In a galaxy far, far away ... did we find life?

By F D Flam

In the last few years, astronomers have discovered that our galaxy is teeming with planets unlike anything in our solar system. One such exotic world, K2-18b, made the news recently over what some scientists claim is tentative evidence of what could be signs of life. Others say it’s far too soon to tell.

K2-18b weighs in at more than eight times the mass of Earth, orbits a red dwarf star every 33 days, and just might be covered with a massive ocean and blanketed by an atmosphere complete with water vapor and rain clouds, according to work done by two teams of researchers.

Researchers now say they’ve detected hints of two compounds that would make K2-18b smell like the sea. Dimethyl sulfide (DMS) and dimethyl disulfide (DMDS) are produced on Earth by phytoplankton and other marine microorganisms. The evidence isn’t strong enough to declare alien life — yet — but with more observations, it might be.

Scientists do agree on the astonishing fact that it’s possible to use tiny changes in light reaching us to measure what’s in the atmospheres of planets orbiting stars trillions of miles away. Previous observations of K2-18b with the Hubble Space Telescope detected carbon dioxide, methane and water vapor in its atmosphere. The newer findings, published in the Astrophysical Journal Letters, were done using the James Webb Space Telescope and strengthened the case for the sea-scented sulfur compounds.

With a mass and size in between that of Earth and Neptune, it’s called a sub-Neptune and fits into a size range that’s not found in our solar system. The red dwarf it orbits is much dimmer than our sun but is closer, so it’s warm enough for water to remain liquid. Models show it might be covered in an ocean 600 miles deep, more than 100 times the depth of our oceans. However, scientists say they can’t rule out that K2-18b is a molten hellscape or a gaseous planet with no surface.

The possible detection of DMS and DMDS is exciting because they constitute a biosignature — a chemical unlikely to have formed without life. Abundant oxygen here on Earth would be a similar giveaway to life-seeking alien astronomers since we acquired our oxygen-rich atmosphere only after the evolution of photosynthesis.

Back in 1999, Sarah Seager, an astrophysicist at MIT, proposed a way to search for life by looking for such biosignature compounds in the atmospheres of planets around other stars — even though most such planets are invisible even to the most powerful telescopes and have to be detected indirectly by changes in stellar brightness or motion.

Seager’s idea would only work in cases where a planet’s orbit takes it between us and its parent star, causing a slight periodic dip in starlight, like a mini eclipse. In such cases, the planet's atmosphere would alter the starlight that passes through, like a flashlight passing through fog, Seager told me. She proposed back then that scientists could observe changes in the spectrum of starlight to infer which atmospheric gases were present.

Since then, Seager said, astronomers have been surprised to find hundreds of these mid-sized sub-Neptunes. “We have no solar system counterparts,” she said, “yet it appears to be the most common planet in our galaxy.”

And while studying distant planets isn’t the primary purpose of the James Webb telescope, it’s given astronomers a new window into their compositions and potential habitability. “It's just absolutely gratifying to see the telescope being used to study untold numbers of exoplanet atmospheres…atmospheres of all kinds that we never anticipated existing,” she said.

One of her graduate students, now at Cambridge University, led this new work on K2-18b. She said they still need stronger evidence that they really detected the dimethyl sulfide and dimethyl disulfide, and then they need to show that these gases couldn’t be produced by some non-biological process.

Meanwhile, other lines of evidence suggest life is abundant in the universe. Microfossils show that Earth was inhabited soon after it cooled enough to form a solid crust, and the building blocks of life have been detected far from Earth. A recent NASA mission to sample an asteroid named Bennu showed it held amino acids, which make up proteins, and nucleotides — the same building blocks humans use to make RNA and DNA.

Detecting ET probably won’t come as a single discovery credited to one group, said astrophysicist Adam Frank of the University of Rochester. Frank compares life detection to the understanding that the universe is expanding. That wasn’t a scientific consensus until it was confirmed with multiple independent lines of evidence over the course of the 20th century.

He said it’s possible to detect not only biosignatures on distant worlds but also “techno-signatures,” including gases unlikely to be produced except through alien technology. That might include chlorofluorocarbons, which started to build up in our atmosphere after we created them to use as refrigerants, propellants for aerosol sprays and other applications.

It may take a space telescope more powerful than the James Webb to determine which planets host life. That’s why NASA is planning to launch a dedicated instrument called the Habitable Worlds Observatory. Given NASA's budget cuts, the future of this project is unknown, but it would be a shame to give up the search now when we’re so close, and the universe has produced so many weird and wonderful planets.