A new analysis of known exoplanets has revealed that Earth-like conditions
on potentially habitable planets may be much rarer than previously thought.
The work focuses on the conditions required for oxygen-based photosynthesis
to develop on a planet, which would enable complex biospheres of the type
found on Earth. The study was recently published in the Monthly Notices of
the Royal Astronomical Society.
The number of confirmed planets in our own Milky Way galaxy now numbers into
the thousands. However, planets that are both Earth-like and in the
habitable zone — the region around a star where the temperature is just
right for liquid water to exist on the surface — are much less common.
At the moment, only a handful of such rocky and potentially habitable
exoplanets are known. However the new research indicates that none of these
has the theoretical conditions to sustain an Earth-like biosphere by means
of ‘oxygenic’ photosynthesis — the mechanism plants on Earth use to convert
light and carbon dioxide into oxygen and nutrients.
Only one of those planets comes close to receiving the stellar radiation
necessary to sustain a large biosphere: Kepler-442b, a rocky planet about
twice the mass of the Earth, orbiting a moderately hot star around 1,200
light-years away.
The study looked in detail at how much energy is received by a planet from
its host star, and whether living organisms would be able to efficiently
produce nutrients and molecular oxygen, both essential elements for complex
life as we know it, via normal oxygenic photosynthesis.
By calculating the amount of photosynthetically active radiation (PAR) that
a planet receives from its star, the team discovered that stars around half
the temperature of our Sun cannot sustain Earth-like biospheres because they
do not provide enough energy in the correct wavelength range. Oxygenic
photosynthesis would still be possible, but such planets could not sustain a
rich biosphere.
Planets around even cooler stars known as red dwarfs, which smolder at
roughly a third of our Sun’s temperature, could not receive enough energy to
even activate photosynthesis. Stars that are hotter than our Sun are much
brighter, and emit up to ten times more radiation in the necessary range for
effective photosynthesis than red dwarfs, however generally do not live long
enough for complex life 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,” comments Prof. Giovanni Covone of the
University of Naples, lead author of the study.
He adds: “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.”
Future missions such as the James Webb Space Telescope (JWST), due for
launch later this year, will have the sensitivity to look to distant worlds
around other stars and shed new light on what it really takes for a planet
to host life as we know it.
Reference:
Giovanni Covone et al., Efficiency of the oxygenic photosynthesis on
Earth-like planets in the habitable zone, Monthly Notices of the Royal
Astronomical Society (2021). DOI:
10.1093/mnras/stab1357
Tags:
Space & Astrophysics