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) were terrestrial (also known as rocky) and comparable in size to Earth – ie not “super-earths”. And even fewer have been found that orbit within the Circumolar Habitable Zone (HZ) of their parent star.
In the years to come, this is likely to change when next-generation instruments (like James Webb) are able to observe smaller planets orbiting closer to their stars (there, Earth-like planets are more likely). However, according to a new study by researchers from the University of Naples and the Italian National Institute of Astrophysics (INAF), Earth-like biospheres for exoplanets could be very rare.
The study, entitled “Efficiency of Oxygen Photosynthesis on Earth-Like Planets in the Habitable Zone,” was recently published in the Monthly Notices of the Royal Astronomical Society. Under the direction of astrophysicist Prof. Giovanni Covone from the University of Naples, the team focused on whether the exoplanets discovered so far received enough photosynthetically active radiation (PAR) to enable the development of complex biospheres.
This artist’s impression shows the planet orbiting the sun-like star HD 85512 in the southern constellation Vela (The Sail). Photo credit: ESO / M. Grain knife
This work builds on what we know about the evolution of the Earth’s biosphere, which has changed dramatically over time. From what scientists have been able to put together from geological records, climatological studies, and fossilized remains, it is theorized that the first life forms appeared on Earth about 4 billion years ago, only 500 million years after the planet emerged from the protoplanetary disk that surrounded our sun.
These unicellular microbes used photosynthesis to create nutrients and molecular oxygen (O2) from sunlight and carbon dioxide – which at the time made up a significant portion of the Earth’s atmosphere. In the Paleoproterozoic (about 2.4 to 2.0 billion years ago) this led to the “Great Oxygenation Event”, in which molecular oxygen slowly accumulated in the earth’s atmosphere and enabled the emergence of more complex forms of life.
Photosynthetic organisms in particular relied on solar radiation, which ranges from 400 to 700 nanometers in the electromagnetic spectrum, to carry out “oxygen-rich photosynthesis” – which roughly corresponds to the range of light that the human eye can perceive – aka. visible light. This is of great concern to astrobiologists as sun-like stars (yellow type G dwarfs) are rare, with an estimated 4.1 billion in the Milky Way (between 1 and 4%).
They are M-type red dwarfs of the main order that make up the majority of the stars in our universe and about 75% in our galaxy alone. Compared to sun-like stars, red dwarfs are cooler and less luminous and are known for their increased flare activity and produce a significant amount of radiation in the ultraviolet range. In addition, based on the current rocky exoplanet count, red dwarfs are considered the most likely place where Earth-like planets can be found.
Artist’s impression of the potentially habitable planet Kepler 422-b (left) compared to Earth (right). Photo credit: Ph03nix1986 / Wikimedia Commons
For their study, Covone and his colleagues examined how much energy known terrestrial exoplanets receive and whether this would be sufficient to produce nutrients and molecular oxygen. As Prof. Covone summarized in a press release from the Royal Astronomical Society:
“Since red dwarfs are by far the most common type of star in our galaxy, this result suggests that Earth-like states may be far less common on other planets than we could hope for. This study severely restricts the parameter space for complex life, so that unfortunately it appears that the “sweet spot” for housing a rich, earth-like biosphere is not that big. “
They found that of all known rocky exoplanets, only one gets anywhere near the amount of PAR that it would need to sustain a large biosphere. This was Kepler-442b, a rocky planet about twice as massive as Earth (also known as Super-Earth) orbiting in the HZ of an orange K-type dwarf about 1,206 light-years away. They also found that stars with half the surface temperature of our Sun – 5,778 K (5500 ° C; 9940 ° F) – or less cannot sustain Earth-like biospheres.
This is true for many orange-colored dwarf stars of type K, which have surface temperatures of 3,900 to 5,200 K (3625 to 4925 ° C; 6560 to 8900 ° F). While planets orbiting them could still photosynthesize oxygen, they would not be able to sustain rich biospheres. Meanwhile, any type M red dwarfs – which are in the 2,000 to 3,900 K (1725 to 4925 ° C; 3140 to 8900 ° F) range – would not be getting enough energy to even activate photosynthesis.
NASA’s James Webb Telescope, featured in this artist’s conception, will provide more information on previously discovered exoplanets. After 2020, many more next-generation space telescopes are expected to build on the discoveries. Photo credit: NASA
Meanwhile, stars fall in the O, B, A, or F spectral range (which are generally blue or white) and have surface temperatures in excess of 30,000 K (29,725 ° C; 53,540 ° F) to a minimum of 5,200 K (4925 ° F) .). ° C; 8,900 ° F). While planets orbiting within the HZs of these stars could produce photosynthetic organisms, they would not be able to sustain biospheres long enough for complex life to develop.
These results are reminiscent of previous research by Manasvi Lingam and Abraham Loeb, a postdoctoral fellow and Frank B. Baird Jr. Professor of Science at Harvard University (each). In a 2019 study titled “Photosynthesis on Inhabitable Planets Around Low Mass Stars,” they showed how planets orbiting red dwarf stars may not receive enough photons to support photosynthesis.
In November 2021, the James Webb Space Telescope (JWST) will be launched into space, where it will use its advanced infrared imaging capabilities to detect smaller planets orbiting closer to their stars, particularly red dwarfs. The Nancy Grace (RST) Roman Space Telescope is expected to follow by 2024, and with its sophisticated optics and large field of view (100 times the size of Hubble’s), it will discover more exoplanets than ever before.
These and other sophisticated observatories will exponentially increase the number of confirmed exoplanets and shed new light on what it takes for a planet to be habitable (at least for life as we know it). With luck, we will discover planetary environments capable of supporting life in a way we are not familiar with, thus expanding the scope of our search.
Further reading: Royal Astronomical Society, MNRAS