The search for extraterrestrial life has entered a new phase, with University of Arizona researchers evaluating the potential of next-generation telescopes to detect biosignatures on distant worlds. Biosignatures are detectable signs of past or present life, such as specific molecules, isotopes, or structures. 

A new study published in the Astronomical Journal explores how the U.S. Giant Magellan Telescope and the European Extremely Large Telescope, both under construction in Chile, could transform scientists' quest for life beyond Earth. 

Central to the research is the "habitable zone oxygen hypothesis" – the idea that planets orbiting within a star's habitable zone are more likely to possess significant oxygen in their atmospheres, potentially indicating the presence of life. 

Kevin Hardegree-Ullman, the study's lead author, was a postdoctoral associate in the NASA-funded Alien Earths group at the U of A Steward Observatory at the time the study was conducted. He is now a staff scientist at the NASA Exoplanet Science Institute. Hardegree-Ullman talked to University of Arizona News about the complexities of this cutting-edge astronomical pursuit and its implications for understanding life in the cosmos.

Q: What was the goal of this study?

A: This study focuses on planning for upcoming giant telescopes. To effectively plan, we conducted simulations to determine the kind of science we can perform with them. One of the main promises of these extremely large telescopes, including the European Extremely Large Telescope and the Giant Magellan Telescope – which the University of Arizona is a major partner in and is creating all the mirrors for – is discovering signs of life on exoplanets.

While this has been a significant goal, there haven't been comprehensive simulations to determine if we can actually detect these biosignatures. On Earth, we have various indicators of life, such as oxygen from plant photosynthesis and methane from waste byproducts. We aim to find similar biosignatures that we believe are created by biological processes on other planets.

To achieve this, we need to simulate how we can detect these biosignatures and consider all the factors involved. This includes modeling the telescope, the instruments, and performing realistic simulations based on exoplanet population statistics from other missions. These simulations can then inform our expectations for future observations.

The survey simulation is designed to determine whether biosignatures can be detected with these upcoming telescopes. In this paper, oxygen was specifically examined. The number of planets that could be surveyed for present-day Earthlike levels of oxygen was assessed.

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Q: What were your main findings?

A: We've found that identifying oxygen will be challenging but not impossible. We're using direct imaging with high-resolution spectroscopy. Direct imaging systems block most of the light from a star to try to see their much fainter planets. Our team is heavily involved in many direct imaging projects and building instruments to block out the host star's light so we can see a pinpoint of light from the planet.

The exoplanet Proxima Centauri b orbits our closest neighboring star, and this exoplanet is slightly more massive than Earth. This will be a primary target for this direct imaging. Once we suppress the starlight, we can use a spectrograph to analyze the planet's atmosphere. Modern spectrographs are a close cousin to a prism, splitting up a beam of light from a target like a star or exoplanet into distinct colors or wavelengths. The spectrum generated from a spectrograph gives the unique signature or fingerprint of the exoplanet's atmosphere, allowing us to figure out what gases are present.  It's still very challenging, and we have some things to work out to make these instruments precise enough to measure the atmospheres of Earthlike exoplanets. We hope these simulations will drive the requirements for building these instruments and hopefully secure funding. If we want to be the first to detect potential signs of life out there, we need to start building these instruments now.

Our survey simulation anticipates observing multiple exoplanets that could be good candidates. We're estimating a 10-year survey. With the GMT, we expect to survey about five to seven Earthlike planets for Earthlike oxygen levels. The ELT can survey about three times more due to its greater light-collecting power.

Both the GMT and ELT will be in Chile, and we could use them for two purposes: independently checking each other's findings to avoid false positives and potentially combining their signals to observe more planets in less time.

We want to survey many planets, but we won't be the only ones using these telescopes. They'll be oversubscribed, so we need to make a convincing case that the search for life out there is one of the biggest goals and worth spending all this time on.

Q: Does this study give us any insights about the existence of biosignatures in any specific Earth-like exoplanet?

A: We don't necessarily have an ideal Earthlike candidate yet. There are ongoing searches to find another very Earthlike planet we can survey. 

We have about six planets that are pretty good candidates to search for oxygen. Probably the best one is Proxima Centauri b, our closest neighboring planet. Most telescopes are designing their instruments to try to detect an atmosphere on Proxima Centauri b.

All of these best target planets are orbiting M dwarf stars, a class of stars about half the size of the sun or smaller. We're still not sure if planets in the habitable zone around M dwarfs are truly going to be Earthlike. We're trying to figure this out with the James Webb Space Telescope right now, searching for atmospheres on planets orbiting M dwarfs. But it's very challenging because these M dwarfs have star spots and flares that make interpretation of the results difficult.

We have some exoplanet candidates to search for atmospheres and biosignatures, but ongoing searches for more Earthlike planets are going to be crucial in the next few years to find the best targets for biosignature searches.