Astronomers have confirmed that there are 4,422 extrasolar worlds within 3,280 star systems. There are another 7,445 candidates still to be confirmed. Only 165 of these have been classified as terrestrials (aka. These are rocky and similar in size to Earth.
This will likely change in the future as next-generation instruments, such as James Webb, can observe smaller planets orbiting closer to their stars. This is where Earth-like worlds are most likely to reside. Researchers from the University of Napoli (INAF) have found that Earth-like biospheres are very rare in exoplanets.
The study, entitled Efficiency of the oxygenic photosynthesis on Earth-like planets within the habitable zone was published in the Monthly Notices of the Royal Astronomical Society. Giovanni Covone, an astrophysics professor at the University of Napoli led the team. The study examined whether exoplanets have enough Photosynthetically Active Radiation to enable the creation of complex biospheres.
This artist's impression shows the planet orbiting HD 85512, a Sun-like star in the southern constellation Vela (The Sail). Credit: ESO/M. Kornmesser
This research builds upon what we know about Earth's evolution over time. Scientists have been able gather information from geological records, climate studies and fossilized remains to suggest that the Earth's first lifeforms appeared on Earth around 4 billion years ago. This is just 500 million years after our Sun formed the protoplanetary disk.
These single-celled microbes used photosynthesis to produce nutrients and molecular oxygen from sunlight and carbon dioxide, which made up a large portion of the Earth's atmosphere at that time. This led to the Great Oxygenation Event (c.a. 2.4 to 2.0 Billion years ago). It saw molecular oxygen slowly accumulate in the Earth's atmosphere, allowing for more complex lifeforms to emerge.
Photosynthetic organisms rely on solar radiation from 400 to 700 nanometers in the electromagnetic spectrum to produce oxygenic photosynthesis. This corresponds roughly with the range of light the human eye can see aka. visible light. Astrobiologists are concerned about this because Sun-like stars (G type yellow dwarfs) are very rare (between 1% & 4%).
Main sequence M-type red giants make up approximately 75% of the stars in our Universe. Red dwarfs are less bright than Sun-like stars and have a lower flare activity. They also produce a lot of ultraviolet radiation. Based on current data, red dwarfs are the most likely to find Earth-like planets.
Kepler 422-b, an artistic representation of a potentially habitable planet (left) and Earth (right). Credit: Ph03nix1986/Wikimedia commons
Covone and his coworkers examined the amount of energy exoplanets known to receive, and whether this would be sufficient to produce nutrients or molecular oxygen. Professor Covone summarized this in a Royal Astronomical Society press release.
This result suggests that Earth-like conditions may not be as common on other planets, given that red dwarfs are the most common type star in our galaxy. This study places strong constraints on the parameters space for complex life. It appears that the sweet spot to host a rich Earth-like biosphere may not be as wide.
They discovered that only one of the known rocky exoplanets receives the PAR required to sustain large biospheres. Kepler-442b was a rocky planet twice as large as Earth (aka. A Super-Earth orbits within the HZ a K-type orange dwarf, approximately 1,206 light years away. Further, they discovered that stars with half of the Sun's surface temperature (5500 C; 994 F) or less could not sustain Earth-like biospheres.
This is true for many K-type orange dwarf star K-types, which have surface temperatures ranging from 3,900 to 5,200 K (3625-2925 C; 6560- 8900 F). Although planets orbiting them might still be capable of oxygenic photosynthesis they wouldn't be able sustain rich biospheres. All M-type red dwarfs ranging from 2,000 to 3900 K (between 1725 and 4925 C; 3140 à 8900 F) wouldn't receive enough energy to activate photosynthesis.
NASA's James Webb Telescope (shown in this artist's conception) will provide additional information about exoplanets previously detected. Many more space telescopes of the next generation will build on the discoveries made by NASA's James Webb Telescope beyond 2020. Credit to NASA
Stars that fall within the O, B or A spectral range (which is generally blue or white), have surface temperatures ranging between over 30,000 K (29.725 C; 53.540 F) and a low of 5.200 K (4925 C, 8,900 F). Although planets that orbit within these HZs could produce photosynthetic organisms from their surface, they wouldn't be able sustain complex life for long enough to support biospheres.
These results are similar to previous research by Manasvi Lingam and Abraham Loeb (a postdoctoral researcher) and the Frank B. Baird Jr. Professor of Science at Harvard University (respectively). They showed that planets orbiting red dwarf stars might not receive enough photons to support photosynthesis in a new study titled Photosynthesis on Habitable Planets Around Low-Mass Stars.
The James Webb Space Telescope will launch into space in November 2021. It will use its advanced infrared imaging capabilities to find smaller planets orbiting closer to their stars, especially red dwarfs, and will return to Earth in November 2021. It will be followed by Nancy Grace Roman Space telescope (RST) in 2024. This will make use of its advanced optics and wide field view (100 times Hubble's), to detect more exoplanets.
These sophisticated observatories and others will exponentially increase the number exoplanets confirmed. This will shed new light on what it takes to make a planet habitable (for life as it is known). We will, with any luck, find planetary environments capable of supporting life as it is not yet known. This will expand the scope of our search efforts.
Further Reading: Royal Astronomical Society (MNRAS)