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COOKING UP ALIEN ATMOSPHERES

Scientists are recreating the air that clings to distant worlds. Could their work help to reveal the presence of extraterrestrial life? by PHILIP BALL

ow do you spot an alien? Some scientists The answer to this question may lie in intriguing are looking for communication signals new research that’s replicating alien air on Earth, beamed out into space. Others propose cooking up exotic brews in laboratories. Meanwhile, looking for dips in starlight that other scientists are simulating the weather and might be caused by huge alien-built circulation of alien atmospheres inside powerful megastructures orbiting a distant sun. supercomputers to find out just how hospitable But perhaps the promising line distant worlds may be. This research is already of enquiry lies with probing the layer providing some tantalising clues about where of gases surrounding alien worlds. If the next generation of alien hunters should be H we were to watch Earth from afar, we focusing their attention. would be able to infer our existence by analysing the make-up of our SOMETHING IN THE AIR . There’s only one process we know Since the first detection of an – a planet of that could keep it so rich in : life. around another star – in 1992, more than 4,000 If aliens live on other worlds, they’d probably have been identified, mostly by observing the subtle also imprint a signature of their existence in their but regular dimming in starlight as the planet atmospheres. While we are able to peer at alien passes across its parent star and blocks some of atmospheres through the latest space telescopes, its light (a ). More than half of there’s a catch: we don’t really know what we’re were detected in this way by NASA’s Kepler space looking for. We only have Earth as a comparison telescope (active from 2009 to October 2018). In – but what if there are other combinations of April 2018, NASA launched a successor to Kepler:

GETTYIMAGES gases that can reveal the presence of life? the Transiting Exoplanet Survey Satellite (TESS). 2 ILLUSTRATION:HOLCROFT JOHN

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2 In order to study an exoplanet’s phenomenon that’ll be familiar to anyone who’s spent time ABOVE Super-Earths atmosphere, astronomers look at how in a big city: haze. like Kepler 62e are a common type of the atmosphere absorbs the starlight exoplanet, with thick and passing through it. Different molecules COOKING WITH GAS misty atmospheres of gas will absorb different wavelengths The two most common types of exoplanet have no equivalent of light, so researchers can analyse the in our own . One is the ‘super-Earth’: rocky, RIGHT Dr Sarah Hörst star’s filtered light spectrum during with a diameter 1.25 to two times that of Earth. The other and assistant research a transit to pin down which gases is the ‘mini-’: about two to four times the size of scientist Chao He examine a sample of simulated are present. In this way, astronomers our planet, with a thick blanket of gases (mostly exoplanet atmosphere made the first direct detection and and helium) over a dense core of rock or ice. chemical analysis of an exoplanet Astronomers have discovered that the atmospheres of atmosphere in 2001 – finding super-Earths and mini- are rather thick and in the atmosphere of a known misty: light doesn’t get through them easily. This could as HD 209458 b. be because they’re full of clouds (perhaps made from Since then, several exoplanets have droplets condensed from vapour or other gases such had their atmospheres analysed, as ), or it might be due to haze: tiny, dust-like solid revealing the presence of water vapour, particles, like the pall over cities caused by traffic fumes. methane, dioxide, and even Hörst is trying to find out. She says that there might be small amounts of oxygen around some ramifications for whether planets like this could support of these worlds. None of these gases life. According to Hörst, haze particles in an atmosphere alone signals life, however – not even can have a huge impact on how starlight moves through oxygen, as we know of processes that it. “This can affect things like how much and what sort of can create small amounts of it without energy is available at the surface of a planet for life, and involving living organisms. what the of the surface is,” she says. This is where the work of planetary We have only the sketchiest information about the chemistry scientist Dr Sarah Hörst comes in. At of these atmospheres, so Hörst carries out simulations for Johns Hopkins University in Baltimore, a wide range of possible compositions, including all the US, she is leading a team of scientists common gases likely to be found around worlds like these: who are brewing lab simulations of water vapour, , , nitrogen, the gases likely to be in exoplanet hydrogen, helium and methane. Hörst mixes up different atmospheres, in order to find out what proportions of these gases at between about they might produce. So far, Hörst’s 25°C and 325°C, mimicking the conditions thought to exist work has focused on an atmospheric on super-Earths and mini-Neptunes.

