Does habitable zone take tidally locked bodies into account?

Does habitable zone take tidally locked bodies into account?

Imagine a tidally locked planet orbiting a red dwarf, where habitability is not situated close to terminator zone, but on a "small" cap normal to incident starlight (zenith) with permanent average +15 Celsius over the year, variations through year depending on orbit's eccentricity.

If there was in our solar system one (maybe not spherical) tidally locked body orbiting some close to circular orbit around the sun, how far from the sun would it be to meet those +15C average zenith cap temperature? Does it make habitable zone larger than it is?

Does habitable zone take tidally locked bodies into account? - Astronomy

While we know that yellow dwarf stars like our sun are capable of supporting life, there’s another star type that is a prime hunting ground for potentially habitable exoplanets.

M-dwarf stars are extremely common in the Universe and a typical one is relatively small and dim, making it easy for astronomers to detect a passing planet. If orbiting planets huddle close enough to an M-dwarf, in theory they could fall within the habitable zone where surface liquid water, and thus life, is possible.

Artist’s impression of a M dwarf star surrounded by planets.

Yet, an M-dwarf’s habitable zone is poorly understood. It is not clear how far away the planets need to be orbiting from the star for surface liquid water to be possible. Because planets in this range orbit so close to an M-dwarf they may be tidally locked, said Ravi Kumar Kopparapu, an assistant research scientist at the NASA Goddard Space Flight Center in Maryland.

“They’re always facing the same side of the star, just like the Moon does around the Earth,” he said.

This position could potentially stabilize the climate for life, but on the other hand, the side facing the star might be very hot while the side facing away is very cold.

Kopparapu said a better understanding of habitable zones around M-dwarfs needs to come quickly because of upcoming missions in exoplanet research. NASA’s Transiting Exoplanet Survey Satellite ( TESS ) is scheduled to launch next year to observe more planets and to serve as a guidepost for NASA’s James Webb Space Telescope in 2018. James Webb can provide higher resolution data that can tell us about what kind of gases are present in the atmosphere of a planet orbiting an M-dwarf star. This data can bring out details such as a planet’s temperature, revealing the potential for the right conditions to exist for life.

A paper based on Kopparapu’s research, “The inner edge of the habitable zone for synchronously rotating planets around low-mass stars using general circulation models,” was recently published in The Astrophysical Journal.

Transiting Exoplanet Survey Satellite (TESS) will look for planets around close, bright stars.

Refined climate model

Kopparapu previously came up with a one-dimensional climate model of habitable zones around all stars, including M-dwarfs. This model did not take into account tidal locking around the star, but instead found two types of habitability limits. The first is a moist greenhouse limit, where a planet that is close enough to a star would have water vapor dominated atmosphere rendering the planet uninhabitable due to high temperatures. The second limit is a runaway greenhouse effect, where the energy from the star is so intense (higher than the energy absorbed in the moist greenhouse limit) that it causes oceans to evaporate.

The new model, explained in the most recent paper, simulates a water-rich planet (roughly Earth’s size). Previous research using this same model found that the climate of such a planet would depend on atmospheric circulation, which in turn depends on the Coriolis force (created by the planet’s rotation). In slowly rotating planets near the inner edge of the habitable zone, the Coriolis force is weak, the clouds stay fairly stationary, and the planet has lower temperatures than predicted by one-dimensional models because the clouds reflect the light of the star. This results in the habitable zone being much closer to the star to take into account this cooling effect.

Kopparapu’s team was able to reproduce those results, but there was one key difference.

“Our habitable zones are a little farther away from the star than what they get from their model because the planets get warmer faster,” he said. “This means the width of the habitable zone around M-dwarf stars is not as wide as previously thought.”

The estimated habitable zones of A stars, G stars and M stars are compared in this diagram. More refinement is needed to better understand the size of these zones.

Kopparapu said his team took into account Kepler’s third law of motion, which is a fundamental part of physics and astronomy. The law, simply put, says the time it takes a planet to orbit its star is roughly proportional to the size of its orbit. The older study assumed a constant orbital period of 60 days at the inner edge of the habitable zone, but the orbital and rotational period did not match what Kepler’s law predicts.

More study is planned to refine the size of these habitable zones. Kopparapu has funding from NASA’s Habitable Worlds Program. NASA’s Habitable Worlds Program includes elements of the Astrobiology Program, the Mars Exploration Program, and the Outer Planets Program. His proposal will update our understanding of how water vapor can absorb incoming radiation from the star. This can influence the warmth of a planet and further reduce the width of the habitable zone.

Funding sources for the work include the NASA Astrobiology Institute’s Virtual Planetary Laboratory, the NASA Planetary Atmospheres Program, the Center for Exoplanets and Habitable Worlds, the National Science Foundation and Penn State Astrobiology Research Center.

Habitable Zones Near Red Dwarf Stars Smaller than Previously Thought

A newly published study reveals that habitable zones near red dwarf stars may be smaller than previously thought. The researchers believe that the exoplanets orbiting the red dwarfs may be mostly covered by icy crusts, save for somewhat lobster-shaped oceans on their day sides.

Alien planets circling the most common stars in the universe may often have strange lobster-shaped oceans on their surfaces, researchers in China now say.

