Over the last several years, thousands of faraway planets have been discovered scattered throughout the Milky Way. Tantalisingly, some of these planets have been found to be rocky, like the Earth, and to be orbiting their parent stars in the so-called habitable zone – the region in which the temperature is just right for liquid water to exist on the surface. This suggests they may have the correct conditions to harbour life.
However, a new study carried out at the University of Naples has found that none of the exoplanets discovered so far have the conditions needed to form an Earth-like biosphere – the area on Earth that is home to all of the planet’s living organisms.
The researchers’ analysis is based on determining the conditions necessary to kickstart oxygen-based photosynthesis – the mechanism plants on Earth use to convert light and carbon dioxide into oxygen and nutrients. If these conditions are not met, then the formation of the kind of complex biosphere found on Earth is highly unlikely.
Key among these conditions is the amount of energy beamed at the planet by its parent star. According to the team’s calculations, stars that are around half the temperature of the Sun can provide enough energy in the correct range of wavelengths – so-called photosynthetically active radiation – to kickstart photosynthesis, but not enough to establish and maintain an Earth-like biosphere.
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Planets in orbit around red dwarfs, even cooler stars that burn at about one-third of the temperature of the Sun, don’t even receive enough energy to kickstart photosynthesis in the first place. While brighter stars that are hotter than the Sun do produce enough photosynthetically active radiation, their short lifespans mean they are likely to burnout long before complex life has the time to evolve.
“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,” said lead author, Giovanni Covone, professor of astrophysics at the University of Naples. “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.”
The only exoplanet discovered so far that comes anyway near close to receiving the correct amount of energy to maintain a large biosphere is Kepler-442b – a rocky planet about twice the mass of the Earth, orbiting a moderately hot star located around 1,200 light-years away, the researchers say.
Future space missions such as the James Webb Space Telescope (JWST), which is due for launch later this year, will be fitted with instruments capable of looking at distant planets in unprecedented detail. This will allow scientists to further investigate what it takes for a planet to host life as we know it.