One of the best ways to find life beyond the Earth is to study the atmospheres of nearby stars, a role the JWST was designed for. However, even the best telescope is no use if the information it provides is misinterpreted, and one team of astronomers fears that is what will happen.
Life has changed Earth’s atmosphere, releasing molecular oxygen and resulting ozone and absorbing most of the carbon dioxide. Planets with abundant life might not precisely replicate our combination of gasses, but astrobiologists hope to find signatures distinctive enough to tell a world teaming with life from one that is mostly or completely dead.
The problem, according to Dr Julien de Wit of MIT, is we risk overestimating the precision with which we can calculate molecular abundance from JWST data. In a new paper, de Wit and co-authors explain why that could lead to wrong conclusions about this very important question.
“There is a scientifically significant difference between a compound like water being present at 5 percent versus 25 percent, which current models cannot differentiate,” de Wit said in a statement.
We can study the atmospheres on other planets by observing what happens to light shining through them. Any gas will absorb electromagnetic radiation at distinctive wavelengths. When the spectrum of light from a more distant source is diminished at those wavelengths it means the gas in question must be present there.
However, the quantity of a gas is as important as its presence. Astronomers use what they call an opacity model to translate dimming at particular wavelengths into estimates of gas abundance. The authors argue the best opacity model yet developed was capable of processing the limited data Hubble could provide on atmospheric absorption, but not what we are starting to get from the JWST. Massive telescopes like the Extremely Large Telescope (ELT) currently under construction in Chile will have similar problems.
This isn’t just speculation, de Wit and co-authors argue. They created a spectra JWST might produce while observing a planet and then created eight “perturbed versions” and fed them all into the model. The model couldn’t distinguish if a planet was at a tropical 27°C (80°F) from a near-Venusian 300°C (572°F), whether atmospheric pressure was similar to Earth’s or double that, nor determine the abundance of gasses to a factor of five.
“Now that we’re going to the next level with Webb’s precision, our translation process will prevent us from catching important subtleties, such as those making the difference between a planet being habitable or not,” de Wit said.
In keeping with the adage “It Ain’t What You Don’t Know That Gets You Into Trouble. It’s What You Know for Sure That Just Ain’t So,” the biggest problem could be the false sense of confidence astronomers may develop. “We found that there are enough parameters to tweak, even with a wrong model, to still get a good fit, meaning you wouldn’t know that your model is wrong and what it’s telling you is wrong,” de Wit explained.
Few things would do more to damage confidence in science than astronomers announcing the discovery of a planet not merely habitable but inhabited, before needing to withdraw that claim.
Consequently, the first message of the paper is to take care in interpreting what comes out of the model. The paper also provides some ideas for creating better models, but neither de Wit nor his co-authors have a superior version ready to go. For that, we’ll need to measure a lot of planetary atmospheres with the JWST and compare them, rather than jumping to conclusions about the first results we get.
“There is so much that could be done if we knew perfectly how light and matter interact,” said MIT graduate student and the paper's lead, Prajwal Niraula. “We know that well enough around the Earth’s conditions, but as soon as we move to different types of atmospheres, things change, and that’s a lot of data, with increasing quality, that we risk misinterpreting.”
The study was published in Nature Astronomy.