These findings suggest the habitable zones where life as we know it might dwell around these stars is smaller than previously thought, scientists added.

The most common type of star in the universe is the red dwarf. These stars, also known as M dwarfs, are small and faint, about one-fifth as massive as the sun and up to 50 times dimmer. They make up to 70 percent of the stars in the cosmos, a vast number that potentially makes them valuable places to look for extraterrestrial life. Indeed, recent findings from NASA&rsquos Kepler space observatory reveal that at least half of these stars host rocky planets that are one-half to four times the mass of Earth.

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

What the day side of a tidally locked exoplanet orbiting a red dwarf might look like, given atmospheric carbon dioxide levels similar to modern-day Earth. On the top frame, white represents ice while blue represents open water on the bottom frame, colors represent surface air temperatures. The top image in each frame represents a computer model that does not take ocean heat flow into account the bottom image in each frame does take such heat flow into account. Credit: Yongyun Hu

The habitable zones around red dwarfs are close to such stars because of how dim they are, often closer than the distance Mercury orbits the sun. This makes it relatively easy for astronomers to detect worlds in a red dwarf&rsquos habitable zone since the orbits of these exoplanets are small, they complete their orbits quickly and often, and scientists can in principle readily detect the way these worlds dim the light of these stars by passing in front of them.

When a planet orbits a star very closely, the star&rsquos gravitational pull can force the world to become &ldquotidally locked&rdquo to it. When a planet is tidally locked to its star, it will always show the same side to its star just as the moon always shows the same side to Earth, so that the planet will have one permanent day side and one permanent night side.

The uneven heating that tidally-locked planets experience could make them profoundly different from Earth. For instance, prior research speculated the dark sides of tidally-locked planets would become so cold that their atmospheres would freeze, leaving even the sunlit sides with little air. However, more recent models of atmospheric circulation have shown that winds on these planets would cause heat to flow enough to avoid this atmospheric collapse for terrestrial planets in the habitable zones around red dwarfs.

Recently astrobiologists suggested that tidally-locked exoplanets around red dwarfs might resemble giant eyeballs. Their night sides would be covered with icy, frozen shells, while their day sides would host giant oceans of liquid water constantly basking in the warmth of their stars.

However, planetary scientists Yongyun Hu and Jun Yang at Peking University in Beijing noted that past research into how tidally-locked exoplanets around red dwarfs might look did not consider the way in which heat might circulate within the oceans of such worlds.

Now these researchers find that when computer models account for the role that ocean heat transport can play, tidally locked exoplanets around red dwarfs might not resemble giant eyeballs at all. Instead, they might be mostly covered by icy crusts, save for somewhat lobster-shaped oceans on their day sides.

The simulations involved a computer model that comprehensively accounted for both atmospheric circulation and oceanic circulation and how they might influence one another on a planet orbiting a red dwarf about 5,660 degrees F (3,125 degrees C). The model used the same planetary parameters as that of an exoplanet called Gliese 581g located about 20 light years away, which may be the first known potentially habitable alien world &mdash this world is a &ldquosuper-Earth,&rdquo a rocky planet 1.5 times wider than Earth. The researchers assumed the planet would have a global ocean about 13,125 feet (4,000 meters) deep, the average depth of Earth&rsquos oceans.

Because of the way ocean heat flows, the amount of open water on the day sides of these planets might be substantially larger than before thought. It also efficiently warms their night sides, preventing atmospheric collapse. If the starlight is bright enough, or if there are sufficiently high levels of heat-trapping greenhouse gases such as carbon dioxide, ocean heat flow could actually lead to a complete lack of ice on the planet&rsquos surface, even on the night side.

&ldquoThis is the first work to demonstrate how a dynamic ocean can change the climate state of super-Earth-like exoplanets,&rdquo Hu said.

Assuming this tidally locked exoplanet has roughly as much carbon dioxide in its atmosphere as modern-day Earth, it would have an ocean on its day side surrounded by ice. However, the computer model suggested this ocean would not be perfectly round like an eyeball&rsquos iris, but would rather have a vaguely lobster-like shape, with two &ldquoclaws&rdquo on either side of the equator and a long &ldquotail&rdquo along the equator.

&ldquoThe lobster shape is created by ocean currents,&rdquo Hu said.

It might be possible to see if these planets really do possess lobster-shaped oceans in the future, although current telescopes cannot do it, he added.

The claws are caused by ocean currents that rotate like cyclones, while the long tail is the result of something called a Kelvin wave, &ldquowhich can be simply understood as a result of an equatorial ocean jet stream,&rdquo Hu said.

Because the jet stream is eastward, it transports warm water from the day side to the night side, and transports cold water from the night side to the day side, explaining why the ocean is not symmetrical when one looks from east to west.

Although ocean heat flow suggests tidally-locked exoplanets orbiting red dwarfs might have more habitable open water on their surfaces than before thought, it could also mean that red dwarf habitable zones are actually narrower than previously suggested.

