Regarding the search for alien life, experts have traditionally focused on younger stellar bodies. However, recent studies suggest that old stars may be the best places to search for signs of life beyond Earth.
In 1995, Swiss astronomers Didier Queloz and Michael Mayor announced the first discovery of an exoplanet that orbits a distant star called 51 Pegasi. Since then, over 5,500 planets have been found outside our solar system. New observations of 51 Pegasi were also recently made, which led experts to conclude that the magnetic condition of a star may affect the formation of complex life.
Magnetic Braking
When stars are born, they spin rapidly and create a strong magnetic field that can violently erupt, bombarding their planetary systems with harmful radiation and charged particles. Over billions of years, the rotation of aging stars gradually slows as its magnetic field drags through a wind that flows from its surface. This process is known as magnetic braking.
The slower rotation of stars produces weaker magnetic fields. These two properties continue to decline together, each feeding off the other. Astronomers have also assumed that magnetic braking is a process that continues indefinitely.
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New observations have started to challenge previous assumptions about magnetic braking. According to senior research scientist Travis Metcalfe from White Dwarf Research Corporation in Golden, Colorado, their team made a study that gives new insight into how rotation and magnetism in older stars change beyond the middle of their lifetimes.
The result of their study is discussed in the paper "Weakened Magnetic Braking in the Exoplanet Host Star 51 Peg." Metcalfe believes that their research has important consequences for stars with planetary systems and their prospects for developing advanced civilizations. This is because weakened magnetic braking throttles the stellar wind, making eruptive events less disruptive.
Previous observations from NASA's Kepler space telescope suggested that the magnetic braking might substantially weaken beyond the sun's age, affecting the close relationship between rotation and magnetism in older stars. However, the evidence for such change is indirect and relies only on the rotation rate measurements for stars with a wide range of ages.
These underlying causes can only be established by directly measuring a star's magnetic field. In 2018, the TESS mission started collecting measurements of the nearest and brightest stars in the sky, including 51 Pegasi. Over recent years, astronomers have been using PEPS on the LBT to assess the magnetic fields of different TESS targets. This gradually brings a new insight into the changes in the magnetism of aging stars.
The findings revealed that magnetic braking suddenly changes in stars slightly younger than the sun. These stellar bodies become ten times weaker at that point and seem to diminish further as the stars cage.
The experts attributed these changes to an unexpected shift in the strength and complexity of the magnetic field and the influence of that shift on the stellar wind. The measured characteristics of 51 Pegasi indicate that this host star has transitioned to weakened magnetic braking, just like our own sun.
In our solar system, the transition of life from the oceans onto land coincided with the time when magnetic braking began to weaken in the sun. This indicates that young stars bombard their planets with radiation and charged particles, which are hostile to the development of complex life. Meanwhile, older stars appear to offer a more stable environment. This led Metcalfe to conclude that the best places to look for outside of our solar system might be around middle-aged or older stars.
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