17 billion earth-like planets, and My God, we are probably and profoundly alone!
In January the Harvard-Smithsonian Center for Astrophysics had released and estimation that there are 17 billion earth sized planets in our galaxy based on NASA’s Keppler mission Data.[i] NASA repeated the findings and continued to talk of Keppler’s mission to find earth-like planets.[ii] Soon, almost every internet news site was announcing 17 billion ‘earth-like’ planets.
The problem is their definition of ‘earth-like’ is any terrestrial planet whose size is smaller than Neptune, with some of those over three of four times the size of Earth. NASA did admit that the vast majority of those known ‘earth-like’ planets are too close to their suns, making them inhospitable.
Fine, so let’s assume 10% of those planets are in the goldilocks zone, the zone in orbit where it’s not too hot and not too cold for liquid water to exist, but just right. This 10% also incorporates a habitable zone for stars. Stars also can get too close to the center of the galaxy, exposing their worlds to extreme cosmic radiation.[iii] That still leaves us nearly 1.7 billion, more reasonably labeled, ‘earth-like’ planets.
The goldilocks zone is still a rather vague criterion for ‘habitability’ though. Our solar system has three planets in the goldilocks zone. And so far, we have discovered only one other system with three planets in the goldilocks zone. Here at Sol, we have Venus which is surrounded by a hot, dense blanket of carbon dioxide and temperatures that can melt lead; Venus is totally hostile to life. We also have Mars which is too cold, dry, and naked to cosmic radiation for any perceptible life. It may have had conditions more favorable for life before, but there is, so far, nothing to show life is or even was on the 4th planet. These two neighbors of ours are most likely the norms. If Mars is so small it couldn’t support a dense atmosphere and therefore life, then any planet smaller probably can’t either. If Venus, being smaller than earth, still accumulates a dense hostile atmosphere, then any of those planets larger than earth are very likely tobe susceptible to the same problem. But let us assume 10% of the 1.7 billion worlds might still harbor life or 170 million planets.
While our planet’s habitability was threatened by cataclysmic events, one of those events can actually be credited to life’s continued history. Slammed by a mars size planet early in our history, we consequently were given a moon whose gravitational tidal effects, maintains our planet’s tectonic activity, and possibly its long lasting magnetic field.[iv] This magnetic field is essential for protecting life and Earth is the only known terrestrial planet that has a field of any strength.[v] So of those 170 million potentially habitable planets we currently have, 10% are truly habitable for life (I think I am being generous). That still gives us 17 million truly ‘earth-like’, life-harboring planets.
The history of life on this planet has been tumultuous. There have been mass extinctions at the rate of every 100 million years during our turbulent 3.5 billion year life bearing history. An asteroid, only a little larger then the one that killed off the dinosaurs, could have wiped out ALL of life. Not all threats are extra-terrestrial either. The planet itself has nearly destroyed life in its far distant past with super-volcanoes and ice-sheets that covered the entire globe.[vi] There is evidence that life had nearly destroyed itself by releasing a toxic substance earlier in its history, (that toxic substance was called oxygen, but more on that later). Let’s say that 10% of the ‘earth-like’ planets actually maintain to sustain life. That means there are 1.7 million earth-like planets that have life in our galaxy. That is an awesome thought and an exciting premise.
Now our history as a living planet, is mostly microbial. If there is life on mars (or was), it is almost certainly microbial. Of the life forms on earth, even now, the vast majority are the much smaller, less complex prokaryotal (bacterial) single-cell life forms. Cell for cell there are 10 times more bacteria in your last stool then the number of cells that makes up your body. It was only after 1.5 billion years of life that the larger more complex single cell life form, called eukaryotic cells seemed to have evolved. These larger, more complex life forms fed off mostly the prokaryotes, and each other. It was in these life forms that the DNA Double Helix evolved. About a half a billion years after they had emerged in our world, a eukaryotic cell went to absorb a prokaryotal cell, but it did not digest it. Instead, for some unexplained reason, it assimilated it. They not only lived together symbiotically, but the DNA structure of the host cell eventually altered to reproduce the embedded prokaryotal when it reproduced itself. This assimilated prokaryotal cell structural was the first mitochondria.[vii] The mitochondria, complete with their own DNA are essential for eukaryotic life to survive and even thrive under an oxygenated environment. The ability of the cells to use oxygen had raised their energy states, a mitochondrion enhanced living cell has a much shorter life, but it is a very dynamic and energized life. The eukaryotic life flourished, and the high metabolism allowed the development of coordinating, multi-cellular life forms. Multi-cellular life is a very lucky by-product of a very lucky symbiotic relationship. It’s fair to say this would be a very rare event. It took the earth 2.5 billion years of single cell life before multi-cellular life could evolve. Given the unusual and complex symbiotic relationship and the time frame, I estimate there are only 1% of the planets mentioned with complex life, life developed into a multi-cellular form. This estimate is probably very generous considering it took half of life’s 5 billion year history to reach this point.
