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  • richardmitnick 5:04 pm on February 8, 2017 Permalink | Reply
    Tags: Ion escape, , Oxygen escape, , Proxima b is subjected to torrents of X-ray and extreme ultraviolet radiation from superflares occurring roughly every two hours., Red dwarf/M Dwarf stars, Stellar eruptions such as flares and coronal mass ejections – collectively called space weather, We have pessimistic results for planets around young red dwarfs in this study   

    From Goddard: “NASA Finds Planets of Red Dwarf Stars May Face Oxygen Loss in Habitable Zones” 

    NASA Goddard Banner

    NASA Goddard Space Flight Center

    Feb. 8, 2017
    Lina Tran
    kathalina.k.tran@nasa.gov
    NASA’s Goddard Space Flight Center, Greenbelt, Md.

    3
    Credit: NASA

    The search for life beyond Earth starts in habitable zones, the regions around stars where conditions could potentially allow liquid water – which is essential for life as we know it – to pool on a planet’s surface. New NASA research suggests some of these zones might not actually be able to support life due to frequent stellar eruptions – which spew huge amounts of stellar material and radiation out into space – from young red dwarf stars.

    Now, an interdisciplinary team of NASA scientists wants to expand how habitable zones are defined, taking into account the impact of stellar activity, which can threaten an exoplanet’s atmosphere with oxygen loss. This research was published in The Astrophysical Journal Letters on Feb. 6, 2017.

    “If we want to find an exoplanet that can develop and sustain life, we must figure out which stars make the best parents,” said Vladimir Airapetian, lead author of the paper and a solar scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “We’re coming closer to understanding what kind of parent stars we need.”

    To determine a star’s habitable zone, scientists have traditionally considered how much heat and light the star emits. Stars more massive than our sun produce more heat and light, so the habitable zone must be farther out. Smaller, cooler stars yield close-in habitable zones.

    But along with heat and visible light, stars emit X-ray and ultraviolet radiation, and produce stellar eruptions such as flares and coronal mass ejections – collectively called space weather. One possible effect of this radiation is atmospheric erosion, in which high-energy particles drag atmospheric molecules – such as hydrogen and oxygen, the two ingredients for water – out into space. Airapetian and his team’s new model for habitable zones now takes this effect into account.


    In this artist’s concept, X-ray and extreme ultraviolet light from a young red dwarf star cause ions to escape from an exoplanet’s atmosphere. Scientists have developed a model that estimates the oxygen ion escape rate on planets around red dwarfs, which plays an important role in determining an exoplanet’s habitability.
    Credits: NASA Goddard/Conceptual Image Lab, Michael Lentz, animator/Genna Duberstein, producer

    The search for habitable planets often hones in on red dwarfs, as these are the coolest, smallest and most numerous stars in the universe – and therefore relatively amenable to small planet detection.

    “On the downside, red dwarfs are also prone to more frequent and powerful stellar eruptions than the sun,” said William Danchi, a Goddard astronomer and co-author of the paper. “To assess the habitability of planets around these stars, we need to understand how these various effects balance out.”

    Another important habitability factor is a star’s age, say the scientists, based on observations they’ve gathered from NASA’s Kepler mission. Every day, young stars produce superflares, powerful flares and eruptions at least 10 times more powerful than those observed on the sun. On their older, matured counterparts resembling our middle-aged sun today, such superflares are only observed once every 100 years.

    “When we look at young red dwarfs in our galaxy, we see they’re much less luminous than our sun today,” Airapetian said. “By the classical definition, the habitable zone around red dwarfs must be 10 to 20 times closer-in than Earth is to the sun. Now we know these red dwarf stars generate a lot of X-ray and extreme ultraviolet emissions at the habitable zones of exoplanets through frequent flares and stellar storms.”

    Superflares cause atmospheric erosion when high-energy X-ray and extreme ultraviolet emissions first break molecules into atoms and then ionize atmospheric gases. During ionization, radiation strikes the atoms and knocks off electrons. Electrons are much lighter than the newly formed ions, so they escape gravity’s pull far more readily and race out into space.

    Opposites attract, so as more and more negatively charged electrons are generated, they create a powerful charge separation that lures positively charged ions out of the atmosphere in a process called ion escape.

