The exoplanet K2–18b is in the habitable zone of the red dwarf star K2–18, at a distance of 124 light-years away from Earth. The planet’s orbital radius, 16% the Earth-Sun separation, provides it with an irradiance of 1.4 kilowatts per meter squared, similar to the value received by Earth from the Sun. The exoplanet size and mass are 2.3 and 8.6 times their values for the Earth, yielding a mean density of 2.7 grams per cubic centimeter, intermediate between the density values for Earth and Neptune.
In 2023, the Webb telescope detected the spectral fingerprints of carbon dioxide and methane in K2–18b’s atmosphere as it transited the face of its host star. The data suggested a liquid water ocean or a magma ocean under a hydrogen-rich atmosphere. Life-as-we-know-it could survive in a hydrogen-rich atmosphere.
K2–18b’s atmosphere also shows a weak spectral feature for dimethyl sulfide (DMS), which is a potential biosignature because it is produced by phytoplankton in terrestrial marine environments. However, the statistical significance of the DMS signature is weak and not sufficiently elevated above the noise for its detection to be regarded as robust.
Moreover, the mass spectrometer on the European Space Agency’s Rosetta spacecraft had recently detected DMS in the cloud of gas and dust shed by the lifeless comet 67P/Churyumov-Gerasimenko. This suggests that ultraviolet light and cosmic rays can trigger the synthesis of complex organic molecules to produce DMS on a comet, which could in turn deliver DMS to an exoplanet like K2–18b without any biological origin.
This ambiguity highlights the challenge of using molecular fingerprints in exoplanet atmospheres as biosignatures. Oxygen, methane or carbon dioxide could be produced by non-biological processes.
An alternative approach is to search for spectral techno-signatures, such as the CF4, CCl3F and CFC molecules produced by industrial pollution on Earth. In a 2014 paper that I wrote with my student Henry Lin and the researcher Gonzalo Gonzales-Abad, we demonstrated that the Webb telescope could detect pollution that exceeds by orders of magnitude the terrestrial levels. Such pollution could be intentional if the exoplanet resides outside the habitable zone and is too cold to support life without a blanket of industrial pollution. Finding the spectral fingerprints of industrial pollution would not only indicate that life exists on an exoplanet but also that this life form is intelligent enough to develop a technological infrastructure. In other words, the detection would suggest that we have an intelligent cosmic neighbor whom we might wish to communicate with.
Of course, we could also search for space trash from a cosmic neighbor as a techno-signature. Recently, Project Lyra envisioned a space mission to catch-up with the weird interstellar object `Oumuamua, which might represent such space trash. The problem with this mission concept is that `Oumuamua is by now a hundred million times fainter than it was close to Earth. A spacecraft attempting to get close to it would need to employ a giant telescope to find its faint glow among the numerous objects of its size in the solar system. A better approach is to design a future mission that will rendezvous with another `Oumuamua-like object along its path towards Earth. The Rubin observatory in Chile has the sensitivity to find several `Oumuamua-like objects every year, starting in 2025. Together with the Galileo Project team, I published a recent paper that worked out the parameters of the required space mission to come close to an `Oumuamua-like object as it approaches Earth.
The race for discovery is ongoing. The interesting question is whether extraterrestrial bio-signatures will be found before techno-signatures or vice versa. It would be prudent to split funding between the two approaches in order to hedge our bets. May the best strategy win.
ABOUT THE AUTHOR
Avi Loeb is the head of the Galileo Project, founding director of Harvard University’s — Black Hole Initiative, director of the Institute for Theory and Computation at the Harvard-Smithsonian Center for Astrophysics, and the former chair of the astronomy department at Harvard University (2011–2020). He is a former member of the President’s Council of Advisors on Science and Technology and a former chair of the Board on Physics and Astronomy of the National Academies. He is the bestselling author of “Extraterrestrial: The First Sign of Intelligent Life Beyond Earth” and a co-author of the textbook “Life in the Cosmos”, both published in 2021. His new book, titled “Interstellar”, was published in August 2023.
