http://www.sci-news.com/astronomy/active-barnards-star-09011.html
This illustration shows radiation from flares from a red dwarf star like Barnard’s star eroding the atmosphere of an orbiting, rocky planet.
Image credit: NASA / CXC / M. Weis.
Barnard’s star, otherwise known as Gliese 699 or GJ 699, is an M3.5 dwarf star located in the constellation Ophiuchus.
The star is the next closest star to our Sun after the Alpha Centauri triple stellar system.
It is much smaller than the Sun and hosts at least one exoplanet — a super-Earth with a mass of 3.2 times that of Earth.
Dubbed Barnard’s star b, the planet orbits around the parent star once every 233 days and lies at a distant region known as the ‘snow line.’
The planet’s surface temperature is estimated to be around minus 170 degrees Celsius (minus 274 degrees Fahrenheit) meaning it is likely to be a frozen world which is uninviting to Earth-like life.
“Red dwarfs are the most numerous types of stars, and their small sizes make them favorable for studying orbiting planets,” said lead author Dr. Kevin France, an astronomer at the University of Colorado in Boulder.
“We are interested in understanding what the prospects are for habitable planets around red dwarfs.”
“Barnard’s star is a great case study for learning about what happens around older red dwarfs in particular.”
An artist’s impression of Barnard’s star b.
Image credit: Sci-News.com.
For the study, Dr. France and colleagues analyzed data gathered by NASA’s Chandra X-ray Observatory and the NASA/ESA Hubble Space Telescope.
The Hubble observations, taken in March 2019, revealed two ultraviolet high-energy flares, and Chandra observations in June 2019 uncovered an X-ray one. Both observations were about seven hours long.
“About 25% of the time, Barnard’s star unleashes scorching flares, which may damage the atmospheres of planets closely orbiting it,” the astronomers said.
“While Barnard’s star b does not have habitable temperatures, the study adds to evidence that red dwarfs may present serious challenges for life on their planets.”
“If these snapshots are representative of how active Barnard’s star is, then it is pumping out a lot of harmful radiation,” said co-author Dr. Girish Duvvuri, also of the University of Colorado.
“This amount of activity is surprising for an old red dwarf.”
The researchers then studied what these results mean for potential planets orbiting in the habitable zone of a red dwarf like Barnard’s star.
Any atmosphere formed early in the history of a habitable-zone planet was likely to have been eroded away by high-energy radiation from the star during its volatile youth.
Later on, however, planet atmospheres might regenerate as the star becomes less active with age.
This regeneration process may occur by gases released by impacts of solid material or gases being released by volcanic processes.
However, the onslaught of powerful flares like those reported here, repeatedly occurring over hundreds of millions of years, may erode any regenerated atmospheres on rocky planets in the habitable zone.
This would reduce the chance of these worlds supporting life.
“It may turn out that most red dwarfs are hostile to life,” said co-author Dr. Tommi Koskinen, an astronomer at the University of Arizona in Tucson.
“In that case the conclusion might be that planets around more massive stars, like our own Sun, might be the optimal location to search for inhabited worlds with the next generation of telescopes.”
Because of these surprising flare findings, the team considered other possibilities for life on planets orbiting old red dwarfs like Barnard’s star.
Although planets in the traditional habitable zone may not be able to hold onto their atmospheres because of flares, astronomers can extend their searches for planets out to greater distances from the host star, where the doses of high-energy radiation are smaller.
At these greater distances, it is possible that a greenhouse effect from gases other than carbon dioxide, such as hydrogen, allows liquid water to exist.
“It’s hard to say what the likelihood is of any one planet in any one system being habitable either today or in the future,” said co-author Dr. Allison Youngblood, an astronomer at the University of Colorado.
“Our research shows one important factor that needs to be considered in the complicated question surrounding whether or not a planet can support life.”
The team’s results will be published in the Astronomical Journal.
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Kevin France et al. 2020. The High-Energy Radiation Environment Around a 10 Gyr M Dwarf: Habitable at Last? AJ, in press; arXiv: 2009.01259
This article is based on a press-release provided by the National Aeronautics and Space Administration
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