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Atmosphere key to cosmic ray protection

Life protector Having a good atmosphere is just as important as being in the right place, when it comes to looking for planets that could harbour life, according to a new study.

The research, reported in the journal Astrobiology, found the thickness of a planet's atmosphere plays an important role in shielding life from dangerous cosmic radiation.

With hundreds of worlds now discovered orbiting stars other than the Sun, planet hunting astronomers are focusing their attention on those orbiting in a habitable zone.

The habitable zone is the region around a star, where the temperature would allow liquid water to exist on the surface of a planet or moon.

When people talk about habitability, they usually concentrate on surface temperature, according to the study's lead author Dr Dimitra Atri of the Blue Marble Space Institute of Science in Seattle, and the Tata Institute of Fundamental Research in Mumbai.

"But there are many other things which are important to life, such as the level of radiation exposure," says Atri.

"There could be some cases on extrasolar planets where the temperature is good enough to have a biosphere, but radiation levels from high energy cosmic ray particles, are too high for a thriving biosphere."

Cosmic ray particles interact with the atmosphere to produce subatomic particles, such as muons, neutrons, electrons and photons, which rain down to the surface.

Shields up

The geomagnetic field generated by the Earth was always thought to provide most of our planet's shielding from cosmic ray particles. However Atri and colleagues show this not the case.

"We thought the main difference between Earth and Mars is that we have good shielding from our magnetic field, while the Martian magnetic field is almost negligible, allowing particles to strike the surface making it dangerous for any kind of life," says Atri.

The authors used theoretical models to calculate the strength and penetration of cosmic ray particles under different magnetic and atmospheric conditions, to determine how much radiation reached the surface.

"On Earth if you measure the radiation dose on the surface and then remove the magnetic field, the radiation dose we get will be doubled," says Atri.

"But if we keep the same magnetic field and instead reduce the atmosphere by 50 per cent, the radiation dose increases by a factor of ten. So the main shielding is provided by the atmosphere, and the magnetic field is a secondary factor."

"This was the most surprising thing about the study, because we always felt the magnetic field deflects all these charged particles, and so it should be most important. But we found the biggest factor deciding the radiation dose was the thickness of the atmosphere."

Red dwarf danger

The findings also challenge the long-held idea that planets orbiting small stars called red dwarfs are a good place to look for life, because red dwarfs are the most common type of star in the galaxy.

Red dwarfs are much cooler than stars like the Sun, meaning their habitable zones are much closer to the star.

Being closer to its star makes it easier to detect an orbiting planet, however Atri says it also means significantly more radiation.

"Any planet in the habitable zone would be closer to the star and subjected to higher levels of radiation," says Atri.

He says that calculations show these planets have weaker magnetic fields and less radiation shielding.

Red dwarfs also have strong magnetic activity, which makes them relatively bright in X-ray and ultra violet radiation, causing them to flare frequently and generate stellar winds that could strip away a nearby planet's atmosphere.

"These are going to hit you directly without any protection, so it's twice as dangerous as living on a place like Earth," says Atri.