New research adds uncertainty to the famous Drake equation, suggesting it's even less likely that humans will find intelligent extraterrestrial life than previously thought.

The hunt for E.T. may have gotten more difficult. New research suggests that alien life may not be as widespread as we had hoped.

When it comes to hunting for alien civilizations, a key question is how plentiful intelligent extraterrestrials are in the universe — but the answer to that question depends on a lot of knowledge scientists don't have yet.

In 1960, Frank Drake, an astronomer and hunter of extraterrestrial intelligence, devised an equation to calculate the probability of hearing from an intelligent, communicating alien civilization. The Drake equation relies on the values of several constants to determine how widespread such civilizations might be, how likely they are to evolve and how likely they are to have broadcast when we were able to detect. While some of the numbers, such as how many stars have planets around them, are fairly well-known, others, such as the fraction of those worlds with life, remain uncertain. [The Father of SETI: Q&A with Astronomer Frank Drake]

Over the years, scientists have attempted to "solve" the Drake equation. But the uncertain quantities required estimation. Optimists tended to put in numbers that would reflect their thoughts — life on other planets is plentiful! Civilizations last for millions of years! Pessimists skew their results the other way, assuming life is rare and civilizations quickly burn out.

Searching for a more accurate answer to the question 'Are we alone?' the new study's researchers have included the uncertainties of the numbers — how confident scientists are in them. Rather than giving each component a hard-and-fast amount, they attempted to gauge the strength of the research into these questions.

"We can show that, given current scientific uncertainty, we get a distribution that could make both the optimists and pessimists happy at the same time: a fair chance of several alien civilizations, but also a fair chance of no aliens within the visible universe," Anders Sandberg told Space.com by email. Sandberg, a philosopher at the University of Oxford, is the lead author on the new research.

"The uncertain sky should not be surprising given our level of uncertainty," Sandberg said. The study, which is available on the preprint site Arxiv, has been submitted to the journal Royal Society of London A.

Alone in the universe?

In 1950, Italian-American physicist Enrico Fermi looked to the skies and asked, "Where are they?" If the universe is filled with alien civilizations, why have none of them contacted Earth? The question, referred to as the Fermi paradox, provided the fuel for the Drake equation.

The Drake equation has never sought a definite number. Instead, it has been used to make a rough estimate of the number of detectable civilizations in the Milky Way (N). According to the equation,

N = Rf p n c f l f i f c L

That number is based on the rate of star formation per year (R), the fraction of stars with planets (f p ), the number of habitable planets per system of planets (n c ), the fraction of those planets with life (f l ), the fraction of life that is intelligent (f i ), the fraction of intelligent civilizations that are detectable (f c ), and the average lifetime of such civilizations in years (L).

Observations of distant stars, with instruments such as NASA's Kepler telescope, have revealed that planets are plentiful around stars, and habitable worlds are spread across the galaxy. All the other variables remain up in the air. [The Most Intriguing Alien Planet Discoveries of 2017]

Sandberg and his colleagues decided to change the inputs for the unknown parts of the equation. Rather than estimating a single number, they included the range. For instance, saying that there is a 1/100 chance for life to evolve doesn't make it clear whether the odds are exactly 1 out of 100, between 1/1000 and 1/10, or between one and one in a googol (10^100), Sandberg said.

"One of the features that differs in [the new research] from previous Fermi paradox analyses is that the current authors tackle the problem of order-of-magnitude uncertainties in each component of Drake's equation in a less-biased, more robust way," Ian Jordan, an astronomer and engineer at the Space Telescope Science Institute in Baltimore, told Space.com in an email. Jordan is not part of the new research.

By factoring in the scientific uncertainty for components like how often life evolves, the researchers determined that the odds that we are the only intelligent life in the Milky Way range between 53 and 99.6 percent. The odds get a bit better when they include the observable universe — the chance that humanity is alone ranges between 39 and 85 percent. The research was published on the journal preprint server arXiv.

The new numbers mean there's a good chance humanity is the only detectable intelligent civilization around. Sandberg doesn't necessarily think that's a bad thing.

"I think this is good news, in a sobering way," he said. "So much more stuff for us to use. So much of the universe we are responsible for. Maybe we will evolve to be the aliens as we spread far and wide in the vastness of the future."

Improving our understanding

Plugging numbers into the Drake equation is an evolving science — actually, it's several branches of science. Astronomy isn't the only one on the hook. Biology and chemistry are both tied into how life, especially intelligent life, evolves. Understanding human history may help us to determine how long a communicating civilization may last.

Today, no one understands how life evolves, but plenty of scientists are searching. Once we determine the biochemical process involved in the emergence of life from inorganic materials, it should become easier to understand how easy it might be for nature to kickstart life on its own (at least, life as we know it), Sandberg said.

Another outstanding question is how easy it is for life to hit a dead end as it invents its genetic code, Sandberg said. If life hits a wall even after it evolves, that could slow down its distribution.

"I am optimistic that we will fill in nearly all [of the variables] over time," Sandberg said. "It might be that we need to settle a few galaxies before we have really solid statistics for life and intelligence, but I think we will be able to figure them out."

Understanding these numbers is only one (admittedly challenging) step. It's also important to continue hunting for other civilizations, because their existence could help us understand how common they are, Sandberg said.

"We should be looking and listening for aliens," he said. "After all, seeing some remote civilization, detecting an alien megastructure or just some ruins somewhere would make us instantly update our probabilities enormously."

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