    “We know oxygen ion escape happens on Earth at a smaller scale since the sun exhibits only a fraction of the activity of younger stars,” said Alex Glocer, a Goddard astrophysicist and co-author of the paper. “To see how this effect scales when you get more high-energy input like you’d see from young stars, we developed a model.”

    The model estimates the oxygen escape on planets around red dwarfs, assuming they don’t compensate with volcanic activity or comet bombardment. Various earlier atmospheric erosion models indicated hydrogen is most vulnerable to ion escape. As the lightest element, hydrogen easily escapes into space, presumably leaving behind an atmosphere rich with heavier elements such as oxygen and nitrogen.

    But when the scientists accounted for superflares, their new model indicates the violent storms of young red dwarfs generate enough high-energy radiation to enable the escape of even oxygen and nitrogen – building blocks for life’s essential molecules.

    “The more X-ray and extreme ultraviolet energy there is, the more electrons are generated and the stronger the ion escape effect becomes,” Glocer said. “This effect is very sensitive to the amount of energy the star emits, which means it must play a strong role in determining what is and is not a habitable planet.”

    Considering oxygen escape alone, the model estimates a young red dwarf could render a close-in exoplanet uninhabitable within a few tens to a hundred million years. The loss of both atmospheric hydrogen and oxygen would reduce and eliminate the planet’s water supply before life would have a chance to develop.

    “The results of this work could have profound implications for the atmospheric chemistry of these worlds,” said Shawn Domagal-Goldman, a Goddard space scientist not involved with the study. “The team’s conclusions will impact our ongoing studies of missions that would search for signs of life in the chemical composition of those atmospheres.”

    Modeling the oxygen loss rate is the first step in the team’s efforts to expand the classical definition of habitability into what they call space weather-affected habitable zones. When exoplanets orbit a mature star with a mild space weather environment, the classical definition is sufficient. When the host star exhibits X-ray and extreme ultraviolet levels greater than seven to 10 times the average emissions from our sun, then the new definition applies. The team’s future work will include modeling nitrogen escape, which may be comparable to oxygen escape since nitrogen is just slightly lighter than oxygen.

    The new habitability model has implications for the recently discovered planet orbiting the red dwarf Proxima Centauri, our nearest stellar neighbor. Airapetian and his team applied their model to the roughly Earth-sized planet, dubbed Proxima b, which orbits Proxima Centauri 20 times closer than Earth is to the sun.

    Considering the host star’s age and the planet’s proximity to its host star, the scientists expect that Proxima b is subjected to torrents of X-ray and extreme ultraviolet radiation from superflares occurring roughly every two hours. They estimate oxygen would escape Proxima b’s atmosphere in 10 million years. Additionally, intense magnetic activity and stellar wind – the continuous flow of charged particles from a star – exacerbate already harsh space weather conditions. The scientists concluded that it’s quite unlikely Proxima b is habitable.

    “We have pessimistic results for planets around young red dwarfs in this study, but we also have a better understanding of which stars have good prospects for habitability,” Airapetian said. “As we learn more about what we need from a host star, it seems more and more that our sun is just one of those perfect parent stars, to have supported life on Earth.”

    See the full article here.

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    NASA’s Goddard Space Flight Center is home to the nation’s largest organization of combined scientists, engineers and technologists that build spacecraft, instruments and new technology to study the Earth, the sun, our solar system, and the universe.

    Named for American rocketry pioneer Dr. Robert H. Goddard, the center was established in 1959 as NASA’s first space flight complex. Goddard and its several facilities are critical in carrying out NASA’s missions of space exploration and scientific discovery.

    NASA Goddard campus
    NASA/Goddard Campus
    NASA image

     
  • richardmitnick 9:36 am on February 9, 2016 Permalink | Reply
    Tags: , , , Red dwarf/M Dwarf stars   

    From PALE RED DOT: “Living in Twilight: An Overview of our Closest and Smallest Stellar Neighbors” 

    Pale Red Dot

    Pale Red Dot

    February 2, 2016
    Sergio Dieterich, Carnegie Institution for Science

    When members of our research group go observing at [NOAO] Cerro Tololo Inter-American Observatory in the Chilean Andes we spend most of our time in a cozy heated control room.

