Earth-like biospheres on other planets may be rare

What are the conditions for an exoplanet to host a rich and complex biosphere like that on Earth?

A new study by a group of Neapolitan astrofixers led by Professor Giovanni Covone of the Federico II Department of Physics has revealed that Earth-like conditions on potentially habitable planets may be much rarer than previously thought. The team is rounded out by Riccardo Maria Ienco and Luca Cacciapuoti, students in the Master of Science degree in Physics at the Federico II, and Dr. Laura Inno, a researcher at Parthenope University.

Although it is difficult to extrapolate from the only planet with life that we know of, scientists believe that the essential conditions for life are the availability of chemical elements such as carbon, hydrogen, oxygen and nitrogen, a solvent (such as water), a generous source of energy (such as light from a star) and an efficient biochemical mechanism to convert inorganic material into biomass.

 

On Earth, bacteria and plant organisms have evolved oxygen photosynthesis to produce organic elements by harvesting light from the Sun in the region of the electromagnetic spectrum from 400 to about 700 nm. Photosynthesis is essential for sustaining Earth's rich and lush biosphere. In fact, it plays a dual role: it is a source of organic food and a source of molecular oxygen, which is essential for animal metabolism.

 

"The process of photosynthesis depends strongly on the type of light radiated by the star around which a planet orbits." - explains Professor Covone - "We studied in detail how the process of oxygen photosynthesis depends on the characteristics of the host star. In particular, we compared the amount of radiation useful for photosynthesis that each star can send to the surface of rocky planets in the so-called habitable zone."

 

The habitable zone around a star is that orbital belt where a planet with an atmosphere could host liquid water on its surface.

Stars cooler than the Sun emit far less useful ration for photosynthesis than the Sun does, while hotter stars do not live long enough to allow the evolution of complex life as we know it.

 

The work, published in the journal Monthly Notices of the Royal Astronomical Society, began with the results of a three-year dissertation by Frederick student Riccardo Maria Ienco, the first thesis in astrobiology in the Department of Physics.

 

"In my thesis work, I compared the conditions present in the Earth-Sun system with those of other planetary systems with Earth-like planets," Ienco explains. "Currently, nearly 4500 extrasolar planets are known, but only 10 of them are Earth-like. And on none of them would photosynthesis be so efficient as to support a lush biosphere as on Earth."

 

Only one of these planets comes close to receiving the stellar radiation needed to support a large biosphere: Kepler-442b, a rocky planet with about twice the mass of Earth, about 1,200 light-years away from us.

 

"Red dwarf stars are by far the most common type of star in the Galaxy,"Luca Cacciapuoti comments , "but in our work we have found that they do not offrono suitable conditions for an Earth-like biosphere on the planets they orbit."

 

On most exoplanets in the habitable zone, oxygenic photosynthesis would be possible, but none of them could support a rich biosphere. "In the Galaxy, Earth-like conditions may be much less common than one might have hoped," comments Professor Giovanni Covone.

 

Future missions such as the James Webb Space Telescope (JWST), scheduled to be launched later this year, will offrifit the chance to study the atmospheres of Earth-like planets and shed new light on what it really takes for a planet to harbor life as we know it.

 

More information

The work appears in, ""Efficiency of the oxygenic photosynthesis on Earth-like planets in the

habitable zone,"" G. Covone, R.M. Ienco, L. Cacciapuoti and L. Inno, Monthly Notices of the Royal Astronomical Society (2021), in press (DOI: 10.1093/mnras/stab1357)