Since it’s discovery was announced in August of 2016, Proxima b has been an endless source of wonder and the target of many scientific studies. As the closest extra-solar planet to our Solar System – and a terrestrial planet that orbits within Proxima Centauri’s circumstellar habitable zone (aka. “Goldilocks Zone”) – scientists have naturally wondered whether or not this planet could be habitable.
Unfortunately, many of these studies have emphasized the challenges that life on Proxima b would likely face, not the least of which is harmful radiation from its star. According to a recent study, a team of astronomers used the ALMA Observatory to detect a large flare emanating from Proxima Centauri. This latest findings, more than anything, raises questions about how habitable its exoplanet could be.
For the sake of their study, the team used data obtained by the Atacama Large Millimeter/submillimeter Array (ALMA) between January 21st to April 25th, 2017. This data revealed that the star underwent a significant flaring event on March 24th, where it reached a peak that was 1000 times brighter than the star’s quiescent emission for a period of ten seconds.
Astronomers have known for a long time that when compared to stars like our Sun, M-type stars are variable and unstable. While they are the smallest, coolest, and dimmest stars in our Universe, they tend to flare up at a far greater rate. In this case, the flare detected by the team was ten times larger than our Sun’s brightest flares at similar wavelengths.
Along with a smaller preceding flare, the entire event lasted fewer than two minutes of the 10 hours that ALMA was observing the star between January and March of last year. While it was already known that Proxima Centauri, like all M-type stars, experiences regular flare activity, this one appeared to be a rare event. However, stars like Proxima Centauri are also known to experienced regular, although smaller, X-ray flares.
All of this adds up to a bad case for habitability. As MacGregor explained in a recent NRAO press statement:
“It’s likely that Proxima b was blasted by high energy radiation during this flare. Over the billions of years since Proxima b formed, flares like this one could have evaporated any atmosphere or ocean and sterilized the surface, suggesting that habitability may involve more than just being the right distance from the host star to have liquid water.”
MacGregor and her colleagues also considered the possibility that Proxima Centauri is circled by several disks of dust. This was suggested by a previous study (also based on ALMA data) that indicated that the light output of both the star and flare together pointed towards the existence of debris belts around the star. However, after examining the ALMA data as a function of observing time, they were able to eliminate this as a possibility.
As Alycia J. Weinberger, also a researcher with the Carnegie Institution for Science and a co-author on the paper, explained:
“There is now no reason to think that there is a substantial amount of dust around Proxima Cen. Nor is there any information yet that indicates the star has a rich planetary system like ours.”
To date, studies that have looked at possible conditions on Proxima b have come to different conclusions as to whether or not it could retain an atmosphere or liquid water on its surface. While some have found room for “transient habitability” or evidence of liquid water, others have expressed doubt based on the long-term effects that radiation and flares from its star would have on a tidally-locked planet.
In the future, the deployment of next-generation instruments like the James Webb Space Telescope are expected to provide more detailed information on this system. With precise measurements of this star and its planet, the question of whether or not life can (and does) exist in this system may finally be answered.
And be sure to enjoy this animation of Proxima Centauri in motion, courtesy of NRAO outreach:
Ever since the Kepler space telescope began discovering thousands of exoplanets in our galaxy, astronomers have been eagerly awaiting the day when next-generation missions are deployed. These include the much-anticipated James Webb Space Telescope, which is scheduled to take to space in 2019, but also the many ground-based observatories that are currently being constructed.
The Transit Method (aka. Transit Photometry) consists of monitoring stars for periodic dips in brightness. These dips are caused by planets passing in front of the star (aka. transiting) relative to the observer. In the past, detecting planets around M-type stars using this method has been challenging since red dwarfs are the smallest and dimmest class of star in the known Universe and emit the majority of their light in the near-infrared band.
However, these stars have also proven to be treasure trove when it comes to rocky, Earth-like exoplanets. In recent years, rocky planets have been discovered around star’s like Proxima Centauri and Ross 128, while TRAPPIST-1 had a system of seven rocky planets. In addition, there have been studies that have indicated that potentially-habitable, rocky planets could be very common around red dwarf stars.
Unlike other facilities, the ExTrA project is well-suited to conduct surveys for planets around red dwrfs because of its location on the outskirts of the Atacama Desert in Chile. As Xavier Bonfils, the project’s lead researcher, explained:
“La Silla was selected as the home of the telescopes because of the site’s excellent atmospheric conditions. The kind of light we are observing – near-infrared – is very easily absorbed by Earth’s atmosphere, so we required the driest and darkest conditions possible. La Silla is a perfect match to our specifications.”
In addition, the ExTrA facility will rely on a novel approach that involves combining optical photometry with spectroscopic information. This consists of its three telescopes collecting light from a target star and four companion stars for comparison. This light is then fed through optical fibers into a multi-object spectrograph in order to analyze it in many different wavelengths.
This approach increases the level of achievable precision and helps mitigate the disruptive effect of Earth’s atmosphere, as well as the potential for error introduced by instruments and detectors. Beyond the goal of simply finding planets transiting in front of their red dwarf stars, the ExTrA telescopes will also study the planets it finds in order to determine their compositions and their atmospheres.
In short, it will help determine whether or not these planets could truly be habitable. As Jose-Manuel Almenara, a member of the ExTrA team, explained:
“With ExTrA, we can also address some fundamental questions about planets in our galaxy. We hope to explore how common these planets are, the behaviour of multi-planet systems, and the sorts of environments that lead to their formation,”
The potential to search for extra-solar planets around red dwarf stars is an immense opportunity for astronomers. Not only are they the most common star in the Universe, accounting for 70% of stars in our galaxy alone, they are also very long-lived. Whereas stars like our Sun have a lifespan of about 10 billion years, red dwarfs are capable of remaining in their main sequence phase for up to 10 trillion years.
