# Do we know of any tidally-locked planets with atmospheres?

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If not, is there any reason such a planet couldn't exist?

I ask only because that planet would literally have a twilight zone and I want to know that that's a thing somewhere.

As we haven't measured the atmosphere of many rocky planets yet, there aren't many examples. But I found 55 Cancri e: It is a super-earth with very close tidally locked orbit and an atmosphere that has been measured by Hubble. A year on the exoplanet lasts for only 18 hours and temperatures on the surface are thought to reach around 2000 degrees Celsius. The detected atmosphere seems to consists mainly of hydrogen and helium. So you do have a constant Twilight Zone, just a rather hot one.

Sources: - Beeing tidally locked - Detection of the Atmosphere

You asked if we know of any such planets, this raises the question of how sure do you want to be. There are definitely a variety of planets located close to their stars which have atmospheres (hot Jupiters, hot Neptunes, various volatile-rich super-Earths), that have short tidal spindown timescales. These planets are generally assumed to be in a 1:1 spin-orbit state for this reason. As far as I'm aware none of these planets has had their rotation measured, so while it may be reasonable to assume they are tidally locked, we don't know for sure.

This then brings up a substantial caveat: atmospheres can act against tidal locking. The usual example given here is Venus, where the atmospheric "thermal tides" make the current rotation state more favourable than the tidally-locked one. A hypothetical airless Venus might well have ended up tidally locked. You don't necessarily need an atmosphere as massive as the one Venus has: according to Leconte et al. (2015) "Asynchronous rotation of Earth-mass planets in the habitable zone of lower-mass stars" an Earth-like atmosphere would be sufficient to prevent an Earth-mass planet from being tidally locked around stars down to about 0.5-0.7 solar masses.

Even below this limit, planet-planet interactions may act to keep planets out of the synchronous state. According to Vinson et al. (2019) "The Chaotic Nature of TRAPPIST-1 Planetary Spin States" several of the TRAPPIST-1 planets may be in non-synchronous rotation states as a result of this, despite being subject to strong tides from the host star.

Thermal tides are also relevant for hot Jupiters and hot Neptunes: again these planets are usually considered to be in synchronous rotation but this is not necessarily the case. For example, see Auclair-Desrotour & Leconte (2018) "Semidiurnal thermal tides in asynchronously rotating hot Jupiters".

So I'd say the answer is a definite maybe. It is likely that the list of known exoplanets does contain some examples of synchronously-rotating planets with atmospheres, unfortunately at the current time we cannot say for sure whether an individual planet is actually in the synchronous rotation state.

## Proxima Centauri and tidally locked planets

Proxima Centauri may have a rocky, earth like planet close to its dim sun. Tidally locked, the sun facing side may have a temperature up to 30 degrees Celsius and a dark side of -30 C. This would make one side temperate with liquid water, and the other frozen like antarctica - cold, but not unbelievably so.

Assuming it holds on to its atmosphere, what sort of weather systems might we find on such a planet, where one side is perpetually day time (with moderate weather) and the other a frozen night?

Taking this further, if I may, would we expect unique adaptations from life originating on such a planet?

## New Clues to Compositions of TRAPPIST-1 Planets

This illustration shows the seven Earth-size planets of TRAPPIST-1. The image does not show the planets’ orbits to scale, but highlights possibilities for how the surfaces of these intriguing worlds might look. Image Credit: NASA/JPL-Caltech

The seven Earth-size planets of TRAPPIST-1 are all mostly made of rock, with some having the potential to hold more water than Earth, according to a new study published in the journal Astronomy and Astrophysics. The planets’ densities, now known much more precisely than before, suggest that some planets could have up to 5 percent of their mass in water — which is 250 times more than the oceans on Earth.

The form that water would take on TRAPPIST-1 planets would depend on the amount of heat they receive from their star, which is a mere 9 percent as massive as our Sun. Planets closest to the star are more likely to host water in the form of atmospheric vapor, while those farther away may have water frozen on their surfaces as ice. TRAPPIST-1e is the rockiest planet of them all, but still is believed to have the potential to host some liquid water.

“We now know more about TRAPPIST-1 than any other planetary system apart from our own,” said Sean Carey, manager of the Spitzer Science Center at Caltech/IPAC in Pasadena, California, and co-author of the new study. “The improved densities in our study dramatically refine our understanding of the nature of these mysterious worlds.”