The scientists found that a dynamic ocean would be more likely to force planets to enter what is known as a runaway greenhouse effect. In this scenario, a planet absorbs enough heat from its star to cause too much water to vaporize. Steam is a greenhouse gas, and so that planet would trap even more heat from its star, causing still more water to vaporize. Eventually, all the water on that world boils away, rendering them uninhabitable &mdash a phenomenon that could explain why Venus is so desolately dry today.

The researchers suggest this increased vulnerability to the runaway greenhouse effect means the inner edge of red dwarf habitable zones may be slightly farther away from these stars than previously thought, although Hu noted they were uncertain exactly how much smaller the habitable zones would be.

Astrobiologist Jim Kasting at Pennsylvania State University at University Park, Penn., who did not take part in this study, noted he would like to see the researchers calculate where the edges of habitable zones might be for tidally-locked exoplanets around red dwarfs. He would also like to see the scientists model the effects that a greater range of brightnesses from the stars would have on these planets.

Publication: Yongyun Hu and Jun Yang, &ldquoRole of ocean heat transport in climates of tidally locked exoplanets around M dwarf stars,&rdquo PNAS, 2014, vol. 111, no. 2, 629&ndash634: doi: 10.1073/pnas.1315215111

Gliese 581: one planet might indeed be habitable

More than 10 years after the discovery of the first extrasolar planet, astronomers have now discovered more than 250 of these planets. Until a few years ago, most of the newly discovered exoplanets were Jupiter-mass, probably gaseous, planets. Recently, astronomers have announced the discovery of several planets that are potentially much smaller, with a minimum mass lower than 10 Earth masses: the now so-called super-Earths.

In April, a European team announced in Astronomy & Astrophysics the discovery of two new planets orbiting the M star Gliese 581 (a red dwarf), with masses of at least 5 and 8 Earth masses. Given their distance to their parent star, these new planets (now known as Gliese 581c and Gliese 581d) were the first ever possible candidates for habitable planets.

Contrary to Jupiter-like giant planets that are mainly gaseous, terrestrial planets are expected to be extremely diverse: some will be dry and airless, while others will have much more water and gases than the Earth. Only the next generation of telescopes will allow us to tell what these new worlds and their atmospheres are made of and to search for possible indications of life on these planets. However, theoretical investigations are possible today and can be a great help in identifying targets for these future observations.

In this framework, Astronomy & Astrophysics now publishes two theoretical studies of the Gliese 581 planetary system. Two international teams, one led by Franck Selsis and the other by Werner von Bloh, investigate the possible habitability of these two super-Earths from two different points of view. To do so, they estimate the boundaries of the habitable zone around Gliese 581, that is, how close and how far from this star liquid water can exist on the surface of a planet.

F. Selsis and his colleagues compute the properties of a planet’s atmosphere at various distances from the star. If the planet is too close to the star, the water reservoir is vaporized, so Earth-like life forms cannot exist. The outer boundary corresponds to the distance where gaseous CO2 is then unable to produce the strong greenhouse effect required to warm a planetary surface above the freezing point of water. The major uncertainty for the precise location of the habitable zone boundaries comes from clouds that cannot currently be modeled in detail. These limitations also occur when one looks at the Sun’s case: climate studies indicate that the inner boundary is located somewhere between 0.7 and 0.9 AU, and the outer limit is between 1.7 and 2.4 AU. Figure 1 illustrates the Sun’s habitable zone boundaries, compared to the case for Gliese 581 as computed both by Selsis and von Bloh.

W. von Bloh and his colleagues study a narrower region of the habitable zone where Earth-like photosynthesis is possible. This photosynthetic biomass production depends on the atmospheric CO2 concentration, as much as on the presence of liquid water on the planet. Using a thermal evolution model for the super-Earths, they have computed the sources of atmospheric CO2 (released through ridges and volcanoes) and its sinks (the consumption of gaseous CO2 by weathering processes). The main aspect of their model is the persistent balance (that exists on Earth) between the sink of CO2 in the atmosphere-ocean system and its release through plate-tectonics. In this model, the ability to sustain a photosynthetic biosphere strongly depends on the age of the planet, because a planet that is too old might not be active anymore, that is, would not release enough gaseous CO2. In this case, the planet would no longer be habitable. To compute the boundaries of the habitable zone as illustrated by Figure 1, von Bloh assumed a CO2 level of 10 bars.

Figure 1 illustrates the boundary of the habitable zone as computed using both models and, for comparison, the boundary of the Sun’s habitable zone. Both teams found that, while Gliese 581 c is too close to the star to be habitable, the planet Gliese 581 d might be habitable. However, the environmental conditions on planet d might be too harsh to allow complex life to appear. Planet d is tidally locked, like the Moon in our Earth-Moon system, meaning that one side of the planet is permanently dark. Thus, strong winds may be caused by the temperature difference between the day and night sides of the planet. Since the planet is located at the outer edge of the habitable zone, life forms would have to grow with reduced stellar irradiation and a very peculiar climate.

Figure 1 also illustrates that the distance of planets c and d to the central star has strong variations due to the eccentricity of their orbits. In addition, being close to the star, their orbital periods are short: 12.9 days for planet c and 83.6 days for planet d. Figure 1 shows that planet d might temporarily leave and re-enter the habitable zone during its journey. However, even under these strange conditions, it might still be habitable if its atmosphere is dense enough. In any case, habitable conditions on planet d should be very different from what we encounter on Earth.

