A look into the future of exoplanet science suggests the coming European extremely large telescope (ELT) will give us the best chance of detecting nearby biosignatures over the next twenty years rocky worlds orbiting other stars. That’s the conclusion of a new study that simulated what it would take to characterize worlds beyond our solar system with the tantalizing prospect of harboring life, such as Proxima Centauri b.
This study will allow astronomers to focus on important exoplanetary targets in the 2030s and beyond.
In addition to measuring exoplanetary bulk properties – mass, radius and orbital period – astronomers learn about these extrasolar worlds by studying their atmospheres. The James Webb Space Telescope (JWST) does this, for example, via transit spectroscopy. As a planet moves past its star (in other words, moving in front of its star) from the telescope’s viewpoint, some of the star’s light filters through the planet’s atmosphere. Any atmospheric molecules present in that atmosphere can absorb the starlight. Importantly, different molecules absorb at certain wavelengths, making each wavelength the signature of a specific molecule. For example, the JWST recently discovered hints of methane and carbon dioxide in the atmosphere of the exoplanet K2-18bFor example.
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However, our machines only manage to see a small fraction of the planets passing through their stars, meaning scientists will have to use another way to characterize exoplanetary atmospheres. This applies to worlds that do not pass at all, but also to worlds that may pass without us noticing.
One option is direct imaging, but directly imaging exoplanets is a difficult proposition.
Although dozens of exoplanets have been imaged so far, such as HD 950086bThey are all young, large worlds that are still hot from their formation processes. They therefore shine brightly in infrared light while at large angular distances from their parent star. In other words, we can’t see any detail of these worlds – the planets look like just pinpricks of light – but within their light patterns are hidden absorption lines associated with atmospheric molecules. However, retrieving these spectral details requires a very large telescope to obtain a sufficiently high signal-to-noise ratio of the planet’s light against background data.
With a trio of massive ground-based observatories set to see first light over the next decade, astronomers Huihao Zhang, Ji Wang and Michael Plummer, all at Ohio State University, wanted to test how well these emerging looks at the universe would characterize exoplanets via direct imaging. The goal was to see how much better they perform the task compared to transit spectroscopy performed by the JWST’s 6.5-meter mirror.
These three next-generation ground-based telescopes are the 39-meter (128-foot) European Extremely Large Telescope (ELT), the 98-foot Thirty Meter Telescope (TMT) on Mauna Kea, and the 24-meter (79-foot) telescope. Giant Magellan telescope (GMT). The ELT and GMT are here built at separate locations in Chile’s Atacama Desert; The first mirror segments for the ELT were shipped to South America last December.
“It is difficult to say whether space telescopes are better than ground-based telescopes because they are different,” Zhang said in a press statement. “They have different environments, different locations, and their observations have different influences.”
Zhang, Wang and Plummer simulated the performance of two instruments on the ELT: the Mid-Infrared ELT Imager and Spectrograph (METIS) and the High Angular Resolution Monolithic Optical and Near-infrared Integral field spectrograph (HARMONI). They modeled these instruments after ten real exoplanets orbiting nearby red dwarf stars, testing how well they can detect potential biosignatures such as molecular oxygen, carbon dioxide, methane and water through direct imaging.
“Not every planet is suitable for direct imaging,” says Zhang. “But that’s why simulations give us a rough idea of what the ELT would have delivered and the promises they should hold when built.”
The results were mixed. A world called GJ887bwhich is a super-Earth four times as massive as our planet which orbits the brightest red dwarf in our sky, about 11 light-years away, performed best in the simulations. METIS in particular proved capable of detecting the above-mentioned biosignature gases in its atmosphere. In the simulations, METIS was also able to detect the same biosignatures on the exoplanets Proxima b and Wolf 1061c, while HARMONI was able to perform the same detections but required longer exposure times.
However, the ELT will likely struggle to directly visualize and characterize the world’s seven worlds TRAPPIST-1 system, the team says, due to “atmospheric visibility limitations” that hamper efforts to resolve important small-angular distance measurements of the planets from their star. In this case, JWST’s transit spectroscopy performs better here, but even the $10 billion observatory is struggling.
Preliminary results from the JWST suggest that the inner planets of TRAPPIST-1, worlds b and c, have no atmospheres. It could take years for the JWST to collect enough data to draw conclusions about the other five TRAPPIST-1 worlds, including planets d, e, and f within the world. habitable zone.
The capabilities and limitations of the ELT and the JWST will not be the final word on exoplanet characterization. The most recent ten-year study for astronomy from the National Academy of Science has developed a new, giant space telescope with a mirror of at least eight meters would be accelerated to a launch date in the 2040s. Such a telescope would be optimized to detect, image and characterize rocky worlds in habitable zones around nearby stars, including Proxima b.
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Until then, Zhang, Wang and Plummer believe that the next generation of large ground-based telescopes, and the JWST, can bring astronomers together to explore exoplanets and learn whether some of them can actually support life – as we know it, at least.
Their results were published in December 2023 The astronomical magazine.