If astronomers see isoprene within the environment of an alien world, there’s a good probability that there’s life there

By On Apr 3, 2021

It is no exaggeration to say that the study of extrasolar planets has exploded in the past few decades. So far, 4,375 exoplanets have been confirmed in 3,247 systems, and a further 5,856 candidates are awaiting confirmation. In recent years, exoplanet studies have started moving from the discovery process to the characterization process. This process is expected to accelerate once the next-generation telescopes are operational.

As a result, astrobiologists are working to compile comprehensive lists of potential “biosignatures” relating to chemical compounds and processes associated with life (oxygen, carbon dioxide, water, etc.) from the Massachusetts Institute of Technology (MIT), another potential one The biosignature to look out for is a hydrocarbon called isoprene (C5H8).

The study “Assessment of isoprene as a possible biosignature gas in exoplanets with anoxic atmospheres”, which describes its results, was recently published online and accepted for publication by the journal Astrobiology. For their study, the MIT team looked at the growing list of possible biosignatures astronomers will look out for in the years to come.

An array of 3 exoplanets to study how atmospheres can look different depending on the chemistry in place and the incoming flow. Photo credit: Jack H. Madden

So far, the vast majority of exoplanets have been discovered and confirmed using indirect methods. For the most part, astronomers have relied on the transit method (transit photometry) and the radial velocity method (Doppler spectroscopy) alone or in combination. Only a few could be detected with direct imaging, which makes the characterization of exoplanet atmospheres and surfaces very difficult.

Only in rare cases have astronomers been able to obtain spectra with which they could determine the chemical composition of the atmosphere of this planet. This was either the result of light passing through an exoplanet’s atmosphere as it passed in front of its star, or in the few cases where direct imaging occurred and light reflected from the exoplanet’s atmosphere could be examined .

Much of this has to do with the limitations of our current telescopes, which do not have the resolution necessary to observe smaller, rocky planets orbiting closer to their star. Astronomers and astrobiologists believe that it is these planets that are most likely to be habitable, but any light reflected from their surfaces and atmospheres is overwhelmed by the light from their stars.

However, that will soon change when next-generation instruments like the James Webb Space Telescope (JWST) fly into space. Sara Seager, professor of physics and planetary science at MIT from 1941, heads the responsible research group (also known as the Seager Group) and was a co-author of the paper. As she emailed Universe Today:

“With the upcoming James Webb Space Telescope launch in October 2021, we will be able to search for biosignature gases for the first time – but it will be difficult because the atmospheric signals of a small rocky planet are so weak at first. With the JWST on the horizon, the number of people working on site has increased enormously. Studies like this one provide new potential biosignature gases and other work showing potential false positives for gases like oxygen as well. “

The artist’s impression shows the planet Proxima b orbiting the red dwarf star Proxima Centauri, the star closest to the solar system. Photo credit: ESO / M. Kornmesser

Once deployed and operational, the JWST can observe our universe at longer wavelengths (in the near and mid-infrared range) and with greatly improved sensitivity. The telescope will also rely on a number of spectrographs to get compositional data, as well as coronagraphs to block out the dark light of the parent stars. With this technology, astronomers can characterize the atmosphere of smaller rocky planets.

This data, in turn, will allow scientists to restrict the habitability of an exoplanet much more strictly and could even lead to the detection of known (and / or potential) biosignatures. As mentioned earlier, these “biosignatures” encompass the chemical indications associated with life and the biological process, not to mention the types of conditions favorable to life.

This includes oxygen gas (O2), which is essential for most life forms on earth and is produced by photosynthetic organisms (plants, trees, cyanobacteria, etc.). The same organisms metabolize carbon dioxide (CO2), which releases oxygen-metabolizing life as a waste product. There is also water (H2O), which is essential for all life as we know it, and methane (CH4), which is released when organic matter breaks down.

