The research focuses on the circumstances required for oxygen-based photosynthesis to flourish on a planet, allowing for complex biospheres similar to those present on Earth.
The number of known planets in our own Milky Way galaxy is currently in the thousands. Planets that are both Earth-like and in the habitable zone - the region surrounding a star where the temperature is just right for liquid water to exist on the surface - are uncommon.
Only a few such rocky and potentially habitable exoplanets are known at the moment. The current research, however, reveals that none of these have the theoretical criteria to sustain an Earth-like biosphere through 'oxygenic' photosynthesis (a method that plants on Earth utilize to convert light and carbon dioxide into oxygen and nutrition). Only one of those planets receives the solar radiation required to support a massive biosphere: Kepler-442b, a rocky planet nearly twice the size of Earth orbiting a moderately bright star around 1,200 light-years away.
Nothng like Earth
The research looked into how much energy a planet receives from its host star and if living things would be able to successfully generate nutrients and molecular oxygen, both of which are required for complex life as we know it, through conventional oxygenic photosynthesis.
The scientists determined that stars half the warmth of our Sun cannot maintain Earth-like biospheres because they can not give enough energy in the relevant wavelength range by estimating the quantity of photosynthetically active radiation (PAR) that a planet receives from its star. Oxygenic photosynthesis would still be possible, but such planets would be incapable of supporting a diverse biosphere.
Planets orbiting even cooler stars (known as red dwarfs), which continue to burn at around one-third the temperature of our Sun, might not receive enough energy to start photosynthesis. Hotter stars are much brighter and release up to 10 times more light in the required range for successful photosynthesis than red dwarfs, but they often do not live long enough for advanced life to form.
"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," says Giovanni Covone, lead author of the study.
The research is published in Monthly Notices of the Royal Astronomical Society.
