Unique Planet

Alone in the Universe is John Gribbin's argument for why ours is the only technological civilization in the universe (well, galaxy anyway).  The book is more or less an expansion on the famous Drake equation which has been used to estimate the number of civilizations in the galaxy. Estimates have historically ranged from a few hundred to several million.

Gribbin goes into great detail with a lot of science about galaxy and planet formation, the history of the earth and the evolution of life.  I had been aware of habitable zones around stars due to the need for liquid water for life to develop, but I hadn't realized that galaxies also have those zones--regions where sufficient amounts of metals exist due to previous generations of stars fusing the heavier elements before the stars exploded.

The details build up--each one slicing away at the likelihood of us being here:  earth-like planets are rare, it's unusual for stars to form singly, our moon is unusually large compared to the planet, the tilt of the earth was from the collision that formed the moon, the moon stabilizes the earth's rotation, the tilt gives us seasons, our large metal core gives us a magnetic field, the thinness of the crust gives us plate tectonics, climate changes prompted and accelerated the evolution of our species.  The extent of uniqueness is staggering.

And if all that isn't enough, Gribbin ends with a discovery from about two and a half years ago I hadn't been aware of--an orange dwarf star called Gliese 710 is on a collision course with the solar system.  Sure, it will take about 1.4 million years and will likely pass through the outer regions of the solar system (Oort Cloud).  But it turns out that the sun typically has a "close" approach with another star about every two million years.  And the Oort Cloud is where comets come from.  If Gliese 710 also has its own cluster of comets, the near pass could send a bombardment of objects toward the earth.  Large impacts have caused mass extinctions before; future impacts could destroy civilization (if any then still exist) or even destroy all of life.

It's strange to consider how contingent our presence is on being at the right place at the right time.  I can't help but believe that simple life exists in many places.  Of course, we don't know for sure.  To be truly alone flies in the face of a lot of science fiction.  Our human history is full of great achievement but also great disappointment--surely this can't be all the universe (or galaxy) has to offer?

Climate change split our ancestors into two groups--one stayed in the trees and one learned to live on the grasslands.  Gribbin mentions that all humans on earth are descended from a group of only one thousand individuals.  Is the intelligence we ascribe to ourselves merely a freak occurrence, a lethal mutation?  Anthropogenic climate change tests the limits of that intelligence. We have the technology, but not the political smarts to solve the problem.  Obviously, Gliese 710 is the least of our concerns.  Perhaps we would have been better off staying in the trees.

Update (August 21):  BBC has an interactive version of Drake's equation.

Update (January 7, 2013):  Perhaps Earth-like planets are not so rare, but most orbit stars much different from ours.

Update (July 30, 2013):  A study from the University of Victoria finds that the habitable zones around stars are narrower than previously assumed.

Update (November 4, 2013):  Based on a survey of 42,000 stars by the Kepler Space Telescope, there may be as many as 4.4 billion Earth-sized planets in the habitable zones of sun-like stars in the Milky Way.  Still unknown is the probability of any planet for supporting life.  MIT astrophysicist Sara Seager points out that "Earth-sized" doesn't mean "Earth-like".  And Washington Post commenter 'wolfeja' lists several other factors relevant to life on Earth such as a molten iron core, gas giants in the solar system, a moon, active tectonic plates, chemical composition, and an axis tilt.  The same sort of factors discussed by Gribbin.

Update (April 4, 2014):  The Cassini spacecraft has confirmed the presence of liquid water between the rocky core and the surface ice of Saturn's moon Enceladus.

Update (August 20, 2014):  A complete ecosystem of microbes is discovered under the Antarctic ice sheet.

Update (August 27, 2014):  Caleb Scharf, author of The Copernicus Complex, points out that just because an improbable series of events lead to humans on Earth doesn't mean there aren't other paths that could lead to similar outcomes on other planets.

Update (January 5, 2015):  A study by Coryn Bailer-Jones of The Max Planck Institute for Astronomy identifies a second dwarf star called Hip 86505 that could reach the edge of the solar system within one-quarter to half million years.

Update (March 18, 2015):  A study projects that billions of stars in the Milky Way galaxy have one to three planets in their habitable zones.

Update (April 25, 2015):  A survey of 100,000 nearby galaxies lead by The Pennsylvania State University concluded that there were no signs of advanced civilizations.  At least according to our assumptions about how advanced civilizations would collect energy.

Update (July 23, 2015):  NASA announced the discovery of the most Earth-like planet so far. Kepler-452b is 1400 light-years away, is 60 percent larger is diameter, and orbits a similar star to our Sun within that star's habitable zone.

Update (October 23, 2015):  Is it possible that most potentially Earth-like planets haven't been formed yet?

Update (January 22, 2016):  A paper by Aditya Chopra and Charley Lineweaver from the Australian National University hypothesizes that life on most planets goes extinct very quickly. Early life forms need to evolve fast enough to be able to regulate the planet's temperature. Lineweaver:

The mystery of why we haven’t yet found signs of aliens may have less to do with the likelihood of the origin of life or intelligence and have more to do with the rarity of the rapid emergence of biological regulation of feedback cycles on planetary surfaces.

Update (May 1, 2016):  A paper by Adam Frank and Woodruff Sullivan modifies the Drake equation and places a lower bound on the probability that humans are unique in the Universe. They argue that other technological civilizations are likely to have evolved before us, but are also probably extinct.

Update (August 2, 2016):  Low mass stars are much more common and a hypothesis suggests that if they can support life, it may be too early for sentient organisms to have evolved.

Update (July 16, 2017):  If there were no earthquakes, life would not be possible. Earthquakes are caused by circulating magma. The iron in moving magma generates a magnetic field. The magnetic field protects the atmosphere from being blown away by solar wind. Atmospheric pressure prevents the oceans from boiling away. Also, fault lines where earthquakes occur are areas where resources are brought to the surface. Mineral rich soil and water made civilization possible.

Update (February 18, 2019):  Joe Scott discusses the Rare Earth Hypothesis.

Update (October 14, 2019):  In an interview with Keith Spencer, Erik Asphaug discusses the possibility that life on Earth would not exist without the moon.

Update (December 23, 2020):  A radio signal from Proxima Centauri was detected in the spring of 2019. 

The researchers studying the wave emission have not yet been able to identify any Earthly origin, whether a satellite in orbit or something on the ground. As a result, scientists at the Breakthrough Listen project — an organization based at the University of California, Berkeley that searches for radio signals from intelligent extraterrestrial life forms in the universe — believe that the radio signal could originate from extraterrestrial intelligent life.