolving on other planets. I'm sure most people reading this understand no-one can make any legitimate postulation until the sample size is greater than 1. I'm glad the research is moving forwards but I'm getting sick of the same tired analysis.

Heck I am glad I had some coffee in me before I started reading that. Wow, the circular logic involved in that article twisted my neurons. :) I can't believe two guys actually spent that much time trying to statistically show that you cannot statistically determine the likelihood of life when the sample size is one.
So if I understand their logic (not saying I actually do) statistically speaking there is a high probability that there is no life on Earth. Can you imagine what Kierkegaard would have to say about this article????

The reason for the circular logic is that the basic reason for the research is not scientific, but religious. This is a Religious Quest. If there is no life out there, then life here is more of a miracle and some god or other is more necessary (although not essential) to get life started.
Someone famous said, "Either Life is Common or it is a Miracle; it is too complex to be anything else."
Since logically one cannot prove something does not exist, the search goes ever on, a wasted Quest.

There is absolutely no reason to believe that life wouldn't evolve exactly as it has here given the enormity of the universe. It will be carbon based and have DNA coding very similar to ours. Why? Because that is the way chemistry works and the chemicals are the same everywhere in the universe. So it really is a situation where the chemicals are everywhere and the chemistry is unified, so the only real variable is environment.

I would go so far as to speculate species very similar to what we have here on Earth. We should expect to see similar adaptations (perhaps solved with different morphology) in those life forms as well.

The bigger question is whether we will ever find any life we can actually "communicate" with. Even if it exists, the scale of the universe and the limiting factor of light speed will greatly hinder that ability.

No discussion of intelligent life on other planets is complete without at least a mention of the Fermi Paradox.


Given the size of the known universe and the amount of Earth-like planets there are (we are already discovering many canidates in our own galaxy!), even IF life were some kind of mircale at the hands of a god... why would it pick Earth and only Earth out of the trillions of possibilities?? If a god put life here I am sure it put life somewhere else. It would be harder than eating 1 potato chip! With such a selection I am sure the gods would want to try something different or better or even the exact same elsewhere to see how different they evolved.

While all the debate goes on, the absence of evidence is not evidence of absence. With the incredible numbers of galaxies and the cross section of possiblity, There is/has been life somewhere else in the universe. Our presence here bodes well for the same circumstances to have existed elsewhere is very high but the time frame would lead to narrowing the field considerably. The time/space boundary makes it impossible to achieve any real proof of a present or prior existance of any kind of life. Our solar system is our only concern. Learning how and why the solar system came about will lead to answers of how to best use what we have been given. Much work has to be done to understand our own existance and how to maintain that existance long enough to breach the boundaries that keep us here.

as long as all religions talk about men with special powers came from the sky, i bet there is something true in this.

There are actually two questions that are being posed. What is the possibility that there is any kind of life out there and what is the possibility that any of it is sentient?
Let’s go for, just life first. Life, for all its complexity, is still basically a chemical reaction, not much different than any star, that eats, ages, reproduces, expels and dies. It would seem that what is being said is that life cannot be asserted to exist elsewhere because we only know it exists here. This is like saying that chemical reactions only happen where we can see them to verify the process but any scientist worth his own DNA will tell you that we know chemical reactions happen without being witnessed. Also, just over a century ago people didn’t think there was any way that life could exist in some places right here on earth, but we have found that life exists at every extreme place on our planet. How far must we go? When we find life in our solar system, will it then be said that the possibility of life in other solar systems is low because there is only one solar system we know of that has life. We don’t have to stop there. Take it to the local group, our galaxy and eventually our universe is the only universe out of the multiverse ect….
The second question obviously is a little more difficult, but the previous paragraph answers a majority of it. It comes down to genetics verses environment. If the primitive life forms are diverse to the 100th power then sentient life should arise for survival purposes.

There are some who consider the possibility that, had the chicxilub asteroid not struck earth, certain saurians might have already become an advanced intelligent presence on this planet. We will probably never know for sure, but I think it interesting, nonetheless as I ponder possible successors, such as raccoons, with their remarkable opposable thumbs and rudimentary intelligence, to our extinction.

It might be good news if life on Earth was unusually quick to develop and most other planets with similar temperature and composition have not developed life. That would mean less competition for us once we're ready to move out in the stars. Can we be so lucky?

Until we get more than one point to plot on our graph, the question of life outside our planet is going to be a philosophical one, not a scientific one.
I've personally spent more time wondering why there where so many centuries between the discovery of paper and the theory of flight. You would have thought that at some point before the 1900's some bored soul would have folded a paper air plane simply by mistake, and everything would have taken off from then. Hind sight is always 20/20. Some day, we might discover life, and when we do so, will wonder again how at one time millions were willing to beleive in the chances of buying a lottery ticket to get rich, yet with countless chances in the cosmos, where still skeptical as to whether or not Earth could be the only place in the Universe where a soap bubble is round.

There have beem any number of mass-extinction events on our own planet, over hundreds of millions of years and yet each recovery saw the flourishing of even greater levels of bio-diversity.

Furthermore it seems not unlikely that at least some considerable proportion of our oceans as well as the biochemical antecedents to our earliest evolving species arrived in the form of visitations from space, meteorites from places unknown.

While I don't believe in alien abduction theories, I do believe that once established, life takes hold with astonishing tenacity and variability. Statistically it is near impossible to imagine contact with an alien [technologically endowed] society but it is a sustainable hope that we will detect organic molecules and even micro-organisms [the most tenacious life-forms]... if we haven't already done so.

The Creator has confirmed that life is a cosmic aberration.

The Creator has posted a video on YouTube. The message comes to you directly, without the need for translation, transcription or interpretation by iron-age scribes, or analysis by a quantum physicist.

The video tells you how you came to be here, outlines the meaning and purpose of your life, and gives you an insight into the future of humankind.

See ' God says sorry. '

we are all limited to what our brain decides. What our brain decides need not to be true. Universe is unimaginable. Exploring through human brain is some what testing the sea with a needle.

'..a point so soon after the planet's formation 4.5 billion years ago..' Is it really so soon? Perhaps if there hadn't been quite so many extinction events during the course of Earth history, some other intelligent critters might've evolved even sooner?

dividing forms such as a star and a moon, a man and a dog, a fish and a bird - human - forget it.
Cosmology , for me is about unconventional thinking.
My mathematical thinking has lost pace with cosmic thinking. forget it. Maths is of no part any star.
as much it is any part any fish.
true intelligent beings find math as a hinderance to progress. sitting on a bomb to get into space is hardly intelligent. all mans technology amounts to brute force stupor problem solving paths.
for instance - without any use of math - could a star radiate with a diameter of only a few metres -
stars are one appearance of one form as we are all part.
Other star forms live. light is a byproduct of some non-carbon lifeforms. eyes take on a different form with a self-glowing property

The Earth, the Galaxy and the Universe are all full of life.
BUT, life does not mean intelligent life. Gaugain's comment above came close.

If we live for another million years, we will never find an extra terrestrial that can talk to us, face to face. We may find out that many planets in the Milky Way Galaxy have life, but none that can even build a fire.

The bad news is that the intelligent life that IS out there is so far away that we cannot even communicate with them, much less stand next to it.

Conclusion: Life is common but intelligent life is vary rare.
The distance to intelligent life prevents us from talking to it. Like half a billion light years distant.

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