None of the potentially habitable Earth-like exoplanets known to astronomers today have the right conditions to sustain life as we know it on Earth, with a rich biosphere of plants, microbes and animals, a new study has found.
The study, published in the journal Monthly Notices of the Royal Astronomical Society on Wednesday (June 23), assessed the basic conditions for oxygen-based photosynthesis on ten Earth-like exoplanets with known masses that orbit in the so-called habitable zones around their stars.
The habitable zone is a region around a star with the right temperature for the presence of liquid water, a major prerequisite for the existence of life as we know it on Earth. However, the study, by a team of astronomers from the University of Naples, Italy, found that being in the habitable zone by itself is not enough.
Photosynthesis, the life-giving process that allows plants and some microorganisms to convert light into organic matter, producing oxygen as a by-product, requires the right amount of sunlight. Not all stars can provide that.
The researchers calculated how much photosynthetically active radiation (PAR) — radiation in the wavelength range between 400 to 700 nanometers that photosynthetic organisms can use — the planets receive from their stars. They found that the planets orbit frequently around stars that are too cool to provide enough PAR. For example, a star about half the temperature of the sun would provide enough PAR to power some photosynthesis but not enough to create such a rich biosphere as Earth has.
In fact, only one of the planets in the studied sample, Kepler-442b, a super Earth orbiting a star some 1,200 light years away in the constellation Lyra, came close to receiving enough PAR to sustain a large biosphere, the scientists said in a statement.
Even though the study was done only on a very small sample of planets, astronomers know enough about the nature of stars in the Milky Way to assume that the right conditions for photosynthesis-driven life might be rare. Most of the stars in the galaxy are the so-called red dwarfs, dim stars about a third of the sun's temperature, too cool to generate any photosynthetic activity on the planets in their vicinity.
"Since red dwarfs are by far the most common type of star in our galaxy, this result indicates that Earth-like conditions on other planets may be much less common than we might hope," Professor Giovanni Covone, lead author of the study, said in the statement.
For example, out of the 30 stars in the sun's immediate neighborhood, 20 are believed to be red dwarfs.
But stars hotter than the sun are not ideal either. Bright stars generally burn up quickly and even though they might be producing enough PAR to trigger enough photosynthetic activity on a planet with water and carbon, they would probably die before any forms of complex life could evolve on those planets, the scientists added.
"This study puts strong constraints on the parameter space for complex life, so unfortunately it appears that the 'sweet spot' for hosting a rich Earth-like biosphere is not so wide," Covone added.
Astronomers have detected thousands of exoplanets in the Milky Way. But they know relatively little about them. It seems, however, that it is not that common to find Earth-like rocky planets in habitable zones where water can exist, the scientists said in the statement.
Future missions, such as the James Webb Space Telescope (JWST), scheduled for launch later this year, might be able to reveal more about the distant worlds around other stars and the possibility of the existence of complex forms of life on them.