In the new movie "Project Hail Mary," a dimming sun proves an existential threat to life on Earth - and Erid and many other life-containing planets in the story. A new study, however, has revealed that the reality of dimming stars is likely a benefit for the prospects for life on orbiting planets around these stars, not a threat.
A paper describing the study was published April 13 in tn Astrophysical Journal.
The research team, which was led by Penn State scientists, used NASA's Chandra X-ray Observatory to find that stars still in their astronomical infancy are calming down and dimming in their X-ray output more quickly than previously thought. Astronomers use Chandra and other telescopes to monitor how powerful radiation from young stars - often in the form of high-powered X-rays - can pummel planets surrounding them. They did not know, however, how long this high-energy barrage continued. The new study looked at eight clusters of stars that are considered young in astronomical terms, between the ages of 45 million and 750 million years old. In these clusters, stars similar in size to the sun unleashed only about a quarter to a third of the X-rays they expected.
"While science fiction - like the microbes in 'Project Hail Mary' - imagines alien life that dims stellar output by consuming its energy, our real observations reveal a natural quieting of young sun-like stars in X-rays," said Konstantin Getman, research professor of astronomy and astrophysics in the Penn State Eberly College of Science and lead author of the new study. "This is not because an outside force is consuming their light, but because their internal generation of magnetic fields becomes less efficient."
In fact, the researchers said, this calming could be a boon to the formation of life on planets around stars that are younger versions of the sun, which is about 4.6 billion years old and considered roughly "middle-aged" in astronomical terms, significantly older than the baby stars in this study. This is because large amounts of X-rays can erode a planet's atmosphere and prevent formation of the molecules necessary for organic life, based on elements like carbon, oxygen and hydrogen. On average, 3-million-year-old stars with a mass equal to the sun produce about a thousand times more X-rays than today's sun. Meanwhile, 100-million-year-old sun-sized stars are about 40 times brighter in X-rays than the present sun.
"It's possible that we owe our existence to our sun doing the same thing, several billion years ago, that we see these young stars doing now," said co-author Vladimir Airapetian of NASA Goddard Space Flight Center. "This real-world dimming echoes the dramatic stellar change in fiction, but it may be even more fascinating because it highlights our own sun's actual history."
The researchers found that stars with about the same mass as the sun quieted down relatively rapidly - after a few hundred million years - while ones with less mass kept up their high levels of X-ray emission for longer. The decrease in the energy of the X-rays combined with a disappearance of energetic particles - high-speed charged particles, such as electrons and protons, that are ejected from a star along with X-rays during periods of strong magnetic activity - the sun-sized stars appear to be better suited to host planets with robust atmospheres and the ingredients for life than previously thought.
"We can only see our sun at this current snapshot in time, so to really understand its past we must look to other stars with about the same mass," said co-author Eric Feigelson, distinguished senior scholar and professor of astronomy and astrophysics and of statistics at Penn State. "By studying X-rays from stars that are hundreds of millions of years old, we have filled in a large gap in our understanding of their evolution."
The research team also used data from the European Space Agency's Gaia satellite and X-ray data from the ROSAT mission, a joint German, U.S. and British X-ray satellite, which collected data between 1990 and 1999. This data allowed them to identify the stars that were members of the clusters - not foreground or background stars. To measure the X-ray output from the stars, they made new Chandra observations of five clusters with ages between 45 million and 100 million years and used Chandra and ROSAT data from archives to study three slightly older clusters, in their "teenage" years with ages between 220 and 750 million years. For context, many stars in the Milky Way galaxy are elderly at over 10 billion years old.
Historically, it has been challenging to study the X-ray output of stars in this teenage range, the researchers said. Most astronomers relied on sparse data to predict the X-ray emission that young stars should produce based on their ages and rates of spin. Older and more slowly rotating stars are usually fainter in X-rays, but the team found that X-ray output drops off about 15 times more rapidly than predicted during this specific adolescent phase.
"While we are still investigating the cause of this slower-than-expected activity, we think the process that generates magnetic fields in these stars may become less efficient," Getman said. "This would lead to the stars to become quieter in X-rays more quickly, as they age, but we will continue to look at this and other potential causes for the rapid dimming of young sun-like stars."
NASA's Marshall Space Flight Center in Huntsville, Alabama, manages the Chandra program. The Smithsonian Astrophysical Observatory's Chandra X-ray Center controls science operations from Cambridge, Massachusetts, and flight operations from Burlington, Massachusetts.
