When greenhouse gases are useful

An exoplanet that presents atmospheric and surface conditions supporting the presence of liquid water on the surface makes it a world that could harbour life. Hypothetically speaking, an intelligent and advanced extraterrestrial civilisation may wish to create such life-supporting conditions on an otherwise challenging or uninhabitable planet through climate engineering. For example, artificial greenhouses gases could be used in a process of planetary terraforming or to heat up a world that's about to enter an ice age.

A team of researchers led by the University of California, Riverside in the US has just published a study in The Astrophysical Journal that investigated what type of detectable imprint extraterrestrial climate engineering may leave on observable exoplanets. Encouragingly, the paper concludes that the ability to detect relatively low concentrations of greenhouse gases in the atmospheres of nearby exoplanets is within reach of the instruments on board the James Webb Space Telescope (JWST). Study co-author Dr Daniel Angerhausen, a researcher in the Institute for Particle Physics and Astrophysics and member of the National Centre of Competence (NCCR) PlanetS, focussed on the simulations showing that this detection capability will be further boosted by the next generation of space-based telescopes under development, of which the Large Interferometer For Exoplanets (LIFE) mission led by ETH Zurich is a prime example.

The right pick of gases

Greenhouse gases are dangerous pollutants, and their emission and atmospheric concentrations on Earth must be controlled to prevent harmful climate effects. However, these gases may be used intentionally on an exoplanet precisely for one of the reasons that makes them so dreaded on our world – their warming power.

Some greenhouse gases are also not known to occur in significant quantities in nature, hence detecting their presence in an exoplanet's atmosphere would be a strong sign of intelligent life forms with access to advanced technology: astrophysicists refer to this type of evidence as technosignatures. For their simulations, the paper's authors chose five artificial greenhouse gases that are used in industrial applications such as the fabrication of computer chips: fluorinated versions of methane, ethane and propane, as well as species made of nitrogen and fluorine or of sulfur and fluorine.

One reason for picking these artificial gases is that their warming power is incredibly effective. A relatively small amount of sulfur hexafluoride, for instance, could heat up a freezing planet to the point where liquid water would persist on its surface. Another advantage of the gases considered by the researchers – at least for the purpose of detecting technosignatures – is that they're exceptionally long-lived, which means they would persist in an Earth-like atmosphere for up to 50000 years. "The long lifetime makes these gases excellent technosignatures to systematically search for in comparison to shorter-lived signals. These signatures might even outlive their civilisation if their geo-engineering experiments were to fail," explains Angerhausen.

Technosignature detection

Artificially produced greenhouses gases modify the climate of a terrestrial planet because they absorb the infrared light emitted and reflected by the planet, thus preventing it from escaping into space and trapping heat in the planet's atmosphere. This strong mid-infrared absorption creates a characteristic spectroscopic signature in the atmospheres of terrestrial planets with temperate climates. Spectroscopic observations of exoplanets are a common way to determine the chemical composition of their atmospheres, and the importance of the thermal mid-infrared window is widely recognised by the research community.

The study's authors simulated the emission spectra of different exoplanetary candidates to establish how successful the detection of greenhouse gases in exoplanetary atmospheres could be with present-day as well as planned technology. They first considered a hypothetical terrestrial planet in the TRAPPIST-1 system, which is about 40 light-years away from Earth and contains several rocky planets. TRAPPIST-1 is one of the most studied exoplanetary systems and a realistic observation target for existing space-based telescopes such as JWST. The team found that the instruments on board JWST would be capable of detecting artificially produced greenhouse gases at the abundance levels required for climate engineering.

The researchers then looked at what technosignatures would be detectable with LIFE on a wider range of hypothetical planets that would not be transiting as is the case in the TRAPPIST-1 system. Angerhausen's knowledge of the mission concept and of LIFEsim, a publicly available software tool that can be used to simulate LIFE observations, were crucial to this part of the study. In this case, the team found that the threshold values for detecting the chosen greenhouse gases are even more favorable than those expected for standard biosignatures such as ozone and methane. Ideally, this means that terraformed atmospheres could be identified during searches for standard biosignatures without further requirements on the instrumentation or on the observation time windows. "Our findings show how powerful our next-generation telescopes are. We're the first generation in history that has the technology to systematically look for life and intelligence in our galactic neighborhood," says Angerhausen.

As LIFE will be able to directly image planets in the infrared range, it will greatly expand the exoplanet discovery space compared to what is accessible with JWST, which studies planets as they transit in front of their host stars. LIFE will use not one but four telescopes, combining their light via interferometry onto a fifth spacecraft to provide exquisite resolution of distant objects. LIFE builds on the heritage of ESA's Darwin or NASA's TPF-I mission concepts while taking advantage of the latest discoveries in exoplanet research and the latest technological developments. "With the support of the NCCR PlanetS and the Swiss Space Office we're currently working on NICE, a laboratory-based version of the instrument that will allow us to show that the technology is mature enough to turn the concept into reality," explains Professor Sascha Quanz, who leads the mission concept at ETH Zurich.

This article is based on external page the news piece published by the University of California, Riverside.