Large, hot, gas-filled loops after a solar frare has erupted. (Photo by NASA via Getty Images)
Our 4.5-billion-year-old yellow-dwarf star used to be a rambunctious whippersnapper, much more active in its youth than middle age. But with age and maturity often comes stellar opportunity and a new paper hints just how.
In a paper published last month in The Astrophysical Journal Letters, the authors argue that life here on Earth and potentially on earthlike planets that circle other nearby sunlike stars may have benefited from a solar magnetic transition to relative quiescence.
We pin down the age at which this magnetic transition takes place, sometime between 2.6 and 3.7 billion years for a sunlike star, Travis Metcalfe, the paper’s lead author and an astronomer at the White Dwarf Research Corporation in Golden, Colo., told me.
This confirms that the Sun must have entered the transition to magnetic quiescence around the same time that life emerged from the oceans and onto land, he says. This suggests that the best places to look for complex life might be stars in the second half of their lifetimes, he notes.
In the paper, Metcalfe and colleagues detail spectroscopic observations of a few sunlike stars made from the Large Binocular Telescope (LBT) in Arizona. The goal was to measure and set limits on the presence of large-scale magnetic fields, from the polarization of the starlight to a precision of less than 0.01 percent, the authors write.
To help explain this stellar transition, the team points to a process known as ‘magnetic braking.’
Magnetic fields are created by the interaction of rotation with near-surface boiling motions (convection) in sunlike stars, a mechanism known as a stellar dynamo, says Metcalfe. But rotation rates of sunlike stars slow over time because of how their stellar winds interact with magnetic fields, he says.
Stars stream of material that flow away from their surfaces. But because this material is electrically charged, it moves along the magnetic field lines out to a distance where the magnetism becomes too weak to influence it. There, it effectively breaks free from the magnetic field and carries away some of the angular momentum of the star, gradually slowing it down over time. This so-called ‘magnetic braking’ is analogous to a spinning ice skater extending their arms to slow down.
When a star’s rotation becomes too slow compared to the convective motions, the stellar dynamo can only organize magnetic fields on smaller scales. These small-scale fields do not interact as strongly with the stellar winds, so very little angular momentum is lost, and magnetic braking is weakened.
Why is this important?
Before weakened magnetic braking was discovered in old stars observed by the Kepler Space Telescope, astronomers assumed that magnetic braking continued throughout the entire lives of sunlike stars, says Metcalfe. We now know that there is a plot twist in the middle of the story, and our own Sun is already making the transition from normal to weakened magnetic braking, he says.
When did the Sun enter this transition phase?
Our best estimates from observations of other sunlike stars is that the Sun probably entered this new phase of magnetic evolution several hundred million years ago, says Metcalfe. It may just be a coincidence, but this is roughly when life on Earth emerged from the oceans onto land, he says.
Is magnetic braking really linked to the onset of life?
Weakened magnetic braking is one consequence of a fundamental shift in how a star builds and maintains its magnetic field, says Metcalfe. That shift may also create a more stable “space weather” environment for any orbiting planets and whatever life exists on them, he notes. It could improve the chances for developing complex life, and facilitate the advancement of technological civilizations, he says.
As for how solar evolution influenced the emergence of life on our own planet?
The younger Sun regularly bombarded the Earth with charged particles and radiation, which is less of a problem for life that is well-shielded underwater, says Metcalfe. If the transition to weakened magnetic braking made the “space weather” environment of the Earth more hospitable, then complex life could begin to take hold on the surface, he says.
What’s the bottom line?
“If we want to find complex life and other technological civilizations, maybe we should be looking around stars that are in the second half of their lifetimes,” said Metcalfe.
