It is no exaggeration to mention that the study of extrasolar planets has exploded in recent decades. To date, 4,375 exoplanets are confirmed in 3,247 systems, with another 5,856 candidates awaiting confirmation.
In recent years, exoplanet studies have began to transition from the-process of discovery to at least one of characterization.
This process is predicted to accelerate once next-generation telescopes become operational.
As a result, astrobiologists are working to make comprehensive lists of potential “biosignatures,” which refers to chemical compounds and processes that are related to life (oxygen, CO2 , water, etc.)
But consistent with new research by a team from the Massachusetts Institute of Technology (MIT), another potential biosignature we should always get on the lookout for is-a hydrocarbon called isoprene (C5H8).
The study that describes their findings, “Assessment of Isoprene as a Possible Biosignature Gas in Exoplanets with Anoxic Atmospheres,” recently appeared online and has been accepted for publication by the journal Astrobiology.
For the sake of their study, the MIT team checked out the growing list of possible biosignatures that astronomers are going to be on the lookout for within the coming years.
To date, the overwhelming majority of exoplanets are detected and confirmed using indirect methods.
For the foremost part, astronomers have relied on the Transit Method (Transit Photometry) and therefore the Radial Velocity Method (Doppler Spectroscopy), alone or together in-combination. Only a couple of are detectable using Direct Imaging, which makes it very difficult to characterize exoplanet atmospheres and surfaces.
Only on rare occasions have astronomers been ready to obtain spectra that allowed them to work out the chemical composition of that planet’s atmosphere. This was either the results of light passing through an exoplanet’s atmosphere because it transited ahead of its star or within the few cases where Direct Imaging occurred and light-reflected from the exoplanet’s atmosphere might be studied.
Much of this has had to try to to with the limits’ of our current telescopes, which don’t have the necessary resolution to observe-smaller, rocky planets that orbit closer to their star.
Astronomers and astrobiologists believe that it’s these planets that are presumably to be potentially habitable, but any light reflected from their surfaces and atmospheres is overpowered by the light coming from their stars.
However, which will change soon as next-generation instruments just like the James Webb Space Telescope (JWST) takes to space. Sara Seager, the category of 1941 Professor of Physics and Planetary Sciences at MIT, leads the research group responsible (aka the Seager Group) and was a co-author on the paper.
As she told Universe Today via email, “With the upcoming October 2021 launch of the James Webb Space Telescope we’ll have our first capability of checking out biosignature gases – but it’ll be tough because the atmospheric signals of small rocky planet are so weak to start with. With the JWST on the horizon, the amount of individuals working within the field has grown tremendously. Studies like this one arising with new potential biosignature gases, and other work showing potential false positives even for gases like oxygen.”
Once it’s deployed and operational, the JWST are going to be ready to observe our Universe at longer wavelengths (in the near- and mid-infrared range) and with greatly improved sensitivity.
The telescope also will believe a series of spectrographs to get composition data, also as coronagraphs to dam out the obscuring light of parent stars. This technology will enable astronomers to characterize the atmospheres of smaller rocky planets.
In turn, this data will allow scientists to put much tighter constraints on an exoplanet’s habitability and will even cause the detection of known (and/or potential) biosignatures.
As noted, these “biosignatures” include the chemical indications related to life and bio-logical process , to not mention the kinds of conditions that are favorable thereto .
These include oxygen gas (O2), which is important to most sorts of life on Earth and is produced by photosynthetic organisms (plants, trees, cyanobacteria, etc.). These same organisms metabolize CO2 (CO2), which oxygen-metabolizing life emits as a waste. There’s also water (H2O), which is important to all or any life as we all know it, and methane (CH4), which is emitted by decaying organic matter.
Since volcanic activity is believed to play a crucial role in planetary habitability, the chemical byproducts related to volcanism – sulfide (H2S), sulphur dioxide (SO2), carbon monoxide gas (CO), hydrogen gas (H2), etc. – also are considered biosignatures.
To this list, Zhan, Seager, and their colleagues wished to feature another possible biosignature – isoprene.