74 HOW TO ANALYSE AN ALIEN ATMOSPHERE

Star Many light-years

3 Exoplanet Transmied Earth starlight

4 Space telescope

1 Starlight 2 Atmosphere

5 Absorption spectrum

1 The star’s light is made 2 The starlight passes 3 The atmosphere absorbs specific 4 During the exoplanet’s ‘transit’ across 5 Gaps in the star’s up of many individual through the wavelengths of starlight, depending its star, a space telescope such as the spectrum indicate the wavelengths exoplanet’s on which gas molecules are present – Hubble – or soon the James Webb presence of particular atmosphere leaving a ‘fingerprint’ of those Space Telescope – detects the filtered gas molecules in the molecules in the transmied light light and analyses its spectrum using planet’s atmosphere a device called a spectrograph

It’s a kind of cosmic cookery: throw together the ingredients, bake at moderate heat, and see what comes out. There’s another crucial ingredient, too: energy to kick off chemical reactions by breaking molecules apart. On exoplanets, this could come from high- energy ultraviolet rays in the starlight, or from electrically charged particles formed by cosmic rays flooding into the upper regions of the atmosphere. The researchers simulate these energy sources using either an ultraviolet lamp or an electrical discharge like that in a fluorescent lighting tube. Most of the mixtures that Hörst and her team have studied generated brownish, smog-like haze, similar to what we see on ’s moon . The amount of haze varied widely, though, depending on the composition of the mixture. For example, two of the experiments with plenty of water vapour and methane produced the most haze, but a third experiment also generated fine particles with no methane present at all. More work is needed to find out what the detection of haze on a distant exoplanet would mean for the likelihood of finding life. Hörst says that in some cases haze might block

NASA, WILL KIRK/JHU ILLUSTRATION: ACUTE GRAPHICS ACUTE ILLUSTRATION: KIRK/JHU WILL NASA, harmful radiation (as the ozone layer 2

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The answer doesn’t appear to be oxygen “ITWON’T BE LONGBEFORE alone. Hörst and colleagues saw oxygen form in their simulation experiments, purely from chemical reactions induced by ultraviolet light. They also saw organic molecules like ethanol and formaldehyde, which rules these out WE’RE ABLETO STUDYTHE as definite biosignatures, too. One possible biosignature is the simultaneous existence of ozone and methane, says Hörst. This is a chemically unstable blend of gases, and there’s no known ATMOSPHERESOF EVER geological process that could sustain them. “It would be really hard to have them together in the same atmosphere without some source replenishing them,” says Hörst. On Earth, the biosphere is ultimately the source of both of these MORE EXOTIC EXOPLANETS” gases in our atmosphere. Some of the most enticing worlds to look for biosignatures such as these, Hörst feels, are the set of planets 2 does on Earth), but it could also lead to cooling of the surface and a detected in 2015 around a dim star lack of liquid water. “We need to know more about the planet and its called TRAPPIST-1, located 40 light- atmosphere to be able to understand what the conditions might be like years away in the constellation of on the surface, and what processes would have led to the formation of Aquarius. Seven of the worlds orbiting haze,” she says. this star are vaguely Earth-like, and most In the meantime, the holy grail of this area of research is to identify of them are potentially habitable, having some molecule – or a set of molecules – that can exist only if there is the right conditions for liquid water on life, i.e. a ‘biosignature’ of alien life. But what might that be? their surfaces. By looking at the light

Think Earth’s weather is unpredictable? Here are some of the WEIRD WEATHER strange phenomena we’ve spo￿ed elsewhere in the Universe

