Sun, Earth Are Unlikely Pair to Support Life

by Anne Minard on August 10, 2009

The violent youth of solar proxies. Courtesy of IAU.

The violent youth of solar proxies. Courtesy of IAU.

We don’t know how lucky we are — really.

We know the interaction between Earth and Sun is a rarity in that it allowed life to form. But scientists working to understand the possibility that it could have happened elsewhere in the Universe are still far from drawing conclusions.

What is becoming clearer is that life probably shouldn’t have formed here; the Earth and Sun are unlikely hosts.
A series of presentations at this year’s meeting of the International Astronomical Union meeting, in Brazil last week, focused on the role of the Sun and Sun-like stars in the formation of life on planets like Earth.

Edward Guinan, a professor of astronomy and astrophysics at Villanova University in Pennsylvania, and his collaegues have been studying Sun-like stars as windows into the origin of life on Earth, and as indicators of how likely life is elsewhere in the cosmos. The work has revealed that the Sun rotated more than ten times faster in its youth (over four billion years ago) than today. The faster a star rotates, the harder the magnetic dynamo at its core works, generating a stronger magnetic field, so the young Sun emitted X-rays and ultraviolet radiation up to several hundred times stronger than it does today.

A team led by Jean-Mathias Grießmeier from ASTRON in the Netherlands looked at another type of magnetic fields — that around planets. They found that the presence of planetary magnetic fields plays a major role in determining the potential for life on other planets as they can protect against the effects of both stellar particle onslaughts.

“Planetary magnetic fields are important for two reasons: they protect the planet against the incoming charged particles, thus preventing the planetary atmosphere from being blown away, and also act as a shield against high energy cosmic rays,” Grießmeier said. “The lack of an intrinsic magnetic field may be the reason why today Mars does not have an atmosphere.”

All things considered, the Sun does not seem like the perfect star for a system where life might arise, added Guinan.

“Although it is hard to argue with the Sun’s ‘success’ as it so far is the only star known to host a planet with life, our studies indicate that the ideal stars to support planets suitable for life for tens of billions of years may be a smaller slower burning ‘orange dwarf’ with a longer lifetime than the Sun — about 20-40 billion years,” he said.

Such stars, also called K stars, “are stable stars with a habitable zone that remains in the same place for tens of billions of years,” he added. “They are 10 times more numerous than the Sun, and may provide the best potential habitat for life in the long run.”

Not are planets like Earth the best places to harbor life, he said. Planets double or triple the size of Earth would do a better job of hanging onto an atmosphere and maintaining a magnetic field: “Furthermore, a larger planet cools more slowly and maintains its magnetic protection.”

Manfred Cuntz, an associate professor of physics at the University of Texas at Arlington, and his collaborators have examined both the damaging and the favorable effects of ultraviolet radiation from stars on DNA molecules. This allows them to study the effect on other potential carbon-based extraterrestrial life forms in the habitable zones around other stars. Cuntz says: “The most significant damage associated with ultraviolet light occurs from UV-C, which is produced in enormous quantities in the photosphere of hotter F-type stars and further out, in the chromospheres, of cooler orange K-type and red M-type stars. Our Sun is an intermediate, yellow G-type star. The ultraviolet and cosmic ray environment around a star may very well have ‘chosen’ what type of life could arise around it.”

Rocco Mancinelli, an astrobiologist with the Search for Extraterrestrial Life (SETI) Institute in California, observes that as life arose on Earth at least 3.5 billion years ago, it must have withstood a barrage of intense solar ultraviolet radiation for a billion years before the oxygen released by these life forms formed the protective ozone layer. Mancinelli studies DNA to delve into some of the ultraviolet protection strategies that evolved in early life forms and still persist in a recognizable form today. As any life in other planetary systems must also contend with radiation from their host stars, these methods for repairing and protecting organisms from ultraviolet damage serve as models for life beyond Earth. Mancinelli says “We also see ultraviolet radiation as a kind of selection mechanism. All three domains of life that exist today have common ultraviolet protection strategies such as a DNA repair mechanism and sheltering in water or in rocks. Those that did not were likely wiped out early on.”

The scientists agree that we do yet know how ubiquitous or how fragile life is, but as Guinan concludes: “The Earth’s period of habitability is nearly over — on a cosmological timescale. In a half to one billion years the Sun will start to be too luminous and warm for water to exist in liquid form on Earth, leading to a runaway greenhouse effect in less than 2 billion years.”

Why is the Sun yellow?

Source: International Astronomical Union (IAU). A link to the meeting is here.

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