3rd May 2016

Are we alone? Setting some limits to our uniqueness

A new paper suggests a way to simplify the famous Drake equation, based on observations of exoplanets discovered in the past two decades.

Are humans unique and alone in the vast universe? This question – summed up in the famous Drake equation – has for a half-century been one of the most intractable and uncertain in science. A new paper, however, shows that the recent discoveries of exoplanets combined with a broader approach to the question makes it possible to assign a new empirically valid probability to whether any other advanced technological civilisations have ever existed.

The authors show that, unless the odds of advanced life evolving on a habitable planet are astonishingly low, then humankind is not the universe's first technological, or advanced, civilisation. The paper, published in Astrobiology, also shows for the first time just what "pessimism" or "optimism" mean when it comes to estimating the likelihood of extraterrestrial life.

"The question of whether advanced civilisations exist elsewhere in the universe has always been vexed with three large uncertainties in the Drake equation," said Adam Frank, professor of physics and astronomy at the University of Rochester and co-author of the study. "We've known for a long time approximately how many stars exist. We didn't know how many of those stars had planets that could potentially harbour life, how often life might evolve and lead to intelligent beings, and how long any civilisations might last before becoming extinct."

"Thanks to NASA's Kepler satellite and other searches, we now know that roughly one-fifth of stars have planets in habitable zones where temperatures could support life as we know it. So one of the three big uncertainties has now been constrained."

The third big question – how long civilisations might survive – is still completely unknown. "The fact that humans have had rudimentary technology for roughly ten thousand years doesn't really tell us if other societies would last that long, or perhaps much longer," he said. But Frank and co-author, Woodruff Sullivan at the University of Washington, found they could eliminate that term altogether by simply expanding the question.

"Rather than asking how many civilisations may exist now, we ask 'Are we the only technological species that has ever arisen? This shifted focus eliminates the uncertainty of the civilisation lifetime question and allows us to address what we call the 'cosmic archaeological question' – how often in the history of the universe has life evolved to an advanced state?" explained Sullivan.

That still leaves huge uncertainties in calculating the probability for advanced life to evolve on habitable planets. It's here that Frank and Sullivan flip the question around. Rather than guessing at the odds of advanced life developing, they calculate the odds against it occurring in order for humanity to be the only advanced civilisation in the entire history of the observable universe. From that, Frank and Sullivan then calculated the line between a universe where humanity has been the sole experiment in civilisation and one where others have come before us.

"Of course, we have no idea how likely it is that an intelligent technological species will evolve on a given habitable planet," says Frank. "But using our method we can tell exactly how low that probability would have to be for us to be the ONLY civilisation the universe has produced. We call that the pessimism line. If the actual probability is greater than the pessimism line, then a technological species and civilisation has likely happened before."

By using this approach, Frank and Sullivan calculated how unlikely advanced life would have to be if there has never been another example among the universe's ten billion trillion stars, or even among our own Milky Way galaxy's hundred billion. The result? Applying the latest exoplanet data to the universe's 2 x 10 to the 22nd power stars, Frank and Sullivan found that human civilisation is likely to be unique in the cosmos only if the odds of a civilisation developing on a habitable planet are less than about one in 10 billion trillion, or one part in 10 to the 22nd power.

"One in 10 billion trillion is incredibly small," says Professor Frank. "To me, this implies that other intelligent, technology producing species very likely evolved before us. Think of it this way. Before our result you'd be considered a pessimist if you imagined the probability of evolving a civilisation on a habitable planet were, say, one in a trillion. But even that guess – one chance in a trillion – implies that what has happened here on Earth with humanity has in fact happened about 10 billion other times over cosmic history!"

For smaller volumes the numbers are less extreme. For example, an alien civilisation has likely evolved in our own Milky Way galaxy if the odds against it evolving on any one habitable planet are better than one chance in 60 billion.

But if those numbers seem to give ammunition to the "optimists" about the existence of intelligent aliens, Sullivan points out that the full Drake equation – which calculates the odds that other civilisations are still around today – may give solace to the pessimists.

In 1961, astrophysicist Frank Drake wrote an equation to estimate the number of civilisations likely to be present in our galaxy. His formula (top row) has proven to be a durable framework for research, and space technology has advanced scientists' knowledge of several variables. But it is impossible to do anything more than guess at variables such as L, the probable longevity of other advanced civilisations.

Click to enlarge

In their new research, Adam Frank and Woodruff Sullivan offer a new equation (bottom row) to address a slightly different question: What is the number of advanced civilisations likely to have developed over the history of the observable universe? Frank and Sullivan's equation draws on Drake's, but it eliminates the need for L.

Their argument hinges upon the recent discovery of how many planets exist and how many lie in the "habitable zone" – regions around a star that are just the right temperature for water to be present in liquid form and where life could therefore exist. This allows Frank and Sullivan to define a number they call N_ast, which is the product of N*, the total number of stars; f_p, the fraction of those stars that form planets; and n_p, the average number of those planets in the habitable zones of their stars.

They set out what they call the "Archaeological-form" of the Drake equation, which defines A as the "number of technological species that have ever formed over the history of the observable universe."

Their equation, A=N_ast*f_bt, describes A as the product of N_ast – the number of habitable planets in a given volume of the universe – multiplied by f_bt – the likelihood of a technological species arising on one of these planets. The volume considered could be, for example, the entire universe, or just our galaxy.

"The universe is more than 13 billion years old," said Sullivan. "That means that even if there have been a thousand civilisations in our own galaxy, if they live only as long as we have been around – roughly ten thousand years – then all of them are likely already extinct. And others won't evolve until we are long gone. For us to have much chance of success in finding another "contemporary" active technological civilisation, on average they must last much longer than our present lifetime."

"Given the vast distances between stars and the fixed speed of light, we might never really be able to have a conversation with another civilisation anyway," explains Frank. "If they were 20,000 light years away, then every exchange would take 40,000 years to go back and forth."

But, as Frank points out – even if there aren't other civilisations in our galaxy to communicate with now, the new result still has a profound scientific and philosophical importance: "From a fundamental perspective, the question is 'has it ever happened anywhere before?' Our result is the first time anyone has been able to set any empirical answer for that question and it is astonishingly likely that we are not the only time and place that an advanced civilisation has evolved."

According to Frank and Sullivan, their result has a practical application as well. As humanity faces its crisis in sustainability and climate change we can wonder if other civilisation-building species went through a similar bottleneck and made it to the other side. As Frank puts it: "We don't even know if it's possible to have a high-tech civilisation that lasts more than a few centuries." With Frank and Sullivan's new result, scientists can begin using everything they know about planets and climate to begin modelling the interactions of an energy-intensive species with their home world, knowing that a large sample of such cases has already existed in the cosmos.

"Our results imply that our evolution has not been unique and has probably happened many times before," says Frank. "The other cases are likely to include many energy-intensive civilisations dealing with their feedbacks onto their planets as their civilisations grow. That means we can begin exploring the problem using simulations to get a sense of what leads to long-lived civilisations and what doesn't."

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