Earth-like atmosphere may be rare

What makes Earth special compared to other planets and the outer planets in the universe? The most obvious answer would be life, even if this concept is just the tip of the iceberg: on Earth, in fact, there is liquid water (because it is in the habitable zone of the solar system); The planet is surrounded by a thick atmosphere and a magnetosphere that protects it from sunlight; There is a great availability of elements such as carbon, hydrogen, oxygen and nitrogen, an abundant source of energy (sunlight) and an efficient biochemical mechanism for converting inorganic materials into biomass. However, there is also a phenomenon without which animal and plant life does not exist: photosynthesis of oxygen. In a study published in the journal Monthly notifications of the Royal Astronomical Society, A research group led by astronomers from the University of Naples Federico II and Parthenope University affiliated with the National Institute of Astrophysics (INAF) hypothesized that this process also exists on other habitable terrestrial (or rocky) exoplanets, evaluating which type of star has the ideal thermodynamic properties necessary for life. From the data collected, and the analysis of a few known, habitable, rocky exoplanets, it appears that Earth-like conditions could be even rarer than previously thought.

“Photosynthesis plays a dual role in the existence of the terrestrial biosphere: it is a source of organic food and a source of molecular oxygen for metabolism,” explains the first author of the article. Giovanni CovoniFederico II, professor at the University of Naples and associated with the INAF. “On Earth, living organisms exploit oxygen photosynthesis to produce organic elements, collecting sunlight mostly in the visible range of the electromagnetic spectrum, from 400 to about 700 nanometers, and producing molecular oxygen, another essential component of life. It is complex as we know it.”

Appearing on Earth more than 2.4 billion years ago, photosynthesis includes chemical elements that should be common on rocky exoplanets, according to the authors. paper, This mechanistic biochemistry could be a universal process of biomass production anywhere in the universe. Schiff adds, “We asked ourselves: Could oxygenic photosynthesis support a large biosphere on a rocky exoplanet in the habitable zone of its star whose radiation differs from that of the Sun?” The answer does not appear to be positive.

“We calculated the photon flux received from the 10 known habitable rocky planets: none of them had the theoretical conditions for maintaining an Earth-like biosphere through oxygenic photosynthesis,” Kovon comments. “Only one planet in this sample receives a useful flux close to that required to sustain a large biosphere: Kepler-442b, a rocky giant planet about twice the mass of Earth, orbiting a temperate star 1,200 light-years away from us.”

The habitable area It’s the region around a star where the surface of an Earth-like planet could contain liquid water, which could lead to life. At present, only ten of the exoplanets are classified as habitable. The study related the quantity and quality (ie, efficiency) of stellar radiation hitting a rocky planet in the interstellar habitable zone with the star’s temperature (and thus its brightness) and the distance of the stellar planet. The surface temperature of a star actually determines the brightness of the star and the spectrum of radiation emitted, the two primary factors that direct oxygenic photosynthesis.

When studying the properties of stars, the line between habitability and sterility on a planet is very thin: cold stars do not provide large amounts of photons and have low efficiency, while stars warmer than about 8000 degrees K do not live long enough. To allow life to evolve into complex systems. The Sun has a temperature of approximately 6000 degrees Kelvin and emits about 40% of its energy in the range active light radiation, in short it is called PAR. Hotter stars provide better opportunities for complex biosphere evolution, because they are brighter and therefore more radiation is emitted into the PAR.

Researchers have found that stars that are cooler than the Sun are dimmer and emit most of their near-infrared radiation. Stars below 3,700 K can’t support an Earth-like biosphere because they don’t provide enough photons in the PAR band — oxygenic photosynthesis would still be possible, but it couldn’t support life as we know it on Earth. Planets around cooler stars (red dwarfs below 2,600 degrees K) may not receive enough photons to activate photosynthesis – this is the case with some planets that make up the Trappist-1 system.

Since red dwarfs are the most common star in the Milky Way, “unfortunately, it appears that optimal conditions for hosting an Earth-like biosphere are not widely distributed,” Covon concludes.

Article “Efficiency of oxygenic photosynthesis on Earth-like planets in the habitable zone”, by Giovanni Covoni, Ricardo M. Enco, Luca Cacciabuti and Laura Eno, to be published in the Journal Monthly Notices of the Royal Astronomical Society.

Top image: Artist’s impression of the rocky exoplanet Kepler-442 b, a super-Earth planet orbiting a K-type star at a distance of 0.409 AU. It has a mass of 2.36 Earth masses, and it takes 112.3 days to complete an orbit around its star (Credits: NASA)