DIAMOND RAIN METAL MONSOON INFERNAL GEMSTONE CLOUDS The atmospheres of some gas The hot HD 209458 b, also 55 Cancri e is a rocky planet about HAT-P-7b, a located giants contain plenty of carbon, known as ‘Osiris’, has atmospheric twice the diameter of the Earth 1,000 light-years away, has created when lightning reacts temperatures of several thousand that orbits so close to its star that clouds on its cool, night side with methane. At high pressures degrees. It’s likely that rains of its surface has a temperature of made of condensed aluminium and temperatures inside the molten iron or silicate glass pelt around 2,000°C. Molten lava oxide – the stu that forms the atmosphere, this might condense down there, falling in wind speeds spouts into the atmosphere to fall basis of minerals like rubies and into crystals of diamond, some as of several kilometres per second as fiery rain, and this turbulent, sapphires. Winds carry these big as blueberries. Experts (compared to a mere 70 metres sooty atmosphere is filled with clouds to the planet’s hot estimate that, on Saturn, about per second for severe hurricanes electrically charged particles, side, where temperatures of 1,000 tonnes of diamonds are on Earth). The result is a lacerating producing immense, planet-wide close to 2,000°C quickly

produced every year. blast of hot metal and rock. lightning . destroy them. LIBRARY PHOTO SCIENCE ESA, X2, NASA/JPL .

76 RIGHT By studying transmitted through their atmospheres, their atmospheres, we can confirm if some of the planets Hörst and coworkers surrounding the have found that some TRAPPIST-1 star have the right of them may have clouds conditions for liquid or haze, though it’s hard water to exist to be more precise at this stage about which option is more likely. Hörst thinks that these intriguing planets should be an early target for NASA’s James Webb Space Telescope (JWST), the planned successor to the , which will take a closer look at exoplanet atmospheres once its been launched in 2021.

EXO-CLIMATES Life on Earth relies not only on having can mix up the chemistry of the hot and cool sides of the the right kind of atmosphere, but also atmosphere, changing the blend of gases in some places on the whole climate system: how and thus the amount of starlight that’s transmitted to the air, oceans and heat circulate, and surface. While hot are not likely to harbour life, how clouds form. If we found an this shows how the circulation of the atmosphere can play exoplanet with an identical atmospheric a crucial role in the surface conditions on a planet – with composition to Earth, it might still be important implications for their habitability. inhospitable to life if it lacked a similar Add water to the equation and things get even more climate system. A new research area interesting. Some potentially habitable planets like the called ‘exoclimatology’ is aiming to TRAPPIST-1 group are also likely to be tidally locked to understand exoplanet climates – and their star. If these exoplanets have liquid water on their their implications for life – by applying surface, then water on the hot day side will evaporate, the computer models used to simulate eventually condensing into rain or snow on the cooler the Earth’s weather and climate to night side. “Land covering the day side would quickly dry other worlds. out and the moisture would be transported to the night So far, much of the work has focused side,” says Mayne. “But if there’s an ocean, the water can on another common exoplanet type: circulate back” – creating a giant conveyor belt of water ‘hot Jupiters’ – gas giants like our own between the two sides of the planet. This could make the Jupiter but which orbit much closer to difference between a barren planet divided into halves – their parent star. They tend to either each too extreme for life to exist – and a planet where water rotate very slowly or be ‘tidally locked’ circulation creates a more moist and clement environment. so that – as with the Moon orbiting the With the latest generation of space telescopes, it won’t Earth – the same side always faces the be long before we’re able to study the atmospheres of ever star. This gives the planets a temperature more exotic exoplanets. “The combination of TESS and the DISCOVER MORE difference between the ‘day’ side and the JWST should provide us with a lot of compelling worlds ‘night’ side which drives atmospheric to study,” says Hörst. Work by the likes of her and Mayne circulation, much as the temperature is set to become crucial to astronomers who want to know LISTEN difference between Earth’s equator and whether the exoplanet gases and weather patterns that the poles powers our own climate. they’re detecting are the symptoms of a sterile planet, or In this episode of The Curious Cases Of Computer models of this circulation maybe – just maybe – hints of life… Rutherford & Fry, find indicate that hot Jupiters have a kind of out if we’ll ever discover atmospheric jet stream, says Dr Nathan by PHILIP BALL alien life. Mayne, who leads the exoclimatology Philip is a freelance science writer, specialising in bit.ly/find_alien_life group at the University of Exeter. This physics and maths

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