Last but not least, the possible habitability of one of these planets is particularly interesting because of the central star, which is a red dwarf, M-type star. About 75% of all stars in our Galaxy are M stars. They are long-lived (potentially tens of billion years), stable, and burn hydrogen. M stars have long been considered as poor candidates for harboring habitable planets: first because planets located in the habitable zone of M stars are tidally locked, with a permanent dark side, where the atmosphere is likely to condense irreversibly. Second, M stars have an intense magnetic activity associated with violent flares and high X and extreme UV fluxes, during their early stage that might erode planetary atmospheres. Theoretical studies have recently shown that the environment of M stars might not prevent these planets from harboring life. M stars have then become very interesting for astronomers because habitable planets orbiting them are easier to detect by using the radial-velocity and transit techniques than are the habitable planets around Sun-like stars.

Both studies definitely confirm that Gliese 581c and Gliese 581d will be prime targets for the future ESA/NASA space mission Darwin/Terrestrial Planet Finder (TPF), dedicated to the search for life on Earth-like planets. These space observatories will make it possible to determine the properties of their atmospheres.

A third paper on the Gliese 581 planetary system has recently been accepted for publication in Astronomy & Astrophysics. In this paper, H. Beust and his team study the dynamical stability of the Gliese 581 planetary system. Such studies are very interesting in the framework of the potential habitability of these planets because the long-term evolution of the planetary orbits may regulate the climate of these planets. Mutual gravitational perturbations between different planets are present in any planetary system with more than one planet. In our solar system, under the influence of the other planets, the Earth's orbit periodically evolves from purely circular to slightly eccentric.

This is actually enough to trigger the alternance of warm and glacial eras. More drastic orbital changes could well have prevented the development of life. Beust and his colleagues computed the orbits of the Gliese 581 system over 100 Myr and find that the system appears dynamically stable, showing periodic orbital changes that are comparable to those of the Earth. The climate on the planets is expected to be stable, so it at least does not prevent life from developing, although it does not prove it happened either.

Which habitable zones are the best to actually search for life?

Credit: NASA

Looking to the future, NASA and other space agencies have high hopes for the field of extra-solar planet research. In the past decade, the number of known exoplanets has reached just shy of 4000, and many more are expected to be found once next-generation telescopes are put into service. And with so many exoplanets to study, research goals have slowly shifted away from the process of discovery and toward characterization.

Unfortunately, scientists are still plagued by the fact that what we consider to be a "habitable zone" is subject to a lot of assumptions. Addressing this, an international team of researchers recently published a paper in which they indicated how future exoplanet surveys could look beyond Earth-analog examples as indications of habitability, and adopt a more comprehensive approach.

The paper, titled "Habitable Zone predictions and how to test them," recently appeared online and was submitted as a white paper to the Astro 2020 Decadal Survey on Astronomy and Astrophysics. The team behind it was led by Ramses M. Ramirez, a researcher with the Earth-Life Science Institute (ELSI) and the Space Science Institute (SSI), who was joined by co-authors and co-signers from 23 universities and institutions.

The purpose of the decadal survey is to consider previous progress in various fields of research and to set priorities for the coming decade. As such, the survey provides crucial guidance to NASA, the National Space Foundation (NSF), and the Department of Energy as they plan their astronomy and astrophysics research goals for the future.

At present, many of these goals focus on the study of exoplanets, which will benefit in the coming years from the deployment of next-generation telescopes like the James Webb Space Telescope (JWST) and the Wide-Field Infrared Space Telescope (WFIRST), as well as ground-based observatories like the Extremely Large Telescope (ELT), the Thirty Meter Telescope, and the Giant Magellan Telescope (GMT).

One of the overriding priorities of exoplanet research is looking for planets where extra-terrestrial life could exist. In this respect, scientists designate planets as being "potentially habitable" (and therefore worthy of follow-up observations) based on whether or not they orbit within their stars' habitable zones (HZ). For this reason, it is prudent to take a look at what goes in to defining a HZ.

As Ramirez and his colleagues indicated in their paper, one of the major issues with exoplanet habitability is the number of assumptions made. To break it down, most definitions of HZs assume the presence of water on the surface since this is the only solvent currently known to host life. These same definitions assume that life requires a rocky planet with tectonic activity orbiting a suitably bright and warm star.

However, recent research has cast doubt on many of these assumptions. This includes studies that indicate that atmospheric oxygen does not automatically mean the presence of life – especially if that oxygen is the result of chemical dissociation and not photosynthesis. Other research has shown that the presence of oxygen gas during the early periods of a planet's evolution could prevent the rise of basic life forms.

Also, there have been recent studies showing that plate tectonics may not be necessary for life to emerge, and that so-called "water worlds" may not be able to support life (but still could). On top of all that, you have theoretical work that suggests that life could evolve in seas of methane or ammonia on other celestial bodies.

The key example here is Saturn's moon Titan, which boasts an environment that is rich in prebiotic conditions and organic chemistry, which some scientists think could support exotic lifeforms. In the end, scientists search for known biomarkers like water and carbon dioxide because they are associated with life on Earth, the only known example of a life-bearing planet.