    CTIO large
    NOAO CTIO

    Modern astronomical observing is done mostly by monitoring computer screens and entering commands to tell the telescope where to point next. If we have to put on our winter jackets and climb the flight of stairs to where the telescope is—under the open dome—it is because something went wrong and we are frantically trying to fix the problem and minimize the loss of precious telescope time. There is one exception. Our group’s tradition dictates that when we are training a new student, and the season and time of night is just right, we will go up to the dome and have our new colleague look through the telescope’s eyepiece. Photography does not do justice to the sight that emerges: a bright ruby red speck of light floats seemingly in front of a vast ocean of fainter and whiter stars. That red speck is Proxima Centauri, the closest star to us other than the Sun, the subject of the Pale Red Dot project, and a typical low mass star. Stars like Proxima Centauri, or just Proxima for short, are amongst the smallest but also the most common types of stars in the Galaxy. Let’s take a few minutes to understand our smallest and closest stellar neighbors a little better.

    Imagine for a moment that we drop a large ceramic dinner plate on a hard kitchen floor. The plate shatters into many, many, pieces, of all different sizes. We then look down and examine the results of our carelessness. Our attention is first drawn to the handful of large fragments. After a more careful look we see that for every one of those large ceramic fragments there are dozens, if not hundreds, of much smaller pieces. Further, we soon realize that if we have any hope of reconstructing the original plate or figuring out what happened we cannot simply ignore those smaller pieces and sweep them under the rug. This unfortunate kitchen accident is a rough analogy to the stellar formation process, and it sheds some light on how the Milky Way Galaxy ended up with the stellar population we observe today. Stars are formed when clouds of interstellar gas and dust, called giant molecular clouds, are somehow perturbed—causing the cloud to start collapsing under its own gravitational pull. Several points in the collapsing cloud achieve higher and higher density, and therefore exert an even greater gravitational force. Over the course of hundreds of thousands of years these high density regions consume enough gas and become compact enough to form stellar embryos, or protostars. When the protostar’s core becomes hot enough to ignite and sustain nuclear fusion, a star is born. In a manner similar to what happens with our shattering plate, but for different physical reasons, the result of this cloud collapse mechanism heavily favors the production of stars whose masses are anywhere from about 60% to about only 8% of our Sun’s mass. When fully formed and contracted these are tiny stars, with the majority having radii between 20% and only 10% our Sun’s radius. The smallest are very close in size (but not in mass or density!) to the planet Jupiter. What these small stars lack in terms of size they make up for in their sheer numbers. Indeed, out of the 366 stars whose accurately measured distances place them within 32.6 light-years (or 10 parsecs, in astronomical lingo) of our Solar System, 275 belong to this type. These objects are commonly known as red dwarfs, or M dwarfs, in the stellar classification system used by professional astronomers. Using the fair assumption that our solar neighborhood is typical of much of the Milky Way Galaxy, that means that about 75% of the stars in our galaxy are M dwarfs. The M dwarf class is sometimes subdivided, with stars having about 20% or less the mass of our Sun being called Very Low Mass, or VLM stars. Proxima is in the upper mass range of the VLM stars.

    What are red dwarfs like as stars, and how does their energy output compare to our Sun’s? These stars are incredibly faint, and not even Proxima can be seen with the naked eye despite its proximity of only 4.25 light-years. To put this distance in context, the best estimates for the diameter of the Milky Way Galaxy place it at anywhere between 100,000 to 180,000 light-years; if our galaxy were a city 10 km across Proxima would be so close to us as to be knocking on our front door! And yet stars that are intrinsically more luminous can be seen with the naked eye from distances almost one fifth of the way across the galaxy. If a representative sample of red dwarfs were all placed at the same distance to us as the Sun the brightest ones would shine only about 7 percent as bright as the Sun. Recent research by our group indicates that the faintest of the VLM stars would shine with only about 0.016 percent, or about 1/6,000th , the brightness of our Sun. Proxima has a total energy output about 0.2% that of our Sun.

