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What to Look for in Red Dwarf Real Estate:
by WKT
November 20, 2007 ::
Should you buy property on a planet around a red dwarf star?
In years past, the answer to the above question would have been "Probably not, if you want to live there."
The thinking was, because of the feeble power output of red dwarf stars - a few percent or less than that of the Sun - that conditions on any planet around such a star would be unearthly and inhospitable to life as we know it.
Red dwarfs compared to the sun:
* Red dwarfs have masses ranging from 8 percent to about 50 percent that of the sun.
* Lower mass means that a lower central pressure and temperature is required to support the star against gravity.
* Lower central pressure and temperature leads to much smaller nuclear fusion rates in the interior of the star.
* Lower nuclear fusion rates yield lower total energy output ranging from 0.2 percent to 6 percent of the luminosity of the sun.
* Lower energy output means a red dwarf can maintain a steady power output for tens of billions of years (high mass red dwarf) to trillions of years (low mass red dwarf) much longer than the lifetime for the sun.
A planet too far away from its red dwarf star would be too cold. In contrast, one close enough to keep warm - about a tenth of the distance of the Earth from the Sun, or about three times closer than Mercury is to the Sun - would get locked into an orbit with one side of the planet always facing the star, and the other forever in the dark.
According to astrobiologists, the "Goldilocks" or habitable zone - the zone where water can remain liquid on a planet around a red dwarf - would be nonexistent or at best unearthly.
Recently, though, astronomers and astrobiologists have been reappraising the habitability of planets around red dwarf stars, which have a mass somewhere between about a tenth and half that of the Sun.
New sophisticated climate modeling suggests that heat transfer from the hot, bright side to the cold, dark side of a red dwarf planet could prevent freeze-out. Meanwhile, back on Earth, the discovery of "extremophiles" that can flourish under extraordinarily harsh conditions, has led some scientists to speculate that red dwarfs may be the most likely place to find extraterrestrial life.
This is good news for those who are searching for extraterrestrial intelligence, or perhaps for future refugees from Earth looking for a new place to settle. Red dwarfs are plentiful. They make up about 75% of all stars in the Galaxy, and there are about 100 red dwarf systems within 25 light years of Earth. Another attractive feature is the lack of an impending energy crisis. The power output of a red dwarf is predicted to remain steady for hundreds of billions or even trillions of years.
So, if life can establish itself around a red dwarf, it will have a long time to evolve intelligence capable of creating telescopes, cell phones, talk radio, and real estate bubbles. Maybe, just maybe, "red dwarfians" have evolved into a wise species that have avoided or solved some of the serious problems we encounter on Earth.
One potential difficulty for life around a red dwarf is that the atmospheres of red dwarf stars are highly turbulent and prone to outbursts of high-energy radiation and particles. A recent Chandra study of red dwarfs in the Orion cluster shows that red dwarfs flare almost continuously. The larger flares can be as energetic as solar flares.
Although we are protected by the Earth's atmosphere and magnetic field, large solar flares can create annoying disruptions of radio communications. They also pose a lethal danger to astronauts floating unprotected in space, and an unhealthy increase in hazardous cosmic ray background radiation at high altitudes.
Since any potentially habitable planet around a red dwarf must be very close to the star - about ten times closer than the Earth is to the Sun - red dwarf flares could cause serious damage. Computations show that such flares can bathe the planetary surface in a deadly rain of biologically damaging high-energy radiation. In extreme cases flares could strip the entire atmosphere from the exoplanet.
Magnetic fields could mitigate the problem, but probably not enough for an Earth-sized planet. However, a larger and more massive terrestrial-type exoplanet could have a stronger magnetic field, which combined with its stronger gravity, would allow it to hold onto its atmosphere.
So, before you buy property on a planet around a red dwarf, even though the price is right and the view is great, be aware that it's not just location, location, location. Size also matters.
Red Dwarf Flares:
The low mass of a red dwarf star leads to a weak central energy source. This causes the interior to undergo a rapid convective motion, similar to boiling water. This motion, when combined with rapid rotation, can produce large, severely twisted magnetic fields. The unwinding of these magnetic fields could heat the upper atmosphere of as star by releasing the pent-up energy either gradually, or explosively in flares.
To examine the frequency and intensity of red dwarf flares, a group composed of astronomers from Palermo University in Italy, Harvard-Smithsonian Center for Astrophysics, and Penn State University used data from a 13-day Chandra survey of the Orion Nebula star cluster. Their study of 165 red dwarf stars showed that the X-ray emission is consistent with an overlapping series of weak flares, punctuated by an occasional intense one.
This result is an important step forward in the quest to understand the mechanism for converting magnetic energy to heat and high energy particles in the atmospheres of stars. It also shows that living around a red dwarf star could be a hazardous proposition.
References:
J. Tarter et al. 2007, Astrobiology 7, 30- 65, "A Reappraisal of The Habitability of Planets around M Dwarf Stars"
M. Caramazza et al. 2007 Astron. & Astrophys. 471, 645, "X-ray flares in Orion: low-mass stars"
H. Lammer, et al. 2007 Astrobiology, Volume 7, 185-207.
"Coronal Mass Ejection (CME) Activity of Low Mass M Stars as An Important Factor for The Habitability of Terrestrial Exoplanets. II. CME-Induced Ion Pick Up of Earth-like Exoplanets in Close-In Habitable Zones"
