Behold, Countless Earths Sail the Galaxies ... That Is, if You Would Only Believe
Once cosmologists had hit on the Copernican Principle (Earth cannot be special by definition), it was instant orthodoxy. Michael Rowan-Robinson, president of the Royal Astronomical Society (2006-2008), opined in 1996:
It is evident that in the post-Copernican era of human history, no well-informed and rational person can imagine that the Earth occupies a unique position in the universe.1
For one thing, the Principle, also called the Principle of Mediocrity, solved so many problems around lack of evidence. What could not be demonstrated could merely be asserted.
In 2010, NASA's Deep Space Camera had, we were told, located a "host of 'Earths'" -- up to 140 in six weeks, orbiting stars in our galaxy.
"The figures suggest our galaxy, the Milky Way [which has more than 100 billion stars], will contain 100 million habitable planets, and soon we will be identifying the first of them," said Dimitar Sasselov, professor of astronomy at Harvard University and a scientist on the Kepler Mission. "There is a lot more work we need to do with this, but the statistical result is loud and clear, and it is that planets like our own Earth are out there."
Actually, from present data, these "100 million habitable planets" are assumed to be Earth-like only in size and rockiness. A current official definition of habitable planets is "in the zone around the star where liquid water could exist," but the ones discovered so far are unsuitable in many other ways.
Then a new cosmology term hit the media, "super-Earths." It means "bigger than Earth," but smaller than gas giant Neptune. Super-Earths could be the most numerous type of planet, in tight orbits around their star -- which is actually bad news for life. Nonetheless, some insist, they may be more attractive to life than Earth is. Indeed, the Copernican Principle allows us to assume that some are inhabited already.
In reality, even the rocky exoplanets (known as of early 2013) that are Earth-sized are not Earth-like. For example, the Kepler mission's first rocky planet find is described as follows: "Although similar in size to Earth, its orbit lasts just 0.84 days, making it likely that the planet is a scorched, waterless world with a sea of lava on its starlit side." As space program physicist Rob Sheldon puts it, Earth is a rocky planet but so is a solid chunk of iron at 1300 degrees orbiting a few solar radii above the star. In any event, a planet may look Earth-like but have a very different internal structure and atmosphere." Could exoplanets support life that has a different chemical composition? Absent details about the composition, who knows? Despite all this, an Earth Similarity Index has been compiled, offered with the caution that life might also exist under unearthly conditions, a caution that renders the Index's value uncertain.
All this aside, planet scientists felt free to assert in 2011 that there were billions of worlds. Some bid as high as ten billion or tens of billions: We learn that "Every star twinkling in the night sky plays host to an average of 1.6 planets, a new study suggests." "That implies there are some 10 billion Earth-sized planets in our galaxy." And "Using a technique called gravitational microlensing, an international team found a handful of exoplanets that imply the existence of billions more."2
Inspired by the Kepler mission's science chief William Borucki, one reporter enthused:
How's this for an astronomical estimate? There are at least 50 billion exoplanets in our galaxy. What's more, astronomers estimate that 500 million of these alien worlds are probably sitting inside the habitable zones of their parent stars.
A 2013 estimate pegged alien planets that could support life at 60 billion. And we are getting ever closer to the "Goldilocks planet," one space scientist says, momentarily forgetting that we are very close indeed to the one known Goldilocks planet, the one under our feet.
He reminds us that longshoreman philosopher Eric Hoffer, who worked on the San Francisco docks for 25 years, noted that intellectuals of the past century had done all in their power
"to denude the human entity of its uniqueness"; to demonstrate that we are "not essentially distinct from other forms of life." He contrasted Pascal's comment that "the firmament, the stars, the earth are not equal in value to the lowest human being," with that of "the humanitarian" Bertrand Russell: "the stars, the wind in waste places mean more to me than even the human beings I love best." Somehow, we take that as a sign of our maturity. Our philosophers want to rub our noses in the dust. Thou art dust!"
But some prominent scientists want to rub our noses too. And, like the conjurors of old, they despise mere lack of evidence. Indeed, they immediately proceeded to conjure the inhabitants of these planets.
(1) Rowan-Robinson, Michael (1996). Cosmology (3rd ed.). Oxford University Press. pp. 62-63.
(2) See also "'Tens of billions' of planets in habitable zones," Yahoo News, Mar 29, 2012.
Photo credit: Earth Rise; NASA, Apollo 8 Crew.