Since the extent of the system was revealed in February 2017, researchers have been working hard to better characterize these planets and collect more information about them. The new study offers better estimates than ever for the planets’ densities.

This artist’s concept shows what the TRAPPIST-1 planetary system may look like, based on available data about the planets’ diameters, masses and distances from the host star, as of February 2018. Credit: NASA/JPL-Caltech

TRAPPIST-1 is named for the Transiting Planets and Planetesimals Small Telescope (TRAPPIST) in Chile, which discovered two of the seven planets we know of today — announced in 2016. NASA’s Spitzer Space Telescope, in collaboration with ground-based telescopes, confirmed these planets and uncovered the other five in the system.

Since then, NASA’s Kepler space telescope has also observed the TRAPPIST-1 system, and Spitzer began a program of 500 additional hours of TRAPPIST-1 observations, which will conclude in March. This new body of data helped study authors paint a clearer picture of the system than ever before — although there is still much more to learn about TRAPPIST-1.

The TRAPPIST-1 planets huddle so close to one another that a person standing on the surface of one of these worlds would have a spectacular view of the neighboring planets in the sky. Those planets would sometimes appear larger than the Moon looks to an observer on Earth. They may also be tidally locked, meaning the same side of the planet is always facing the star, with each side in perpetual day or night. Although the planets are all closer to their star than Mercury is to the Sun, TRAPPIST-1 is such a cool star, some of its planets could still, in theory, hold liquid water.

In the new study, scientists led by Simon Grimm at the University of Bern in Switzerland created computer models to better simulate the planets based on all available information. For each planet, researchers had to come up with a model based on the newly measured masses, the orbital periods and a variety of other factors — making it an extremely difficult, 󈬓-dimensional problem,” Grimm said. It took most of 2017 to invent new techniques and run simulations to characterize the planets’ compositions.

This chart shows, on the top row, artist concepts of the seven planets of TRAPPIST-1 with their orbital periods, distances from their star, radii, masses, densities and surface gravity as compared to those of Earth. Credit: NASA/JPL-Caltech

What might these planets look like?

It is impossible to know exactly how each planet looks, because they are so far away. In our own solar system, the Moon and Mars have nearly the same density, yet their surfaces appear entirely different.

“Densities, while important clues to the planets’ compositions, do not say anything about habitability. However, our study is an important step forward as we continue to explore whether these planets could support life,” said Brice-Olivier Demory, co-author at the University of Bern.

Based on available data, here are scientists’ best guesses about the appearances of the planets:

TRAPPIST-1b, the innermost planet, is likely to have a rocky core, surrounded by an atmosphere much thicker than Earth’s. TRAPPIST-1c also likely has a rocky interior, but with a thinner atmosphere than planet b. TRAPPIST-1d is the lightest of the planets — about 30 percent the mass of Earth. Scientists are uncertain whether it has a large atmosphere, an ocean or an ice layer — all three of these would give the planet an “envelope” of volatile substances, which would make sense for a planet of its density.

Scientists were surprised that TRAPPIST-1e is the only planet in the system slightly denser than Earth, suggesting it may have a denser iron core than our home planet. Like TRAPPIST-1c, it does not necessarily have a thick atmosphere, ocean or ice layer — making these two planets distinct in the system. It is mysterious why TRAPPIST-1e has a much rockier composition than the rest of the planets. In terms of size, density and the amount of radiation it receives from its star, this is the most similar planet to Earth.

TRAPPIST-1f, g and h are far enough from the host star that water could be frozen as ice across these surfaces. If they have thin atmospheres, they would be unlikely to contain the heavy molecules of Earth, such as carbon dioxide.

“It is interesting that the densest planets are not the ones that are the closest to the star, and that the colder planets cannot harbor thick atmospheres,” said Caroline Dorn, study co-author based at the University of Zurich, Switzerland.

This graph presents known properties of the seven TRAPPIST-1 exoplanets (labeled b thorugh h), showing how they stack up to the inner rocky worlds in our own solar system. Credit: NASA/JPL-Caltech

Scientists are able to calculate the densities of the planets because they happen to be lined up such that when they pass in front of their star, our Earth- and space-based telescopes can detect a dimming of its light. This is called a transit. The amount by which the starlight dims is related to the radius of the planet.

