The Milky Way may host over 100 million planets supporting complex life
A survey conducted by astronomers at Cornell University has taken into account the characteristics of 637 known exoplanets and elaborated a Biological Complexity Index (BCI) to assess the relative probability of finding complex life on them. Their data supports the view that as many as one hundred million planets scattered around the Milky Way, and perhaps more, could support life beyond the microbial stage.
The Biological Complexity Index
We know that organic molecules are present in star-forming regions, protoplanetary disks, meteorites, comets, and even deep space; moreover, water is among the most common molecules in the Universe, and energy is available in many forms both on the surface and deep within a planet. These reasons lead us to believe that may be other forms of life even within our galaxy; however, the question of just how many might be there has been a topic of speculation for decades.
In 1961, American astronomer Frank Drake proposed a formula that could be used to roughly estimate of the number of intelligent and technologically advanced extraterrestrial civilizations in the Milky Way. However, we have so little data on the world outside our solar system that estimating the parameters of this formula accurately is next to impossible. Depending on your initial assumptions, the number of advanced civilizations in our galaxy according to Drake's equation could range from virtually zero to a whopping 36.4 million.
Now, a group of scientists at Cornell University led by associate professor Alberto Fairén have proposed a formula that takes into account the characteristics of over 600 known exoplanets to help estimate how likely we are to find complex life on them. By "complex life," the researchers don't mean necessarily technologically advanced or even highly intelligent life, but rather life forms that are above the microbial level and form stable food chains like those found in ecosystems on Earth.
The researchers took into account the density, temperature, substrate, chemistry, distance from its star and age of a given exoplanet, combining these parameters into a unique metric that they call the Biological Complexity Index (BCI).
The index doesn't represent an absolute statistical prediction of whether complex life could be present on a planet; rather, it can be used to estimate the relative likelihood of life having evolved there, based on the conditions that we know are compatible with the evolution of complex life forms on a planet, and assuming that no further information is available.
In essence, an outside observer could use the index to compare two planets or moons which are light-years away and with only limited, easily detectable information at his disposal, tell which one is the most likely to harbor life.
Life on other planets
Prof. Fairén and colleagues have used the BCI index to assess the habitability of 637 known exoplanets for which they had access to all the necessary parameters. According to their report, 11 of those exoplanets (1.7 percent of the sample) have a BCI above that of Europa, and five (0.8 percent of sample) have a score higher than Mars. Although that number might seem small, when extrapolating it to the entire galaxy this means there may be north of 100 million planets in our Milky Way alone on which complex life has plausibly evolved.
Of course, the accuracy of this estimation is constrained by the limited amount of data that we have on those planets. For instance, our instruments aren't currently powerful enough to detect Earth-size planets that are very far away, and this might mean that the estimate is actually a conservative one. On the other hand, some planets that might look hospitable from light-years away may not look as good after a closer look.
According to some astronomers, worlds that are larger, warmer, and older than Earth, orbiting dwarf stars, are probably the most likely candidates for hosting complex life. The results from Fairén team's survey are in accordance with this theory, as all five exoplanets detected with a BCI value higher than Mars have exactly these characteristics.
Curiously enough one of the planets, Gliese 581c, has an even higher BCI value than Earth. Again, this is not a comment on the absolute likelihood of finding complex life there; rather, it means that if an external observer (such as a technically advanced alien civilization) were to observe both Earth and Gliese 581c from light-years away, with only limited information at their disposal, they might be led to conclude that Gliese 581c is the more likely candidate for hosting life – at least, if they were using the same formula.
We know quite well from looking at the fossil record that life appeared on Earth very soon after the environmental conditions were favorable on the surface. Therefore, a further refinement might be to combine the BCI with a second metric that takes into account how similar a planet is compared to Earth. The researchers have therefore proposed an "Earth Similarity Index" (ESI) rates the similarity of extrasolar planets to Earth on the basis of mass, size, and temperature.
Overall, the data produced by the researchers supports the idea that the evolution of complex life on other worlds is relatively rare across our galaxy, but still extremely large in terms of absolute numbers. So, even though they may very well be countless other advanced forms of life in the Milky Way, we are so far from one another that we are unlikely to make the trip there in the foreseeable future.
When the James Webb Space Telescope – an instrument so powerful that it could easily detect a firefly from a distance of 240 thousand miles (385,000 km) – launches in 2018, we will be able to gather much more accurate data on which to base our estimations.
The researchers describe their findings in an open-access paper published on the journal Challenges.
Source: Cornell University
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Can't say I know what the others don't, but the fact that we have yet to discover any radio broadcast should prune these optimistic assumptions. Even if we were to say only intelligent life is rare, one has to wonder what the probability of using radio waves would be just as a biologic process by relatively unintelligent creatures. Animals in the deep ocean use light to communicate, doesn't seem that implausible that a creature on another world could use radio to communicate with members of its own species. Such radio broadcasts would not have the speculated to be short period of intelligent life. They should be uninterrupted for millions and millions of years. My guess is that 1 in 5 worlds with nervous system level life probably would have animals with radio communication capability. The fact that we have not picked up a one, is not encouraging.
It still could be that life is all over, but just not at the animal level (something with a nervous system and ability to move and make choices in actions). More like single cellular stuff or plants and fungus level stuff.
It is also possible that our radio antennas are just too insensitive to detect radio waves from biological sources across space.
These models also seem to always leave out most of the unique things about our planet that make life possible. If our planet didn't have trees for building and fuel along with easily obtainable minerals, we would still be in the stone age. If our percentage of oxygen was a little higher or lower we wouldn't have fire or it would be uncontrollable. So many requirements must be met for higher life forms to advance. Even with millions of earth sized planets in the Goldilock's zone, the odds of intelligent life are slim to none. I suspect most of them are like Venus or Mars.
Per Drake's Equation, I found the notion that civilizations have some kind of end of civilization life, most troublesome, thus limiting the number of civilizations based on what is pulled out of thin air per a X variable. It could be that if a civilization exists long enough, it will be hard to snuff it out.
I did suggest that a civilization/people might de-evolve. See the movie, Idiocracy.
Per radio waves: We discussed this in depth. The "WOW" signal --1977-- had been detected by then, but never found again. We speculated that distant signals, unless willfully directed, would be hard to detect anyway. Recently, as in this year, quantum jump transmissions have been played with. Arecibo would be useless here. There was discussion on how much more advanced means to detect and to put together distant radio signals, might be achieved. We are just really beginning so it seems to me.
It could be that we have bits of signals smattering all around us and we just don't know it --yet--. All exciting!
But remember, the Milky Way is made up of some 300 BILLION stars. That's a pretty thin scattering of 'maybe' complex life. Here's hoping, but don't expect to be shaking appendages with aliens anytime soon.
No search for ETs was possible until last century. Space travel, exploration, and ET contact is a very low priority compared to killing each other. While TPTB waste the masses wealth on building weapons of species extinction, the masses cheer them on. People who specialize in killing other people and breaking things get standing ovations. Scientists, not so much. There are no memorial graveyards for them. The same can be said for the everyday family businessperson and the industrial giants. History emphasizes war, not science or business. How many businesspersons have gotten the Metal of Honor? Even the ones who were considered the pillars of society were vilified later by pseudo sociologists as "robber barons".
And last but not least, in fact the most crucial people to survival, the philosophers who gave us the sciences, mathematics, ethics, and economics, are unsung in school compared to people who contribute little or nothing.