Maybe it is not impossible. If there really is extraterrestrial life, it is really difficult to confirm its existence!

There could be life lurking on other planets, but trapped on them, how could we be sure it was there? One good idea is to look for compounds known to be the key ingredients for life as we know it on other worlds. Detecting these so-called biosignatures, compounds known to be produced by living organisms, would be strong evidence that a planet could contain life. But extracting chemicals from such distant planets, and choosing the right compounds to look for, is complicated. Professor Ignas Snellen of Leiden University in the Netherlands has been perfecting the technique of combining data from the largest ground-based telescopes with high-contrast imaging.

These imaging techniques can reveal faint objects such as planets. Telescopes use high-precision spectroscopy to examine the different wavelengths of light they detect from space, filtering out as much actual starlight as possible to make any information that can be obtained from exoplanets visible. By examining the light that penetrates the planet's atmosphere and reaches Earth, it is possible to deduce the types of gases present. While telescopes are not yet large enough to examine the spectra of Earth-sized planets, scientists are exploring larger exoplanets, honing the method on so-called hot Jupiters, which are too hot to support life as we know it. These are gas giant exoplanets that orbit very close to their parent stars.

Exoplanetary systems

In fact, they are so close that they are tidally locked, and like Earth's Moon, the exoplanets rotate only once in each orbit around their star. Since one side of these planets is always in light and the other in darkness, the light side gets so hot that the atmosphere can boil off, creating a wind in which material flows away from the planet, a bit like a comet's tail. In the EXOPLANETBIO project, Professor Snellen and the research team used high-precision spectroscopy for the first time to confirm the amount of helium in the atmosphere of a hot Jupiter using ground-based telescopes, which could shed light on how this process is going on.

"This is a breakthrough for these hot Jupiters," said Dr. Carolyn Loeb, a professor of astronomy and a co-author of the study. "These types of exoplanet tails are known, but they have been difficult to observe because they can only be seen by detecting hydrogen, which cannot be detected through the Earth's atmosphere, so the Hubble Space Telescope was used. Now, with the stronger helium line, it is possible to do this very well from the ground with a telescope. Understanding whether a hot Jupiter sheds its atmosphere, and how long it might take, could explain how all exoplanet atmospheres change over time. Such atmospheric escape processes are not very important now, but they were important in the early days of the Solar System when the Sun was much more active."

Climate of exoplanets

Using these new techniques, the research team was also able to achieve another first, detecting the rotation rate (how fast a planet spins) and orbital speed of an exoplanet. The spin rates on hot Jupiters are usually quite low because they are usually tidally locked, which can reveal some information about the climate and associated weather on exoplanets. When a planet spins faster, it gets belted structures like Jupiter. Earth spins slower and has some belted structures, but it is still dominated by low-pressure systems. Now, if you have a hot Jupiter that spins even more slowly, you don't get any belted structures, but you get very different weather systems.

"We have been able to observe winds high up in the atmospheres of these planets as energy from the hotter, eternal day side is spun to the cooler night side. An upgrade to the CRIRES (Cryogenic High Resolution Infrared Spectrometer) instrument, which will be networked on the European Southern Observatory's (ESO) Very Large Telescope next year, will be able to spot compounds such as methane on cooler planets, which could be a component of life if found in an Earth-sized planet. Astronomers are now learning methods that could one day be applied to Earth-like planets, and (ESO's) Very Large Telescope should be ready by 2026, when we can start detecting Earth-like planets."

Signs of life

However, even with good samples from rocky, Earth-sized planets, how do you know if a compound is really a sign of life? Kevin Heng, a professor at the University of Bern in Switzerland, said: "Geology is very good at making things that look like life, such as methane. If you consider biosignatures, they have to meet various conditions. They can't be simulated by geology. They have to persist in the atmosphere for a long time, which means they are very stable or replenished in some way. And they have to be detectable. As part of the EXOKLEIN project, Professor Heng is studying such compounds;

"If you look at a planet like Jupiter... they look a little bit like our sun. They're made of hydrogen, they have trace amounts of metallic elements and so on. Based on the differences between this planet and this star, astronomers can figure out how it formed, and it will preserve a fossil record of how it formed."

But for smaller planets, the atmospheres change significantly over time, such as the carbon cycle. The research team has spent 8 to 10 years studying how to use climate models designed for Earth (exoplanets) and how to adjust and modify them. When instruments are able to measure smaller planets, these models will be used to provide potential explanations for the data collected to understand whether the compounds are truly biosignatures or can be explained as geological. Extraordinary signatures require extraordinary standards of proof, so if something is consistent with not needing biology, it will be said that there is no biology.

We're also modeling planets that could have a more dramatic fate. For asteroids around red stars to support life, they'd need to have a very tight orbit that makes them tidally locked like hot Jupiters. That means the night side is really cold, maybe cold enough that gases in the atmosphere condense into ice. So, you get runaway condensation and no atmosphere -- the atmosphere collapses. Such a collapse would make the planet cold and lifeless, like Mars. While this research is only theoretical right now, upcoming missions like the European Space Agency's Cheops satellite and NASA's James Webb Space Telescope should produce data that matches up with the theory.

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Boco Garden｜Text: Ethan Bilby/Horizon

Reference journal EU Research and Innovation

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