Monday, June 23, 2014

Red Dwarfs May Be Unfit Parent Stars

Red dwarf stars are the smallest true stars dancing around in our Galaxy, as well as the most abundant. Because of their small size--by star standards, that is--they can "live" for trillions of years on the hydrogen-burning main-sequence, and the Universe itself is "only" about 13.8 billion years old. For these reasons, many astronomers have suggested that most of the exoplanets in our Milky Way Galaxy circle "tiny" red dwarf stars--making these planetary systems prime targets in the hunt for life on other, distant worlds. However, a team of astronomers announced in June 2014 that life in the Universe may be rarer than previously believed, because their study found that harsh space weather might tear the atmosphere off any rocky world circling within a red dwarf's life-friendly habitable zone. The team of astronomers announced their discovery during a press conference at the 2014 summer meeting of the American Astronomical Society (AAS) held in Boston, Massachusetts.

"A red dwarf planet faces an extreme space environment, in addition to other stresses like tidal locking," commented Dr. Ofer Cohen to the press on June 2, 2014. Dr. Cohen is of the Harvard-Smithsonian Center for Astrophysics (CfA) in Cambridge, Massachusetts.

Our own planet is protected from violent solar eruptions and space weather by its magnetic field, which essentially works like the shields of the Starship Enterprise of Star Trek. Earth's magnetic field serves to deflect approaching--and potentially destructive--blasts of energy. Our planet is also protected by its distance from the Sun, since it circles it at a comfortable 93 million miles!

Because a red dwarf's habitable zone is much further in towards its seething star than the Earth's distance from the Sun, any planet circling it would be subjected to more powerful and destructive space weather originating from its fiery stellar parent. The habitable zone of a star is that comfortable "Goldilocks" distance where the temperature is not too hot, not too cold, but just right for water to exist on its surface in a life-sustaining liquid state. Where liquid water exists, the potential for life as we know it, also exists.

Red Dwarfs

Relatively small red dwarf stars compose the vast majority of stellar inhabitants of our large, majestic, barred-spiral Galaxy, the Milky Way--which sparkles with the fires of at least 100 billion stars. There are approximately 100 red dwarf systems dwelling within 25 light-years of Earth. These tiny stars are very faint, and because they emit such a comparatively puny amount of radiation, they can lurk in interstellar space quite secretively, well-hidden in our Galaxy, where they cannot be easily detected by the prying eyes of curious astronomers.

Red Dwarf Star
 Red dwarfs are, therefore, the coolest, tiniest, and most common type of star. Estimates of their abundance range from 70% of all the stellar denizens of a spiral galaxy to more than 90% of all stars dwelling in elliptical galaxies. Usually, the median figure quoted is that red dwarfs account for 73% of all the stars dancing around in our Milky Way. Because of their relatively feeble energy output, these faint stars are never visible with the unaided human eye from Earth. The closest red dwarf to our Star, the Sun is Proxima Centauri, and it is a glittering member of a triple system of sister stars. Proxima Centauri (which is also the Sun's nearest stellar neighbor), is much too dim to be viewed from Earth with the naked eye--as is the closest solitary red dwarf named Barnard's star.

In recent years, astrobiologists and astronomers have been considering the possibility of life dwelling on alien worlds circling these tiny and very dim stars. A red dwarf sports the relatively small mass of only about one-tenth to one-half that of our Sun, and determining how their various characteristics affect the potential habitability of the planets that circle them may reveal to scientists the frequency of extraterrestrial life and intelligence.

Because red dwarf planets orbit so close to their parent stars, they are subjected to powerful tidal heating--which is certainly a major impediment to the evolution of delicate living things within these systems. Other tidal effects also make the development of life in such planetary systems difficult. For example, there are extreme temperature variations that result from the fact that one side of habitable zone red dwarf planets permanently face the star--while the other side is perpetually turned away. There are also non-tidal impediments to the evolution of tender living creatures on red dwarf worlds, such as small circumstellar habitable zones resulting from small light output. Other non-tidal impediments include extreme stellar variation, as well as spectral energy distributions that are shifted to the infrared part of the electromagnetic spectrum relative to our Sun.

However, many scientists have considered that several factors actually increase the chances for life to evolve on red dwarf worlds. For example, vigorous cloud formation on the star-facing side of a tidally locked alien world may lessen overall thermal flux, thus reducing equilibrium temperature variations between the two sides of the exoplanet. Furthermore, the sheer abundance of these faint little stars increases the number of potentially habitable alien worlds that may be circling them. As of 2013, scientists calculated that approximately 60 billion red dwarf worlds inhabit our Galaxy.

On our own planet Earth, the discovery of a vast and diverse array of bizarre creatures, collectively termed extremophiles, has encouraged some exobiologists to speculate that these cool and very abundant little stars may be the most likely alien worlds to finally discover extraterrestrial life. Extremophiles are organisms that can thrive under conditions that human beings find hostile--such as extremely hot environments, extremely cold environments, extremely acidic environments, and extremely dry environments.

Red Dwarfs Unfit To Be Parent Stars?

Earlier studies have focused on the impact of stellar flares that are violently hurled out by red dwarfs in the direction of a close-in, unfortunate exoplanet. However, the new study that was announced in June 2014 at the AAS summer meeting, instead examines the effect of persistent gusts of fierce stellar wind. The team of astronomers used a supercomputer model created at the University of Michigan to represent a trio of known red dwarf worlds orbiting a simulated, middle-aged star.

The team found than even a magnetic field, like that of our own planet, would not necessarily be able to protect a habitable zone alien world from its seething red dwarf star's persistent bombardment. Although there were intervals when the unfortunate planet's magnetic shield functioned effectively, it spent far too much time with weak shields than strong shields.

"The space environment of close-in exoplanets is much more extreme than what the Earth faces," study co-author Dr. Jeremy Drake told the press on June 2, 2014. Dr. Drake, of CfA, is a study co-author. "The ultimate consequence is that any planet potentially would have its atmosphere stripped over time," he continued to explain.

The ferocious and extreme space weather could also create breathtaking Northern Lights, or aurorae. The aurora lighting a red dwarf world's sky could be a whopping 100,000 times more powerful than those seen on Earth--and they could extend from the poles halfway to the equator.

"If Earth were orbiting a red dwarf, then people in Boston would get to see the Northern Lights every night. On the other hand, we'd also be in constant darkness because of tidal locking, and blasted by hurricane-force winds because of the dayside-nightside temperature contrast. I don't think even hardy New Englanders want to face that kind of weather," Dr. Cohen commented to the press on June 2, 2014.

More Information for you:

Harvard-Smithsonian Center for Astrophysics
Harsh Space Weather May Doom Potential Life on Red-Dwarf Planets

Harsh Space Winds Diminish Hopes For Life On Red-Dwarf Planets
Potential Life on Red Dwarf Planets In Peril Due To Extreme Space Weather

About the Author

Judith E. Braffman-Miller is a writer and astronomer whose articles have been published since 1981 in various newspapers, magazines, and journals. Although she has written on a variety of topics, she particularly loves writing about astronomy because it gives her the opportunity to communicate to others the many wonders of her field. Her first book, "Wisps, Ashes, and Smoke," will be published soon.

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