Since volcanic activity is believed to play an important role in the habitability of the planet, so are the chemical by-products associated with volcanism – hydrogen sulfide (H2S), sulfur dioxide (SO2), carbon monoxide (CO), hydrogen gas (H2), etc. – available as biosignatures. To that list, Zhan, Seager, and their colleagues wanted to add another possible bio-signature – isoprene. As Zhan explained to Universe Today via email:

“Our research group at MIT focuses on using a holistic approach to investigate all possible gases as potential biosignature gas. Our previous work resulted in the creation of the database for all small molecules. We filter the ASM database to identify the most plausible biosignature gas candidates, one of which is isoprene, using machine learning and data-driven approaches – Dr. Zhuchang Zhan. “

The picture was taken by a crew member of Expedition 13 of the ISS and shows the eruption of the Cleveland volcano in the Aleutian Islands in Alaska. Photo credit: NASA

Like its cousin methane, isoprene is an organic hydrocarbon molecule that is produced as a secondary metabolite by various species here on earth. In addition to deciduous trees, isoprene is also produced by a large number of evolutionarily distant organisms such as bacteria, plants and animals. This, as Seager explained, makes it promising as a potential biosignature:

“Isoprene is very promising because life on earth produces it in enormous qualities – just like methane production! In addition, a variety of life forms (from bacteria to plants and animals) that are evolutionarily distant from one another produce isoprene, suggesting that this could be some kind of key building block that life elsewhere could form as well. “

While isoprene is about as common here on earth as methane, isoprene is destroyed by interaction with oxygen and oxygen-containing radicals. Because of this, Zhang, Seager and their team focused on anoxic atmospheres. These are environments that mainly consist of H2, CO2 and nitrogen gas (N2), similar to the primordial atmosphere of the earth.

According to their findings, a primordial planet (on which life begins to arise) would have plenty of isoprene in its atmosphere. This would have been the case on earth 4 to 2.5 billion years ago, when unicellular organisms were the only life and photosynthetic cyanobacteria slowly converted the earth’s atmosphere into an oxygen-rich one.

2.5 billion years ago this culminated in the “Great Oxygenation Event” (GOE), which proved to be toxic to many organisms (and metabolites such as isoprene). During this time complex life forms (eukaryotes and multicellular organisms) also emerged. In this regard, isoprene could be used to characterize planets that are in the midst of a great evolutionary change and to lay the foundation for future animal phyla.

But as Zhang noted, filtering out this potential biosignature will be a challenge even for the JWST:

“The reservations about isoprene as a biomarker are as follows: 1. 10x-100x the isoprene production rate of the earth is required for detection; 2. The detection of isoprene spectral features in the near infrared can be hindered by the presence of methane or other hydrocarbons. The unique detection of isoprene will be challenging with JWST, as many hydrocarbon molecules have similar spectral characteristics in wavelengths in the near infrared. However, future telescopes that focus on the mid-IR wavelength will be able to capture isoprene spectral features in a unique way. “

In addition to JWST, the Roman space telescope Nancy Grace (successor to the Hubble mission) will also fly into space by 2025. This observatory will have the power of “One Hundred Hubbles” and its recently updated infrared filters will enable it to characterize exoplanets alone and in collaboration with the JWST and other “large observatories”.

Several ground-based telescopes based on sophisticated spectrometers, coronographs and adaptive optics (AOs) are currently being built here on earth. These include the Extrem Large Telescope (ELT), the Giant Magellan Telescope (GMT) and the 30-meter telescope (TMT). These telescopes can also perform direct imaging studies of exoplanets. The results are expected to be groundbreaking.

Relative sizes of the planets of the habitable Kepler zone discovered from April 18, 2013. From left to right: Kepler-22b, Kepler-69c, Kepler-62e, Kepler-62f and Earth (with the exception of Earth, these are artist representations). Photo credit: NASA / Ames / JPL-Caltech.

Between improved instruments, rapidly improved data analysis and techniques, and improvements in our methodology, the study of exoplanets is expected to only accelerate further. In addition to the tens of thousands of study opportunities (many of which will be rocky and “earth-like”), the unprecedented views allow us to see how many habitable worlds there are.

Whether or not this will lead to the discovery of extraterrestrial life in our lifetimes remains to be seen. One thing is clear, however. When astronomers begin sifting through all new data on exoplanet atmospheres in the years to come, they will have an extensive list of biosignatures to guide them.

Seager and Zhan’s earlier work includes a concept for a Martian greenhouse that can provide a crew of four astronauts with all the food they need for up to two years. Known as the Biosphere Engineered Architecture for Viable Alien Residences (BEAVER), this greenhouse took second place in the 2019 NASA BIG Idea Challenge. You can read more about it here.

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