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Three Frontiers in the Search for Signs of Life Beyond Earth

For thousands of years, inspired by the starry night sky, humanity has wondered whether life exists beyond Earth. In the past three decades, this ancient question has moved from philosophy to data. Astronomers have discovered thousands of exoplanets orbiting stars other than the Sun, revealing that small rocky worlds are common in our galaxy. Astronomers now routine study exoplanet atmospheres, and have begun to search for gases that could be signs of life. Yet many observationally accessible rocky exoplanets may be hotter, more chemically aggressive, or otherwise unlike Earth. If so, the search for life must include environments beyond water. Venus offers a nearby test case, where its clouds have the right conditions for life: liquid, suitable temperatures, and an energy source (the Sun). The Venus clouds, however, are not made of liquid water but are composed of concentrated sulfuric acid—an aggressive chemical that is toxic for Earth life. We have found that some key biomolecules—including amino acids, short peptides, and even a genetic-like polymer—can remain stable in concentrated sulfuric acid under Venus-like conditions. This work has opened a third frontier: alternative solvents for life. Sulfuric acid may be only one example of a broader solvent frontier. Ionic liquids—polar, non-volatile solvents long known in chemistry—may form naturally from planetary materials and remain stable where water cannot exist. By considering environments very different from Earth, we open a frontier that draws together astronomy, planetary science, chemistry, biomolecular physics, and astrobiology—and forces us to ask what life truly requires.