New research from the Hubble Space Telescope and the ESO’s Very Large Telescope is dampening some of the enthusiasm in the search for life. Observations by both ‘scopes suggest that the raw materials necessary for life may be rare in solar systems centered around red dwarfs.
And if the raw materials aren’t there, it may mean that many of the exoplanets we’ve found in the habitable zones of other stars just aren’t habitable after-all.
In the course of searching for extra-solar planets, some very interesting finds have been made. Some of them have even occurred within our own galactic neighborhood. Just two years ago, astronomers from the Red Dots and CARMENES campaigns announced the discovery of Proxima b, a rocky planet that orbits within the habitable zone of our nearest stellar neighbor – Proxima Centauri.
This rocky world, which may be habitable, remains the closest exoplanet ever discovered to our Solar System. A few days ago (on Nov. 14th), Red Dots and CARMENES announced another find: a rocky planet orbiting Barnard’s star, which is just 6 light years from Earth. This planet, Barnard’s Star b, is now the second closest exoplanet to our Solar System, and the closest planet to orbit a single star.
In of August of 2016, astronomers from the European Southern Observatory (ESO) confirmed the existence of an Earth-like planet around Proxima Centauri – the closest star to our Solar System. In addition, they confirmed that this planet (Proxima b) orbited within its star’s habitable zone. Since that time, multiple studies have been conducted to determine if Proxima b could in fact be habitable.
Unfortunately, most of this research has not been very encouraging. For instance, many studies have indicated that Proxima b’s sun experiences too much flare activity for the planet to sustain an atmosphere and liquid water on its surface. However, in a new NASA-led study, a team of scientists has investigated various climate scenarios that indicate that Proxima b could still have enough water to support life.
Since its discovery was announced in August of 2016, Proxima b has been an endless source of wonder and the target of many scientific studies. In addition to being the closest extra-solar planet to our Solar System, this terrestrial planet also orbits within Proxima Centauri’s circumstellar habitable zone (aka. “Goldilocks Zone”). As a result, scientists have naturally sought to determine if this planet could actually be home to extra-terrestial life.
Many of these studies have been focused on whether or not Proxima b could retain an atmosphere and liquid water on its surface in light of the fact that it orbits an M-type (red dwarf) star. Unfortunately, many of these studies have revealed that this is not likely due to flare activity. According to a new study by an international team of scientists, Proxima Centauri released a superflare that was so powerful, it would have been lethal to any life as we know it.
As they indicate in their study, solar flare activity would be one of the greatest potential threats to planetary habitability in a system like Proxima Centauri. As they explain:
“[W]hile ozone in an Earth-like planet’s atmosphere can shield the planet from the intense UV flux associated with a single superflare, the atmospheric ozone recovery time after a superflare is on the order of years. A sufficiently high flare rate can therefore permanently prevent the formation of a protective ozone layer, leading to UV radiation levels on the surface which are beyond what some of the hardiest-known organisms can survive.”
In addition stellar flares, quiescent X-ray emissions and UV flux from a red dwarf star can would be capable of stripping planetary atmospheres over the course of several billion years. And while multiple studies have been conducted that have explored low- and moderate-energy flare events on Proxima, only one high-energy event has even been observed.
As the team indicates in their study, the March 2016 superflare was the first to be observered from Proxima Centauri, and was rather powerful:
“In March 2016 the Evryscope detected the first-known Proxima superflare. The superflare had a bolometric energy of 10^33.5 erg, ~10× larger than any previously-detected flare from Proxima, and 30×larger than any optically measured Proxima flare. The event briefly increased Proxima’s visible-light emission by a factor of 38× averaged over the Evryscope’s 2-minute cadence, or ~68× at the cadence of the human eye. Although no M-dwarfs are usually visible to the naked-eye, Proxima briefly became a magnitude-6.8 star during this superflare, visible to dark-site naked-eye observers.”
The superflare coincided with the three-month Pale Red Dot campaign, which was responsible for first revealing the existence of Proxima b. While monitoring the star with the HARPS spectrograph – which is part of the 3.6 m telescope at the ESO’s La Silla Observatory in Chile – the campaign team also obtaining spectra on March 18th, 08:59 UT (just 27 minutes after the flare peaked at 08:32 UT).
The team also noted that over the last two years, the Evryscope has recorded 23 other large Proxima flares, ranging in energy from 10^30.6 erg to 10^32.4 erg. Coupled with rates of a single superflare detection, they predict that at least five superflares occur each year. They then combined this data with the high-resolution HARPS spectroscopy to constrain the superflare’s UV spectrum and any associated coronal mass ejections.
The team then used the HARPS spectra and the Evryscope flare rates to create a model to determine what effects this star would have on a nitrogen-oxygen atmosphere. This included how long the planet’s protective ozone layer would be able to withstand the blasts, and what effect regular exposure to radiation would have on terrestrial organisms.
“[T]he repeated flaring is sufficient to reduce the ozone of an Earth-like atmosphere by 90% within five years. We estimate complete depletion occurs within several hundred kyr. The UV light produced by the Evryscope superflare therefore reached the surface with ~100× the intensity required to kill simple UV-hardy microorganisms, suggesting that life would struggle to survive in the areas of Proxima b exposed to these flares.”
Essentially, this and other studies have concluded that any planets orbiting Proxima Centauri would not be habitable for very long, and likely became lifeless balls of rock a long time ago. But beyond our closest neighboring star system, this study also has implications for other M-type star systems. As they explain, red dwarf stars are the most common in our galaxy – roughly 75% of the population – and two-thirds of these stars experience active flare activity.
As such, measuring the impact that superflares have on these worlds will be a necessary component to determining whether or not exoplanets found by future missions are habitable. Looking ahead, the team hopes to use the Evryscope to examine other star systems, particularly those that are targets for the upcoming Transiting Exoplanet Survey Satellite (TESS) mission.
“Beyond Proxima, Evryscope has already performed similar long-term high-cadence monitoring of every other Southern TESS planet-search target, and will therefore be able to measure the habitability impact of stellar activity for all Southern planetsearch-target M-dwarfs,” they write. “In conjunction with coronal-mass-ejection searches from long- wavelength radio arrays like the [Long Wavelength Array], the Evryscope will constrain the long-term atmospheric effects of this extreme stellar activity.”
For those who hoped that humanity might find evidence of extra-terrestrial life in their lifetimes, this latest study is certainly a letdown. It’s also disappointing considering that in addition to being the most common type of star in the Universe, some research indicates that red dwarf stars may be the most likely place to find terrestrial planets. However, even if two-thirds of these stars are active, that still leaves us with billions of possibilities.
It is also important to note that these studies help ensure that we can determine which exoplanets are potentially habitable with greater accuracy. In the end, that will be the most important factor when it comes time to decide which of these systems we might try to explore directly. And if this news has got you down, just remember the worlds of the immortal Carl Sagan:
“The universe is a pretty big place. If it’s just us, seems like an awful waste of space.”
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.
In February of 2017, NASA scientists announced the existence of seven terrestrial (i.e. rocky) planets within the TRAPPIST-1 star system. Since that time, the system has been the focal point of intense research to determine whether or not any of these planets could be habitable. At the same time, astronomers have been wondering if all of the system’s planets are actually accounted for.
For instance, could this system have gas giants lurking in its outer reaches, as many other systems with rocky planets (for instance, ours) do? That was the question that a team of scientists, led by researchers from the Carnegie Institute of Science, sought to address in a recent study. According to their findings, TRAPPIST-1 may be orbited by gas giants at a much-greater distance than its seven rocky planets.
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.