For these reasons, there are those who think that M-type stars are our best bet for finding habitable planets in the long run. At the same time, there are unresolved questions about whether or not planets that orbit red dwarf stars can stay habitable for long, owing to their variability and tendency to flare up. But with ExTrA and other next-generation instruments entering into service, astronomers may be able to address these burning questions.
“With the next generation of telescopes, such as ESO’s Extremely Large Telescope, we may be able to study the atmospheres of exoplanets found by ExTra to try to assess the viability of these worlds to support life as we know it. The study of exoplanets is bringing what was once science fiction into the world of science fact.”
Red dwarf stars have become a major focal point for exoplanet studies lately, and for good reason. For starters, M-type (red dwarf) stars are the most common type in our Universe, accounting for 75% of stars in the Milky Way alone. In addition, in the past decade, numerous terrestrial (i.e rocky) exoplanets have been discovered orbiting red dwarf stars, and within their circumstellar habitable zones (“Goldilocks Zones”) to boot.
This has naturally prompted several studies to determine whether or not rocky planets can retain their atmospheres. The latest study comes from NASA, using data obtained by the Mars Atmosphere and Volatile Evolution (MAVEN) orbiter. Having studied Mars’ atmosphere for years to determine how and when it was stripped away, the MAVEN mission is well-suited when it comes to measuring the potential habitability of other planets.
Launched in November 18th, 2013, the MAVEN mission established orbit around Mars on September 22nd, 2014. The purpose of this mission has been to explore the Red Planet’s upper atmosphere, ionosphere and its interactions with the Sun and solar wind for the sake of determining how and when Mars’ atmosphere went from being thicker and warmer in the past (and thus able to support liquid water on the surface) to thin and tenuous today.
Since November of 2014, MAVEN has been measuring Mars’ atmospheric loss using its suite of scientific instruments. From the data it has obtained, scientists have surmised that the majority of the planet’s atmosphere was lost to space over time due to a combination of chemical and physical processes. And in the past three years, the Sun’s activity has increased and decreased, giving MAVEN the opportunity to observe how Mars’ atmospheric loss has risen and fallen accordingly.
Because of this, David Brain – a professor at the Laboratory for Atmospheric and Space Physics (LASP) at the CU Boulder is also a MAVEN co-investigator – and his colleagues began to think about how these insights could be applied to a hypothetical Mars-like planet orbiting around an red dwarf star. These planets include Proxima b (the closest exoplanet to our Solar System) and the seven planet system of TRAPPIST-1.
“The MAVEN mission tells us that Mars lost substantial amounts of its atmosphere over time, changing the planet’s habitability. We can use Mars, a planet that we know a lot about, as a laboratory for studying rocky planets outside our solar system, which we don’t know much about yet.”
To determine if this hypothetical planet could retain its atmosphere over time, the researchers performed some preliminary calculations that assumed that this planet would be positioned near the outer edge of the star’s habitable zone (as Mars is). Since red dwarf’s are dimmer than our Sun, the planet would have to orbit much closer to the star – even closer than Mercury does to our Sun – to be within this zone.
They also considered how a higher proportion of the light emanating from red dwarf stars is in the ultraviolet wavelength. Combined with a close orbit, this means that the hypothetical planet would be bombarded with about 5 times more UV radiation the real Mars gets. This would also mean that the processes responsible for atmospheric loss would be increased for this planet.
Based on data obtained by MAVEN, Brain and colleagues were able to estimate how this increase in radiation would affect Mars’ own atmospheric loss. Based on their calculations, they found that the planet’s atmosphere would lose 3 to 5 times as many charged particles through ion escape, while about 5 to 10 times more neutral particles would be lost through photochemical escape (where UV radiaion breaks apart molecules in the upper atmosphere).
Another form of atmospheric loss would also result, due to the fact that more UV radiation means that more charged particles would be created. This would result in a process called “sputtering”, where energetic particles are accelerated into the atmosphere and collide with other molecules, kicking some out into space and sending others crashing into neighboring particles.
Lastly, they considered how the hypothetical planet might experience about the same amount of thermal escape (aka. Jeans escape) as the real Mars. This process occurs only for lighter molecules such as hydrogen, which Mars loses at the top of its atmosphere through thermal escape. On the “exo-Mars”, however, thermal escape would increase only if the increase in UV radiation were to push more hydrogen into the upper atmosphere.
In conclusion, the researchers determined that orbiting at the edge of the habitable zone of a quiet M-type star (instead of our Sun) could shorten the habitable period for a Mars-like planet by a factor of about 5 to 20. For a more active M-type star, the habitable period could be cut by as much as 1,000 times. In addition, solar storm activity around a red dwarf, which is thousands of times more intense than with our Sun, would also be very limiting.
However, the study is based on how an exo-Mars would fair around and M-type star, which kind of stacks the odds against habitability in advance. When different planets are considered, which possess mitigating factors Mars does not, things become a bit more promising. For instance, a planet that is more geologically active than Mars would be able to replenish its atmosphere at a greater rate.
Other factors include increase mass, which would allow for the planet to hold onto more of its atmosphere, and the presence of a magnetic field to shield it from stellar wind. As Bruce Jakosky, MAVEN’s principal investigator at the University of Colorado (who was not associated with this study), remarked:
“Habitability is one of the biggest topics in astronomy, and these estimates demonstrate one way to leverage what we know about Mars and the Sun to help determine the factors that control whether planets in other systems might be suitable for life.”
