One of the best ways to find life beyond Earth is to study the atmospheres of nearby stars, a role for which the JWST was designed. However, even the best telescope is of no use if the information it provides is misinterpreted, and a team of astronomers fears that is exactly what will happen.
Life has altered the Earth’s atmosphere by releasing molecular oxygen and resulting ozone and absorbing most of the carbon dioxide. Life-abundant planets may not exactly replicate our combination of gases, but astrobiologists hope to find signatures distinctive enough to distinguish a world teeming with life from one that’s mostly or entirely dead.
The problem, according to Dr. MIT’s Julien de Wit argues that we risk overestimating the accuracy with which we can calculate molecular abundance from JWST data. In a new article, de Wit and co-authors explain why this could lead to incorrect conclusions on this very important question.
“There is a scientifically significant difference between a compound like water, which is 5 percent present, and 25 percent, which current models cannot distinguish,” de Wit said in a statement.
We can study the atmospheres of other planets by observing what happens to light shining through them. Every gas absorbs electromagnetic radiation at certain wavelengths. If the spectrum of light from a more distant source is reduced at these wavelengths, it means that the gas in question must be present there.
However, the amount of a gas is just as important as its presence. Astronomers use what is known as an opacity model to translate dimming at specific wavelengths into gas abundance estimates. The authors argue that the best opacity model developed so far has been able to handle the limited data Hubble was able to provide on atmospheric absorption, but not what we’re starting to get from JWST. Massive telescopes like the Extremely Large Telescope (ELT) currently under construction in Chile will face similar problems.
This is not just speculation, argue de Wit and co-authors. They created a spectrum that JWST could produce when observing a planet, and then created eight “disturbed versions” and fed them all into the model. The model could not distinguish if a planet had a tropical 27°C (80th°F) from a 300 near Venus°C(572°F) whether the atmospheric pressure was similar to or twice that of Earth, nor determine the abundance of gases to a factor of five.
“Now that we’re taking Webb’s precision to the next level, our translation process will prevent us from capturing important subtleties, such as those that make the difference between a planet being habitable and not,” de Wit said.
As the saying goes, “It’s not what you don’t know that gets you in trouble.” It’s what you know for sure, it just isn’t.” The biggest problem may be the false confidence astronomers can develop. “We found that even with a wrong model, there are enough parameters that need to be adjusted to still get a good fit, which means you wouldn’t know your model was wrong and what it was telling you, is wrong,” explained de Wit.
Few things would damage confidence in science more than astronomers announce the discovery of a planet that is not only habitable but inhabited before having to retract that claim.
Consequently, the first message of the paper is to be careful in interpreting what comes out of the model. The paper also contains some ideas for creating better models, but neither de Wit nor his co-authors have a better version ready to use. For this we need to measure and compare many planetary atmospheres with the JWST, rather than jumping to conclusions from the first results we get.
“There is so much that could be done if we knew exactly how light and matter interact,” said Prajwal Niraula, a PhD student at MIT and the paper’s director. “We know this well enough about Earth’s conditions, but as we move into different types of atmospheres things change, and that’s a lot of data of increasing quality that we risk misinterpreting.”
The study was published in Nature Astronomy.