Imagine receiving a powerful message from extraterrestrial life, like in the movie "Contact". While it's a thrilling thought, the truth is that if we ever find signs of life on other worlds, the discovery is more likely to come in the form of a biological signature in an exoplanet's atmosphere.
However, a recent study reveals that this search for extraterrestrial life might be even more challenging than anticipated.
Unlocking the secrets of alien atmospheres
But don't lose hope just yet. Scientists have already made progress in directly imaging some exoplanets and detecting molecules in their atmospheres. But these observations have been so far limited to gas giants. Earth-sized planets in the habitable zone of a star remain elusive. Nevertheless, astronomers have a clever trick up their sleeves to study the atmospheres of these planets.
Most exoplanets discovered so far have been found using the transit method. When a planet passes in front of its star, the observed brightness of the star slightly dips. At this point some of the starlight passes through the planet's atmosphere before reaching the eyes of the observer.
By observing and analyzing the spectrum of the star during transit, scientists can determine what light was absorbed by the atmosphere. This will tell the composition of the exoplanet's atmosphere.
Is this effect significant enough to assist astronomers?
This is the question that the recent study aimed to answer. For current observatories, the answer is a clear no. Therefore, the focus shifted to simulations of future telescopes with enormous mirrors, ranging from 10 to 50-100 meters in size.
These advanced telescopes should be capable of detecting various molecules in exoplanet atmospheres, with oxygen being a particularly interesting candidate. Oxygen is highly reactive and, on Earth, is constantly replenished by living organisms. Its detection in an exoplanet's atmosphere could indicate the presence of life.
The study suggests that these large telescopes of the future would be capable of observing oxygen in exoplanetary atmospheres, under the condition that the atmosphere is reasonably transparent and dense. However, there's a catch.
Hundreds of transits needed: example of TRAPPIST-1
Astronomers will face a challenge when it comes to distinguishing the signal from the noise. Even with these powerful telescopes, they will only capture a few pixels of light from a star. Additionally, atmospheric observations can only occur during planetary transits. This means that to identify the signal out of the noise, astronomers will have to observe multiple transits – possibly hundreds of them.
To illustrate the complexity of this endeavor, let's consider the TRAPPIST-1 planetary system, located approximately 40 light-years away. This system boasts seven Earth-sized planets, with four orbiting within the star's habitable zone.
However, due to TRAPPIST-1 being a red dwarf star, the habitable zone is relatively close to the star. The system’s planets have short orbital periods ranging from 4 to 12 days.Let’s assume these planets have Earth-like atmospheres rich in oxygen. It would still require thousands of transits to obtain a positive result.
Using telescopes that have not even been constructed yet, confirming atmospheric oxygen levels would take between 16 to 55 years of continuous observations.
The team conducting the study emphasizes the importance of future advancements in telescope technology, particularly the highly anticipated next-generation observatories like the James Webb Space Telescope.
The search for extraterrestrial life is an ongoing exploration, filled with uncertainty and excitement. Even if we cannot detect life on TRAPPIST-1 or other distant worlds just yet, the quest itself expands our understanding of the cosmos and our place within it.