To get the density, scientists take advantage of what are called “transit timing variations.” If there were no other gravitational forces on a transiting planet, it would always cross in front of its host star in the same amount of time — for example, Earth orbits the Sun every 365 days, which is how we define one year. But because the TRAPPIST-1 planets are packed so close together, they change the timing of each other’s “years” ever so slightly. Those variations in orbital timing are used to estimate the planets’ masses. Then, mass and radius are used to calculate density.

All seven planets discovered in orbit around the red dwarf star TRAPPIST-1 could easily fit inside the orbit of Mercury, the innermost planet of our solar system. In fact, the proportions of the TRAPPIST-1 system look more like Jupiter and its moons. Image Credit: NASA/JPL-Caltech

The next step in exploring TRAPPIST-1 will be NASA’s James Webb Space Telescope, which will be able to delve into the question of whether these planets have atmospheres and, if so, what those atmospheres are like. A recent study using NASA’s Hubble Space Telescope found no detection of hydrogen-dominated atmospheres on planets TRAPPIST-1d, e and f — another piece of evidence for rocky composition — although the hydrogen-dominated atmosphere cannot be ruled out for g.

Illustrations of these worlds will change as ongoing scientificinvestigations home in on their properties.

“Our conceptions of what these planets look like today may change dramatically over time,” said Robert Hurt, senior visualization scientist at the Spitzer Science Center. “As we learn more about these planets, the pictures we make will evolve in response to our improved understanding.

## Why do planets/moons rotate?

Do we know of any examples of extrasolar planets that do not rotate, i.e. their day and year are equivalent? What about tidally locked planets? Would life have been able to evolve on Earth if either of these conditions was true for it?

(Sorry for the many questions just curious)

Because an object in motion, stays in motion, unless acted on by an outside force. Rock planets/moons form through collision in accretion disks there is no counter to the momentum of things hitting each other.

Furthermore, in space, there is no force. As Neuton stated, an object in motion will stay in motion as long as no forces are applied upon it. Spacial bodies exemplify this perfectly because unlike earth, where gravity plays a key role in everything, objects which have been impacted in someway will continue to revolve, to rotate forever until some other force acts upon it.

Conservation of angular momentum. As a dust cloud collapses due to gravity, the momentum that exists in the cloud in transferred to the planets (both in terms of rotation around the sun and in the spin of the planet).

There would be no planets that would form without spin. Some may stop spinning due to tidal locking. Yes I know that the day and year are the same but I maintain that that is not really a spin as the spin is really just a consequence of the tidal lock and not any inherent angular momentum.

As far as life forming in a tidally locked planet, my guess is yes. The weather patterns would be strange but there would still be a wide band that could potentially have liquid water. And of course life may form near hydrothermal vents etc that would not be affected by the planet's rotation.

## Do we know of any tidally-locked planets with atmospheres? - Astronomy

I was just curious. do the gas giant planets (Neptune, Uranus, Jupiter and Saturn) have at least a thin band of oxygen in the atmosphere which would make it possible to live in the sky. (floating cities)

There is only one planet where gaseous oxygen is found: Earth! And the only reason that Earth has oxygen is because Earth has plants that do photosynthesis. There is no other natural process that we know of that will put significant amounts of oxygen into a planet's atmosphere.

So that is bad news if you'd like to live on another planet in our Solar System. However, it does mean that if we ever find a planet around another star that has oxygen in its atmosphere, we can be fairly confident that there is life of some kind on that planet.

#### Britt Scharringhausen

Britt studies the rings of Saturn. She got her PhD from Cornell in 2006 and is now a Professor at Beloit College in Wisconson.

## Alien Life On Exoplanets May Be 'More Abundant And Active' Than On Earth, Say Scientists

This artist's concept shows what the TRAPPIST-1 planetary system may look like, based on available . [+] data about the planets' diameters, masses and distances from the host star, as of February 2018. 3 of the 7 exoplanets are in the “habitable zone”, where liquid water is possible.

