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Markus Pössel
Markus Pössel
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In-depth description: Traces of life on nearest exoplanets may be hidden in equatorial trap, study finds

The search for exoplanets – planets orbiting stars other than the Sun – is one of the most fruitful areas of astronomical research over the past decades. It also promises answers to one of the most fundamental questions of science: are we alone in the universe? Or is there other life out there? 

With the next generations of telescopes, and improved observational techniques, answers to these questions might be forthcoming within the next few decades. Studies combining results from planetology, astronomy, atmospheric chemistry and biology have demonstrated a number of possibilities for tracing life on other planets via observations of exoplanet atmospheres. But now, a study led by Ludmila Carone of the Max Planck Institute for Astronomy has shown that when it comes to the search for life in the universe, matters are likely not as simple as had commonly been assumed.

The explanation involves concepts such as weather phenomena and jet streams that are familiar from terrestrial meteorology, but have only recently come to be included in realistic models of exoplanets – and it turns out to be particularly important for a number of Earth-like exoplanet candidates that are rather close to our own solar system, and thus the natural first candidates for in-depth observations in search of life: Proxima Centauri b, whose discovery was announced in August 2016, and the planets of the TRAPPIST-1 system, announced in early 2017. In the TRAPPIST-1 system, Carone and her colleagues focused on TRAPPIST-1d in particular, as the most promising potentially habitable planet of the system. For comparison purposes, the researchers also applied their models to two other objects: the planet TRAPPIST-1b and the planet candidate GJ 667 Cf.

In the observation plans for the search for life on other planets, oxygen plays a key role. Oxygen is highly reactive; in chemical equilibrium, when chemical reactions have had sufficient time to run their course, one would expect oxygen to occur mostly in tightly bound molecules, having reacted with other elements, such as carbon, nitrogen, or various metals. But an alien astronomer studying Earth's atmosphere would find ample amounts of oxygen, and in particular of ozone, a particularly short-lived variation of oxygen (consisting of three oxygen atoms). And they would immediately ask why that should be so.

Not only is the ozone layer, located in Earth's stratosphere at a height of between 20 and 30 kilometers above ground, an important protection for life on Earth, shielding as it does harmful ultraviolet radiation. It is also a direct consequence of the presence of life: Up until 2.45 billion years ago, Earth's atmosphere was practically devoid of oxygen molecules (O2) and ozone (O3). It was only through the rise of oxygen-producing cyanobacteria, which produce oxygen via photosynthesis like modern plants. Once the oxygen content was sufficiently high, ozone was produced in the atmosphere's higher layers, with UV light splitting oxygen molecules into oxygen atoms, and single oxygen atoms bonding with oxygen molecules O2 to form ozone.

Since oxygen molecules are depleted as the oxygen reacts with other molecules in the atmosphere, an ozone layer will only be present if it is constantly replenished. On modern Earth, plants are responsible for this. Photosynthesis, by which plants produce energy-rich carbohydrates and molecular oxygen from carbon dioxide, water, and sunlight, ensures a steady supply of new oxygen molecules.

In the search for life, oxygen and ozone are key players – although not the only ones. Typical search strategies combine various different telltale clues from the spectrum of an exoplanet atmosphere, such as oxygen, ozone, water, and methane, to ensure that a given chemical imbalance really indicates the presence of oxygen-producing life, and to exclude non-biological reactions that could be responsible for at least some atmospheric oxygen content.

So far, so good. But planetary atmospheres are not merely the locus of chemical reactions. Instead, atmospheres are in constant and complex motion, as we all know from everyday life, subject as we are to changing weather. Atmospheric dynamics turns out to be crucial for our atmosphere's ozone content, as well.

On Earth, most of the ozone is produced over the equator, where sunlight hits the atmosphere straight on, perpendicular to the atmosphere's layered structures. In regions far from the tropics, sunlight hits the atmosphere at more oblique angles, which makes for a less dense stream of photons and also means that photons have to traverse a larger portion of the higher atmospheric layers before reaching the stratosphere. Luckily for us, there is a large-scale flow, a gigantic "transport belt" for air which then carries most of the ozone in the direction of the poles. Also, Earth has seasons; as the Earth orbits around the Sun, either the Northern or the Southern hemisphere is tilted towards the Sun, receives a greater amount of sunlight and, in consequence, produces more ozone. Seasons and the transport belt are how Earth manages to have a global ozone shield, as opposed to a localized ozone concentration in the equatorial regions.

But this is also where it gets complicated for planets like Proxima Centauri b or the TRAPPIST 1 planets. All of these planets are very close to their respective stars: Proxima Centauri b is a mere 0.05 astronomical units from Proxima Centauri (the closest star to Earth), in other words: a mere 0.05 of the average Earth-Sun distance. The seven known planets in the TRAPPIST 1 system are between 0.01 and 0.06 astronomical units from their host star.

Such proximity almost inevitably leads to what astronomers call "bound rotation;" the planets themselves are said to be "tidally locked": At such close distances, the differences in gravity on a planet's far and near side are sufficiently great for this gravity to rotate the planet into a "preferred orientation"; in consequence, there will be one side of the planet that always faces the host star ("eternal day-side") and another side that always faces away from the star ("eternal night"). The same effect is at work in the Earth-Moon system; there, too, gravity is sufficiently strong for one side of the Moon to remain turned towards Earth, while the far side of the Moon remains invisible to an observer on Earth.

MPIA's Ludmila Carone is a specialist for exactly this kind of situation. A geophysicist by training, with a background in astronomy and having run hundreds of three-dimensional climate simulations for various types of planet, Carone uses her unique skill set to investigate questions at the interface of astronomy, geophysics, chemistry and atmospheric physics.