The “Goldilocks” zone around a star is where a planet is neither too hot nor too cold to support liquid water. Credit: Petigura/UC Berkeley, Howard/UH-Manoa, Marcy/UC Berkeley.

But as Ramirez explained to Universe Today via email, this mindset (where Earth analogues are considered suitable for life) is still fraught with problems:

"The classical habitable zone definition is flawed because its construction is mainly based on Earth-centric climatological arguments that may or may not be applicable to other potentially habitable planets. For instance, it assumes that multi-bar CO2 atmospheres can be supported on potentially habitable planets near the habitable zone outer edge. However, such high CO2 levels are toxic to Earth plants and animals, and thus without a better understanding of the limits of life, we do not know how reasonable this assumption is.

"The classical HZ also assumes that CO2 and H2O are the key greenhouse gases sustaining potentially habitable planets, but several studies in recent years have developed alternative HZ definitions using different combinations of greenhouse gases, including those that, although relatively minor on Earth, could be important for other potentially habitable planets."

In a previous study, Dr. Ramirez showed that the presence of methane and hydrogen gas could also cause global warning, and thus extend the classical HZ somewhat. This came just a year after he and Lisa Kaltenegger (an associate professor with the Carl Sagan Institute at Cornell University) produced a study showing that volcanic activity (which releases hydrogen gas into the atmosphere) could also extend a star's HZ.

Luckily, researchers will have the opportunity to test these definitions, thanks to the deployment of next-generation telescopes. Not only will scientists be able to test some of the longstanding assumptions on which HZs are based, they will able to compare different interpretations. According to Dr. Ramirez, a good example is provided by the levels of CO2 gas that are dependent on a planet's distance from its star:

Exoplanet Kepler 62f would need an atmosphere rich in carbon dioxide for water to be in liquid form. Credit: NASA Ames/JPL-Caltech/T. Pyle

"Next-generation telescopes could test the habitable zone by searching for a predicted increase in atmospheric CO2 pressure the farther away that potentially habitable planets are from their stars. This would also test whether the carbonate-silicate cycle, which is what many believe has kept our planet habitable for much of its history, is a universal process or not."

In this process, silicate rocks are converted to carbon rocks through weathering and erosion, while carbon rocks are converted to silicate rocks through volcanic and geological activity. This cycle ensures the long-term stability of Earth's atmosphere by keeping CO2 levels consistent over time. It also illustrates that water and plate tectonics are essential to life as we know it.

However, this type of cycle can only exist on planets that have land, which effectively rules out "water worlds." These exoplanets – which may be common around M-type (red dwarf) stars – are believed to be up to 50 percent water by mass. With this amount of water on their surfaces, "water worlds" are likely to have dense layers of ice at their core-mantle boundary, thus preventing hydrothermal activity.

But as noted already, there is some research that indicates that these planets could still be habitable. While the abundance of water would prevent the absorption of carbon dioxide by rocks and suppress volcanic activity, simulations have shown that these planets could still cycle carbon between the atmosphere and the ocean, thus keeping the climate stable.

If these types of ocean worlds exist, says Dr. Ramirez, scientists could detect them through their lower planetary density and high pressure atmosphere. And then there is the matter of various greenhouse gases, which are not always an indication of warmer planetary atmospheres, depending on the type of star.

Artist’s depiction of a waterworld. A new study suggests that Earth is in a minority when it comes to planets, and that most habitable planets may be greater than 90% ocean. Credit: David A. Aguilar (CfA)

"Although methane warms our planet, we found that methane actually cools the surfaces of habitable zone planets orbiting red dwarf stars," he said. "If that is the case, high atmospheric methane amounts on such planets could mean frozen conditions that are perhaps unsuitable for hosting life. We will be able to observe this in planetary spectra."

Speaking of red dwarfs, the debate rages on as to whether or not planets that orbit these stars would be capable of maintaining an atmosphere. In the past few years, multiple discoveries have suggested that rocky, tidally locked planets are common around red dwarf stars, and that they orbit within their stars' respective HZs.

However, subsequent research has reinforced the theory that the instability of red dwarf stars would likely result in solar flares that would strip the atmospheres of any planets orbiting them. Lastly, Ramirez and his colleagues raise the possibility that habitable planets could be found orbiting main sequence type-A stars, which have until recently been considered unlikely candidates. Main sequence type-A stars Sirius A, Altair, and Vega were thought to be too bright and hot to be habitable.

Ramirez says, "I am also interested in finding out if life exists on habitable zone planets orbiting A-stars. There has not been a lot of published assessments of A-star planetary habitability, but some next-generation architectures plan to observe them. We will soon learn more about the suitability of A-stars for life."

Ultimately, studies like this one, which question the definition of the "habitable zone," will come in handy when next-generation missions commence science operations. With higher-resolution, more sensitive instruments, they will be able to test and validate many of the predictions that have been made by scientists.