Earth is the only place we know of in the entire Universe that sustains any kind of life. That doesn’t make it the only place per se, and it certainly doesn’t necessarily make it the perfect place to support life. That's according to a new study that suggests that ocean currents may be critical in the search for life beyond Earth.

The authors even suggest that Earth could be considered sub-optimal, with distant exoplanets much better suited to supporting life that’s both more abundant and more active.

“NASA’s search for life in the Universe is focused on so-called 'habitable zone' planets, which are worlds that have the potential for liquid water oceans,” Dr. Stephanie Olson, T.C. Chamberlin Postdoctoral Fellow at the University of Chicago, is expected to say on August 23 at keynote lecture at the Goldschmidt Geochemistry Congress in Barcelona, Spain. “But not all oceans are equally hospitable—and some oceans will be better places to live than others due to their global circulation patterns.”

What do we know about distant ocean-worlds?

Not much. Astronomers’ understanding of oceanography beyond our solar system is presently rudimentary. During Exo-oceanography and the search for life in uncharted waters, Olson–who studies Earth history, ocean-atmosphere evolution and astrobiology–will describe the search to identify the best environments for life on exoplanets, with the study demonstrating that some exoplanets may have greater variety of life than exists on Earth. “This is a surprising conclusion”, says Olson. “It shows us that conditions on some exoplanets with favourable ocean circulation patterns could be better suited to support life that is more abundant or more active than life on Earth.”

The key is the sea

Olson’s team modelled likely conditions on different types of exoplanets using ROCKE-3D software developed by NASA’s Goddard Institute for Space Studies (GISS). They simulated the climates and ocean habitats on different types of exoplanets, and were able to define which exoplanet types stand the best chance of developing and sustaining thriving biospheres.

“Our work has been aimed at identifying the exoplanet oceans which have the greatest capacity to host globally abundant and active life”, says Olson. “Life in Earth’s oceans depends on upwelling–upward flow–which returns nutrients from the dark depths of the ocean to the sunlit portions of the ocean where photosynthetic life lives.”

### In Photos: The ‘Super Strawberry Moon’ Sparkles As Summer’s First, Biggest And Brightest Full Moon Hangs Low

An artist's concept allows us to imagine what it would be like to stand on the surface of the . [+] exoplanet TRAPPIST-1f, located in the TRAPPIST-1 system in the constellation Aquarius. Because this planet is thought to be tidally locked to its star, meaning the same face of the planet is always pointed at the star, there would be a region called the terminator that perpetually divides day and night. If the night side is icy, the day side might give way to liquid water in the area where sufficient starlight hits the surface. One of the unusual features of TRAPPIST-1 planets is how close they are to each other -- so close that other planets could be visible in the sky from the surface of each one. In this view, the planets in the sky correspond to TRAPPIST1e (top left crescent), d (middle crescent) and c (bright dot to the lower right of the crescents). TRAPPIST-1e would appear about the same size as the moon and TRAPPIST1-c is on the far side of the star. The star itself, an ultra-cool dwarf, would appear about three times larger than our own sun does in Earth's skies. The system has been revealed through observations from NASA's Spitzer Space Telescope as well as other ground-based observatories, and the ground-based TRAPPIST telescope for which it was named after. (Photo digital Illustration by NASA/NASA via Getty Images)

Why is ocean circulation so important?

The process is pretty simple. More upwelling means more nutrient resupply, which means more biological activity. “These are the conditions we need to look for on exoplanets”, says Olson. “We have used an ocean circulation model to identify which planets will have the most efficient upwelling and thus offer particularly hospitable oceans.”

The study found that higher atmospheric density, slower rotation rates, and the presence of continents all yield higher upwelling rates. However, perhaps the most startling implication of the study is that Earth might not be optimally habitable. Life elsewhere may be present on a planet that is even more hospitable than our own.

Why are 'exo-oceans' important?

We can’t visit exoplanets and search for life. Not us, not robotic probes. They’re just too far away. So we point telescopes at them in an effort to understand what conditions prevail. To compare exoplanets–and to recognise which of them may host life–scientists need sophisticated models of their climates and evolution.

Though life may come in many forms and in many environments, we only know for sure that life exists where a planet's temperature allows liquid water oceans. So in their search for life on the 4,000+ exoplanets astronomers must target those that will be most favourable to large, globally active biospheres. On these planets life will be easiest to detect, and if it’s not, then we’ll learn more about where to look next (and where not to).