    Red dwarfs are not only faint, but the little light they do emit is also very different from the warm sunlight we enjoy on a Caribbean beach on Earth. The surface of our Sun shines at a temperature of approximately 5,500 degrees Celsius (10,000 F). At that temperature most of the light is emitted in the yellow-green region of the visible light spectrum. It therefore makes sense that the human eye has evolved to be the most sensitive to the yellow-green light that most strongly bathes our planet. Low mass stars have significantly cooler surface temperatures: about 3,500 C (6,400 F) for the hottest red dwarfs and approximately 1,800 C (3,300 F) for the smallest and faintest VLM stars. At these temperatures not only does the star emit considerably less light overall, but the light emitted is also shifted to longer wavelengths, which we perceive as redder colors. The color spectrum of the hottest red dwarfs has its peak at a deep red color that is just at the limit of the detection range of the human eye. For the faintest VLM stars the color spectrum peaks in the near infrared range of the electromagnetic spectrum, well beyond the detection capabilities of the human eye. In both cases the human eye’s enhanced sensitivity to yellow-green light will shift the perceived colors to shorter wavelengths than the peak color emission. A future interstellar voyager who sees a hot red dwarf up close will likely perceive a distinctive orange hue, whereas one of the cooler red dwarfs may appear to be a lively red (Figure 2ab). To make these faintest of faint stars even more unusual, there is evidence to suggest that they have strong surface magnetic fields. These magnetic fields would cause dark spots analogous to sunspots, but they may be more numerous and larger—perhaps covering a substantial portion of the star’s surface.

    2cc73-reddwarfstar
    2cc73 Red Dwarf

    Astronomers currently think that as many as 1/3 of red dwarfs may harbor rocky planets with compositions similar to Earth’s. Could life evolve on these planets, and what would life around a red dwarf be like? The idea of life evolving on planets around red dwarfs is extremely exciting. If for no other reason, their sheer numbers means that the question of red dwarf habitability has tremendous implications in determining whether we live in a Universe teeming with life or whether life is a sparse occurrence. Despite this huge potential, the notion of life on low mass star systems is not without its challenges.

    Because of their lower mass and consequentially weaker gravitational pull, red dwarfs take a very long time to settle into their fully contracted configuration, once they stop accreting material from their parent star forming cloud. Similarly, the comparatively slow rate of nuclear reactions in a low mass star’s core causes these stars to have extremely long lives when compared to more massive stars. Their slow evolution and long lives are both a blessing and a curse for the possibility of life. Once fully formed and contracted, red dwarfs change very little for hundreds of billions of years. The oldest red dwarfs may therefore have provided a stable environment for life for as long as they have existed, roughly 10 billion years based on current estimates for the age of the Galaxy. Compare that with only 4.1 billion years of biological evolution on Earth. Even if evolution around a planet hosting red dwarf happened slower and hit a few dead ends, the final result might still mean a complex and diverse ecosystem. However, the prospect of a prolonged period of stability suitable for biological evolution is only exciting if we assume that the right conditions for life were present to begin with, and that is where a red dwarf’s life in the slow lane becomes a problem. Liquid water is essential for life as we know it on Earth, and liquid water can only exist if the temperature on a planet’s surface allows it. A planet’s temperature is governed primarily by the planet’s orbital distance from its parent star and the star’s intrinsic luminosity. Astronomers call the range of orbital radii allowing the existence of liquid water the ‘habitable zone’ around a star. All stars are significantly brighter during their initial contraction phase, when most of the star’s energy comes from its gravitational collapse and not from nuclear fusion. For red dwarfs this initial period of increased luminosity may last up to 3 billion years, which is well beyond the formation time for planets. Any planet that forms in what will eventually become the star’s habitable zone will be subject to scorching heat during its early life. Calculations suggest that this fiery youth may cause all water to evaporate away, thus effectively sterilizing the planet. A possible way out of this scenario involves the retention of water in minerals called chondrites. If chondrites are present in sufficient amounts in the rocky material that coalesces to form planets, the fully formed planets could have substantial water reserves in their interiors. The water could then be released from the planet’s interior by volcanic activity at later times when the surface temperature is right for liquid water. Whether or not this scenario is likely is an area of active research.

