A Breakthrough May Be Close in Search for Exoplanet Biosignatures
JWST and CRIRES+ covers the same wavelengths and came into operation about the same time. By combining the strengths of both instruments it may now be possible to make qualified analyses of the atmospheres of Earth like planets.
What is CRIRES+?
CRIRES+ is an abbreviation for CRyogenic InfraRed Echele Spectrography. It is a spectrographic instrument mounted on top of the ESO Paranal Observatory in the Atacama Desert in the Andes. The + sign means that it is an upgrade from a previous instrument installed already in 2014.
CRIRES+ can break up the starlight that is passing through the atmosphere of a planet transiting in front of the hosting star. By measuring the fraction of stellar light able to penetrate the atmosphere at different wavelengths, the chemical composition of the atmosphere can be inferred. This method is called the Transit Spectroscopy Method.
The light effects that are analysed are in the near infrared (NIR) spectrum, which covers longer wavelengths close to visible light. This spectrum has optical characteristics that will help us identify molecules of interest for exoplanetary atmospheres, for example water (H20), carbon oxide (CO2) and methane (CH4). Oxygen and methane are both considered as important biosignatures for life.
Combining JWST and CRIRES+
CRIRES+ may be a very advanced instrument but it has to compensate for Earth’s atmosphere. JWST is located 1,5 million kilometers away from Earth and has its own spectrograph operating in almost the same spectrum. One of JWTS’s main tasks is to study the atmospheres of exoplanets.
While JWST operates in an environment that is free from any Earthly disturbance, the spectrographic instrument of CRIRES+ has a resolution that is 100 times higher than the one that is mounted on JWST.
According to Nikolai Piskunov, who leads the work with CRIRES+, a combination of data from these two instruments could allow for a much deeper understanding of the chemical composition of the atmospheres around exoplanets, but it could also help analysing the physiological characteristics of the surfaces on these planets.
Oxygen and methane are good signs of life
As far as we have learned from the processes here on Earth, planets with free oxygen will most likely host some kind of life and oxygen does not arise through natural geological processes.
But there is much that we don’t know about how different energy sources initiate chemical reactions and how those reactions can create biosignatures like oxygen. Recent studies suggest that even if exoplanets have atmospheres with oxygen, it doesn’t mean there’s life there.
In a study conducted at the Johns Hopkins University, a team of researchers created nine different gas mixtures to simulate exoplanet atmospheres. The team was able to demonstrate that oxygen gas and the raw materials from which life could emerge could both be created through simple chemical reactions.
Another study, from UC Santa Cruz, suggests that methane can be a compelling biosignature. Finding a large amount of methane in an exoplanet’s atmosphere might even be our most reliable indication that life’s at work there. There are abiotic sources of methane, but for the most part, methane comes from life.
But to understand methane as a potential biosignature, we need to understand it in a planetary context. In planetary atmospheres, methane exists in relation to other gases. So identifying abiotic methane sources is only part of the picture. How do other gases like carbon monoxide and carbon dioxide fit into a methane-rich atmosphere? How do they affect one another?
Methane is also interesting because it doesn’t last long in an atmosphere. Photochemical reactions destroy it, so detecting a lot of it means something is constantly replenishing it. There has to be a large and consistent source, such as life.
JWST and CRIRES+ are tasked to look for both methane and oxygen biosignatures. Astronomer Maggie Thompson at UC Santa Cruz puts it like this:
“Methane is one piece of the puzzle, but to determine if there is life on a planet you have to consider its geochemistry, how it’s interacting with its star and the many processes that can affect a planet’s atmosphere on geologic timescales.”
