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Are We Alone in the Universe?

Are We Alone in the Universe?

The scientific search for extraterrestrial (ET) life has been in full swing for a long time. Since the founding of the SETI (“search for extraterrestrial intelligence”) Institute in 1984, the idea that we might actually be able to contact or detect intelligent alien beings has worked its way into the popular consciousness. But despite hundreds of millions of dollars invested into various approaches to validate the belief that ET life is out there somewhere, there is still no proof.

SETI researchers and a panoply of other astronomers, astrophysicists, and astrobiologists have worked hard to convince the public (and the science community) that the existence of intelligent ET life is quite likely. Surely with the billions of other solar systems and planets in outer space there are some that harbor intelligent life. After all, the same raw ingredients at the heart of the evolution of life on earth are present, even abundant, elsewhere in space. The mathematical probability that life could develop elsewhere must be fairly high, so optimism about ET life is reasonable.

But is the probability really that high? Are these and other assumptions based on sound data? Confidence in the idea that there’s certainly intelligent life somewhere in the universe has become as certain in the minds of so many people that it seems silly to challenge the notion. But a small group of scientists aren’t optimistic at all—and with good reason.

Are We Alone in the Universe? The Math Doesn’t Add Up

The idea that there is a high mathematical probability that life exists elsewhere in the universe is based to a significant degree on the Drake Equation, an equation postulated in 1961 by astronomer Frank Drake:

N = R* • fp • ne • fl • fi • fc • L

The equation’s values are identified as follows:

  • N = The number of civilizations in the Milky Way galaxy whose electromagnetic emissions are detectable.
  • R* =The rate of formation of stars suitable for the development of intelligent life.
  • fp = The fraction of those stars with planetary systems.
  • ne = The number of planets, per solar system, with an environment suitable for life.
  • fl = The fraction of suitable planets on which life actually appears.
  • fi = The fraction of life bearing planets on which intelligent life emerges.
  • fc = The fraction of civilizations that develop a technology that releases detectable signs of their existence into space.
  • L = The length of time such civilizations release detectable signals into space.

Though the discussion of the probability of ET life has been steered by this equation for over fifty years, it is (and always has been) vulnerable to significant criticism. In reality, each element of the equation is little more than a guess. Several elements have no empirical basis at all. In her essay, “The Drake Equation is Obsolete,” Lynnette Cook pulls no punches about the flaws of the famous equation:

The integers that are plugged into this equation are often subject to wide interpretation and can differ significantly from scientist to scientist. Even the slightest change can result in vastly different answers. Part of the problem is that our understanding of cosmology and astrobiology is rapidly changing and there is often very little consensus among specialists as to what the variables might be.

Consequently, the Drake formula relies on 'stabs in the dark.' This makes it highly imprecise and unscientific. The margin of error is far beyond what should be considered acceptable or meaningful.

Another major problem of the Drake Equation is that it does not account for two rather important variables: cosmological developmental phases and time. . . . For example, the equation asks us to guess the number of Earth-like planets, but it does not ask us when there were Earth-like planets.

Tom Hartsfield of echoes Cook’s estimation only in harsher language. In his essay, “Why the Drake Equation is Useless,” Hartsfield goes so far as to identify five of the equation’s elements “truly useless,” and asserts that the equation “may actually misrepresent the search for ET and limit our ideas about it.“

Are We Alone in the Universe? The Science Says It’s Unlikely

The contrarian position to the idea that ET life is likely is known as the “rare earth” hypothesis. The term is often associated with geologist Peter Ward and astrobiologist Donald Brownlee, who co-authored a book by that name. In simplest form, the rare earth thesis draws attention to an underlying assumption of those optimistic about ET life: “Just because life happened here, on what basis would we conclude it has happened elsewhere?” Ward and Brownlee outlined a number of conditions or parameters that made life possible on earth and then look for evidence that those same conditions and parameters on allegedly habitable extra-solar planets. The data are not encouraging.

Two Princeton University scientists have followed up on Ward and Brownlee’s book. Edwin Turner (professor of astrophysical sciences) and David Spiegel (computational and theoretical astrophysics),

. . . analyzed what is known about the likelihood of life on other planets in an effort to separate the facts from the mere expectation that life exists outside of Earth. The researchers used a Bayesian analysis — which weighs how much of a scientific conclusion stems from actual data and how much comes from the prior assumptions of the scientist — to determine the probability of extraterrestrial life once the influence of these presumptions is minimized. (Kelly, “Expectation”)

In a report to the National Academy of Science, the two scientists concluded that:

. . . the idea that life has or could arise in an Earth-like environment has only a small amount of supporting evidence, most of it extrapolated from what is known about abiogenesis, or the emergence of life, on early Earth. Instead, their analysis showed that the expectations of life cropping up on exoplanets — those found outside Earth’s solar system — are largely based on the assumption that it would or will happen under the same conditions that allowed life to flourish on this planet. . . . [T]he current knowledge about life on other planets suggests that it’s very possible that Earth is a cosmic aberration where life took shape unusually fast. If so, then the chances of the average terrestrial planet hosting life would be low. (Kelly, “Expectation”)

Some of the issues that lead to the pessimism are surprisingly straightforward. For example (Kelly, “Expectation”):

  • The right chemical elements for life are too rare in abundance.
  • Most everywhere else in the universe, the radiation levels are too high for life.
  • The rain of killer meteorites is too intense for life ever to have evolved into advanced communities. . . . the rate of terrestrial impacts [on earth] could be as much as 10,000 times higher, but for the presence of Jupiter.

In support of the pessimism, in 2016 astrophysicist Erik Zackrisson from Uppsala University used a computer model simulating the universe’s evolution since the Big Bang to conclude that life on earth was 1 in 700 quintillion (Scharping).

The bottom line is, as much as the discovery that intelligent ET life exists would fascinate, right now confidence in the idea is little more than a faith statement. Perhaps the odds will improve in the future. For now, there is little reason to be confident.


Elizabeth Howell, “The Drake Equation: Estimating the Odds of Finding E.T.,”, March 26, 2014

Lynette Cook, “The Drake Equation is Obsolete,”, May 31, 2007

Tom Hartsfield, “Why the Drake Equation is Useless,”, March 11, 2015

Morgan Kelly, “Expectation of extraterrestrial life built more on optimism than evidence, study finds,” news, April 26, 2012

Nathaniel Scharping, “Earth May Be a 1-in-700-Quintillion Kind of Place,” Discover Magazine (Feb 22, 2016)