Looking for ET: Astronomers narrow down search for aliens by honing in on planets most likely to host life

They found that planets around inactive, or quiet stars are more likely to maintain life-sustaining liquid water


                            Looking for ET: Astronomers narrow down search for aliens by honing in on planets most likely to host life

Looking for signs for life outside our planet could be easier when astronomers know where to focus their attention. And a new tool developed by astronomers might now do exactly that by narrowing down the search for alien life. 

The researchers from multiple institutions refined the search or conditions that make a planet habitable by focusing on how much radiation a star gives out. They found that planets around inactive, or quiet stars are more likely to maintain life-sustaining liquid water. According to the scientists, planets orbiting active stars —  that emit a lot of ultraviolet (UV) radiation — lose significant water to vaporization, making it unlikey to support life.

“‘Are we alone?’ is one of the biggest unanswered questions. If we can predict which planets are most likely to host life, then we might get that much closer to answering it within our lifetimes.” says Howard Chen, the study’s first author.

In this quest, the astronomers from the Northwestern University, University of Colorado Boulder, NASA’s Virtual Planet Laboratory and the Massachusetts Institute of Technology turned their attention to the diminutive M dwarf stars – which make up 70% of our galaxy.

The prevalence of M dwarfs means that astronomers have numerous planets that may host alien life. “There are a lot of stars and planets out there, which means there are a lot of targets,” says Daniel Horton, senior author of the study. “Our study can help limit the number of places we have to point our telescopes,” adds Horton.

But only a few planets belong to the habitable zone: if planets are too close to its star, water will vaporize completely and if a planet is too far, then water will freeze, and the greenhouse effect will be unable to keep the surface warm enough to support life.

Planets that orbit active stars are vulnerable to losing significant amounts of water due to vaporization and may not be suitable for hosting life.(NASA/Harvard-Smithsonian Center for Astrophysics/D. Aguilar)

Lying between the two extremes is the habitable zone and it is in this region that Horton and team explored within M dwarf stellar systems. The research team developed a tool to explore the habitability of planets around M dwarf stars. 

Their tool helped them understand how UV radiation from a star interacts with gases, including water vapor and ozone in the planet’s atmosphere. Active stars emit a lot of UV radiation. Planets that orbit these stars are vulnerable to losing significant amounts of water due to vaporization and may not be suitable for hosting life.

Another factor that further narrowed down the search for life is the ozone layer. The team found that many planets in the habitable zone could not support life due to their thin ozone layers despite having habitable surface temperatures. These planets were ruled out as they were allowing too much UV radiation to pass through and penetrate to the ground, which is hazardous for surface life.

Horton and Chen believe this information will help astronomers in the hunt for life elsewhere. Instruments, such as the Hubble Space Telescope and James Webb Space Telescope, have the capability to detect water vapor and ozone on exoplanets. They just need to know where to look, they add.

The research will be published online on November 14 in the Astrophysical Journal. The researchers have reported their findings in a preprint version.

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