anomalien.com
Why Webb May Never Be Able to Find Evidence of Life on Another World
An artist’s rendering of the James Webb Space Telescope. (Credit: NASA GSFC/CIL/A. M. Gutierrez)The exoplanet K2-18b is generating headlines because researchers announced what could be evidence of life on the planet. The JWST detected a pair of atmospheric chemicals that on Earth are produced by living organisms, reports universetoday.com.
The astronomers responsible for the results are quick to remind everyone that they have not found life, only chemicals that could indicate the presence of life. The results beg a larger question, though: Will the JWST really ever detect life?
The JWST was developed with four overarching science themes, and one of them is Planetary Systems and the Origins of Life. Early design documents and science papers developed this theme, though they were cautious in predicting what the telescope would find.
Much of the writing acknowledged that the JWST would struggle to identify definitive biosignatures. Instead, the telescope was characterized as an intermediate step between the Hubble and the Spitzer, and future telescopes that could reliably detect biosignatures.
In a new paper, well-known planetary scientist Sara Seager from MIT and her co-authors from the USA, the UK, and Europe remind us how difficult it is for the JWST to provide definitive proof of life on distant exoplanets. The paper is titled “Prospects for Detecting Signs of Life on Exoplanets in the JWST Era,” and will be published in the Proceedings of the National Academy of Sciences.
“The search for signs of life in the Universe has entered a new phase with the advent of the James Webb Space Telescope (JWST),” the authors write. “Detecting biosignature gases via exoplanet atmosphere transmission spectroscopy is in principle within JWST’s reach.” The question is, how reliable are those detections? Are public expectations out of line with the telescope’s actual capabilities?
This image illustrates the recent strong detection of potential biosignatures DMS and DMDS in the atmosphere of the exoplanet K2-18b. Image Credit: A. Smith, N. Madhusudhan (University of Cambridge)
Definitive evidence for life on a distant exoplanet like K2-18b was never going to jump up and announce itself. That planet is about 125 light-years away. Planetary atmospheres are complex, and the great distance makes understanding them more difficult.
Transmission spectroscopy is a powerful tool, but it faces great challenges. The light from the star can pollute spectroscopic results, and data retrieval is tricky. For these reasons and others, Seager and her colleagues suggest we abandon the idea of detecting an atmospheric “silver bullet” that reveals the presence of life.
Instead, the JWST’s main contribution is to build a more comprehensive understanding of exoplanets and their atmospheres. “Characterizing rocky or sub-Neptune-size exoplanets with JWST is an intricate task, and moves us away from the notion of finding a definitive ‘silver bullet’ biosignature gas,” the authors write.
One of the difficulties the JWST faces in transmission spectroscopy of rocky and sub-Neptune size planets is that it’s really only suitable for planets orbiting M dwarfs (red dwarfs). Since these stars are smaller, the signal from transiting exoplanets is more easily detected, whereas larger, brighter stars can introduce a lot of noise into planetary transit signals.
“Since M dwarf stars are half to one-tenth the size of our Sun, the TS (transmission spectroscopy signal) will have signals 4 to 100 times larger than Sun-sized star hosts,” the authors explain.
However, M dwarfs present their own challenges.
The problem is that M dwarfs tend to be more active than Sun-sized stars. “Their stellar magnetic activity, higher than for Solar-type stars, manifests as star spots, faculae, and flares that contaminate the spectra,” the authors write. They mention that in the well-known TRAPPIST-1 system, the M dwarf star contaminates and overwhelms the transmission spectra.
It’s worth noting that K2-18b also orbits an M dwarf.
The authors are reminding us how difficult it is to take a transmission spectroscopy signal and reach concrete conclusions about its meaning. “It may seem a stretch to use spectra to ascertain planetary properties (atmosphere abundances, surface and interior bulk composition, habitability and presence of life, and more).
After all, observed exoplanet spectra represent a highly averaged signal of complex 3D physical and chemical atmospheric processes, reduced to relative changes in the observed wavelength-dependence of the combined star and planet light as a point source,” they explain.
Interpreting transmission spectroscopy signals is not simple. We’re still in the early stages of this type of science, and researchers will only get better at it. Seager and her co-authors explain that there are three criteria for determining if a biosignature detection is reliable:
1. Detection: Is the signal robust?
2. Attribution: Are the spectral features correctly attributed to the appropriate gas(es)?
3. Interpretation: How reliable are the derived planetary properties?
According to the authors, the tentative detection of DMS and/or DMDS fails to meet all three of these criteria.
“The example of the tentative detection of DMS in K2-18 b’s atmosphere is the exoplanet community’s first encounter with a biosignature gas prospect—a claim that fails all three Key Criteria above,” they write.
The authors don’t mince words in their conclusion: “We conclude with the sobering realization that with JWST, we may never be able to definitively claim the discovery of a biosignature gas in an exoplanet atmosphere.”
However, scientists are making progress, and the JWST is a key tool in the effort. By acquiring more observations and data, it is contributing to a better understanding of exoplanets and their atmospheres. Astronomers will continue to find biosignature candidates in exoplanet atmospheres, and each detection will add to their body of knowledge.
“In the years to come, JWST will remain the flagship of this era of discovery and will be remembered as the first telescope that set the first concrete steps toward answering the question: Are we alone?”
The post Why Webb May Never Be Able to Find Evidence of Life on Another World appeared first on Anomalien.com.
News Feed (Home)
Popular Posts 
Events 
Blog 
Market 
Forum 
Headline News
VidWatch
Game Zone
Top PodCasts