As Zhan explained : “Our research group at MIT focuses on employing a holistic approach to explore all possible gases as potential biosignature gas. Our prior work led to the creation of the all small molecules database. We proceed to filter the ASM database to spot the foremost plausible biosignature gas candidates, one among which is isoprene, using machine learning and data-driven approaches.”
Like its cousin methane, isoprene is an organic hydrocarbon molecule that’s produced as a secondary metabolite by various species here on Earth. additionally to deciduous trees, isoprene is additionally produced by a various array of evolutionary-distant organisms – like bacteria, plants, and animals.
As Seager explained, this makes it promising as a possible biosignature. “Isoprene is promising because it’s produced in vast qualities by life on Earth – as-much-as methane production! Furthermore, an enormous sort of life forms (from bacteria to plants and animals), people who are evolutionary distant from one another , produce isoprene, suggesting it’d be some type of key building block that life elsewhere may also-make.”
While isoprene is about as abundant as methane here on Earth, isoprene is destroyed by interaction with oxygen and oxygen-containing radicals. For this reason, Zhang, Seager, and their team chose to specialise in anoxic atmospheres. These are environments that are predominantly composed of H2, CO2, and nitrogen gas (N2), which is analogous to what Earth’s primordial atmosphere was composed of.
According to their findings, a primordial planet (where life is starting to emerge) would have abundant isoprene in its atmosphere.
This would had-been the case on Earth between 4 to 2.5 billion years ago when single-celled organisms were the sole life and photosynthetic cyanobacteria were slowly converting Earth’s atmosphere into one that was oxygen-rich.
By 2.5 billion years ago, this culminated within the “Great Oxygenation Event“ (GOE), which proved toxic to several organisms (and metabolites like isoprene).
It was also during this point that complex lifeforms (eukaryotes and multi-celled organisms) began to emerge. In this respect, isoprene might be used’ to characterize planets that are within the midst of a serious evolutionary shift and laying the groundwork for future animal phyla.
But as Zhang noted, teasing out this potential biosignature are going to be a challenge, even for the JWST.
“The caveats with isoprene as a biomarker are that:
1. 10x-100x the Earth’s Isoprene production rate is required for detection
2. Detecting Near-Infrared isoprene spectral feature are often hindered by the presence of methane or other hydrocarbons. Unique detection of isoprene are going to be challenging with JWST, as many hydrocarbon molecules share similar spectra features in Near-Infrared wavelengths. But future telescopes that specialise in the mid-IR wavelength are going to be ready to detect isoprene spectral features uniquely.”
Beyond the JWST, the Nancy Grace Roman Space Telescope (successor to the Hubble mission) also will be taking to space by 2025. This observatory will have the facility of “One-Hundred Hubbles“ and its recently-upgraded infrared filters will allow it to characterize exoplanets on its own and thru collaborations with the JWST and other “great observatories.”
There also are several ground-based telescopes currently being built here on Earth which will believe sophisticated spectrometers, coronographs, and adaptive optics (AOs). These include the Extremely Large Telescope (ELT), the Gaint Magellan Telescope (GMT), the Thirty Meter Telescope (TMT) These telescopes also will be ready to conduct Direct Imaging studies of exoplanets, and therefore the results are expected to be ground-breaking.
Between improved instruments, rapidly improving data analysis and techniques, and enhancements in our methodology, the study of exoplanets is merely expected to accelerate further.
In addition to having tens of thousands of more available for study (many of which can be rocky and “Earth-like”), the unprecedented views we’ll have of them will allow us to see just what percentage habitable worlds are out there.
Whether or not this may end in the invention of extraterrestrial life within our lifetimes remains to be seen.
But one thing is obvious . within the coming years, when astronomers start combing through all the new data they’re going to wear exoplanet atmospheres, they’re going to have a comprehensive list of biosignatures to guide them.
Seager and Zhan’s previous work include an idea for a Martian greenhouse that would provide all the required food for a crew of 4 astronauts for up to 2 years. This greenhouse, referred to as the Biosphere Engineered Architecture for Viable Extraterrestrial Residence (BEAVER), took second place in 2019 NASA BIG Idea Challenge.
The findings were reported on arxiv
