The habitable zone of a planetary system is based on a simple idea: If a planet is too close to its star, the conditions for life are too hot, and if a planet is too far away, things are too cold. On the whole, it is based on the estimated temperature/distance range for liquid water on the surface of a planet, since life, as we know, needs to need liquid water to exist. The problem with this definition is that it is too rough to be very useful. For example, both Venus and Mars are on the inner and outer edges of the habitable zone of the sun, but also not really habitable. But now that we have observed hundreds of planetary systems, we can define the zone more precisely. One way to do this is to look at the sulfur chemistry.
In a new paper in scientific advances, it is examined how sulfur chemistry can better define the inner border of the habitable zone of a star. The authors find that the key is whether a planet can maintain a surface ice cream. Many inner planets are warm enough to have liquid oceans early, but lose this ocean over time. Venus is a good example of this. Early Venus was probably very earthy, but the lack of a strong magnetic field and a water -rich volcanic activity meant that the early oceans of Venus cooked away.
Even from light years away, the difference between Venus and Earth is striking. If extraterrestrial astronomers observed the atmospheres of both, they would see that the earth has a mixture of nitrogen and oxygen, while Venus has a mostly carbon dioxide atmosphere that is rich in sulfur dioxide. From this they would know that the earth has oceans while Venus does not. Both planets have a lot of sulfur, but the earth prevent the formation of sulfur dioxide, which form large amounts of sulfur dioxide. Chemical dry surface is needed to create sulfur dioxide.
The authors show how the presence of atmospheric sulfur is a marker for an oceanless planet. This could be used for sun -like stars to limit the habitable zone and to select better candidates for foreign life. If an inner planet has a sulfuric atmosphere, you don't have to look further. However, there is a catch.
While dry, warm planets tend to create many sulfur compounds, ultraviolet light tends to break up these molecules. The team demonstrates, while the presence of atmospheric sulfur proves that a planet is dry, the opposite is not always true. A dry planet that circles a star with high UV UV would also have no sulfur compounds. To demonstrate this, the team examined the red dwarf system Trappist-1, which has at least three potentially habitable planets. They found that the UV values for these worlds are too high to use the sulfur test. This is a real crap, since red dwarf planets are most frequently at home for potentially habitable worlds and most of these planets are bathed in much more UV than earth because they circle their star so closely.
This study shows that sulfur chemistry is a useful instrument to find a life, although not as useful as we want. More chemical identifiers will need to narrow down the habitable zones for red dwarfs.
Reference: Jordan, Sean, Oliver Cuttle and Paul B. Rimmer. “Follow the inner edge of the habitable zone with sulfur chemistry.” Science Advances 11.5 (2025): EADP8105.
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