    Another interesting aspect of the idea of life in planets orbiting red dwarfs has to do with the extreme proximity of the star’s habitable zone to the star itself. These stars are so faint that planets in their habitable zones would have orbits smaller than the orbit of Mercury in our Solar System. At such small distances the slight difference in the star’s gravitational pull from the planet’s side facing the star to the planet’s far side causes a phenomenon called tidal locking. In a tidally locked planet the same side of the planet always faces the star, causing it to be much hotter than the side that is perpetually facing away from the star. The Earth-Moon system is a good example of a tidally locked satellite. The habitable conditions in a tidally locked planet may be confined to a narrow ring shaped region where the illuminated side meets the dark side of the planet. This habitable region would be in perpetual twilight, with the star shining low in the horizon. Such low illumination conditions may seem rather depressing to us humans, but low light levels peaking at redder wavelengths are the norm around red dwarfs, and it is quite possible that any existing life form in these otherworldly environments may have evolved to use infrared light in much the same way we utilize the bright yellow-green light of our parent star. Perhaps venturing too close to the planet’s illuminated side would cause these creatures to get a “star burn” from red light in much the same way we get can get a sunburn from the small portion of our Sun’s energy that is emitted as ultraviolet light.

    Finally, a treatment of low mass stars would not be complete without making a connection to their lower mass cousins, the substellar brown dwarfs.

    Brown dwarf
    Artist’s concept of a T-type brown dwarf

    Looking back to our shattered plate analogy of star formation, the cloud collapse process that produces stars with a wide range of different masses can also produce objects whose mass is too small to create the conditions necessary for sustainable core nuclear fusion. These objects are called brown dwarfs. Brown dwarfs look much like their VLM star counterparts in their youth because during that phase gravitational contraction releases a large amount of energy for both stars and brown dwarfs. However, once brown dwarfs are fully contracted they keep cooling down over the course of billions of years. For much of the red dwarf range of temperatures and colors it is difficult to tell whether a given object is a young brown dwarf or a VLM star of any age. Recent research by my collaborators and I indicates that the stellar sequence comes to an end when we reach objects with surface temperatures of about 1,800 C (3,300 F) and luminosities of roughly 1/6,000th that of our Sun (interested in the technical details? read the paper here). We came to this conclusion by performing the observations necessary to estimate the radius of a sample of 63 objects thought to lie close to the end of the stellar sequence. We then noted that for temperatures higher than 1,800 C the objects cover a wide range of radii, including the radii expected for old and fully contracted stars. At cooler temperatures we encountered larger radii that can only be explained if the objects in question are young brown dwarfs that are not yet fully contracted.

    The temperature we obtained for the end of the stellar sequence is substantially higher than that predicted by theoretical models, and we are now trying to pinpoint the root causes of this discrepancy. As a part of this research we have found what we believe to be the smallest known star to date and also a representative of the smallest possible stars. This star is called 2MASS J0523-1403, and shines faintly in the constellation Lepus the hare, under the feet of Orion the hunter. 2MASS J0523-1403 has a radius of only [?] percent the radius of our Sun. That radius makes 2MASS J0523-1403 about 15 percent smaller than the planet Jupiter. Indeed, perhaps coincidentally, the size we calculate for 2MASS J05234-1403 is within 1 percent of the size of the planet Saturn. Therefore while we can say that VLM stars in general have sizes comparable to Jupiter, we can go one step further and say that the smallest stars are Saturn sized. In making these comparisons we must be careful not to confuse volume and mass. While these stars have the volume of giant planets their mass is theoretically predicted to be anywhere from 70 to 80 times the mass of Jupiter, making them incredibly dense. In fact, it is the quantum mechanical limit on the allowed upper density that causes brown dwarfs to stop contracting before nuclear fusion ignites.

    Over the last few decades our knowledge of red dwarfs has gone from simply knowing that they exist, to realizing just how numerous they are, and finally to being able to characterize them and assess their suitability as hosts for habitable planets. This progress is in part due to advances in observational astronomy, such as the substitution of blue sensitive photographic film to red sensitive digital CCD detectors and infrared detectors. Those advances in sensitivity and data management were then utilized to conduct large all-sky surveys that revealed a multitude of new red dwarfs and gave astronomers the unprecedented ability to study them not only as individual objects but also as a population. We now have a good understanding of how red dwarfs contribute to the overall stellar population of the Galaxy and are gaining greater understanding of their promises and challenges as hosts of livable planets. The history of astronomy has taught us that we cannot predict what the next discovery will be and how it will change our understanding of things. It could well be that after thorough study we may realize that the roughly 75 percent of the stars in the Galaxy that we call red dwarfs are not suitable as hosts of living planets. That alone would let us know that life in the Universe might be a bit more special than previously thought and how fortunate we are to have a home on planet Earth. On the opposing view, we know from our experience on Earth that evolution usually finds a way to make life flourish in the most extreme and odd environments. If life forming mechanisms are able to overcome the challenges we discussed here, plus many others that we have not yet even imagined, it is quite possible that our solar neighborhood abounds with beings of unimaginable forms thriving under the soft red twilight of their tiny parent star.

