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It was Stephen Kane looking for stars that could shelter planets with warm, temperate climates, hospitable to life – you know, like Earth – when he saw a young red dwarf called AU Microscopii, which is “only” 32 light years from home.
“The star is a complete baby when it comes to planetary systems. That means we have the opportunity to observe a planet here in the earliest stages of the planet’s development, “he said. So Kane, an astrophysicist at the University of California, Riverside, and his colleagues used the star as a laboratory and as a model for others like her, designing her future life. This helped them understand when the planets orbiting it could fall into the star’s “habitable zone” – a distance that is neither too hot nor too cold to sustain life. They found that the star would first blaze brightly, then calm down and burn less intensely, so that the range of life-friendly places would approach the star by about 30 to 40 percent in the first 200 million years of the star. They published their work this month in The Astronomical Journal.
This is important for Kane and other scientists who hope to one day see a life-friendly world beyond Earth, with green ecosystems teeming with extraterrestrial life, because it suggests that a habitable planet may not be habitable forever. For the best case scenario of “Golden Hair”, everything must be accurate, including the temperature that allows the planet to have liquid water on the surface – a prerequisite for life as we know it. (Life like us do not know that this is another story.) Other factors are also important, such as a breathable atmosphere, a stable climate and sufficient protection from harsh ultraviolet radiation. Mars, for example, is in the habitable zone of our sun, but it lost water and most of its atmosphere eons ago. Venus lies on the inner edge of the zone, but thanks to its veil of carbon dioxide, it is extremely hot.
AU Microscopii gives scientists an insight into how this area can grow or shrink throughout a star’s life. “These red dwarf stars have a very long, very bad teenage phase. It may be hundreds of millions of years before a star like this finally settles as an adult, “said Sarah Sieger, an astrophysicist at the Massachusetts Institute of Technology and former deputy director of science at NASA’s planetary mission TESS.
Kane and his team show that because their red dwarf and other similar stars may behave like teenagers for a while, a currently inhospitable world may become more susceptible to life down the road. But the opposite can happen: “A planet that is now habitable may not be there after the star changes,” he said.
If the host star cools down a lot, the planet could become too cool for any alien who makes a living on it; lakes and rivers will gradually freeze. On the other hand, much older stars usually end up heating up, so aliens who were once in a life-friendly location can eventually see how the water needed for life boils as everything on the surface on their planet it burns to death.
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