A star thought to resemble the Sun at the time life appeared on Earth has shed light on what planets need to become inhabited. In particular, a strong magnetic field looks to be an essential feature if evolution is to get under way.
The idea that a powerful magnetic shield against radiation is an essential feature for life has been around for a while. Unblocked radiation is a menace to DNA. Moreover, it is thought that Mars lost most of its atmosphere to solar radiation that would have been blocked by a field as strong as Earth's. Now the idea has extra rigor, following a study of the star Kappa Ceti.
Like the far more famous Tau Ceti, Kappa is a very Sun-like star in the constellation Cetus and near enough to be easy to study. However, where Tau Ceti is somewhat older than the Sun, Kappa Ceti is estimated by two different methods to be 400 to 600 million years old. This is conveniently about the time in Earth's development where life is thought to have first appeared.
A study in the Astrophysical Journal Letters reports that Kappa Ceti is covered with dark patches many times larger than sunspots, experiences powerful magnetic outbursts, and has a stellar wind 50 times as strong as the Sun's. Very massive stars have powerful stellar winds, but Kappa Ceti's mass is only 2 percent larger than that of the Sun. This activity is attributed to Kappa Ceti's impulsive youth, suggesting that the now middle-aged Sun would once have been similar.
A computer model of Kappa Cet's magnetic filed lines showing the way they twist out from the star's surface. CfA / Do Nascimento et. al. and TCD / A. Vidotto
"To be habitable, a planet needs warmth, water, and it needs to be sheltered from a young, violent Sun," said first author Dr. José-Dias do Nascimento of the Harvard-Smithsonian Center for Astrophysics. He mapped the complex magnetic fields on Kappa Ceti, which change as it goes through its nine-day rotation.
He found an average field strength of 24 Gauss and a maximum of 61 Gauss. By comparison, the Sun's background is 1 to 2 Gauss, although it's much higher for sunspots.
Modeling the effects of such an active star on a planet with a magnetic field of similar strength to Earth, do Nascimento found that the magnetosphere that protects the Earth would have been only a third to half the size it is today, barely sufficient to protect the planet and atmosphere. "The early Earth didn't have as much protection as it does now, but it had enough," he said in a statement. For any planet of Kappa Ceti to be habitable, it would need a magnetic field at least close to the strength of the Earth. A larger sample size will be needed to see if this is the norm for Sun-like stars.
If the Sun's behavior at the equivalent point in its life was anything like Kappa Ceti's, “the early magnetic interaction between the stellar wind and the young-Earth planetary magnetic field may well have prevented the volatile losses from the Earth exosphere and create conditions to support life,” the paper concluded.