In the coming years, astronomers and exoplanet researchers hope to learn more about the planets orbiting nearby red dwarf stars. These efforts are expected to be helped immensely thanks to the deployment of the James Webb Space Telescope, which will be able to conduct more detailed surveys of these star systems using its advanced infrared imaging capabilities.
These studies will allow scientists to place more accurate constraints on exoplanets that orbit red dwarf stars, which will allow for better estimates about their size, mass, and compositions – all of which are crucial to determining potential habitability.
Other panelists that took part in the presentations included Giada Arney and Katherine Garcia-Sage of NASA Goddard Space Flight Center and Stephen Kane of the University of California-Riverside. You can access the press conference materials by going to NASA Goddard Media Studios.
When hunting for potentially habitable exoplanets, one of the most important things astronomers look for is whether or not exoplanet candidates orbit within their star’s habitable zone. This is necessary for liquid water to exist on a planet’s surface, which in turn is a prerequisite for life as we know it. However, in the course of discovering new exoplanets, scientists have become aware of an extreme case known as “water worlds“.
Water worlds are essentially planets that are up to 50% water in mass, resulting in surface oceans that could be hundreds of kilometers deep. According to a new study by a team of astrophysicists from Princeton, the University of Michigan and Harvard, water worlds may not be able to hang on to their water for very long. These findings could be of immense significance when it comes to the hunt for habitable planets in our neck of the cosmos.
This most recent study, titled “The Dehydration of Water Worlds via Atmospheric Losses“, recently appeared in The Astrophysical Journal Letters. Led by Chuanfei Dong from the Department of Astrophysical Sciences at Princeton University, the team conducted computer simulations that took into account what kind of conditions water worlds would be subject to.
This study was motivated largely by the number of exoplanet discoveries have been made around low-mass, M-type (red dwarf) star systems in recent years. These planets have been found to be comparable in size to Earth – which indicated that they were likely terrestrial (i.e. rocky). In addition, many of these planets – such as Proxima b and three planets within the TRAPPIST-1 system – were found to be orbiting within the stars habitable zones.
However, subsequent studies indicated that Proxima b and other rocky planets orbiting red dwarf stars could in fact be water worlds. This was based on mass estimates obtained by astronomical surveys, and the built-in assumptions that such planets were rocky in nature and did not have massive atmospheres. At the same time, numerous studies have been produced that have cast doubt on whether or not these planets would be able to hold onto their water.
Basically, it all comes down to the type of star and the orbital parameters of the planets. While long-lived, red dwarf stars are known for being variable and unstable compared to our Sun, which results in periodic flares up that would strip a planet’s atmosphere over time. On top of that, planets orbiting within a red dwarf’s habitable zone would likely be tidally-locked, meaning one side of the planet would be constantly exposed to the star’s radiation.
Because of this, scientists are focused on determining just how well exoplanets in different types of star systems could hold onto their atmospheres. As Dr. Dong told Universe Today via email:
“It is fair to say that the presence of an atmosphere is perceived as one of the requirements for the habitability of a planet. Having said that, the concept of habitability is a complex one with myriad factors involved. Thus, an atmosphere by itself will not suffice to guarantee habitability, but it can be regarded as an important ingredient for a planet to be habitable.”
To test whether or not a water world would be able to hold onto its atmosphere, the team conducted computer simulations that took into account a variety of possible scenarios. These included the effects of stellar magnetic fields, coronal mass ejections, and atmospheric ionization and ejection for various types of stars – including G-type stars (like our Sun) and M-type stars (like Proxima Centauri and TRAPPIST-1).
With these effects accounted for, Dr. Dong and his colleagues derived a comprehensive model that simulated how long exoplanet atmospheres would last. As he explained it:
“We developed a new multi-fluid magnetohydrodynamic model. The model simulated both the ionosphere and magnetosphere as a whole. Due to the existence of the dipole magnetic field, the stellar wind cannot sweep away the atmosphere directly (like Mars due to the absence of a global dipole magnetic field), instead, the atmospheric ion loss was caused by the polar wind.
“The electrons are less massive than their parent ions, and as a result, are more easily accelerated up to and beyond the escape velocity of the planet. This charge separation between the escaping, low-mass electrons and significantly heavier, positively-charged ions sets up a polarization electric field. That electric field, in turn, acts to pull the positively charged ions along behind the escaping electrons, out of the atmosphere in the polar caps.”
What they found was that their computer simulations were consistent with the current Earth-Sun system. However, in some extreme possibilities – such as exoplanets around M-type stars – the situation is very different and the escape rates could be one thousand times greater or more. The result means that even a water world, if it orbits an red dwarf star, could lose its atmosphere after about a gigayear (Gyr), one billion years.
Considering that life as we know it took around 4.5 billion years to evolve, one billion years is a relatively brief window. In fact, as Dr. Dong explained, these results indicate that planets that orbit M-type stars would be hard pressed to develop life:
“Our results indicate that the ocean planets (orbiting a Sun-like star) will retain their atmospheres much longer than the Gyr timescale as the ion escape rates are far too low, therefore, it allows a longer duration for life to originate on these planets and evolve in terms of complexity. In contrast, for exoplanets orbiting M-dwarfs, they could have their oceans depleted over the Gyr timescale due to the more intense particle and radiation environments that exoplanets experience in close-in habitable zones. If the atmosphere were to be depleted over the timescale less than Gyr, this could prove to be problematic for the origin of life (abiogenesis) on the planet.”
Once again, these results cast doubt on the potential habitability of red dwarf star systems. In the past, researchers have indicated that the longevity of red dwarf stars, which can remain in their main sequence for up to 10 trillion years or longer, make them the best candidate for finding habitable exoplanets. However, the stability of these stars and the way in which they are likely to strip planets of their atmospheres seems to indicate otherwise.