Artist’s concept of Earth-like exoplanets, which (according to new research) need to strike the careful balance between water and landmass. Credit: NASA

These tests will also confirm whether or not life could exist out there only as we know it, or also beyond the parameters that we consider to be "Earth-like." Ramirez says the study that he and his colleagues conducted also highlights just how important it is that we continue to invest in advanced telescope technology:

"Our paper also stresses the importance of a continued investment in advanced telescope technology. We need to be able to find and characterize as many habitable zone planets as possible if we wish to maximize our chances of finding life. However, I also hope that our paper inspires people to dream beyond just the next 10 years so. I really believe that there will eventually be missions that will be far more capable than anything that we are currently designing. Our current efforts are just the beginning of a much more committed endeavor for our species."

The 2020 Decadal Survey meeting is being hosted jointly by Board of Physics and Astronomy and the Space Studies Board of the National Academy of Sciences, and will be followed by a report to be released roughly two years from now.


The habitable zone (HZ) is the circumstellar region where a planet can sustain surface liquid water. Searching for terrestrial planets in the HZ of nearby stars is the stated goal of ongoing and planned extrasolar planet surveys. Previous estimates of the inner edge of the HZ were based on one-dimensional radiative-convective models. The most serious limitation of these models is the inability to predict cloud behavior. Here we use global climate models with sophisticated cloud schemes to show that due to a stabilizing cloud feedback, tidally locked planets can be habitable at twice the stellar flux found by previous studies. This dramatically expands the HZ and roughly doubles the frequency of habitable planets orbiting red dwarf stars. At high stellar flux, strong convection produces thick water clouds near the substellar location that greatly increase the planetary albedo and reduce surface temperatures. Higher insolation produces stronger substellar convection and therefore higher albedo, making this phenomenon a stabilizing climate feedback. Substellar clouds also effectively block outgoing radiation from the surface, reducing or even completely reversing the thermal emission contrast between dayside and nightside. The presence of substellar water clouds and the resulting clement surface conditions will therefore be detectable with the Jamesmore » Webb Space Telescope. « less

Habitable Conditions

Students use an interactive model to explore the zone of liquid water possibility around different star types and determine the characteristics of stars and planets that are most favorable for habitability.

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1. Engage students in a discussion about conditions that are necessary for life.

Introduce the idea that there is a variety of living things on Earth that live in a wide variety of environments—from organisms that live in hot springs to organisms that live in the Antarctic ice. Ask:

  • What conditions are necessary for life to exist on a planet? (Scientists think that liquid water is necessary for life. They also think that a habitable planet should have an atmosphere.) 
  • Do you think that a planet should be exactly like Earth to be able to support life? (Student answers will vary. Students should recognize that there is a wide variety of conditions on Earth that have life, so there could be planets that are very different from Earth that still have some habitable regions. Humans would not survive at the bottom of the ocean, but there are many organisms that thrive there. Some organisms use sulfur compounds for respiration, instead of oxygen humans would die without oxygen.) 

Tell students that scientists look for certain characteristics of planets to assess their potential habitability.

2. Discuss the role of uncertainty in the scientific process.

Introduce students to the concept of uncertainty in the scientific process. Explain that science is a process of learning how the world works and that scientists do not know the “right” answers when they start to investigate a question. Tell students that they can see examples of scientists' uncertainty in determining whether or not the data collected from telescopes show the presence of planets.

Show the Kepler Planet Candidates graph from the NASA Exoplanet Archive. Tell students that the red dots indicate potential planets the Kepler telescope has detected and the blue dots indicate the planets the Kepler telescope detected and have been confirmed by other means. Ask:

  • Why do you think there are more red dots than blue dots (more potential planets than confirmed planets)? (The telescope may detect planets that are not there. The technology may not be good enough to tell the difference between a planet and some other phenomenon.)
  • Why do scientists need to independently confirm the presence of planets? (Scientists need to check the accuracy of the telescope's predictions of a planet. If the telescope shows a planet and the scientists confirm that it is a planet, then the scientists can spend more time trying to learn about the planet.)

Let students know that they will be asked questions about the certainty of their predictions and that they should think about what scientific and model-based data are available as they assess their certainty with their answers. Encourage students to discuss the scientific evidence with each other to better assess their level of certainty with their predictions.

3. Introduce and discuss the use of computational models. 

Explain the concept of computational models, and give students an example of a computational model that they may have seen, such as forecasting the weather. Project the NOAA Weather Forecast Model, which provides a good example of a computational model. Tell students that scientists use planetary models to predict the motion and apparent brightness of stars if planets are present and to predict the habitability of planets. Explain that there are many different types of models and that they will be using simple models of planetary motion in this activity.

Provide students with the link to the Habitable Conditions interactive. Divide students into groups of two or three, with two being the ideal grouping to allow students to share computer workstations. Tell students they will be working through a series of pages of data with questions related to the data. Ask students to work through the activity in their groups, discussing and responding to questions as they go. 

NOTE: You can access the Answer Key for students' questions—and save students' data for online grading—through a free registration on the  High-Adventure Science portal page .

5. Discuss the issues.

After students have completed the activity, bring the groups back together and lead a discussion focusing on the following questions:

  • Why does the habitable zone change around different star types? (The habitable zone, roughly defined as the area where liquid water can exist on a planet's surface, is different around different star types because different stars have different temperatures. Around a cool star, the habitable zone will be closer to the star. Around a hot star, the habitable zone will be farther from the star.)
  • Show the model on page 4 of the activity.

