Mirage Earth Exoplanets: A Burning Issue

Also known as M stars, red dwarfs are faint when compared to stars like our Sun, and they are also much smaller--a mere 10 to 20 percent as massive. However, red dwarfs make up for their puny size by being the most abundant type of true star in our Milky Way Galaxy, accounting for approximately 80 percent of its stellar inhabitants. Because red dwarfs are so abundant, astrobiologists have wondered if they might provide the best opportunity for spotting exoplanets habitable to life as we know it and, indeed, more and more planets are being discovered circling around red dwarf parent stars. However, a study released in December 2014 indicates that even though planets orbiting close to their low-mass parent stars have been prime targets in the search for extraterrestrial life, some of these planets may have long since lost their chance at hosting life because of the roasting heat present during their formative years.

The new research was led by Rodrigo Luger, who is a doctoral student at the University of Washington in Seattle. Red dwarfs are much less luminous than stars like our Sun and, therefore, their habitable zones tend to be quite close in. The habitable zone surrounding a star is that "Goldilocks" region where the temperatures are not too hot, not too cold, but just right for water to exist in its liquid state. Where there is liquid water, there may also potentially be life as we know it.

Planets that circle their parent stars in close orbits are easier for astronomers to discover than their cooler siblings dwelling in more distant orbits. Planet-hunting astronomers spot and measure these faraway alien worlds, circling stars beyond our Sun, by observing the small dips in stellar brightness that occur when a planet floats in front of, or transits, the glaring face of its parent star. Alternatively, distant exoplanets can be found by astronomers measuring the star's tiny "wobble". This extremely small "wobble" occurs as a result of the orbiting planet's gravity slightly tugging on its parent star. This so-called "wobble method" is more technically termed the radial velocity method, and it is the original method used by those planet-hunting astronomers who made the historic discoveries of the first exoplanets a generation ago.

Because a red dwarf's habitable zone is situated much closer-in than the Earth's distance from our Sun, any planet orbiting it would be inflicted with a shower of much more powerful and, therefore, destructive space weather emanating from its glaring stellar parent.

Red Dwarf Stars

Our barred-spiral Milky Way Galaxy sparkles with the seething fires of at least 100 billion stellar inhabitants. There are about 100 red dwarf systems located within 25 light-years of our own planet. These relatively puny stars are very dim, and because they emit such a comparatively small quantity of radiation, they can hide in interstellar space, well-hidden in our own Galaxy, where they cannot be readily discovered by curious observational astronomers.

Red dwarfs, therefore, are the runts of the stellar litter. They are the smallest, coolest, and most abundant type of true star inhabiting our Galaxy. Usually, the median figure, provided by astronomers, is that red dwarfs account for about 73% of all the stars sparkling in our Milky Way. Because of their relatively puny energy output, these dim little stars are never visible with the unaided human eye from our planet. The closest red dwarf to our Sun is Proxima Centauri, and it is one member of a shining triple system of three stellar sisters. Proxima Centauri, our own Star's closest stellar neighbor, is far too faint to be seen from Earth with the unaided human eye--as is the nearest lone red dwarf star dubbed Barnard's star.

Because stellar models suggest that red dwarfs holding less than 35% of our Star's mass are fully convective, the helium manufactured by their thermonuclear fusion of hydrogen is perpetually being remixed throughout the tiny star. This results in the avoidance of a buildup at its core. Therefore, red dwarfs evolve very, very sluggishly, showing a constant luminosity and spectral type for (in theory) trillions of years--before their supply of necessary nuclear fuel is depleted. Because the Universe itself is "only" about 3.8 billion years old, it is thought that no elderly red dwarfs exist. They have not had sufficient time to reach more advanced evolutionary stages since the Big Bang.

Red dwarfs possess relatively low temperatures in their cores and energy is generated at a lazy rate by way of nuclear fusion of hydrogen into helium. Therefore, these stellar runts emit little light--sometimes as little as 1/10,000 that of our own roiling, fiery Star.

