Lab experiments conducted by MIT researchers have shown that micro-organisms can grow and thrive in an atmosphere of nearly 100% hydrogen or helium. Astronomer Sara Seager and her team placed samples of single-celled organisms in small bottles with some nutrient broth and filled different bottles with different gas mixtures (pure hydrogen, pure helium, 80% nitrogen + 20% carbon dioxide, earth air). In the end, the microbes were found to have reproduced and replicated in all the bottles, including the one with 100% hydrogen gas, which is made up of the most abundant element in the known universe.
While this discovery does increase the number of planets being considered as potentially life-supporting, an important thing to note in the experiment is the presence of the nutrient broth, which would need to be present in some shape or form in order to support life, such as a liquid ocean that can exchange chemicals with the rest of the ecosystem.
Using starlight filtration (seeing the changes in the path, colour and intensity of starlight after passing through a planet’s atmosphere), new telescopes in the pipeline such as the James Webb telescope are designed to detect the telltale gas signatures emitted by surfaces of rocky planets; gases that are often released as by-products of various microbial life processes. Being the lightest element, an atmosphere consisting of mostly or completely hydrogen would reach out up to 14x farther than our own, causing greater starlight filtration and making these gas signatures easier to detect.
Life on these planets would be vastly different from that on the Earth, with potential natives demonstrating peculiar adaptations in order to survive and thrive in their environment. Research in this field continues, and each step is more fascinating and raises more questions than the last.