According to this model, what characteristics make a planet suitable for life? (A planet should be rocky, orbit entirely in the liquid water zone, and orbit a M, K, G, or F class star.)

  • Do you think that a planet needs to orbit completely within the zone of liquid water possibility to be able to have life? (Student answers will vary. Students should note that the zone of liquid water possibility means that water can be liquid on the planet's surface. There can still be liquid water below the surface that could support living things. In this case, with liquid water under the surface, life could exist on a planet that orbits in-and-out of the zone of liquid water possibility.)

Informal Assessment

1. Check students' comprehension by asking students the following questions:

  • Why is the habitable zone around an F-class star different than the habitable zone around an M-class star?
  • What type of planet is most suitable for life: a rocky planet, or a gaseous planet?
  • Which type of planet and solar system would you want to explore further for life?

2. Use the answer key to check students' answers on embedded assessments.

Why are the planets of red dwarf stars so commonly tidally locked?

I am an aspiring science fiction writer, and I was wondering if it would be possible to have a planet which was in the habitable zone, was very roughly earth-sized, and wasn't tidally locked.

If anyone has an explanation or thoughts on this, please share!

The habitable zone of red dwarfs is very, very small, and very close to the parent star. Because it's so close it generally means that any body in that zone is going to be tidally locked.

There is a pretty neat thing that can happen with a tidally locked planet in orbit of a red dwarf they can be habitable along the terminator. So you have a blazing hot side facing the star, a freezing cold side facing away from the star, but along the terminator it could be comfortable and would allow for some pretty unique scenery.

Dumb question here as a followup:

Let's assume a planet the size of the Earth were the one in question. How wide would this habitable zone be? Would a city be able to fit into its width?

Because of current detection methods available, you're going to have a bias toward objects close to the star, which corresponds to the temperature range of liquid water at 1 earth atmosphere of pressure with red dwarf stars. That proximity promotes relatively fast tidal synchronization between objects in orbit around each other. For an exact explanation of the mechanics of tidal locking, try this excerpt from "Solar System Dynamics":

Iɽ guess that the main problem with habitability around red dwarfs is the instability of their magnetic fields, not surface temperatures. My understanding of current models is that any atmosphere will equalize temperature differences- I don't know if that holds true for terrestrial style planets. Red dwarf stars tend to have frequent outbursts, similar to sun-like stars in the T-Tauri phase of development. Aside from blasting away the atmosphere, the ionizing radiation is going to make it tough for biological macromolecules to stay around long enough for life to get a foothold before tidal locking takes place. If you're looking for some more background on the subject, explained in layman's terms, Iɽ suggest reading "Rare Earth" by Peter Ward.

The planets of red dwarf stars in the habitable zone are likely to be tidally locked because they are much closer to their sun than a planet in the habitable zone of a bright star like ours. Tidal force is inversely proportional to distance cubed, so getting closer to a massive body greatly increases tidal forces. That's why the Moon, despite being much less massive than the Sun, raises higher tides on the Earth than the Sun does.

if it would be possible to have a planet which was in the habitable zone, was very roughly earth-sized, and wasn't tidally locked.

It's possible, yes, but the planet would need to be fairly young. If it was only, say, a billion years old, there might not have been enough time for the planet to become tidally locked. But such a young planet is less likely to have multicellular life than an older planet.

If you want a planet around a red dwarf star that has complex life and is not tidally locked, Iɽ recommend:

Make the star on the massive end of the red dwarf spectrum. Like M0 or M1. The more massive the star, the brighter it is, and the farther out its habitable zone which means smaller tidal forces.

Make it as young as possible. Something like 3 billion years is probably old enough to reasonably have complex life if it developed a bit faster than Earth's did, yet still young enough that the planet might not be locked yet.

Put it toward the outer edge of the habitable zone. Earth is near the inner edge of its habitable zone, and could be maybe 40% farther away and still be habitable. Earthlike planets regulate their temperature with CO2, so such a planet would have a higher percentage of CO2 in its atmosphere and more even temperatures thanks to the greenhouse effect (less difference between daytime and nighttime, and between tropical and polar zones than Earth).

The above would also help to avoid potentially an even bigger hazard for habitable red dwarf planets than tidal locking: the young red dwarf. Unlike brighter stars like the Sun, red dwarfs start out brighter for their first few hundred million years. This means that an older planet that's currently in the habitable zone may have been interior of the habitable zone when it was young, and may have lost its water. Such a planet may have an atmosphere rich in abiotic oxygen from its lost ocean, but no water or life. Making the star a bright red dwarf and putting the planet toward the outer edge of the habitable zone minimizes the chance of that happening.

Do Atmospheres Spin Worlds to Habitability?

By: Shannon Hall January 22, 2015 0

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The best place to look for nearby Earth-size planets are around the smallest, coolest stars. New research shows that any exoplanets tightly circling their stars might have a better chance of being habitable than previously thought.