In more recent years, astrobiologists and planet-hunting astronomers have contemplated the possibility of life evolving on alien worlds orbiting these small and very faint stars. Because a red dwarf holds the relatively small mass of one-tenth to one-half that of our Sun, determining how their various characteristics influence the potential habitability of the planets that orbit them may reveal to scientists the frequency of extraterrestrial life--including intelligent life.

Because red dwarf planets circle their stellar parents in close orbits, they suffer the effects of strong tidal heating--which is certainly a powerful impediment to the evolution of fragile living tidbits inhabiting these systems. There are additional tidal effects that also hinder the evolution of life in such planetary systems. For example, there are extreme variations in temperature that are caused by one side of a habitable zone red dwarf planet permanently facing its parent star--while the other side is permanently turned away. Alas, there are also an assortment of non-tidal impediments to the development of delicate living creatures on red dwarf planets--including small circumstellar habitable zones resulting from a puny 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 own Star.

Many scientists, however, have suggested several factors that may actually increase the chances for life to emerge on a red dwarf planet. Vigorous cloud formation on the star-facing side of a tidally locked alien planet, for example, may decrease the overall thermal flux. This would reduce equilibrium temperature variations between the two sides of the exoplanet. In addition, the great abundance of these tiny, dim stars increases the number of potentially habitable alien planets that may be in orbit around them.

The discovery of a vast and diverse array of weird creatures--collectively called extremophiles--dwelling on our own Earth, has encouraged some exobiologists to wonder if these abundant, cool little stars may be the most likely type to be circled by exoplanets hosting extraterrestrial life. Extremophiles are life-forms that can thrive under conditions that human beings find extreme and uncomfortable--such as very hot environments, very cold environments, very acidic environments, and very dry environments.

Mirage Earth Exoplanets

In the new research paper published in the journal Astrobiology, doctoral student Rodrigo Luger and co-author Dr. Rory Barnes, who is a University of Washington research assistant professor, say that through computer simulations they have found that some exoplanets close to low-mass stars likely had their water and atmospheres burned away when they were still in the process of forming.

"All stars form in the collapse of a giant cloud of interstellar gas, which releases energy in the form of light as it shrinks. But because of their lower masses, and therefore lower gravities, M dwarfs take longer to fully collapse--on the order of many hundreds of millions of years," Luger explained in a December 2, 2014 University of Washington Press Release.

Luger added that "Planets around these stars can form within 10 million years, so they are around when the stars are still extremely bright. And that's not good for habitability, since these planets are going to initially be very hot, with surface temperatures in excess of a thousand degrees. When this happens, your oceans boil and your entire atmosphere becomes steam."

Also potentially destructive to the forming atmospheres of these alien worlds is the fact that red dwarf stars spew out a large quantity of X-ray and ultraviolet light, which heats the upper atmosphere of an orbiting planet to thousands of degrees and causes gas to expand so rapidly that it actually escapes the tortured world--and is then lost to space.

"So, many of the planets in the habitable zones of M dwarfs could have been dried up by this process early on, severely decreasing their chance of actually being habitable," Luger continued to explain in the University of Washington Press Release.

An additional effect of this destructive process, Lugar and Barnes write, is that ultraviolet radiation can split up water into its component hydrogen and oxygen atoms. The lighter hydrogen zips away from the atmosphere into space, leaving the heavier oxygen atoms behind. Even though some amount of oxygen is obviously very beneficial to the development of life, as on our own planet, too much oxygen has the opposite effect--and can be a negative factor for the evolution of living things.

"Rodrigo has shown that this prolonged runaway greenhouse phase can produce huge atmospheres full of oxygen--like, 10 times denser than that of Venus and all oxygen. Searches for life often rely on oxygen as a tracer of extraterrestrial life--so the abiological production of such huge quantities of oxygen could confound our search for life on exoplanets," Dr. Barnes explained in the December 2, 2014 University of Washington Press Release.

Luger noted that the title of their paper is Mirage Earths. He explained that they had chosen this title "Because of the oxygen they build up, they could look a lot like Earth from afar--but if you look more closely you'll find that they're really a mirage; there's just no water there."

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.

Article Source: http://EzineArticles.com/8833745