That means 17 thousand planets in our galaxy have multi-cellular life. Think about it! 17,000 planets teeming with growing, crawling, swimming, climbing creatures.
Once again, looking at earth’s history, humans are the only creatures we know of that have had sentience. To some that may be unfair, when you consider intelligent animals like the whale or higher apes, but if you consider our writings and our ability to model the earth for our use, you should accept that we are at least different, cognitively. Of the 1 billion years that complex life has swarmed the earth, Homonoids has been here a mere 1 million years of it. If you consider only Homosapiens, excluding Neandrathals and Homo Habilis you could reduce that time frame to 100,000 years. Assuming the entire Homonoid family as a sentient, environment changing creature, that would mean 0.1% of the planets in the galaxy have had time to develop sentient creatures.
That means there are 17 planets in the galaxy that have sentient, environment changing beings.
Again, looking at our history in the last 1 million years, we have only just begun to build a technology that can explore the universe. We only now can send probes into space, can look at the universe through a telescope and realize it is expanding. We have just learned the fundamentals of relativity, electronics, and physics in the last 100 years and use them to explore beyond our frail world. That’s 0.01% of our existence. That means statistically, there is only one planet in our galaxy that has reached the sentient awareness of the universe around them.
One planet, us!
Using the above percentage, that also means one planet per 29,000 galaxies that have life aware of the universe and its origins.
So statistically we are alone, but we are very, very, very, very special.
[i] Aguilar, David and Christine Pulliam, “At least one in six stars has an earth-sized planet”, Harvard-Smithsonian Center for Astrophysics, Harvard University in conjunction with the Smithsonian Astrophysical Observatory, 7 January 2013, WEB, retrieved 29 June 2013, http://www.cfa.harvard.edu/news/2013/p201301.html
[ii] Michele Johnson, “Kepler mission news: At least one in Six stars has an earth-sized planet”, NASA, NASA news,10 January 2013, WEB, Retrieved 29 June 2013, http://www.NASA.gov/mission_pages/kepler/news/17-percent-of-stars-have-earth-size-planets.html
[iii] Mullen, Leslie, “Galactic Habitable Zones”, Astrobiology Magazine, FirstGov - NASA, 18 May 2001, WEB, Retrieved 29 June 2013, http://www.astrobio.net/exclusive/139/
[iv] Canup, Robin and Erik Asphaug,). "Origin of the Moon in a giant impact near the end of the Earth's formation". Nature, Vol. 412 16 August, 2001: 708–712, Retrieved 2013-12-10, http://www.es.ucsc.edu/%7Ercoe/eart206/canup_Moon_Nature_01.pdf
[v] table: http://www.astronomynotes.com/solarsys/plantblb.htm based on NASA planetary fact sheet by Williams, David “Planetary Fact Sheet – Metric”, NSSDC, Goddard Spaceflight Center, 28 September 2012, WEB, retrieved:unknown, http://nssdc.gsfc.nasa.gov/planetary/factsheet/
[vi] Shields-zhou, G.A. with A.C. Hill and B.A. Macgabhann ”Chapter 17: The Cryogenian Period”, University of Maryland – Department of Geology, Felix M. Gradstein, James G. Ogg, Mark Schmitz and Gabi Ogg. Published by Elsevier B.V., 2012, PDF, Retrieved 29 June 2013, http://www.geol.umd.edu/~hcui/NeoproterozoicGeobiology/GeolTimeScale2012/Ch17-Cryogenian.pdf
[vii] Carprette, David R. “Evolutionary Origin of Mitochondria”, Rice University,, 26 May 2005, WEB, Retrieved 29 June 2013, http://www.ruf.rice.edu/~bioslabs/studies/mitochondria/mitorigin.html