Studies such as this one are therefore highly significant in that they help to address just how long a potentially habitable planet around a red dwarf star could remain potentially habitable. As Dr. Dong indicated:
“Given the importance of atmospheric loss on planetary habitability, there has been a great deal of interest in using telescopes such as the upcoming James Webb Space Telescope (JWST) to determine whether these planets have atmospheres and, if so, what their composition are like. It is expected that the JWST should be capable of characterizing these atmospheres (if present), but quantifying the escape rates accurately requires a much higher degree of precision and may not be feasible in the near-future.”
The study is also significant as far as our understanding of the Solar System and its evolution is concerned. At one time, scientists have ventured that both Earth and Venus may have been water worlds. How they made the transition from being very watery to what they are today – in the case of Venus, dry and hellish; and in the case of Earth, having multiple continents – is an all-important question.
In the future, more detailed surveys are anticipated that could help shed light on these competing theories. When the James Webb Space Telescope (JWST) is deployed in Spring of 2018, it will use its powerful infrared capabilities to study planets around nearby red dwarfs, Proxima b being one of them. What we learn about this and other distant exoplanets will go a long way towards informing our understanding of how our own Solar System evolved as well.
When it comes to searching for worlds that could support extra-terrestrial life, scientists currently rely on the “low-hanging fruit” approach. Since we only know of one set of conditions under which life can thrive – i.e. what we have here on Earth – it makes sense to look for worlds that have these same conditions. These include being located within a star’s habitable zone, having a stable atmosphere, and being able to maintain liquid water on the surface.
Until now, scientists have relied on methods that make it very difficult to detect water vapor in the atmosphere’s of terrestrial planets. But thanks to a new study led by Yuka Fujii of NASA’s Goddard Institute for Space Studies (GISS), that may be about to change. Using a new three-dimensional model that takes into account global circulation patterns, this study also indicates that habitable exoplanets may be more common than we thought.
To put it simply, liquid water is essential to life as we know it. If a planet does not have a warm enough atmosphere to maintain liquid water on its surface for a sufficient amount of time (on the order of billions of years), then it is unlikely that life will be able to emerge and evolve. If a planet is too distant from its star, its surface water will freeze; if it is too close, its surface water will evaporate and be lost to space.
While water has been detected in the atmospheres of exoplanets before, in all cases, the planets were massive gas giants that orbited very closely to their stars. (aka. “Hot Jupiters”). As Fujii and her colleagues state in their study:
“Although H2O signatures have been detected in the atmospheres of hot Jupiters, detecting molecular signatures, including H2O, on temperate terrestrial planets is exceedingly challenging, because of the small planetary radius and the small scale height (due to the lower temperature and presumably larger mean molecular weight).”
When it comes to terrestrial (i.e. rocky) exoplanets, previous studies were forced to rely on one-dimensional models to calculate the presence of water. This consisted of measuring hydrogen loss, where water vapor in the stratosphere is broken down into hydrogen and oxygen from exposure to ultraviolet radiation. By measuring the rate at which hydrogen is lost to space, scientists would estimate the amount of liquid water still present on the surface.
However, as Dr. Fujii and her colleagues explain, such models rely on several assumptions that cannot be addressed, which include the global transport of heat and water vapor vapor, as well as the effects of clouds. Basically, previous models predicted that for water vapor to reach the stratosphere, long-term surface temperatures on these exoplanets would have to be more than 66 °C (150 °F) higher than what we experience here on Earth.
These temperatures could create powerful convective storms on the surface. However, these storms could not be the reason water reaches the stratosphere when it comes to slowly rotating planets entering a moist greenhouse state – where water vapor intensifies heat. Planets that orbit closely to their parent stars are known to either have a slow rotation or to be tidally-locked with their planets, thus making convective storms unlikely.
This occurs quite often for terrestrial planets that are located around low-mass, ultra cool, M-type (red dwarf) stars. For these planets, their proximity to their host star means that it’s gravitational influence will be strong enough to slow down or completely arrest their rotation. When this occurs, thick clouds form on the dayside of the planet, protecting it from much of the star’s light.
The team found that, while this could keep the dayside cool and prevent water vapor from rising, the amount of near-Infrared radiation (NIR) could provide enough heat to cause a planet to enter a moist greenhouse state. This is especially true of M-type and other cool dwarf stars, which are known to produce more in the way of NIR. As this radiation warms the clouds, water vapor will rise into the stratosphere.
To address this, Fujii and her team relied on three-dimensional general circulation models (GCMs) which incorporate atmospheric circulation and climate heterogeneity. For the sake of their model, the team started with a planet that had an Earth-like atmosphere and was entirely covered by oceans. This allowed the team to clearly see how variations in distance from different types of stars would effect conditions on the planets surfaces.
These assumptions allowed the team to clearly see how changing the orbital distance and type of stellar radiation affected the amount of water vapor in the stratosphere. As Dr. Fujii explained in a NASA press release:
“Using a model that more realistically simulates atmospheric conditions, we discovered a new process that controls the habitability of exoplanets and will guide us in identifying candidates for further study… We found an important role for the type of radiation a star emits and the effect it has on the atmospheric circulation of an exoplanet in making the moist greenhouse state.”
In the end, the team’s new model demonstrated that since low-mass star emit the bulk of their light at NIR wavelengths, a moist greenhouse state will result for planets orbiting closely to them. This would result in conditions on their surfaces that comparable to what Earth experiences in the tropics, where conditions are hot and moist, instead of hot and dry.
What’s more, their model indicated that NIR-driven processes increased moisture in the stratosphere gradually, to the point that exoplanets orbiting closer to their stars could remain habitable. This new approach to assessing potential habitability will allow astronomers to simulate circulation of planetary atmospheres and the special features of that circulation, which is something one-dimensional models cannot do.
