The findings suggest that only planets with atmospheres rich in greenhouse gases, such as carbon dioxide, and located at significant distances from their stars may sustain life as we know it. This conclusion was based on observations of Wolf 359, a red dwarf star approximately one-tenth the mass of the Sun and only 7.8 light-years away, making it one of the closest stars to our solar system.
"Wolf 359 can help us unlock the secrets around stars and habitability," said Scott Wolk of the Center for Astrophysics | Harvard and Smithsonian (CfA), who led the study. "It's so close and it belongs to such an important class of stars - it's a great combination."
Astronomers have long searched for exoplanets around red dwarfs. Optical telescopes have provided evidence for two potential planets around Wolf 359, though this remains a subject of debate among scientists.
"While we don't have proof of planets around Wolf 359 yet, it seems very possible that it hosts multiple planets," Wolk added. "This makes it an excellent test bed to look at what planets would experience around this kind of star."
The research team used Chandra and XMM-Newton to measure Wolf 359's emissions of X-rays and extreme ultraviolet (UV) radiation, the most harmful forms of radiation for planetary atmospheres. Their analysis revealed that the star produces sufficient radiation to strip away atmospheres unless planets are shielded by greenhouse gases and are far from the star.
"Just being far enough away from the star's harmful radiation wouldn't be enough to make it habitable," said co-author Vinay Kashyap of CfA. "A planet around Wolf 359 would also need to be blanketed in greenhouse gases like Earth is."
The team examined Wolf 359's habitable zone, the region where liquid water could exist on a planet's surface. For this star, the outer limit of the habitable zone is only 15% of the Earth-Sun distance due to its lower brightness. Neither of the two potential planets lies within this zone - one is too close to the star, while the other is too far away.
"If the inner planet is there, the X-ray and extreme UV radiation it is subjected to would destroy the atmosphere of this planet in only about a million years," said co-author Ignazio Pillitteri of CfA and the National Institute for Astrophysics in Palermo, Italy.
The researchers also modeled the effects of radiation on hypothetical planets within the habitable zone. They concluded that an Earth-like planet in the center of the habitable zone could sustain an atmosphere for nearly two billion years, while one at the zone's outer edge might retain its atmosphere indefinitely due to the insulating effects of greenhouse gases.
A further threat comes from X-ray flares - intense bursts of radiation - which were observed 18 times over 3.5 days. These flares, combined with steady radiation, create an environment where atmospheres would struggle to survive long enough for multicellular life to evolve, except potentially at the outermost edge of the habitable zone with a strong greenhouse effect.
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Chandra X-ray Observatory
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