How can we check?

Future telescopes will use models like this one to search for bio signs in the atmospheres of exoplanets. “Ideally this work this will inform telescope design to ensure that future missions, such as the proposed LUVOIR or HabEx telescope concepts, have the right capabilities”, says Olson. “Now we know what to look for, so we need to start looking”.

The Large UV/Optical/IR Surveyor (LUVOIR) is a concept for a highly capable, multi-wavelength space . [+] observatory with ambitious science goals. LUVOIR has the major goal of characterizing a wide range of exoplanets, including those that might be habitable - or even inhabited.

NASA's Goddard Space Flight Center Conceptual Image Lab

What is LUVOIR?

Due to launch in 2039, the Large UV/Optical/IR Surveyor (LUVOIR) will be a bit of an all-rounder, much like the Hubble Space Telescope. Using a 15-meter mirror, it will likely be capable of studying exoplanet atmospheres and searching for bio-signatures such as oxygen and methane. It will even be able to directly photograph exoplanets.

What is HabEx?

Now on the drawing board, the Habitable Exoplanet Observatory (HabEx) space telescope will directly photograph exoplanets despite them being 10 million times dimmer than their host star. That will be thanks to a novel star-shade that will align itself many thousands of miles away from the telescope to suppress the light from faint stars of the exoplanets. HabEx will also study exoplanet atmospheres and look for signs of life.

## How and why to find chemical pollution around distant planets

The team also think that astronomers should look for the presence of chlorofluorocarbons (CFCs) in exoplanet atmospheres, which could indicate the presence of industrial activity.

Astronomers already seek biosignatures in the atmospheres of exoplanets, which are detected as chemicals such as oxygen and methane. “We pollute Earth’s atmosphere with our industrial activity,” said Loeb. “If another civilization had been doing it for much longer than we have, then their planet’s atmosphere might show detectable signs of artificially produced molecules that nature is very unlikely to produce spontaneously, such as CFCs.”

## Differential Equation and Solution(?)

Overall energy balance for the atmopspheric circulation is, after converting angle to time using the 10 m/s wind speed: $frac = 0.0557 - 5.6 imes10^<-8>t - 5.9 imes10^<-12>T^4$ .

After attempting to solve numerically using an Euler method, I discovered that this does not work. My problem is ignoring the potential energy imparted to air molecules to raise them from the lower atmosphere to the upper atmosphere. This takes something like 5e8 W at the 10 m/s flow I calculated and needs to be accounted for. Still working.

I'll try a simple answer. I didn't see the call for hard-science facts, so i thought i can give it a try. I didn't do a master in physics, and i do neither have the time nor the possibility to program a weather simulation that would most certainly be required to give you a correct answer. Let me say this right away: this answer is not really hard-science, but i tried my best.

1) How is temperature distributed on a tidal-locked planet? Do we have real life examples? 2) How does having an atmosphere influence temperature distribution? 3) Which temperatures are required for a planet to carry life? 4) Where is our isotherm point?

## Research Box Title

In April 2018, NASA launched the Transiting Exoplanet Survey Satellite (TESS). Its main goal is to locate Earth-sized planets and larger “super-Earths” orbiting nearby stars for further study. One of the most powerful tools that will examine the atmospheres of some planets that TESS discovers will be NASA’s James Webb Space Telescope. Since observing small exoplanets with thin atmospheres like Earth will be challenging for Webb, astronomers will target easier, gas giant exoplanets first.

Some of Webb’s first observations of gas giant exoplanets will be conducted through the Director’s Discretionary Early Release Science program. The transiting exoplanet project team at Webb’s science operations center is planning to conduct three different types of observations that will provide both new scientific knowledge and a better understanding of the performance of Webb’s science instruments.

“We have two main goals. The first is to get transiting exoplanet datasets from Webb to the astronomical community as soon as possible. The second is to do some great science so that astronomers and the public can see how powerful this observatory is,” said Jacob Bean of the University of Chicago, a co-principal investigator on the transiting exoplanet project.

“Our team’s goal is to provide critical knowledge and insights to the astronomical community that will help to catalyze exoplanet research and make the best use of Webb in the limited time we have available,” added Natalie Batalha of NASA Ames Research Center, the project’s principal investigator.