In the future, the team plans to assess how variations in planetary characteristics -such as gravity, size, atmospheric composition, and surface pressure – could affect water vapor circulation and habitability. This will, along with their 3-dimensional model that takes planetary circulation patterns into account, allow astronomers to determine the potential habitability of distant planets with greater accuracy. As Anthony Del Genio indicated:
“As long as we know the temperature of the star, we can estimate whether planets close to their stars have the potential to be in the moist greenhouse state. Current technology will be pushed to the limit to detect small amounts of water vapor in an exoplanet’s atmosphere. If there is enough water to be detected, it probably means that planet is in the moist greenhouse state.”
Beyond offering astronomers a more comprehensive method for determining exoplanet habitability, this study is also good news for exoplanet-hunters hoping to find habitable planets around M-type stars. Low-mass, ultra-cool, M-type stars are the most common star in the Universe, accounting for roughly 75% of all stars in the Milky Way. Knowing that they could support habitable exoplanets greatly increases the odds of find one.
In addition, this study is VERY good news given the recent spate of research that has cast serious doubt on the ability of M-type stars to host habitable planets. This research was conducted in response to the many terrestrial planets that have been discovered around nearby red dwarfs in recent years. What they revealed was that, in general, red dwarf stars experience too much flare and could strip their respective planets of their atmospheres.
These include the 7-planet TRAPPIST-1 system (three of which are located in the star’s habitable zone) and the closest exoplanet to the Solar System, Proxima b. The sheer number of Earth-like planets discovered around M-type stars, coupled with this class of star’s natural longevity, has led many in the astrophysical community to venture that red dwarf stars might be the most likely place to find habitable exoplanets.
With this latest study, which indicates that these planets could be habitable after all, it would seem that the ball is effectively back in their court!
Ultraviolet light is what you might call a controversial type of radiation. On the one hand, overexposure can lead to sunburn, an increased risk of skin cancer, and damage to a person’s eyesight and immune system. On the other hand, it also has some tremendous health benefits, which includes promoting stress relief and stimulating the body’s natural production of vitamin D, seratonin, and melanin.
And according to a new study from a team from Harvard University and the Harvard-Smithsonian Center for Astrophysics (CfA), ultraviolet radiation may even have played a critical role in the emergence of life here on Earth. As such, determining how much UV radiation is produced by other types of stars could be one of the keys to finding evidence of life any planets that orbit them.
Recent studies have indicated that UV radiation may be necessary for the formation of ribonucleic acid (RNA), which is necessary for all forms of life as we know it. And given the rate at which rocky planets have been discovered around red dwarf stars of late (exampled include Proxima b, LHS 1140b, and the seven planets of the TRAPPIST-1 system), how much UV radiation red dwarfs give off could be central to determining exoplanet habitability.
“It would be like having a pile of wood and kindling and wanting to light a fire, but not having a match. Our research shows that the right amount of UV light might be one of the matches that gets life as we know it to ignite.”
For the sake of their study, the team created radiative transfer models of red dwarf stars. They then sought to determine if the UV environment on prebiotic Earth-analog planets which orbited them would be sufficient to stimulate the photoprocesses that would lead to the formation of RNA. From this, they calculated that planets orbiting M-dwarf stars would have access to 100–1000 times less bioactive UV radiation than a young Earth.
As a result, the chemistry that depends on UV light to turn chemical elements and prebiotic conditions into biological organisms would likely shut down. Alternately, the team estimated that even if this chemistry was able to proceed under a diminished level of UV radiation, it would operate at a much slower rate than it did on Earth billions of years ago.
As Robin Wordsworth – an assistant professor at the Harvard School of Engineering and Applied Science and a co-author on the study – explained, this is not necessarily bad news as far as questions of habitability go. “It may be a matter of finding the sweet spot,” he said. “There needs to be enough ultraviolet light to trigger the formation of life, but not so much that it erodes and removes the planet’s atmosphere.”
Previous studies have shown that even calm red dwarfs experience dramatic flares that periodically bombard their planets with bursts UV energy. While this was considered to be something hazardous, which could strip orbiting planets of their atmospheres and irradiate life, it is possible that such flares could compensate for the lower levels of UV being steadily produced by the star.
This news also comes on the heels of a study that indicated how the outer planets of the TRAPPIST-1 system (including the three located within its habitable zone) might still have plenty of water of their surfaces. Here too, the key was UV radiation, where the team responsible for the study monitored the TRAPPIST-1 planets for signs of hydrogen loss from their atmospheres (a sign of photodissociation).
Compared to higher-mass stars that have shorter life spans, red dwarf stars are likely to remain in their main sequence for as long as six to twelve trillion years. Hence, red dwarf stars would certainly be around long enough to accommodate even a vastly decelerated rate of organic evolution. In this respect, this latest study might even be considered a possible resolution for the Fermi Paradox – Where are all the aliens? They’re still evolving!
But as Dimitar Sasselov – the Phillips Professor of Astronomy at Harvard, the Director of the Origins of Life Initiative and a co-author on the paper – indicated, there are still many unanswered questions:
“We still have a lot of work to do in the laboratory and elsewhere to determine how factors, including UV, play into the question of life. Also, we need to determine whether life can form at much lower UV levels than we experience here on Earth.”
As always, scientists are forced to work with a limited frame of reference when it comes to assessing the habitability of other planets. To our knowledge, life exists on only on planet (i.e. Earth), which naturally influences our understanding of where and under what conditions life can thrive. And despite ongoing research, the question of how life emerged on Earth is still something of a mystery.