    See the full article here.

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    What is PALE RED DOT?

    It is an outreach project to show to the public how scientists are working to address a major question that could affect us all, namely are there Earth-like planets around the nearest stars?

    Why we call it PALE RED DOT?

    In 1990, Voyager 1, on its trek towards interstellar space, sent back a picture of the Inner Solar System on which the Earth occupied less than a pixel. This image of Earth was called Pale Blue Dot, and inspired the late Carl Sagan’s essay ‘Pale Blue Dot : A vision of the human future in Space’, which in turn has been the source of inspiration for a generation of exoplanet hunters. Given that Proxima Centauri — or just Proxima — is a red dwarf star, such a planet would show reddish tints. Even if successful, we will only obtain information about its orbital period and mass — even less than Voyager 1’s pale blue pixel… at least for now!

    What is special about the project?

    Proxima Centauri is the nearest star to the Sun. The discovery of a planet with some characteristics like Earth in our immediate vicinity would be momentous. After years of data acquisition by many researchers and teams, a signal has been identified which may indicate the presence of an Earth-like planet. The Pale Red Dot project will carry out further detailed observations with the aim to confirm or refute the presence of the planet. By broadcasting the progress and results of the observations through all media channels available e.g. press, website, and social media, the Pale Red Dot project aims to promote Science Technology Engineering and Mathematics (STEM) in the broader society, inform the public and hopefully inspire a new generation of scientists.

    How such a scientific program is organized?

    The planned observation campaign is based on a proposal submitted by the involved scientists to ESO, LCOGT and BOOTES observatories. The proposals, in turn, are based on the analysis of data accumulated and obtained over the years by ourselves or by other researchers abroad. Observatories and other advanced research facilities are mostly supported by public resources, large international consortia and private foundations.

    How the results will be reported?

    As in any professional scientific work, final results need to be reviewed by the community before being announced. After the campaign is finished by April 1st, the really tough process of analyzing the data, drawing conclusions and presenting them in a credible manner will begin. After that, the analysis will be summarized in an article and submitted to a scientific journal. At that point, one or more scientists NOT involved in the project will critically revise the work, suggest modifications and even reject its publication if fundamental flaws are spotted. This last step of peer-review can take any time between a few months to a year or two. Hopefully, the data will prove to be high quality and the observations will have a straightforward interpretation, but that is just a hope. A few key milestones of the peer-review process will also be reported on the website, which might remain active at a lower activity level after the observing campaign has finished.

     
  • richardmitnick 7:03 pm on February 9, 2015 Permalink | Reply
    Tags: , , , Red dwarf/M Dwarf stars   

    From astrobio: “Planets Orbiting Red Dwarfs May Stay Wet Enough for Life” 

    Astrobiology Magazine

    Astrobiology Magazine

    Feb 9, 2015
    Charles Q. Choi

    1
    Oceans currents would transport heat to the dark side of tidally-locked exoplanets. Image Credit: Lynette Cook

    Small, cold stars known as red dwarfs are the most common type of star in the Universe, and the sheer number of planets that may exist around them potentially make them valuable places to hunt for signs of extraterrestrial life.

    However, previous research into planets around red dwarfs suggested that while they may be warm enough to host life, they might also completely dry out, with any water they possess locked away permanently as ice. New research published on the topic finds that these planets may stay wet enough for life after all. The scientists detailed their findings online on November 12 in The Astrophysical Journal Letters.

    Red dwarfs, also known as M stars, are roughly one-fifth as massive as the Sun and up to 50 times fainter. These stars comprise up to 70 percent of the stars in the cosmos, and NASA’s Kepler space observatory has discovered that at least half of these stars host rocky planets that are one-half to four times the mass of Earth.

