Can We Develop a More Accurate Habitable Zone Using Sulfur?

The habitable zone of a planetary system is based on a simple idea: if a planet is too close to its star then conditions are too hot for life, and if a planet is too distant then things are too cold. It’s broadly based on the estimated temperature/distance range for liquid water to exist on a planet’s surface, since life as we know it needs liquid water to exist. The problem with this definition is that it’s too crude to be very useful. For example, both Venus and Mars are at the inner and outer edges of the Sun’s habitable zone, but neither are really habitable. But now that we have observed hundreds of planetary systems, we can start to pin down the zone more accurately. One way to do this is to look at sulfur chemistry.

A new paper in Science Advances looks at how sulfur chemistry can better define the inner border of a star’s habitable zone. The authors note that the key is whether a planet can maintain a surface ocean. Many inner planets are warm enough to have liquid oceans early on but lose those oceans over time. Venus is a good example of this. Early Venus was likely very Earth-like, but the lack of a strong magnetic field and water-rich volcanic activity meant Venus’s early oceans boiled away.

Even from light-years away, the difference between Venus and Earth is striking. If alien astronomers were to observe the atmospheres of both, they would see that Earth has a mix of nitrogen and oxygen, while Venus has a mostly carbon dioxide atmosphere rich in sulfur dioxide. From this, they would know that Earth has oceans while Venus does not. Both planets have plenty of sulfur, but Earth’s oceans prevent large amounts of sulfur dioxide from forming. It takes dry surface chemistry to create sulfur dioxide.

The authors show how the presence of atmospheric sulfur is a marker for an oceanless planet. For sunlike stars, this could be used to narrow the habitable zone and select better candidates for alien life. If an inner planet has a sulfur-rich atmosphere, there’s no need to look further. There is, however, a catch.

While dry, warm planets would tend to generate plenty of sulfur compounds, ultraviolet light tends to break these molecules up. So, the team demonstrates, while the presence of atmospheric sulfur proves a planet is dry, the opposite is not always true. A dry planet orbiting a high-UV star would also lack sulfur compounds. To demonstrate this, the team looked at the red dwarf system TRAPPIST-1, which has at least three potentially habitable planets. They found that the UV levels for these worlds are too high to use the sulfur test. This is a real bummer, since red dwarf planets are the most common home for potentially habitable worlds, and most of those planets are bathed in much more UV than Earth since they orbit their star so closely.

So this study shows that sulfur chemistry is a useful tool for finding life, though not as useful as we’d like. It will take more chemical identifiers to narrow down the habitable zones for red dwarfs.

Reference: Jordan, Sean, Oliver Shorttle, and Paul B. Rimmer. “Tracing the inner edge of the habitable zone with sulfur chemistry.” Science Advances 11.5 (2025): eadp8105.

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