John P. Millis, Ph.D. for redOrbit.com – Your Universe Online
Most of the stars in the Universe are classified as red dwarf stars – stars considerably smaller and cooler than our Sun. About 75 percent of the main sequence stars in our galaxy are classified as such, so they represent an exciting population to study in the search for life beyond Earth.
If it were found that Earth-like planets formed around red dwarf stars, then the door would be opened to the possibility we might soon find life elsewhere in the Universe. But new work is casting some doubt on this possibility.
In order for life to exist on a planet, scientists believe certain conditions must be present. Obviously, the planet would have to have a rocky surface, something upon which the life could build up. Also, the planet would need to have sufficient gravity (i.e. be large enough) in order to maintain an atmosphere, but not so large that the gravitational field strength would overwhelm any forming life.
Finally, and crucially, the planet must receive enough energy from its host star that liquid water can exist on the surface. This means the planet must maintain a certain orbital distance from the star, a region known as the habitable zone. This exact size and location of the habitable zone varies depending on the size and brightness of the star.
For cool red dwarfs, the planets would need to be close in to the star, much closer than the Earth is to our Sun, in order to receive enough heat to maintain the needed temperatures. This is actually great news in the attempt to find planets the size of Earth.
Because the star itself is small, the tug from an Earth-like planet so near the star would mean the star would wobble in response to the planet’s gravity. This wobble would be visible to instruments on Earth, and scientists could then identify “new Earths.” A problem exists however.
A team from St. Andrews University has announced this week that another property of red dwarfs could potentially squelch the possibility for life around these stars. While virtually all stars possess magnetic fields, red dwarfs are known to maintain fields that are quite strong, particularly early on in their formation.
Such strong magnetic fields can have a dramatic effect on nearby planetary objects. For a world close enough to maintain liquid water, the planetary magnetic field could be compressed by that of the nearby red dwarf; so much so that the field could be practically extinguished all together.
This is important, because an atmosphere is another element critical for life, and without a magnetic field the atmosphere would have considerably more exposure to cosmic radiation, especially high-energy charged particles. These charged particles can strip away the atmosphere all together. The chance of this is also increased by the fact the planet is so close to the host red dwarf, as the greatest source of charged particle radiation can come from the stellar wind itself.
Hope still exists, however. The team also revealed that the effect is driven by the strength of the red dwarf’s magnetic field, which varies depending on the rotation period of the star, which itself can slow over time.
“Our work suggests that red dwarf stars with rotation periods larger than about one to a few months will have magnetic fields that won’t significantly squash the magnetosphere of an Earth-analogue planet orbiting inside the habitable zone of its host star”, says Aline Vidotto, from the University of St Andrews, and the lead scientist on the research. “Astronomers will have to take this on board in their search for life elsewhere. The conditions for habitability are turning out to be a lot more complex than we thought.”