Transit – An atmospheric spectrum

When a planet crosses in front of, or transits, its host star, the star’s light is filtered through the planet’s atmosphere. Molecules within the atmosphere absorb certain wavelengths, or colors, of light. By splitting the star’s light into a rainbow spectrum, astronomers can detect those sections of missing light and determine what molecules are in the planet’s atmosphere.

For these observations, the project team selected WASP-79b, a Jupiter-sized planet located about 780 light-years from Earth. The team expects to detect and measure the abundances of water, carbon monoxide, and carbon dioxide in WASP-79b. Webb also might detect new molecules not yet seen in exoplanet atmospheres.

Phase curve – A weather map

Planets that orbit very close to their stars tend to become tidally locked. One side of the planet permanently faces the star while the other side faces away, just as one side of the Moon always faces the Earth. When the planet is in front of the star, we see its cooler backside. But as it orbits the star, more and more of the hot day-side comes into view. By observing an entire orbit, astronomers can observe those variations (called a phase curve) and use the data to map the planet’s temperature, clouds, and chemistry as a function of longitude.

The team will observe a phase curve of the “hot Jupiter” known as WASP-43b, which orbits its star in less than 20 hours. By looking at different wavelengths of light, they can sample the atmosphere to different depths and obtain a more complete picture of its structure. “We have already seen dramatic and unexpected variations for this planet with Hubble and Spitzer. With Webb we will reveal these variations in significantly greater detail to understand the physical processes that are responsible,” said Bean.

Eclipse – A planet’s glow

The greatest challenge when observing an exoplanet is that the star’s light is much brighter, swamping the faint light of the planet. To get around this problem, one method is to observe a transiting planet when it disappears behind the star, not when it crosses in front of the star. By comparing the two measurements, one taken when both star and planet are visible, and the other when only the star is in view, astronomers can calculate how much light is coming from the planet alone.

This technique works best for very hot planets that glow brightly in infrared light. The team plans to study WASP-18b, a planet that is baked to a temperature of almost 4,800 degrees Fahrenheit (2,900 K). Among other questions, they hope to determine whether the planet’s stratosphere exists due to the presence of titanium oxide, vanadium oxide, or some other molecule.

Habitable planets

Ultimately, astronomers want to use Webb to study potentially habitable planets. In particular, Webb will target planets orbiting red dwarf stars since those stars are smaller and dimmer, making it easier to tease out the signal from an orbiting planet. Red dwarfs are also the most common stars in our galaxy.

“TESS should locate more than a dozen planets orbiting in the habitable zones of red dwarfs, a few of which might actually be habitable. We want to learn whether those planets have atmospheres and Webb will be the one to tell us,” said Kevin Stevenson of the Space Telescope Science Institute, a co-principal investigator on the project. “The results will go a long way towards answering the question of whether conditions favorable to life are common in our galaxy.”

The James Webb Space Telescope is the world’s premier infrared space observatory of the next decade. Webb will solve mysteries of our solar system, look beyond to distant worlds around other stars, and probe the mysterious structures and origins of our universe and our place in it. Webb is an international program led by NASA with its partners, the European Space Agency (ESA) and the Canadian Space Agency (CSA).

## Even if Exoplanets Have Atmospheres With Oxygen, it Doesn’t Mean There’s Life There

In their efforts to find evidence of life beyond our Solar System, scientists are forced to take what is known as the “low-hanging fruit” approach. Basically, this comes down to determining if planets could be “potentially habitable” based on whether or not they would be warm enough to have liquid water on their surfaces and dense atmospheres with enough oxygen.

This is a consequence of the fact that existing methods for examining distant planets are largely indirect and that Earth is only one planet we know of that is capable of supporting life. But what if planets that have plenty of oxygen are not guaranteed to produce life? According to a new study by a team from Johns Hopkins University, this may very well be the case.

The findings were published in a study titled “ Gas Phase Chemistry of Cool Exoplanet Atmospheres: Insight from Laboratory Simulations “, which was recently published in the scientific journal ACS Earth and Space Chemistry. For the sake of their study, the team simulated the atmospheres of extra-solar planets in a laboratory environment to demonstrate that oxygen is not necessarily a sign of life.