If life should be found on a planet orbiting a red dwarf, or in extreme environments we thought were uninhabitable, it would suggest that life can emerge and evolve in conditions that are very different from those of Earth. In the coming years, next-generation missions like the James Webb Space Telescope are the Giant Magellan Telescope are expected to reveal more about distant stars and their systems of planets.
The payoff of this research is likely to include new insights into where life can emerge and the conditions under which it can thrive.
Studies of low-mass, ultra-cool and ultra-dim red dwarf stars have turned up a wealth of extra-solar planets lately. These include the discoveries of a rocky planet orbiting the closest star to the Solar System (Proxima b) and a seven-planet system just 40 light years away (TRAPPIST-1). In the past few years, astronomers have also detected candidates orbiting the stars Gliese 581, Innes Star, Kepler 42, Gliese 832, Gliese 667, Gliese 3293, and others.
The majority of these planets have been terrestrial (i.e. rocky) in nature, and many were found to orbit within their star’s habitable zone (aka. “goldilocks zone”). However the question whether or not these planets are tidally-locked, where one face is constantly facing towards their star has been an ongoing one. And according to a new study from the University of Washington, tidally-locked planets may be more common than previously thought.
The study – which is available online under the title “Tidal Locking of Habitable Exoplanets” – was led by Rory Barnes, an assistant professor of astronomy and astrobiology at the University of Washington. Also a theorist with the Virtual Planetary Laboratory, his research is focused on the formation and evolution of planets that orbit in and around the “habitable zones” of low-mass stars.
For modern astronomers, tidal-locking is a well-understood phenomena. It occurs as a result of their being no net transfer of angular momentum between an astronomical body and the body it orbits. In other words, the orbiting body’s orbital period matches its rotational period, ensuring that the same side of this body is always facing towards the planet or star it orbits.
Consider Earth’s only satellite – the Moon. In addition to taking 27.32 days to orbit Earth, the Moon also takes 27.32 days to rotate once on its axis. This is why the Moon always presents the same “face” towards Earth, while the side that faces away is known as the “dark side”. Astronomers believe this became the case after a Mars-sized object (Theia) collided with Earth some 4.5 billion years ago.
Aside from throwing up debris that would eventually form the Moon, the impact is believed to have struck Earth at such an angle that it gave our planet an initial rotation period of 12 hours. In the past, researchers have used this 12-hour estimation of Earth’s rotation as a model for exoplanet behavior. However, prior to Barnes’ study, no systematic examinations had ever been conducted.
Looking to address this, Barnes chose to address the long-held assumption that only smaller, dimmer stars could host orbiting planets that were tidally locked. He also considered other possibilities, which included slower or faster initial rotation periods as well as variations in planet size and the eccentricity of their orbits. What he found was that previous studies had been rather limited and only made allowances for one outcome.
“Planetary formation models, however, suggest the initial rotation of a planet could be much larger than several hours, perhaps even several weeks. And so when you explore that range, what you find is that there’s a possibility for a lot more exoplanets to be tidally locked. For example, if Earth formed with no Moon and with an initial ‘day’ that was four days long, one model predicts Earth would be tidally locked to the sun by now.”
From this, he found that potentially-habitable planets that orbit very late M-type (red dwarf) stars are likely to attain highly-circular orbits about 1 billion years after their formation. Furthermore, he found that for the majority, their orbits would be synchronized with their rotation – aka. they would be tidally-locked with their star. These findings could have significant implications for the study of exoplanets formation and evolution, not to mention habitability.
In the past, tidally-locked planets were thought to have extremes climates, thus eliminating any possibility of life. As an example, the planet Mercury experiences a 3:2 spin-orbit resonance, meaning it rotates three times on its axis for every two orbits it completes of the Sun. Because of this, a single day on Mercury lasts as long as 176 Earth days, and temperature range from 100 (-173 °C; -279 °F) to 700 K (427 °C; 800 °F) between the day side and the night side.
For a tidally-locked planets that orbit close to their stars, it was believed this situation would be even worse. However, astronomers have since come to speculate that the presence of an atmosphere around these planets could redistribute temperature across their surfaces. Unlike Mercury, which has no atmosphere and experiences no wind, these planets could maintain temperatures that would be supportive to life.
In any case, this study is one of many that is putting constraints on recent exoplanet discoveries. This is especially important given that the detection and study of extra-solar planets is still in its infancy, and limited to largely indirect methods. In other words, astronomers make estimates of a planet’s size, composition and whether or not it has an atmosphere based on transits and the influence these planets have on their stars.
In the coming years, next-generations missions like the James Web Space Telescope and the Transiting Exoplanet Survey Satellites (TESS) are expected to improve this situation drastically. In addition to conducting more detailed observations on existing discoveries, they are also expected to uncover a wealth of more planets. If Barnes’ study is correct, the majority of those found will be tidally-locked, but that need not mean they are uninhabitable.
In February of 2017, a team of European astronomers announced the discovery of a seven-planet system orbiting the nearby star TRAPPIST-1. Aside from the fact that all seven planets were rocky, there was the added bonus of three of them orbiting within TRAPPIST-1’s habitable zone. As such, multiple studies have been conducted that have sought to determine whether or not any planets in the system could be habitable.
When it comes to habitability studies, one of the key factors to consider is the age of the star system. Basically, young stars have a tendency to flare up and release harmful bursts of radiation while planets that orbit older stars have been subject to radiation for longer periods of time. Thanks to a new study by a pair of astronomers, it is now known that the TRAPPIST-1 system is twice as old as the Solar System.
Back in of August of 2016, the existence of an Earth-like planet right next door to our Solar System was confirmed. To make matters even more exciting, it was confirmed that this planet orbits within its star’s habitable zone too. Since that time, astronomers and exoplanet-hunters have been busy trying to determine all they can about this rocky planet, known as Proxima b. Foremost on everyone’s mind has been just how likely it is to be habitable.
