On the trail of the oxygen signature

This is what an oxygen-rich exoplanet could look like. (Image: NASA / GSFC / Friedlander-Griswold)

Is there any life out there? A new method could now benefit the search for living exoplanets: Scientists present a method for the detection of the vital element oxygen in the atmosphere of distant worlds. The collision of the oxygen molecules could be noticeable in the light that shimmers through their gas shells, the astronomers explain.

The universe is teeming with stars and planets – the successes of planetary hunters in recent years have shown this. The new challenge is to uncover features of these celestial bodies. Some are already showing that they are rock-like planets of earth-like size orbiting their star at distances that could make life-friendly conditions possible. But can other indications of a life-friendly character be uncovered? Possibly in the composition of the atmosphere, researchers say. Insights into the characteristics of the gas envelopes can provide spectral analysis of the light that shimmers through the atmospheres of the exoplanets as they pass in front of their central star. The characteristics of the light that reaches us can then provide information about the presence of certain substances in the planet’s atmosphere.

The life element in sight

The detection of a certain element is particularly interesting: oxygen is closely linked to life as we know it. It is created as part of photosynthesis, through which plants extract energy from sunlight. This energy forms the basis of the food chains and the oxygen in turn serves as an oxidizing agent for the animals in their energy production. The biological production of oxygen is clearly reflected in the content of the earth’s atmosphere. That means: If one were to detect similar amounts of O2 in the atmosphere of an exoplanet as on Earth, this would indicate the existence of life forms as we know them.

But with the previous approaches, the proof of the prominent element turned out to be tricky. The hope now lies in the improved possibilities of the James Webb space telescope, whose launch is planned for 2021. From this sharp astro eye, planetary researchers hope to gain more insight when analyzing the atmospheres of distant worlds. But as far as oxygen is concerned, the prospects with Webb have so far seemed disappointing: “Before our work, it was assumed that oxygen in concentrations similar to those in the Earth’s atmosphere could not be detected with Webb,” says Thomas Fauchez from NASA’s Goddard Space Flight Center in Greenbelt ,

A collision creates a signature

But he and his colleagues have now shown a way with which the signature of oxygen in the atmosphere of distant planets could be determined in the data of the new space telescope. They use the effect that collisions of oxygen molecules leave traces in the infrared range of light in the atmosphere. They block parts of the spectrum, so that certain wavelengths are eliminated. “This oxygen signal has been known from Earth’s atmospheric studies since the early 1980s, but has never been studied for exoplanet research,” says Fauchez. As part of their study, the scientists have now calculated how much light is shielded by these oxygen collisions. As they explain, a characteristic signature is created in the radiation that the Webb telescope can detect. In the future, conclusions about the oxygen content of the atmospheres of exoplanets could be made.

“Oxygen is one of the most exciting molecules that can be discovered because of its connection with life, but we cannot say that life is the only cause of oxygen in an atmosphere,” says co-author Schwieterman of the University of California in Riverside to consider. A high level of concentration can also result from a certain developmental history of exoplanets: If it is too close to its central star or receives too much energy, the atmosphere becomes very warm and saturated with water vapor from the evaporating oceans. This water can then be broken down into atomic hydrogen and oxygen by strong ultraviolet radiation. The hydrogen then escapes into space and an oxygen-rich atmosphere revives itself. For this reason, a lot of atmospheric oxygen does not necessarily mean abundant life, but indicates a loss of water in the past, the scientists explain.

Since astronomers do not yet know how widespread this process could be with exoplanets, the scientists now want to pay attention to this aspect as well. “It is important to know whether and to what extent dead planets have atmospheric oxygen so that we can better recognize when a planet could be alive or not,” says Schwieterman.

Source: University of California – Riverside, technical article: Nature Astronomy, doi: 10.1038 / s41550-019-0977-7

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