Distant planets may be swaddled in a protective bubble of magnetism and charged particles, courtesy of their parent star. The first study to scrutinise these so-called astrospheres shows that some exoplanets are more well shielded than Earth, others not so lucky – and that their protection can be fickle.
Within our solar system, the sun’s wind of charged particles and radiation forms a bubble called the heliosphere, which repels cosmic rays that can affect Earth’s weather, eat away at the ozone layer and damage DNA. Likewise, astrospheres guard faraway worlds from the ravages of the cosmos, says Marshall C. Johnson, an astronomer at the University of Texas in Austin.
An astrosphere’s size is determined partly by the strength of the star’s winds, and partly by the surrounding interstellar medium of gas and dust. The star’s velocity through the galaxy also has an effect.
“It’s just like if you are driving and you stick your hand out the window. You will experience higher pressure on your hand when you are driving fast than when you are driving slowly,” Johnson says. “If there is a greater relative velocity between the star and the interstellar medium, there will be a greater pressure exerted on the astrosphere.”
This can weaken the astrosphere or push it back toward the star, so much so that a planet’s orbit no longer lies within the protective bubble. That could have a big impact on its ability to support life. “Cosmic rays can have a number of effects on the climate and habitability of planets,” Johnson says.
Johnson and his colleagues used data from the exoplanet-hunting Kepler spacecraft to analyse the composition of stars known to have planets. Knowing the density of certain elements present in those stars allowed them to determine the density of the interstellar medium surrounding them. In turn, that gave them clues to the potential size of astrospheres.
They found that one possibly habitable planet, Kepler-186f, might orbit outside its star’s astrosphere, which is smaller than the one puffed out by our sun. But some stars with multiple planets, including Kepler-437 and KOI-4427, might have astrospheres much larger than the sun’s.
Oscillating in tandem
The measurements have some degree of uncertainty, partly because Johnson’s mathematical models make some assumptions, and partly because sunlike stars can exhibit activity cycles. The sun’s lasts 22 years, and the heliosphere expands and contracts in tandem. It’s likely that astrospheres also oscillate, Johnson says.
A star’s size and age also affects the size of its astrosphere, says Sarbani Basu at Yale University. Smaller stars have relatively large star-spots and flares, which generally means stronger winds, she says.
Even though astrospheres offer some protection to planets, having a magnetic field or an atmosphere is much more significant. “If the astrosphere is magnetised the way the heliosphere is, the fraction of galactic cosmic rays could be lower,” Basu says. “But the protection against cosmic rays is more effective when provided by the planet’s own magnetic field.”
By reducing the amount of interstellar gas and dust reaching alien worlds, astrospheres could still affect cloud cover, surface temperatures, the intensity of ultraviolet radiation and more. Johnson notes that when the Voyager 2 spacecraft left the heliosphere in 2013, it picked up an increase in cosmic rays of nearly 10 per cent. If the heliosphere ever shrank enough to uncloak the Earth, the increase in cosmic rays could chemically alter the atmosphere enough to bring on an ice age.
“But [an astrosphere is] certainly not going to be the most important effect for determining whether a planet can sustain life, or even whether life develops,” Johnson says. That requires many more ingredients we still don’t understand.
Journal reference: The Astrophysical Journal, DOI: 10.1088/0004-637X/807/2/162
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