However, numerous studies have emerged since that time that indicate that Proxima b, given the fact that it orbits an M-type (red dwarf), would have a hard time supporting life. This was certainly the conclusion reached in a new study led by researchers from NASA’s Goddard Space Flight Center. As they showed, a planet like Proxima b would not be able to retain an Earth-like atmosphere for very long.
Red dwarf stars are the most common in the Universe, accounting for an estimated 70% of stars in our galaxy alone. As such, astronomers are naturally interested in knowing just how likely they are at supporting habitable planets. And given the distance between our Solar System and Proxima Centauri – 4.246 light years – Proxima b is considered ideal for studying the habitability of red dwarf star systems.
On top of all that, the fact that Proxima b is believed to be similar in size and composition to Earth makes it an especially appealing target for research. The study was led by Dr. Katherine Garcia-Sage of NASA’s Goddard Space Flight Center and the Catholic University of America in Washington, DC. As she told Universe Today via email:
“So far, not many Earth-sized exoplanets have been found orbiting in the temperate zone of their star. That doesn’t mean they don’t exist – larger planets are found more often, because they are easier to detect – but Proxima b is of interest because it’s not only Earth-sized and at the right distance from its star, but it’s also orbiting the closest star to our Solar System.”
For the sake of determining the likelihood of Proxima b being habitable, the research team sought to address the chief concerns facing rocky planets that orbit red dwarf stars. These include the planet’s distance from their stars, the variability of red dwarfs, and the presence (or absence) of magnetic fields. Distance is of particular importance, since habitable zones (aka. temperate zones) around red dwarfs are much closer and tighter.
“Red dwarfs are cooler than our own Sun, so the temperate zone is closer to the star than Earth is to the Sun,” said Dr. Garcia-Sage. “But these stars may be very magnetically active, and being so close to a magnetically active star means that these planets are in a very different space environment than what the Earth experiences. At those distances from the star, the ultraviolet and x-ray radiation may be quite large. The stellar wind may be stronger. There could be stellar flares and energetic particles from the star that ionize and heat the upper atmosphere.”
In addition, red dwarf stars are known for being unstable and variable in nature when compared to our Sun. As such, planets orbiting in close proximity would have to contend with flare ups and intense solar wind, which could gradually strip away their atmospheres. This raises another important aspect of exoplanet habitability research, which is the presence of magnetic fields.
To put it simply, Earth’s atmosphere is protected by a magnetic field that is driven by a dynamo effect in its outer core. This “magnetosphere” has prevented solar wind from stripping our atmosphere away, thus giving life a chance to emerge and evolve. In contrast, Mars lost its magnetosphere roughly 4.2 billion years ago, which led to its atmosphere being depleted and its surface becoming the cold, desiccated place it is today.
To test Proxima b’s potential habitability and capacity to retain liquid surface water, the team therefore assumed the presence of an Earth-like atmosphere and a magnetic field around the planet. They then accounted for the enhanced radiation coming from Proxima b. This was provided by the Harvard Smithsonian Center for Astrophysics (CfA), where researchers determined the ultraviolet and x-ray spectrum of Proxima Centauri for this project.
From all of this, they constructed models that began to calculate the rate of atmospheric loss, using Earth’s atmosphere as a template. As Dr. Garcia-Sage explained:
“At Earth, the upper atmosphere is ionized and heated by ultraviolet and x-ray radiation from the Sun. Some of these ions and electrons escape from the upper atmosphere at the north and south poles. We have a model that calculates how fast the upper atmosphere is lost through these processes (it’s not very fast at Earth)… We then used that radiation as the input for our model and calculated a range of possible escape rates for Proxima Centauri b, based on varying levels of magnetic activity.”
What they found was not very encouraging. In essence, Proxima b would not be able to retain an Earth-like atmosphere when subjected to Proxima Centauri’s intense radiation, even with the presence of a magnetic field. This means that unless Proxima b has had a very different kind of atmospheric history than Earth, it is most likely a lifeless ball of rock.
However, as Dr. Garcia-Sage put it, there are other factors to consider which their study simply can’t account for:
“We found that atmospheric losses are much stronger than they are at Earth, and the for high levels of magnetic activity that we expect at Proxima b, the escape rate was fast enough that an entire Earth-like atmosphere could be lost to space. That doesn’t take into account other things like volcanic activity or impacts with comets that might be able to replenish the atmosphere, but it does mean that when we’re trying to understand what processes shaped the atmosphere of Proxima b, we have to take into account the magnetic activity of the star. And understanding the atmosphere is an important part of understanding whether liquid water could exist on the surface of the planet and whether life could have evolved.”
So it’s not all bad news, but it doesn’t inspire a lot of confidence either. Unless Proxima b is a volcanically-active planet and subject to a lot of cometary impacts, it is not likely be temperate, water-bearing world. Most likely, its climate will be analogous to Mars – cold, dry, and with water existing mostly in the form of ice. And as for indigenous life emerging there, that’s not too likely either.
These and other recent studies have painted a rather bleak picture about the habitability of red dwarf star systems. Given that these are the most common types of stars in the known Universe, the statistical likelihood of finding a habitable planet beyond our Solar System appears to be dropping. Not exactly good news at all for those hoping that life will be found out there within their lifetimes!
But it is important to remember that what we can say definitely at this point about extra-solar planets is limited. In the coming years and decades, next-generation missions – like the James Webb Space Telescope (JWST) and the Transiting Exoplanet Survey Satellite (TESS) – are sure to paint a more detailed picture. In the meantime, there’s still plenty of stars in the Universe, even if most of them are extremely far away!