This artist's conception shows the planet GJ 581g, which has a 37-day orbit right in the middle of the star's habitable zone and is only three to four times the mass of Earth, circling its red dwarf star.
Lynette Cook

In the hunt for Earth 2.0, many astronomers are pointing their telescopes toward smaller, cooler stars. Not only are these so-called red dwarfs the most abundant type of star in the galaxy, but they’re also roughly one-quarter the Sun’s mass, bringing their habitable zones closer in and making it easier to spot any Goldilocks planets, either via their gravitational tugs on the star or when the planet passes in front of the star from our perspective.

There’s just one catch. A planet that orbits close enough to its dim star to be in the habitable zone could become tidally locked. Just as our planet sees one side of the Moon at all times, red dwarfs will only see one side of a close-in planet at all times. So one side of the planet will likely see continuous day and the other perpetual darkness, potentially destabilizing chemical exchanges between the atmosphere and surface or even (in extreme instances) causing the atmosphere to collapse. In short, tidally locked planets are likely uninhabitable.

New research, however, suggests not all is lost for tightly orbiting planets. Jérémy Leconte (University of Toronto and Pierre Simon Laplace Institute, France) and his colleagues think that an atmosphere’s effect might be strong enough to break any tidal locking, allowing the planet to rotate freely and exhibit a day-night cycle similar to Earth’s.

Leconte and his colleagues created a three-dimensional climate model (similar to those used in analyzing climate change on Earth) to predict the effect of a given planet’s atmosphere on the speed of its rotation.

It all goes back to the amount of starlight able to penetrate the planet’s atmosphere and reach the surface. Any temperature differences at the surface — between day and night and between the equator and the poles — drive winds. Those winds constantly push against the planet by running into mountains or creating waves on the ocean. Such friction then influences the rotation rate of the planet, helping to speed it up or slow it down.

“While gravitational tides and their associated torques tend to tidally lock the planet, thermal tides, produced by the star heating the atmosphere of the planet, tend to oppose the gravitational tides, and prevent the planets from becoming tidally locked,” says coauthor Norm Murray (University of Toronto).

Astronomers have long seen this effect on the planet Venus, where the atmosphere’s influence is so powerful that it forces the planet out of synchronous rotation into a slow retrograde rotation: to a Venusian, the Sun rises in the west and sets in the east. But Venus’s large atmosphere weighs in about 90 times heavier than our own, and planetary scientists didn’t think thinner atmospheres like Earth’s could throw their weight around as effectively.

Leconte’s simulations show that thinner atmospheres actually have a larger rotational effect on their planets. With less scattered sunlight, extra heat reaches the deepest atmospheric layer and creates stronger winds. If Venus were to have an atmosphere like Earth’s, it would spin 10 times faster. This is radically different from previous research, which suggested that it would spin 50 times slower.

An unlocked planet should have strong atmospheric mixing and relatively stable temperatures. “This greatly increases the chances for atmospheric stability — and, hence, for life — on any of these bodies, provided they are Earth-like in terms of mass, water content, and maybe their atmospheres,” says exoplanet expert René Heller (McMaster University, Canada).

In addition, it avoids many problems created on tidally locked planets, Take the cold trap, for example. “Liquid water on the sunny side tends to evaporate, and is thence transported by winds (driven by the temperature gradient) to the dark side, where it precipitates as snow and forms large-scale ice sheets,” says Murray. “Since the back side never sees the light of the host star, the ice sheets may well be permanent.” Eventually all the liquid water would move to the dark side, making life impossible.

Although the researchers show that a large number of known terrestrial exoplanets should have a day-night cycle, potentially rendering them habitable, the duration of their days could last between a few weeks and a few months. So Heller cautions that these planets would still be far from Earth-like, with only a few days per year.

Hopefully the theoretical results don’t remain in the observational dark for too long. Astronomers can determine the temperature of exoplanets when they pass behind their host stars. But it won’t be easy to do this for Earth-size worlds. Leconte thinks it might be within reach of the James Webb Space Telescope (slated to launch in 2018) if there is a particularly favorable planet to observe. If not, astronomers might have to wait for the European Extremely Large Telescope, whose first light is tentatively scheduled for 2024.

Learn more about the weird weather on alien worlds in a special report featured in our May 2014 issue.

In the Zone: How Scientists Search for Habitable Planets

This artist's concept shows a Super Venus planet on the left, and a Super Earth on the right. Researchers use a concept known as the habitable zone to distinguish between these two types of planets, which exist beyond our solar system.

Super Venuses and Super Earths are similar to Venus and Earth, respectively, but larger in mass. Similar to our solar system's worlds, a Super Venus would most likely be a dry, toxic wasteland while a Super Earth might host oceans.

The habitable zone is the region around a star where liquid water -- an essential ingredient for life as we know it -- can pool on the surface. Earth lies within the habitable zone around our star. A recent study looked at the planet Kepler-69c, discovered by NASA's Kepler mission and originally thought to lie in its star's habitable zone. The planet is 1.7 times the size of Earth and lies 2,700 light-years away. The analysis showed that this planet lies just outside the inner edge of the zone, making it more of a Super Venus than a Super Earth. That means the planet is not a tropical paradise, but more likely scorching hot, with volcanic eruptions.

The search for planets as small as Earth, situated in the habitable zones of stars like our sun, is ongoing.