On Earth, oxygen gas constitutes about 21% of the atmosphere and emerged as a result of photosynthesis, which culminated in the Great Oxygenation Event (ca. 2.45 billion years ago). This event drastically changed the composition of Earth’s atmosphere, going from one composed of nitrogen, carbon dioxide and inert gases to the nitrogen-oxygen mix we know today.

Because of its importance to the rise of complex life forms on Earth, oxygen gas is considered one of the most important biosignatures when looking for possible indications of life beyond Earth. After all, oxygen gas is the result of photosynthetic organisms (such as bacteria and plants) and is consumed by complex animals like insects and mammals.

But when it comes right down to it, there is much that scientists don’t know about how different energy sources initiate chemical reactions and how those reactions can create biosignatures like oxygen. While researchers have run photochemical models on computers to predict what exoplanet atmospheres might be able to create, real simulations in a laboratory environment have been lacking.

The research team conducted their simulations using the specially designed Planetary HAZE (PHAZER) chamber in the lab of Sarah Hörst, an assistant professor of Earth and planetary sciences at JHU and one of the principle authors on the paper. The researchers began by creating nine different gas mixtures to simulate exoplanet atmospheres.

Artist’s impression of the nearest super-Earth to our Solar System. Credit: ESO/M. Kornmesser

These mixtures were consistent with predictions made about the two most common types of exoplanet in our galaxy – Super-Earths and mini-Neptunes. Consistent with these predictions, each mixture was composed of carbon dioxide, water, ammonia and methane, and was then heated to temperatures ranging from 27 to 370 °C (80 to 700 °F).

The team then injected each mixture into the PHAZER chamber and exposed them to one of two forms of energy known to trigger chemical reactions in atmospheres – plasma from an alternating current and ultraviolet light. Whereas the former simulated electrical activities like lightning or energetic particles, the UV light simulated light from the Sun – the main driver of chemical reactions in the Solar System.

After running the experiment continuously for three days, which corresponds to how long atmospheric gases would be exposed to an energy source in space, the researchers measured and identified the resulting molecules with a mass spectrometer. What they found was that in multiple scenarios, oxygen and organic molecules were produced. These included formaldehyde and hydrogen cyanide, which can lead to the production of amino acids and sugars.

A CO2-rich planetary atmosphere exposed to a plasma discharge in Sarah Hörst’s lab. Credit: Chao He

In short, the team was able to demonstrate that oxygen gas and the raw materials from which life could emerge could both be created through simple chemical reactions. As Chao He, the lead author on the study, explained:

“People used to suggest that oxygen and organics being present together indicates life, but we produced them abiotically in multiple simulations. This suggests that even the co-presence of commonly accepted biosignatures could be a false positive for life.”

This study could have significant implications when it comes for the search for life beyond our Solar System. In the future, next-generation telescopes will give us the ability to image exoplanets directly and obtain spectra from their atmospheres. When that happens, the presence of oxygen may need to be reconsidered as a potential sign for habitability. Luckily, there are still plenty of potential biosignatures to look for!

## Could the Closest Extrasolar Planet Be Habitable? Astronomers Plan to Find Out

The extra-solar planet known as Proxima b has occupied a special place in the public mind ever since its existence was announced in August of 2016. As the closest exoplanet to our Solar System, its discovery has raised questions about the possibility of exploring it in the not-too-distant future. And even more tantalizing are the questions relating to its potential habitability.

Despite numerous studies that have attempted to indicate whether the planet could be suitable for life as we know it, nothing definitive has been produced. Fortunately, a team of astrophysics from the University of Exeter – with the help of meteorology experts from the UK’s Met Office – have taken the first tentative steps towards determining if Proxima b has a habitable climate.

According to their study, which appeared recently in the journal Astronomy & Astrophysics, the team conducted a series of simulations using the state-of-the-art Met Office Unified Model (UM). This numerical model has been used for decades to study Earth’s atmosphere, with applications ranging from weather prediction to the effects of climate change.

