As more planets are discovered outside Earth's solar system, scientists must update estimations about the likelihood that life has formed somewhere other than Earth.

SALT LAKE CITY — What are the odds that alien life exists elsewhere in the universe? At a major physics meeting, experts talked about updates to historic predictions about whether humans are alone in the cosmos.

In 1961, astronomer Frank Drake wrote an equation to quantify the likelihood of finding a technologically advanced civilization elsewhere in the universe. The so-called Drake equation took into account factors such as the fraction of stars with planets around them and the fraction of those planets that would be hospitable to life.

In the years since 1961, scientists have updated the values in the Drake equation to incorporate newly acquired scientific information. For example, when Drake wrote his equation, scientists didn't know for sure if stars other than the sun had planets around them; now, researchers have evidence that most stars host planets. But science wasn't the only thing that influenced Drake — even current events factor into his calculation. [The Father of SETI: Q&A with Astronomer Frank Drake]

A lonely planet?

At the heart of the search for life elsewhere in the universe is the question "Is Earth unique?" said Matthew Stanley, a science historian at New York University. Stanley discussed the history of humanity's evolving view of its place in the cosmos at the American Physical Society April Meeting on Saturday (April 16), in a session focused on recent discoveries in planetary science.

Humans once thought that the Earth was not only unique, but at the center of the entire universe, Stanley said. Scientific investigations eventually showed that our planet is not even at the center of its own solar system — it is one of seven other planets and many smaller bodies orbiting the sun.

On the other hand, in the last 20 years, scientists have discovered thousands of planets around other stars, and most of those planets are not like Earth (they're big and gaseous, like Jupiter). And most solar systems are not like Earth's solar system (big planets orbit close to their parent star, whereas in Earth's solar system, the large planets orbit further out).

Does this suggest that Earth is unique? Stanley said that currently, this question is difficult to answer, because telescopes that search for exoplanets have a selection bias toward large, gas giant planets that orbit very close to their parent stars. With current technologies, these types of planets are easier to detect. With that in mind, scientists are still trying to estimate how many rocky and Earth-like planets are out there.

By one estimation, for every grain of sand on Earth, there could be as many as 10 Earth-like planets in the universe. That's according to Peter Behroozi, a Hubble fellow at the University of California, Berkeley, who presented during the same session as Stanley. (Of course, it is important to remember that the universe is a very big place, and at the moment scientists can search for life only on planets within the Milky Way galaxy.)

An artist's concept of planet system Kepler-47. As scientists learn more about the cosmos, estimations of the likelihood that life exists beyond Earth are changing. (Image credit: NASA)

Behroozi is working to link galaxy formation with planet formation. In a paper published in 2015 in the Monthly Notices of the Royal Astronomical Socity, he and his co-author showed that larger galaxies produce a greater number of Earth-like planets than do smaller galaxies such as, for example, the Milky Way. But because there are many more Milky Way-size galaxies in the universe, that's where most Earth-like planets in the universe should be found. Thus, Earth's location in a Milky Way-size galaxy is not unique.

This work not only helps scientists make estimations about how many planets are currently in the universe, but how many will form, assuming the universe continues to grow and evolve in the same way it has in the recent past. In the 2015 paper, Behroozi and his colleague look far ahead into the future and estimate that "the universe will form over 10 times more planets than currently exist." The paper summary goes on to say that there is "at least a 92 percent chance that we are not the only civilization the universe will ever have."

A historical perspective

During his talk, Stanley re-traced the history of humanity's search for life beyond Earth, and showed how people are influenced by their own times and experiences when trying to predict what lies beyond this planet.

William Herschel, an extremely influential 18th century astronomer, believed that intelligent beings lived on the sun. In the 19th century, mainstream astronomers thought they saw artificial canals built by intelligent creatures on Mars and Venus. Observations of those two planets and the sun by space-based probes have disproved those ideas, but new research has also given rise to updated ideas about how and where life could exist elsewhere in the universe.

For example, Stanley said, in the last 40 years, scientists have adopted a broader view of the conditions under which life can exist. So-called extremophiles are organisms that live in environments that were previously thought inhospitable, like at the bottom of the ocean, under the ice in Antarctica and in areas that receive high doses of radiation.

Stanley said many aspects of the Drake equation need updating not only with new scientific evidence, but also with new perspectives. (It should be noted that there are scientists and writers who have dedicated entire books to updating Drake's predictions.)

Drake's equation, for example includes the variable L, which stands for "the length of time such [technologically advanced] civilizations release detectable signals into space," according to the SETI Institute.

When Drake wrote his equation in the 1960s, the value for L was thought of as the time between when a civilization discovered atomic energy and when that society managed to destroy itself through nuclear annihilation, Stanley said.

"That's a totally reasonable way to think about the length of time of a civilization at the height of the Cold War," he said. "But there's been recent work … arguing that we shouldn't think about 'L' in terms of nuclear war. We should think about it in terms of environmental destruction. … That is, it's the time between the discovery of a steam engine and catastrophic climate change."

The equation also includes the variable fc, which represents the fraction of alien civilizations that "develop a technology that releases detectable signs of their existence" (such as radio communications or television broadcast signals sprayed out into space), the SETI Institute said.

Today, however, many of Earth's communications no longer leak out into space, but are instead passed neatly between ground sources and satellites. There are still projects searching for leaky alien communications, and some scientists have proposed that humans should look for focused, laser-based systems used by alien civilizations to communicate between multiple planets or even multiple star systems. But Stanley's larger point is that to some extent, humanity can only look for alien civilizations that bear some resemblance to our own.

Today, the search for life on other planets is largely focused on telescopes that can study the atmospheres of distant planets and look for signs of biological processes. For example, high levels of methane (produced by many living organisms on Earth) or oxygen in a planet's atmosphere could be due to biological activity. And one day, researchers may be able to search for artificially created atmospheric elements.

"So even if we bomb ourselves back to the Renaissance or the Stone Age, the evidence that a civilization once existed on our planet [would not be] erased," Behroozi told Space.com.

The composition of a planet's atmosphere could even reveal how an intelligent civilization that once lived managed to kill itself, Stanley said.

It may be impossible for humans to be purely objective in their speculation about life the universe, Stanley said. He added that he thinks personal bias and human experiences will always infuse science, but that those things can also help lead to successes in science. Having different perspectives helps people look at things in new ways, which can lead to breakthroughs, he said. That's why, he said, it's actually a good idea for scientists to "talk to people outside your field … listen to marginal people. Get a diversity of people, people from different backgrounds, different genders [and] different kinds of cultures.

"I think it's actually helpful to embrace the fact that this is always how science is done," he said. "And to accept that everybody's different, everybody has weird ideas, and that's actually a source of strength rather than weakness."

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