Ever since scientists confirmed the existence of seven terrestrial planets orbiting TRAPPIST-1, this system has been a focal point of interest for astronomers. Given its proximity to Earth (just 39.5 light-years light-years away), and the fact that three of its planets orbit within the star’s “Goldilocks Zone“, this system has been an ideal location for learning more about the potential habitability of red dwarf stars systems.
This is especially important since the majority of stars in our galaxy are red dwarfs (aka. M-type dwarf stars). Unfortunately, not all of the research has been reassuring. For example, two recent studies performed by two separate teams from Harvard-Smithsonian Center for Astrophysics (CfA) indicate that the odds finding life in this system are less likely than generally thought.
The first study, titled “Physical Constraints on the Likelihood of Life on Exoplanets“, sought to address how radiation and stellar wind would affect any planets located within TRAPPIST-1s habitable zone. Towards this end, the study’s authors – Professors Manasvi Lingam and Avi Loeb – constructed a model that considered how certain factors would affect conditions on the surface of these planets.
This model took into account how the planets distance from their star would affect surface temperatures and atmospheric loss, and how this might affect the changes life would have to emerge over time. As Dr. Loeb told Universe Today via email:
“We considered the erosion of the atmosphere of the planets due to the stellar wind and the role of temperature on ecological and evolutionary processes. The habitable zone around the faint dwarf star TRAPPIST-1 is several tens of times closer in than for the Sun, hence the pressure of the stellar wind is several orders of magnitude higher than on Earth. Since life as we know it requires liquid water and liquid water requires an atmosphere, it is less likely that life exists around TRAPPIST-1 than in the solar system.”
Essentially, Dr. Lingam and Dr, Loeb found that planets in the TRAPPIST-1 system would be barraged by UV radiation with an intensity far greater than that experienced by Earth. This is a well-known hazard when it comes to red dwarf stars, which are variable and unstable when compared to our own Sun. They concluded that compared to Earth, the chances of complex life existing on planets within TRAPPIST-1’s habitable zone were less than 1%.
“We showed that Earth-sized exoplanets in the habitable zone around M-dwarfs display much lower prospects of being habitable relative to Earth, owing to the higher incident ultraviolet fluxes and closer distances to the host star,” said Loeb. “This applies to the recently discovered exoplanets in the vicinity of the Sun, Proxima b (the nearest star four light years away) and TRAPPIST-1 (ten times farther), which we find to be several orders of magnitude smaller than that of Earth.”
Essentially, the team found that TRAPPIST-1, like our Sun, sends streams of charged particles outwards into space – i.e. stellar wind. Within the Solar System, this wind exerts force on the planets and can have the effect of stripping away their atmospheres. Whereas Earth’s atmosphere is protected by its magnetic field, planets like Mars are not – hence why it lost the majority of its atmosphere to space over the course of hundreds of million of years.
As the research team found, when it comes to TRAPPIST-1, this stream exerts a force on its planets that is between 1,000 to 100,000 times greater than what Earth experiences from solar wind. Furthermore, they argue that TRAPPIST-1’s magnetic field is likely connected to the magnetic fields of the planets that orbit around it, which would allow particles from the star to directly flow onto the planet’s atmosphere.
In other words, if TRAPPIST-1’s planets do have magnetic fields, they will not afford them any protection. So if the flow of charged particles is strong enough, it could strip these planets’ atmospheres away, thus rendering them uninhabitable. As Garraffo put it:
“The Earth’s magnetic field acts like a shield against the potentially damaging effects of the solar wind. If Earth were much closer to the Sun and subjected to the onslaught of particles like the TRAPPIST-1 star delivers, our planetary shield would fail pretty quickly.”
As you can imagine, this is not exactly good news for those who were hoping that the TRAPPIST-1 system would hold the first evidence of life beyond our Solar System. Between the fact that its planets orbit a star that emits varying degrees of intense radiation, and the proximity its seven planets have to the star itself, the odds of life emerging on any planet within it’s “habitable zone” are not significant.
The findings of the second study are particularly significant in light of other recent studies. In the past, Prof. Loeb and a team from the University of Chicago have both addressed the possibility that the TRAPPIST-1 system’s seven planets – which are relatively close together – are well-suited to lithopanspermia. In short, they determined that given their close proximity to each other, bacteria could be transferred from one planet to the next via asteroids.
But if the proximity of these planets also means that they are unlikely to retain their atmospheres in the face of stellar wind, the likelihood of lithopanspermia may be a moot point. However, before anyone gets to thinking that this is bad news as far as the hunt for life goes, it is important to note that this study does not rule out the possibility of life emerging in all red dwarf star systems.
As Dr. Jeremy Drake – a senior astrophysicist from the CfA and one of Garraffo’s co-authors – indicated, the results of their study simply mean that we need to cast a wide net when searching for life in the Universe. “We’re definitely not saying people should give up searching for life around red dwarf stars,” he said. “But our work and the work of our colleagues shows we should also target as many stars as possible that are more like the Sun.”
And as Dr. Loeb himself has indicated in the past, red dwarf stars are still the most statistically-likely place to find habitable worlds:
“By surveying the habitability of the Universe throughout cosmic history from the birth of the first stars 30 million years after the Big Bang to the death of the last stars in 10 trillion years, one reaches the conclusion that unless habitability around low-mass stars is suppressed, life is most likely to exist near red dwarf stars like Proxima Centauri or TRAPPIST-1 trillions of years from now.”
If there is one takeaway from these studies, it is that the existence of life within a star system does not simply require planets orbiting within the circumstellar habitable zones. The nature of the stars themselves and the role played by solar wind and magnetic fields also have to be taken into account, since they can mean the difference between a life-bearing planet and a sterile ball of rock!