Peter Ward and Donald Brownlee, the originators of the theory, explain exactly why they think the development of complicated life is extraordinarily rare on other worlds.
Pioneer 4 was the very first spacecraft to discover space beyond Earth’s orbit in 1959. Twenty-five years down the line, in 1984, astronomers Carl Sagan and Jill Tarter founded the Search for Extraterrestrial Intelligence (SETI), a program that is searching the cosmos for signs of alien life ever since.
However neither an international armada of robotic spacefarers nor alien seekers have discovered proof of extraterrestrial life. Even though our exploration of the solar system has been nothing short of astonishing when it comes to scientific data and images, the worlds we have visited beyond Earth seem to be totally sterile.
Even the most dedicated SETI researcher would need to acknowledge that our attempts to find life elsewhere in the universe have met with an uncomfortable stony silence, at least up to this point. But why?
The Rare Earth hypothesis
In 2000, two researchers, Peter Ward and Donald Brownlee, published a book which offered a possible explanation for the apparent aloneness of our species. It is known as a Rare earth. Just why Complex Life is Uncommon in the Universe (Copernicus Books 2000) Paleontologist by training, Ward and Brownlee, an astronomer, combined their efforts to produce what’s currently known as the Rare Earth hypothesis.
Simply stated, the Rare Earth hypothesis suggests that the really special conditions on Earth that made it possible for complex life to arise and flourish are exceptionally unusual, plus they are unlikely to arise generally throughout the universe.
Ward and Brownlee postulated that many fortuitous features of the solar system, our Sun and also the Earth led to our highly favorable & surprisingly stable ecosystem. Although some of these properties had been talked about widely in astronomy circles before, others had not been stated at all.
The Rare Earth hypothesis concentrates on numerous facets of the Earth as well as its surroundings which have played a role in allowing complex life to grow. Several crucial elements that Ward and Brownlee regarded as crucial to the development of complicated living included:
- A planet that exists in a favorable part of the right kind of galaxy, where significant amounts of heavy elements are available and sterilizing radiation sources are located far away.
- An orbit around a star that has a long lifetime (billions of years) but does not give off too much ultraviolet radiation.
- An orbital distance that allows liquid water to exist at or near the planet’s surface.
- An orbital distance that is far enough away to prevent the planet from becoming tidally locked to its host star.
- An orbit that is stable around its host star over cosmic timescales.
- A planetary tilt that allows for seasonal atmospheric changes to be mild, not severe.
- A solar system that includes gas giants capable of preventing debris from polluting the inner solar system, reducing the odds of major cosmic impacts and subsequent mass extinctions.
- A planetary mass large enough to both retain an atmosphere and allow for liquid oceans.
- A moon large enough to help stabilize the tilt of the planet’s axis.
- A molten planetary core that generates a significant global magnetic field, largely protecting the surface from solar radiation.
- The presence of oxygen, and the right amount of oxygen, at the right time for complex life to utilize it.
- The presence of plate tectonics, which build up land masses, create diverse ecosystems, cycle carbon into and out of the atmosphere, prevent a runaway greenhouse effect, and help stabilize the surface temperature worldwide.
Could we really be alone?
In the two years since this book was released, interest in these suggestions has only grown. In the past year, Astronomy caught up with Brownlee and Ward to talk about the Rare Earth hypothesis. Ward recalled during those conversations how the whole idea of the Rare Earth hypothesis originated from a movie-based discussion with Brownlee.
Ward laughed, saying: “We were just talking about how ridiculous the Star Wars bar room scene was. “That’s how it all began. Have a look at the aliens! I think the idea of aliens all over is foisted on the general public, you know?
Brownlee and Ward challenged a number of widely held perceptions that complex life is out there waiting to be found. Although astronomer Carl Sagan frequently opines that our Sun is an unremarkable star, in reality about 80 to ninety five percent of stars are considerably different from our own in terms of mass, luminosity, lifetime and many other factors.
Additionally, previous researchers that had attempted to answer the question of the reason why life is so uncommon in the universe but so plentiful on Earth had not included plate tectonics in their thinking. In Rare Earth, there is a whole chapter dedicated to this subject, detailing the role of plate tectonics in transforming the environment into a great place for living. As far as we are aware, the Earth could be the only body in the solar system possessing active plate tectonics. And there’re many other features of our world which are not replicated anywhere else in the universe.
Does simple life count?
The Rare Earth hypothesis only pertains to the development of complex life, and it is crucial to keep in mind that. Both Brownlee and Ward think that basic life, like bacteria, is prevalent all through the universe, after all, even the most harsh habitats on Earth are home to microbes. The pair thinks, however, that complicated life, metazoans as humans and animals, are extremely rare.
If life is discovered someplace else, it may be microbial, “Brownlee said. “You know, Earth is going to have a lifetime of approximately twelve billion years, but [compared to bacteria], metazoans have a more limited range of environmental criteria they can survive in.” What this means is that the environment of a world is a lot longer favorable to simple life than it’s to complicated life.
“The time period when we’ve oxygen in the environment – co2 to go to oxygen and plants for metazoans – is most likely just like 10 or 20 percent of [Earth’s lifetime]. In case you landed arbitrarily on our world during its whole history, you’d have absolutely nothing to see. “
Ward and Brownlee don’t believe complicated life is prevalent throughout the universe, but that doesn’t mean they do not wish to find it. The 2 observatories are going to welcome new data coming from cutting edge observatories such asRB_IN the James Webb Space Telescope (JWST) that aims to disclose in detail the atmospheres of exoplanets. Additionally there are particular atmospheric signatures which are more intriguing compared to others.
I believe it really is much more important to try to search for oxygen atmospheres, but additionally to search for reflections which suggest chlorophyll. ” “There are merely a number of methods to create specific molecules,” Ward said. “It comes back to the reality that, as David Catling, University of Washington planetary scientist, pointed out, any animal equivalent will require a great deal of oxygen. You can’t have really quickly moving creatures and quickly thinking creatures, which is a kind of movement, and not have oxygen in the environment to get it done. “We aren’t going to have folks living on co2 out there,” he stated.
The Rare Earth theory has its pros and cons, but it continues to be intriguing. A number of the ecological variables determined in the book by Brownlee and Ward came under fire in the past twenty years. One of the most often attacked suggested conditions for complicated life is that a huge planet, like Jupiter, is required to keep the inner solar system fairly free of dangerous debris. A number of scientists say such planets could in fact improve the frequency of planetary impacts. Some other critics have brought up concerns regarding the suggested needs of worldwide magnetic field and plate tectonics.
Ward is understanding and motivates problems to his ideas with regards to these critiques. “Good science does lots of things, but the most crucial thing is it stimulates other science,” says he. Excellent science angers individuals. Many individuals get furious enough to do a thing about it. “
The Rare Earth hypothesis continues to be unproven, though it’s difficult to dismiss the big body of information that Ward and Brownlee have collected in support of the case. The bare and impressive surfaces of Mercury, Venus, and Mars function by comparison as close by reminders of exactly what a fortunate paradise Earth is. And uncommon as it might be, it’s the only home we’ve.