The Space Reviewin association with SpaceNews

TOI 700 e
As astronomers discovery more potentially habitable exoplanets, like TOI 700 e (illustrated above), other scientists see a growing pool of worlds to test hypotheses about the development of life. (credit: NASA/JPL-Caltech/Robert Hurt)

Searching for life and grappling with uncertainty

Bookmark and Share

One of the biggest developments of the last few decades in astronomy has been the explosion of exoplanet discoveries. The first planet orbiting a Sun-like star was discovered only in 1995 (a few had been found earlier orbiting pulsars). Today, the number of known exoplanets exceeds 5,000, with many more potential worlds awaiting confirmation.

Those discoveries started with Michel Mayor and Didier Queloz, who announced the discovery of 51 Pegasi b in 1995, a finding that would result in them winning the Nobel Prize in Physics in 2019.

“It’s not impossible that in the near future, we will have a much better understanding of why life is on this planet and if we have life on other planets,” said Queloz.

Queloz, a graduate student at the time of the discovery and now a professor who splits his time between ETH Zurich and the University of Cambridge, says that avalanche of discoveries proves that exoplanets are not rare. “The universe is full of planets,” he said during a panel at the annual meeting of the American Association for the Advancement of Science (AAAS) earlier this month in Washington. “The question of whether or not there are plenty of planets is essentially solved.”

Now, he’s interested in using those exoplanets to look for evidence of life. “We are on the verge of getting data,” he said. “It’s not impossible that in the near future, we will have a much better understanding of why life is on this planet and if we have life on other planets.”

The idea of looking for life beyond Earth, including exoplanets, is not new. But Queloz and others have organized a new interdisciplinary initiative called the Origins Federation to tie that search for biosignatures to more fundamental questions about the emergence and evolution of life. In other words, is Earth—a habitable and richly inhabited world—the rule or the exception in the galaxy?

The new effort includes ETH Zurich and Cambridge, as well as Harvard and the University of Chicago, “to work together to help promote the field,” Queloz said. “Essentially, to help develop enough sustained funding and curriculum for the next generation of scientists.”

It would, he said, help deal with the challenges faced by interdisciplinary fields that span departments and funding mechanisms. “Sometimes you have to battle the system.”

It draws together scientists from other fields who see the search for life on exoplanets as a way of getting around the “n=1” problem: Earth is the only world we know with life, so everything we know about how it evolves is based on circumstances here that could be unique or commonplace.

“This is where exoplanets are really, really exciting for evolutionary biologists, because these are taking our understanding of life beyond n=1 that we’re stuck with on Earth,” said Emily Mitchell of the department of zoology at Cambridge. “When we have all these different planets, we can start making predictions about what sort of life, what level of biological complexity we can expect.”

Studies of exoplanets could show how life evolved differently on other worlds, based on what biosignatures, like oxygen, are found on their atmospheres as a function of age. “We can start comparing the patterns that we’re seeing.”

It’s possible, she said, that life on other worlds might evolve like what we’ve seen on Earth. More intriguing, though, would be differences, particularly if the increase in complexity is faster or slower.

“Life started really, really quickly on Earth. That implies it’s relatively easy” to form, she said. But, she noted, life remained as microbes for billions of years. “To me, that implies the hard part is making its more complex: making animals, making intelligence.”

“If we ever find a way to detect biosignatures in the universe, we’ll find that the universe is lousy with life, but there is no one else intelligent out there,” predicted Adamala.

Queloz had similar impressions. “The chemistry is the same everywhere in the universe,” he said. Getting to more advanced, even intelligent, life may be more difficult: “maybe a series of unlikely events, one after the other.”

“I think chemistry is just itching to make life,” said Kate Adamala of the University of Minnesota, but more complex life may be difficult. “If we ever find a way to detect biosignatures in the universe, we’ll find that the universe is lousy with life, but there is no one else intelligent out there.”

Adamala is taking a different tack to the question of how life forms and evolves. “We only have one form of life that we know of,” she said, with similar biochemistry. “From the biochemical perspective, all of modern biology has been done on a sample size of one. What kind of science is that?”

She is working in her lab to create synthetic life, with similar cellular structures but all synthesized in the lab. “Until we can build life from scratch with well-known components, we will never understand exactly how life works on the molecular level.”