When Proxima Centauri b and the potentially habitable planets in the TRAPPIST system were discovered, Carone and her team turned their attention to these promising candidates for future observations. Building on previous work on large-scale flows in exoplanets that are in bound rotation, they tested various scenarios for these planets. Assuming the best, that is, assuming that these planets have atmospheres similar to Earth, would they also have a similar ozone layer that might be detected by future observations? What these planets definitely do not have is seasons, seeing that it is always the same hemisphere of the planet that is facing the star. But is there at least an "atmospheric transport belt" similar to that on Earth?

Crucially, Carone and her colleagues found that there is a likely complication. Some planets in bound rotation can indeed have an atmospheric transport belt, which carries air from the equatorial zones towards the poles. In that case, ozone produced near the equator would be distributed equally over the atmosphere, resulting in a global ozone layer. But there is another possibility: the "transport belt" could run in the exact opposite direction, towards the equator. In that case, what forms at the equator – such as ozone! – stays at the equator. Such a transport belt would essentially be a trap for chemical compounds such as ozone, confining them to a narrow portion of the planetary atmosphere.

In their simulations, the scientists found that the crucial factor to determine which scenario applies – trap or global distribution? – is the length of the planet's year, that is, the time it takes the planet to orbit its host star. Planets which take more than about 25 days for one full orbit around their star have ordinary, pole-wards transport belts, just like Earth. Planets with shorter orbits, such as Proxima Centauri b where one "year" takes a mere eleven days, are at risk of developing an equatorial trap.

But even those planets with a proper transport belt, running from the equator towards the poles, can face a potential problem, which literally lurks in the eternal darkness of the planet's night side. The pole-wards transport belt on the Earth-like exoplanets studied by Carone and her colleagues is generally much stronger and thus faster than on Earth.

When it comes to evenly distributing chemicals throughout the atmosphere, that can be too much of a good thing. In fact, overly strong pole-wards circulation might trap the planet's stratospheric ozone on the night side. For astronomers in search of life and, more concretely, in search of information about the chemical composition of the atmosphere, that is an unwelcome complication. These searches use spectroscopy, namely the rainbow-like decomposition of light into different color components to identify tell-tale signs ("spectral lines") characteristic for various atoms and molecules.

But for planets orbiting distant stars, spectral analysis is challenging. One method analyses infrared radiation from the planet's warmer day side. But this will, naturally, not find chemical compounds that occur only on the night side. Another method can be applied to transiting planets, which, from the perspective of an observer on Earth, pass regularly in front of their host star. Once the planet is in front of the star, one can examine how the starlight is changed as it passes through the planet's (thin!) atmosphere. But this method only yields information about atmospheric regions near the day-night border, and can tell us nothing about compounds trapped deep in the night regions. In both cases, detection of chemical compounds that are confined to the night regions would be considerably more difficult than for compounds that are distributed evenly in the atmosphere. 

Even without a global ozone layer, or even an ozone layer altogether, an Earth-like exoplanet like Proxima Centauri b or TRAPPIST-1d might still be habitable. For one, these planets orbit comparatively cool, red stars, which emit very little harmful ultraviolet to begin with – although on the other hand, these stars can also be very temperamental, and prone to violent outbursts of harmful radiation that include such ultraviolet light. The jury is still out on what that means for the possibility of life. 

For more definite results both on the habitability of Earth-like exoplanets without global ozone layers and about the detectability problems, both better observations and more complex models are needed. The former is likely to be provided by the James Webb Space Telescope (JWST), slated for launch in 2019. JWST is going to take infrared spectra with unprecedented accuracy, with higher resolution and over a larger range of wavelength than before. The infrared region is a part of the spectrum where molecules in the atmospheres of exoplanets, including ozone and methane, leave their most characteristic traces. That is why JWST can be expected to take the chemical analysis of exoplanet atmospheres to a whole new level.

Regarding the latter, more complex models, Carone and her colleagues are already hard at work. The study described here focuses on atmospheric dynamics, charting the various streams and "transport belts" and, more generally, air circulation in the atmospheres of exoplanets in bound rotation. This gives valuable indications about what to expect for the distribution of various chemicals, in particular ozone, in the atmospheres of these planets, but it does not include explicit modelling of those chemicals and their changing distributions. A more definite answer as to whether or not ozone might be playing "hide and seek" in the atmospheres of Proxima Centauri b and TRAPPIST-1d will require simulations that include both atmospheric dynamics and the chemistry of compounds such as ozone, and Carone and her colleagues are in the process of running this kind of more complete simulation.

All in all, the present results sound a note of caution when it comes to the search for life in the atmospheres of exoplanets. After all, the apparent absence of ozone in such observations would not mean that there is no ozone, and oxygen-producing life at all. Alternatively, the ozone could be trapped out of sight, making a detection difficult. This would stress the importance of alternative markers of the presence of oxygen, such as molecular oxygen (O2) itself and the highly unstable tetraoxygen (O4), that would not be subject to the same difficulties as ozone. In addition, the possibility of hidden oxygen shows how important it is not to focus on a subset of markers, but instead to obtain a fuller picture of what is happening in the atmosphere. The prevalence of methane and water, direct measurements of the intensity of the ultraviolet light falling onto the atmosphere, and measurements of indicators that show atmospheric temperature and pressure: all these together would allow for a model of chemical reactions within the atmosphere that would show much more definitely whether biological activity is needed to explain the observations or not.

It is a truism that the search for signs of life on other planets is difficult. But as this new study shows, there might be some additional wrinkles to that difficulty – but, in the shape of atmospheric models, also new and promising directions in which the search for life outside Earth might make progress towards its ultimate goal.

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