The Milky Way may host over 100 million planets supporting complex life


June 10, 2014

According to a survey conducted by astronomers at Cornell University, the Milky Way may be host to over 100 million planets hosting life beyond the microbial stage (Image: PHL at UPR Arecibo/NASA/Richard Wheeler @Zephyris)

According to a survey conducted by astronomers at Cornell University, the Milky Way may be host to over 100 million planets hosting life beyond the microbial stage (Image: PHL at UPR Arecibo/NASA/Richard Wheeler @Zephyris)

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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 BCI can use the limited information in our possession to assess the relative likelihood of different planets to host complex life (Image: Cornell University)

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.

The Biological Complexity Index plotted against the Earth Similarity Index (Image: Cornell University)

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

About the Author
Dario Borghino Dario studied software engineering at the Polytechnic University of Turin. When he isn't writing for Gizmag he is usually traveling the world on a whim, working on an AI-guided automated trading system, or chasing his dream to become the next European thumbwrestling champion. All articles by Dario Borghino

Wow millions or none.

Heather Bowman

"We have absolutely no idea" explained scientists.

Jon A.

When the Americas were discovered people speculated about all sorts of things like fountains of youth, enormous stockpiles of gold etc. I think the same speculation nonsense is fueling these sorts of projections.

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.


I know I am not the first to say this but if there are many planets with intelligent life, many would be older than ours and probably advanced much farther than we are. It is much more likely they will come here than any chance that we will go there.

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.


We had to speculate in my Advanced Astronomy class, UT Austin --circa 1980-- as to the very existence of planets as none had been discovered yet. One area that we discussed was that it might be very important for a potential LIFE harboring planet to have a Moon that created tides... Might was the operative word. If Mars has life or once had life, then, never mind.

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!


Another Scientific Wild A$$ Guess. Let's find ONE before we start predicting 100 million!


100 million planets in the Milky Way 'may' support complex life--'may'. Pure conjecture really.

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.


I rather think that advanced intelligent species just don´t live on planets. Technology tends to have a very heavy footprint... one that damages the environment. That should be true for any technologically advanced civilization. You cannot get something for nothing... Advanced civilizations must therefore live in free space or at least in interplanetary space (in habitats a lá O´neill). Most of the time their comms would be strictly a line-of-sight system, like laser. They simply have no need for onmidirectional comms (like radio) and the associated high energy demands. B.T.W, directional comms are a must in a heavily colonized cislunar or interplanetary space!

Alfred Max Hofbauer

If there is no other sentient life in the great beyond, then describing me as the only blue-eyed person on planet earth would not come even close to the rarity of the human species in the universe.

Mel Tisdale

yrag: Here's some "impure conjecture". We know all complex life was killed off here 4 times. We don't know if some had become sentient. For example, if life here went back to "reset" again, which will happen for sure some day, and we have not stopped fighting long enough to establish a colony elsewhere, will evidence of our existence remain? Future sentient species may find no trace of us.

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.

Don Duncan

You can't estimate the probability of ice cream by working out the availability of it's ingredients - yes - ingredients (molecules) magically combine to form life - we're the example - but they need the magic of spectacularly large quantities of time, mixed with the right ingredients, all simmered in the right conditions.

Monkeys can write Shakespeare, but no, you do not need an infinite number. You need to calculate the relative probabilities of monkey-derived typewrite keystrokes, factor in the the wording of Shakespeare's works, decide how long you want to wait for them to finish typing, and you'll reach the actual finite number.

Life is no different. You need to calculate the probabilities of molecules in motion aligning into something that evolves into DNA, and factor in the relative abundance of the elements involved and their motion. We know the time span already. What we don't know, without bothering to put the effort into working it out, is whether we exist here because earth has the right amount of everything we needed to make it so, or, if we exist because the entire universe did.

Big difference - we're either the only life there is, or we're not, and it really doesn't seem that hard to work out properly, if someone tried to get serious about it.

These guys, and Drake alike, do not seem serious. You cannot calculate "probability of life on other planets" without first calculating the probability of life itself.


" 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."

What animals? Some creatures detect polarization changes and can alter the absorption polaization of their bodies but this is not the same as modulating a light beam with information!

As to animals communicating by radio, although both light and radio waves are electromagnetic radiation but differing only in wavelength, the interaction with biological matter is very wavelength dependent - animals are transparent to radio waves so can't interact at these wavelengths. See

Sheldon Cooper
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