Such synthetic life, besides providing scientists with insights into how life formed and evolved, could have practical applications as well. One example she gave was transmitting the instructions for creating a life form to a base on Mars so it could be used, for example, to create medicine for a base there. “It can be used to build support systems for long-term space missions,” she said, as well as understanding the origins of life on other worlds.

That synthetic life, she said, would not depart too far from traditional life: “I like weird, but I also like water.”

Challenge of communicating

A day earlier, another panel at the AAAS meeting addressed a related topic. When the day comes that astronomers detect biosignatures in an exoplanet’s atmosphere—or believe they have detected biosignatures—how do they communicate that to the world?

One issue is that the detection of a biosignature may be ambiguous. The spectral lines that show oxygen or other chemicals associated with life will have noise and error bars, and some will raise doubts the detections are valid. It may also be difficult to replicate the findings, particularly if they’re made with one-of-a-kind telescopes and instruments that push the limits of technology.

“There are a lot hints and false starts, and we learn a lot from those,” said Brooks Hanson, executive vice president for science at the American Geophysical Union and a former deputy editor at the journal Science. “I suspect we’ll see lots of hints before we reach a consensus as a community.”

The search for life beyond Earth, but within our own solar system, has already seen both hints and false starts: the readings from life detection experiments on the Viking landers that most believe to be the result of chemistry rather than biology, and the biosignatures seen in Martian meteorite ALH84001 that later had non-biological explanations. More recently, scientists detected phosphine, a gas that could be a biosignature, in the atmosphere of Venus, only for others to fail to make similar detections with other instruments.

But any uncertainty and nuance in future detections of exoplanet biosignatures will likely get lost when reported in the wider media. “We in the press have inevitably gone overboard,” said Marc Kaufman, a science writer who previously worked for the Washington Post, citing the experience from events like ALH84001.

Any formal discovery of a biosignature, he said, will have to be probabilistic: “We’re 90% sure, we’re 85% sure,” he explained. “From my own experience, that probability will get lost.”

Any formal discovery of a biosignature, Kaufman said, will have to be probabilistic. “From my own experience, that probability will get lost.”

He and others on the panel talked about the importance of “confidence scales” to convey how certain scientists were of any future astrobiological discoveries. Heather Graham, an astrobiologist at NASA’s Goddard Space Flight Center, discussed a “biosignature assessment framework” that scientists could use to demonstrate how effectively that had ruled out abiotic explanations for their evidence and the next steps needed to advance the research.

“That was something that we thought was very important about the biosignature assessment framework: rather than using this as a tool to say someone is right or wrong, how we as a community, representing hundreds of specialists, work together to build up convincing data sets about life detection events,” she said, an “iterative process to build up confidence in any sort of data set.”

That framework, though, was primarily intended for the science community and not the general public. “It’s tough for scientists because we feel like our main mode of communications is to the journal, and then what happens after that it a little amorphous,” she said.

She suggested a version of the “Rio Scale”, a zero-to-ten scale developed more than 20 years ago to communicate the significance of a SETI discovery, but which remains relatively obscure. The Rio Scale is itself based on the Torino Scale, developed in the late 1990s to communicate risks of potentially hazardous asteroids, but which is used largely within the planetary science community rather than the public.

Kaufman said that such a scale, or some other way of expressing nuance about discoveries, could be important to avoid a public backlash if biosignature discoveries don’t pan out. He noted that a key tool for exoplanet astrobiology will be NASA’s proposed Habitable Worlds Observatory, the large space telescope endorsed by the Astro2020 decadal survey the agency would like to launch by the early 2040s, at a cost roughly similar to the James Webb Space Telescope.

“That’s going to cost a lot of money,” he said. “If there are a lot of false starts, I think that it makes it difficult to get the money. That’s why a confidence scale is so important, so that the field doesn’t undercut itself.”

Yet, he noted, the field of astrobiology would not be where it is today without a biosignature discovery that didn’t hold up: ALH84001. “It really gave birth to the field of astrobiology,” he said. “It claimed more than it could back up, ultimately, but it still had an extremely beneficial effect.”

Note: we are using a new commenting system, which may require you to create a new account.