    NASA Kepler Telescope
    Kepler

    Red dwarf planets are potentially key places to search for life as we know it, not just because there are so many of them, but also because of their incredible longevity. Unlike our Sun, which will die in a few billion years, red dwarfs will take trillions of years to burn through their fuel, significantly longer than the age of the Universe, which is less than 14 billion years old. This longevity potentially gives red dwarfs a great deal more time for life to evolve around them.

    Research into whether a distant world might host life as we know it usually focuses on whether or not it has liquid water, since there is life virtually everywhere there is liquid water on Earth, even miles underground. Scientists typically concentrate on habitable zones, the area around a star where it is neither too hot for all its surface water to boil away, nor too cold enough for all its surface water to freeze.

    Recent findings suggest that planets in the habitable zones of red dwarf stars could accumulate significant amounts of water. In fact, each planet could possess about 25 times more water than Earth.

    The habitable zones of red dwarfs are close to these stars because of how dim they are, often closer than the distance Mercury orbits the Sun. This closeness makes them appealing to astrobiologists, since planets near their stars cross in front of them more often, making them easier to detect than planets that orbit farther away.

    However, when a planet orbits very near a star, the star’s gravitational pull can force the world to become “tidally locked” to it. When a planet is tidally locked to its star, it will always show the same side to its star, just as the Moon always shows the same side to Earth. This causes the planet to have one permanent day side and one permanent night side.

    3

    The extremes of heat and cold that tidally locked planets experience could make them profoundly different from Earth. For example, prior research speculated the dark sides of tidally locked planets would become so cold that any water there would freeze. Sunlight would make water on the sunlit side evaporate, and this water vapor could get carried by air currents to the night sides, eventually leading to sheets of ice miles thick on the night sides and removing all water from the sunlit sides. Life as we know it probably could not develop on the day sides of such planets. Although they would have sunlight for photosynthesis, they would have no water to serve as the primordial soup for life to swim in.

    To see how habitable tidally-locked planets really are, scientists devised a 3D global climate model of planets that simulated interactions between the atmosphere, ocean, sea ice, and land, as well as a 3-D model of ice sheets large enough to cover entire continents. They also simulated a red dwarf with a temperature of about 5,660 degrees Fahrenheit (3,125 degrees Celsius), and investigated whether all the water on these planets would indeed get trapped on their night sides.

    “I’ve been interested in trying to make calculations relevant for M-star planet habitability since being convinced by astronomers that these types of planets will likely be closest (in proximity) to Earth,” said study co-author Dorian Abbot, a geoscientist at the University of Chicago.

    For instance, the nearest known star to the Sun, Proxima Centauri, is a red dwarf, and it remains uncertain whether or not it has a planet. The possibility that red dwarf planets might be relatively near to Earth “means that anything geoscientists can tell astronomers about habitability of these planets will be essential for planning future missions.”

    The researchers simulated planets of Earth’s size and gravity that experienced between 63 percent and 77 percent as much sunlight as Earth. They also modeled a super-Earth planet 50 percent wider than Earth with 38 percent stronger gravity, because astronomers have discovered super-Earth worlds around red dwarfs. For instance, Gliese 667Cb, a super-Earth at least 4.5 times the mass of Earth, orbits Gliese 667C, a red dwarf about 22 light years from Earth. They set this super-Earth on an orbit where it received about two-thirds as much as sunlight as Earth.

    The researchers modeled three different arrangements of continents for all these planets. One was a water world with no continents and global oceans of varying depths. Another involved a supercontinent covering the night side and an ocean covering the day side. The last mimicked Earth’s configuration of continents. The planets also had atmospheres similar to Earth’s, but the researchers also tested lower levels of the greenhouse gas, carbon dioxide, which traps heat and helps keep planets warm.

    When it came to super-Earths covered entirely in water, and super-Earths with continental arrangements like Earth’s, the researchers found it was unlikely that all their water would get trapped on their night sides.

    “This is because surface winds transport sea ice to the day side where it is melted easily,” said lead study author Jun Yang at the University of Chicago.

    Moreover, ocean currents transport heat from the day side to the night side on these planets.

    “Ocean heat transport strongly influences the climate and sea ice thickness on our Earth,” Yang said. “We found this may also work on exoplanets.”

