To date, astronomers have confirmed the existence of 4,422 extrasolar planets in 3,280 star systems, with an additional 7,445 candidates awaiting confirmation. Of these, only a small fraction (165) have been terrestrial (aka. rocky) in nature and comparable in size to Earth – i.e., not “Super-Earths.” And even less have been found that are orbiting within their parent star’s circumsolar habitable zone (HZ).
In the coming years, this is likely to change when next-generation instruments (like James Webb) are able to observe smaller planets that orbit closer to their stars (which is where Earth-like planets are more likely to reside). However, according to a new study by researchers from the University of Napoli and the Italian National Institute of Astrophysics (INAF), Earth-like biospheres may be very rare for exoplanets.
The study, titled “Efficiency of the oxygenic photosynthesis on Earth-like planets in the habitable zone,” was recently published in the Monthly Notices of the Royal Astronomical Society. Led by astrophysics Prof. Giovanni Covone of the University of Napoli, the team focused on whether or not exoplanets discovered so far get enough Photosynthetically Active Radiation (PAR) to allow for the development of complex biospheres.
This artist’s impression shows the planet orbiting the Sun-like star HD 85512 in the southern constellation of Vela (The Sail). Credit: ESO/M. Kornmesser
This work builds on what we’ve come to know about the evolution of Earth’s biosphere, which has changed drastically over time. From what scientists have been able to piece together from the geological record, climatological studies, and fossilized remains, it is theorized that the first lifeforms emerged on Earth roughly 4 billion years ago, just 500 million years after the planet formed from the protoplanetary disk that surrounded our Sun.
These single-celled microbes relied on photosynthesis to generate nutrients and molecular oxygen (O2) from sunlight and carbon dioxide – which made up a significant portion of Earth’s atmosphere at the time. By the Paleoproterozoic Era (c.a 2.4 to 2.0 billion years ago), this led to the “Great Oxygenation Event,” where molecular oxygen began to slowly accumulate in Earth’s atmosphere and allowed for the emergence of more complex lifeforms.
Specifically, photosynthetic organisms relied on solar radiation that ranges from 400 to 700 nanometers on the electromagnetic spectrum to carry out “oxygenic photosynthesis” – which corresponds roughly to the range of light that the human eye can perceive – aka. visible light. This is of significant concern to astrobiologists since Sun-like stars (G-type yellow dwarfs) are rare, with an estimated 4.1 billion in the Milky Way galaxy (between 1% and 4%).
It is main sequence M-type red dwarfs that make up the majority of stars in our Universe, accounting for roughly 75% in our galaxy alone. Compared to Sun-like stars, red dwarfs are cooler and less luminous and are known for their elevated flare activity and producing a significant amount of radiation in the ultraviolet band. In addition, based on the current census of rocky exoplanets, red dwarfs are considered to be the most likely place to find Earth-like planets.
Artistic representation of the potentially habitable planet Kepler 422-b (left), compared with Earth (right). Credit: Ph03nix1986/Wikimedia Commons
For the sake of their study, Covone and his colleagues examined how much energy known-terrestrial exoplanets receive and whether it would be enough to