Old stars could be the best places to look for life

Once upon a time in cosmic times, scientists assumed that stars applied an eternal magnetic brake, causing their rotation to slow endlessly. Thanks to new observations and sophisticated methods, they have now discovered the magnetic secrets of a star and discovered that they did not match their expectations. Cosmic hotspots for finding extraterrestrial neighbors could be around midlife crisis stars and beyond.

This unique study, highlighting magnetic phenomena and habitable environments, was published in Letters from the astrophysical journal.

In 1995, Swiss astronomers Michael Mayor and Didier Queloz announced the first discovery of a planet outside our solar system, orbiting a distant sun-like star known as 51 Pegasi. Since then, more than 5,500 so-called exoplanets have been discovered orbiting other stars in our galaxy, and in 2019 the two scientists shared the Nobel Prize in Physics for their pioneering work. This week, an international team of astronomers published new observations of 51 Pegasus, suggesting that the current magnetic environment around the star may be particularly favorable for the development of complex life.

Stars like the Sun are born by rotating rapidly, which creates a powerful magnetic field that can erupt violently, bombarding their planetary systems with charged particles and harmful radiation. Over billions of years, the star’s rotation gradually slows as its magnetic field is driven by wind coming from its surface, a process known as magnetic braking. The slower rotation produces a weaker magnetic field, and the two properties continue to decay together, each feeding off the other.

Until recently, astronomers believed that magnetic braking continued indefinitely, but new observations have begun to question this assumption.

“We’re rewriting the textbooks on how the rotation and magnetism of older stars like the Sun change beyond the middle of their lives,” says team leader Travis Metcalfe, a senior research scientist at the White Dwarf Research Corporation in Golden, Colorado, United States. “Our results have important consequences for stars with planetary systems and their prospects for the development of advanced civilizations.”

Klaus Strassmeier, director of the Leibniz Institute for Astrophysics in Potsdam, Germany and co-author of the study, adds: “This is because weakened magnetic braking also throttles the stellar wind and makes devastating eruptive events.”

The team of astronomers from the United States and Europe combined observations of 51 Pegasi from NASA’s Transiting Exoplanet Survey Satellite (TESS) with cutting-edge measurements of its magnetic field from the Large Binocular Telescope (LBT) in Arizona using the Potsdam polarimetric and spectroscopic scale. Instrument (PEPSI).

Although the exoplanet orbiting 51 Pegasi does not pass in front of its parent star as seen from Earth, the star itself exhibits subtle brightness variations in TESS observations that can be used to measure radius, mass and the age of the star – a technique known as asteroseismology.

During this time, the star’s magnetic field imprints a tiny amount of polarization on the star’s light, allowing PEPSI on the LBT to create a magnetic map of the stellar surface as the star rotates – a technique known as Zeeman-Doppler imaging. Together, these measurements allowed the team to assess the current magnetic environment around the star.

Previous observations from NASA’s Kepler space telescope already suggested that magnetic braking could weaken significantly beyond the age of the sun, breaking the close relationship between rotation and magnetism in older stars. However, the evidence for this change was indirect and relied on measurements of the rotation rates of stars of widely varying ages. It was clear that the rotation had stopped slowing at some point near the age of the Sun (4.5 billion years) and that weakened magnetic braking in older stars could reproduce this behavior.

However, only direct measurements of a star’s magnetic field can establish the underlying causes, and the targets observed by Kepler were too faint for LBT observations. The TESS mission began collecting measurements in 2018, similar to Kepler’s observations, but for the closest and brightest stars in the sky, including 51 Pegasi.

Over the past few years, the team began using PEPSI on the LBT to measure the magnetic fields of several TESS targets, gradually building a new understanding of how magnetism changes in stars like the sun as they are getting older. Observations revealed that magnetic braking changes suddenly in stars slightly younger than the Sun, becoming more than 10 times weaker and decreasing further as the stars continue to age.

The team attributed these changes to an unexpected change in the strength and complexity of the magnetic field, as well as the influence of this change on the stellar wind. The newly measured properties of 51 Pegasi show that, just like our own sun, it has already gone through this transition to weakened magnetic braking.

“It is very gratifying that LBT and PEPSI were able to reveal a new perspective on this planetary system that has played such a central role in exoplanet astronomy,” says Strassmeier, principal investigator of the PEPSI spectrograph. “This research represents an important step in the search for life in our galaxy.”

In our own solar system, the transition of life from oceans to land occurred several hundred million years ago, coinciding with the time when magnetic braking began to weaken in the sun. Young stars bombard their planets with radiation and charged particles hostile to the development of complex life, but older stars seem to provide a more stable environment. According to Metcalfe, the team’s results suggest that the best places to look for life outside our solar system might be around middle-aged and older stars.