How can we optimize our search for extraterrestrial life?
Magnificent illustration of the confirmed rocky and icy worlds. Image based on measured and inferred size, mass, density and temperature data. © Martin Vargic
Today we know between 7334 confirmed exoplanets and 5600, depending on the sources and validation methods. Of this fairly large number, between 5% and 8% are rocky exoplanets comparable to Earth. It seems quite logical that the search for exobiological activity, that is to say extraterrestrial life, focuses on these worlds.
Some will argue that for about 10 years since we have been regularly detecting exoplanets (the first was officially detected in 1995, then many between 2014 and 2018 thanks to the Kepler telescope), the signature of life outside our Earth has been slow to appear. .
It is true that Humanity has not yet shown that a form of life, even simple, exists elsewhere. Two space probes have also set off towards the moon Europa for this purpose, because we know that 700 million kilometers from us there is a promising ocean world (read our report on Europa Clipper here). The journey will nevertheless be long.
Jupiter's moon Europa where extraterrestrial life would be possible depending on the characteristics of its subglacial ocean (real colors). © NASA/Juno
The duration of existence of civilizations may be very short
In fact, this ability to truly search for extraterrestrial life (or exobiology) is recent compared to the 300,000 years of existence of theHomo sapiens. And if we take a step back even further, life has inhabited the Earth for billions of years without even a single animal setting foot on a continent. The possibility of detecting an exoplanet with a life comparable to ours is therefore statistically very low. And, who knows, perhaps human civilization only has 1000 years to live? Or 250 years? We ultimately would not have existed that long, because our planet belonged to bacteria much longer than to animals. This is also a well-known approach to the Fermi paradox: the Great Filter theory.
Recently, researchers from the Carl Sagan Institute showed that primitive bacteria could have been purple using a vitamin A called retinal (we popularized the publication here). Planets supporting life could therefore be purple and not green.
Illustration of a planet where bacteria and algae use a vitamin that colors them purple rather than green, as with current photosynthesis. © Generated on Bing Creator by Brice Haziza
Polarized light gives additional information to the spectra as usually analyzed
Here, they are researchers from Florida and Seti which remind us that the analyzed atmospheres of exoplanets focus on non-polarized light: sunlight passes through the atmosphere and spectrometers determine which chemical species are found there. These researchers believe that it is necessary to add polarimetry, that is to say the light reflected by a surface, therefore polarized, that is to say returned in one direction. It is a technique used in biology, because it gives a lot of information about the object that reflects the light.
To illustrate the principle of polarized light, here is a diagram of the effect of a polarizing filter on non-polarized light. It goes in all directions before passing through the filter. © https://science-questions.org
Goodis Gordon, of the Planetary Sciences Group in Florida and author of the study, explains that “Polarization is a more sensitive tool than flux observations alone, and can improve characterizations of exoplanets. Polarimetry is extremely sensitive to the physical mechanism scattering light, allowing precise characterizations of the properties of a planetary atmosphere and surface”. He adds that“within the Solar System, polarimetric observations have contributed to characterizing the clouds of Titan, Venus and gas giants […] In most of these cases, the characterizing finding was only possible with polarimetry.”
A team therefore decided to focus its research on the polarized signatures of light reflected by the Earth over time, then to study the flow and polarization of the reflected light in order to identify a planet that would shelter primitive life. . For example, we assume that the Proterozoic atmosphere of the Earth, which corresponds to the greatest period of habitability of our planet, i.e. 2 billion years, was very different from ours with 0.1% of O2 (21% currently).
Artist's illustration of the Earth during the Archean, aeon which precedes the Proterozoic. Note the apparent size of the Moon, much closer to Earth at that time. © Tim Bertelink
Scientists have therefore characterized the different light spectra according to different geological eras, as well as the polarized light reflected by an oceanic world, with an icy surface, covered with vegetation, etc.
This type of fundamental study is in fact essential to guide the research and manufacturing of our future telescopes and instruments, such as LUVOIR and the HabeX.
Provisional design of the LUVOIR space telescope. © NASA/Goddard
The future HabEx telescope and its external coronagraph to mask the light of target stars to better observe exoplanets orbiting them. © NASA / Public domain
This video revisits the idea that purple worlds can support life:
