The artist's conception shows a hypothetical ...
November 20, 2013
When it comes to stars in the galaxy, what we can see in the sky at night is only the beginning.
Hidden in the darkness of the universe are billions of small, dim stars scientists call red dwarfs. Though none of them are visible to the naked eye, it’s thought that they compose some 75 percent of the Milky Way galaxy’s approximately 300 billion stars.
The prevalence of red dwarfs makes them key targets for planet hunters seeking small, Earth-size worlds orbiting nearby stars. And because it's easier to find exoplanets orbiting dim stars, it’s likely that many future discoveries will be found around red dwarfs.
The prospects for finding a habitable exoplanet orbiting a red dwarf are intriguing. Because they burn more slowly than larger stars like the sun, red dwarfs are exceptionally long-lived. They last so long, astronomers think, that their life expectancies, exceed the amount of time that the universe has existed. In other words, dying red dwarf stars don’t exist yet because the universe is still too young.
This long lifespan could be a good thing for habitable planets like Earth, giving life more time to develop and survive. While Earth is likely to be burnt to a crisp by the dying sun in 5.4 billion years, a similar planet orbiting a red dwarf star could possibly remain habitable indefinitely.
The dwarfs' dirty secret
However, red dwarf stars have a dirty secret, one that’s being explored in-depth by Aline Vidotto of the University of St. Andrews in Scotland, who’s found that life around a red dwarf star may be a serious challenge.
Stars don’t sit idly in space -- they spin around their axis, just like Earth. The rate at which they spin has to do with how the star was born and evolved. The sun completes one revolution about every month. Some red dwarf stars spin much faster, usually completing a full revolution in a matter of days – or even hours. Generally, the older a star is, the slower its spin.
Though red-dwarf stars are smaller, dimmer, and less energetic than bigger ones, their fast rates of spin cause them to have magnetic fields that can be up to one thousand times more powerful than the sun’s. Red dwarfs emit particles through continuous stellar winds and their fast spins make them more prone to huge eruptions of particles called coronal mass ejections.
M-dwarf stars might be better places for life to form as they age and their spins slow down.
Vidotto’s research has focused on what’s called the “pressure” of a magnetic field. “The effect of the sun’s magnetic pressure on the Earth’s is negligible,” Vidotto said. “This allows the Earth to have a large magnetic field that protects us from the solar wind and coronal mass ejections.”
But Vidotto’s findings suggest that a red dwarf’s magnetic field, which could be hundreds of times stronger than the sun’s, would have a much more pronounced effect on the planets that orbit it. “The strong stellar wind and stellar magnetic field would apply magnetic pressure that would compress the magnetic field of a planet,” she said.
The problem would be exacerbated by the fact that an Earth-like planet would need to orbit quite close to its star in order to be warm enough for liquid water to exist, making it even more vulnerable to the torrents of stellar wind and magnetic pressure coming from its no-so-docile star.
A smaller, tighter magnetic field could be bad news for an Earth-like exoplanet. Inadequate magnetic protection could allow a red dwarf’s stellar wind to rip away the planet’s atmosphere, irradiating the surface and allowing life-giving liquid water to evaporate away.
Hope for habitables
Even if it’s true that habitable planets around red dwarf stars are hard to come by, there’s still hope that, in certain cases, the conditions for life could still exist. “Stars, like people, slow down as they age. A red dwarf that’s older and isn’t spinning as quickly might be less of a danger to a habitable planet. And outgassing from the planet might help replenish planetary atmospheres that had been stripped away.” Research from others suggests that red dwarf stars in pairs slow down each other’s spins and could have habitable zones with even more liquid water than single stars.
Vidotto’s prognosis for habitable planets orbiting red dwarf stars isn’t good, but she cautions that there’s much more research to be done to determine how these mysterious stars work. “Habitable planets around red dwarf stars is a complex problem and we can’t jump to conclusions,” Vidotto said “Red dwarf stars have a very different structure from the sun and we don’t fully understand the dynamo that creates their magnetic fields. It’s an open question.”
Written by Joshua Rodriguez/PlanetQuest