    If a super-Earth has very large continents covering most of its night side, the scientists discovered ice sheets of at least 3,300 feet (1,000 meters) thick could grow on its night side. However, the day sides of these super-Earths would dry out completely only if they received less geothermal heat from volcanic activity than Earth, and had 10 percent of the amount of water on Earth’s surface or less. Similar results were seen with Earth-sized planets.

    “The important implication is that it may be easier than previously thought to keep liquid water on the dayside of a tidally locked planet, where photosynthesis is possible,” Abbot said. “There are many issues that will affect the habitability of M-star planets, but our results suggest at least that water-trapping on the night side will only be a problem for relatively dry planets with large continents on their nightside and relatively low geothermal heat flux.”

    Based on present and near-future technology, Yang said it would be very difficult for astronomers to gauge how thick the sea ice or the ice sheets are on the night sides of red dwarf planets and test whether their models are correct. Still, using current and upcoming technology “it may be possible to know whether the day sides are dry or not,” Yang said.

    See the full article here.

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  • richardmitnick 5:32 pm on August 11, 2014 Permalink | Reply
    Tags: , , , , , Red dwarf/M Dwarf stars   

    From Astrobiology magazine: “Red Dwarf Stars Might Be Best Places to Discover Alien Life” 

    Astrobiology Magazine

    Astrobiology Magazine

    Aug 11, 2014

    Red dwarfs are the most common type of star in the universe, and nearly every one of these stars may have a planet located in its habitable zone where life has the best chance of existing, a new study concludes.

    This discovery may increase the chances that alien life could exist elsewhere in the cosmos, researchers say. They detailed their findings in the International Journal of Astrobiology.

    Red dwarfs, also known as M dwarf stars, are up to 50 times dimmer than the Sun and are just 10 to 20 percent as massive. They make up to 70 percent of the stars in the universe.

    red dwarf
    red dwarf

    The fact that red dwarfs are so common has made scientists wonder if they might be the best places to discover alien life. Astronomers are discovering more and more planets around red dwarfs, and recent findings from NASA’s Kepler space observatory reveal that at least half of these stars host rocky planets that are one-half to four times the mass of Earth. All in all, planets about the size of Earth seem plentiful in the universe, as do other worlds that are smaller than most gas giants, on the order of Neptune (which is 17 times the mass of Earth). Why such worlds are abundant is a mystery.

    NASA Kepler Telescope
    NASA/Kepler

    A leading theory in planetary formation suggests that as embryonic planets develop in the disks of gas and dust surrounding newborn stars, these nascent planets migrate inward as the matter in these proto-planetary disks erodes their orbits. However, migration models suggest Neptune-size planets should be rarer than they actually are.

    Instead, some researchers have suggested these relatively low-mass planets may assemble in situ — that is, they are born and stay in much the same places around their stars their entire lives, with little to no migration toward or away from their stars. Study author Brad Hansen, an astrophysicist at the University of California at Los Angeles, used computer models of in situ planetary formation to see how often red dwarfs might develop Earth-sized worlds, and where these planets might orbit around the stars.

    In his computer simulations, Hansen modeled red dwarfs half the mass of the Sun, with proto-planetary disks extending from 0.05 AU to 1 AU (one astronomical unit is the average distance from the Sun to the Earth) from the stars. The disks contained an amount of gas and dust equal to six times the mass of Earth. He then looked at how many planets developed after 10 million years.

    Of particular interest to Hansen were the so-called habitable zones of these stars, the areas where planets are potentially warm enough to sustain liquid water — and potentially life — on their surfaces. Red dwarfs are relatively cold stars, which means their habitable zones are closer than Mercury is to the Sun — just 0.1 to 0.2 AU.

    Hansen found most of the resulting planetary systems comprise between four and six surviving planets inside 0.5 AU, although the largest number went as high as 10. In addition, the red dwarfs usually possessed one or two planets within their habitable zones, which extended from 0.23 to 0.44 AU.

    “A high frequency of potentially habitable planets makes it more likely that we could actually find one that is habitable,” Hansen said.

    Moreover, Hansen also found that planets in the habitable zones of red dwarf stars could accumulate significant amounts of water. In fact, each could possess roughly 25 times more water than Earth has as a whole. All in all, he noted these results “broadly support the notion that habitable planets are plentiful around M dwarfs in the solar neighborhood.”

    See the full article here.

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