Artist’s impression of the surface of the planet Proxima b orbiting the red dwarf star Proxima Centauri. The double star Alpha Centauri AB is visible to the upper right of Proxima itself. Credit: ESO

With this model, the team simulated what the climate of Proxima b would be like if it had a similar atmospheric composition to Earth. They also conducted simulations on what the planet would be like it if had a much simpler atmosphere – one composed of nitrogen with trace amounts of carbon dioxide. Last, but not least, they made allowances for variations in the planet’s orbit.

For instance, given the planet’s distance from its sun – 0.05 AU (7.5 million km 4.66 million mi) – there have been questions about the planet’s orbital characteristics. On the one hand, it could be tidally-locked, where one face is constantly facing towards Proxima Centauri. On the other, the planet could be in a 3:2 orbital resonance with its sun, where it rotates three times on its axis for every two orbits (much like Mercury experiences with our Sun).

In either case, this would result in one side of the planet being exposed to quite a bit of radiation. Given the nature of M-type red dwarf stars, which are highly variable and unstable compared to other types of stars, the sun-facing side would be periodically irradiated. Also, in both orbital scenarios, the planet would be subject to significant variations in temperature that would make it difficult for liquid water to exist.

For example, on a tidally-locked planet, the main atmospheric gases on the night-facing side would be likely to freeze, which would leave the daylight zone exposed and dry. And on a planet with a 3:2 orbital resonance, a single solar day would most likely last a very long time (a solar day on Mercury lasts 176 Earth days), causing one side to become too hot and dry the other side too cold and dry.

This infographic compares the orbit of the planet around Proxima Centauri (Proxima b) with the same region of the Solar System. Credit: ESO

By taking all this into account, the team’s simulations allowed for some crucial comparisons with previous studies, but also allowed the team to reach beyond them. As Dr. Ian Boutle, an Honorary University Fellow at the University of Exeter and the lead author of the paper, explained in a University press release:

“Our research team looked at a number of different scenarios for the planet’s likely orbital configuration using a set of simulations. As well as examining how the climate would behave if the planet was ‘tidally-locked’ (where one day is the same length as one year), we also looked at how an orbit similar to Mercury, which rotates three times on its axis for every two orbits around the sun (a 3:2 resonance), would affect the environment.”

In the end, the results were quite favorable, as the team found that Proxima b would have a remarkably stable climate with either atmosphere and in either orbital configuration. Essentially, the UM software simulations showed that when both atmospheres and both the tidally-locked and 3:2 resonance configurations were accounted for, there would still be regions on the planet where water was able to exist in liquid form.

Naturally, the 3:2 resonance example resulted in more substantial areas of the planet falling within this temperature range. They also found that an eccentric orbit, where the distance between the planet and Proxima Centauri varied to a significant degree over the course of a single orbital period, would lead to a further increase in potential habitability.

Artist’s depiction of a watery exoplanet orbiting a distant red dwarf star. New research indicates that Proxima b could be especially watery. Credit: CfA

As Dr James Manners, another Honorary University Fellow and one of the co-authors on the paper, said:

“One of the main features that distinguishes this planet from Earth is that the light from its star is mostly in the near infra-red. These frequencies of light interact much more strongly with water vapor and carbon dioxide in the atmosphere which affects the climate that emerges in our model.”

Of course, much more work needs be done before we can truly understand whether this planet is capable of supporting life as we know it. Beyond feeding the hopes of those who would like to see it colonized someday, studies into Proxima b’s conditions are also of extreme importance in determining whether or not indigenous life exists there right now.

But in the meantime, studies such as this are extremely helpful when it comes to anticipating what kinds of environments we might find on distant planets. Dr Nathan Mayne – the scientific lead on exoplanet modelling at the University of Exeter and a co-author on the paper – also indicated that climate studies of this kind could have applications for scientists here at home.

“With the project we have at Exeter we are trying to not only understand the somewhat bewildering diversity of exoplanets being discovered, but also exploit this to hopefully improve our understanding of how our own climate has and will evolve,” he said. What’s more, it helps to illustrate how conditions here on Earth can be used to predict what may exist in extra-solar environments.

While that might sound a bit Earth-centric, it is entirely reasonable to assume that planets in other star systems are subject to processes and mechanics similar to what we’ve seen on the Solar planets. And this is something we are invariably forced to do when it comes to searching for habitable planets and life beyond our Solar System. Until we can go there directly, we will be forced to measure what we don’t know by what we do.

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