30Jul2016

Orange Dwarfs Most Likely To Harbour Intelligent Life

This artist's concept illustrates a young, red dwarf star surrounded by three planets.

This artist’s concept illustrates a young, red dwarf star surrounded by three planets.

Orange  dwarf stars stand the best chance of harbouring intelligent life like our own, say the co-authors of a new paper appearing in The Astrophysical Journal.here Bruce Dorminey examines the probabilities.

The sweet spot for planets suitable for life are cooler, longer-lived stars of the spectral-type G and K; slightly less luminous and slightly cooler than our own Sun, Edward Guinan, the paper’s second author, and an astronomer at Villanova University in Pennsylvania, told me. Stars hotter than the Sun, Guinan says, are not ideal since they have short lifetimes of only three billion years or less.

Our own Sun is a yellow dwarf — by stellar spectral classification, a G-2 star. Thus, the fact that we’re here posing such questions on a planet circling a star that is not even astrobiologically-optimal gives pause to wonder: Is intelligent life in our galaxy even more abundant than anyone could have imagined?

 “We don’t yet have an answer,” Manfred Cuntz, the paper’s lead author and an astronomer at the University of Texas at Arlington, told me. “And due to significant roadblocks to biological evolution, intelligent life could be relatively rare in our galaxy.”

In contrast, some astrobiologists have long argued that extremely long-lived and ubiquitous red dwarf M-type stars would make the best candidates for complex life. They make up some 75 percent of all stars in our galaxy and are expected to have lifetimes of some 50 billion years or more. But in order to be warm enough to be habitable, they must orbit their stars in such close proximity that they would also suffer the effects of intense radiation, stellar flares and winds.

“This can cause red dwarf planets’ to lose their atmospheres and water inventories,” Guinan said. In most cases, this results in dry so-called “Dune” desert-like planets with little or no atmospheres. Or, he says, such red dwarf planets can also end up like Venus, with a huge carbon dioxide atmosphere and also no water. However, because they evolve more slowly than our own Sun, orange dwarfs allow for very stable habitable zones over long timescales, say the authors.

An artist's impression of the Tau Ceti system. (Image by J. Pinfield for the RoPACS network at the University of Hertfordshire, 2012)

An artist’s impression of the Tau Ceti system. (Image by J. Pinfield for the RoPACS network at the University of Hertfordshire, 2012)

Their paper outlined the properties of more than 300 nearby orange dwarf stars in order to determine both the width and stability of what they term the stars’ climatological stellar habitable zones. They analyzed the stars’ stellar habitable real estate — including the numbers of stars as a function of their spectral type; their stellar lifetimes; habitable zone locations and also the orbital width of each such habitable zone.

In the process, they also took into account the long-term effects of stellar flares, x-rays and ultraviolet radiation on possible earthlike planets around such G- and K-type stars. The authors were particularly impressed by the astrobiological potential for life around K-stars, which are two to three times more numerous than stars like the Sun.

“Many K-stars can be much older than our Sun,” Guinan said. “So, if life formed and evolved on habitable zone planet hosted by a old K-star — a few to several billions of years older than the Sun; it could maybe even harbor intelligent life.”

The yellow-orange dwarf Tau Ceti — which lies only 12 light years from Earth — even hosts five possible super-earths. Two of these may be potentially habitable, says Guinan.

With an age of as much as eight billion years, could any of Tau Ceti’s planets harbor intelligent life? After all, this was SETI pioneer Frank Drake’s first target in his initial radio search for extraterrestrial intelligence more than 50 years ago. But after numerous scans for intelligent signals, the planets around Tau Ceti still appear void of extraterrestrial civilizations. Even so, the odds now appear to be shifting in sentient life’s favour.

To wit, says Guinan, suppose both a K-star and a sunlike star each formed some 10 billion years ago and soon spawned habitable earthlike planets. Life on planets around a sunlike star, he says, would have likely long ended, since that star would have already expanded into a Red Giant.

But a K-type star doesn’t expand into a red giant until it is at least 20 billion years old, says Guinan. Thus, for an earthlike planet circling a K-star, he says, life could be “very advanced by now.” Source:Bruce Dorminey Forbes Follow me on Facebook, Twitter and Google +. And like my ‘Distant Wanderers’ exoplanet Facebook page.

 

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