Earth’s Atmosphere Could Be an Extremely Rare Species as a Result of a Single Chemical Process
At the moment, life has a sample size of one. Without an extraterrestrial or two to push biological boundaries, Earth’s evolutionary past limits our expectations of other worlds producing complicated critters like, well, us.
Given that many types of life owe a great deal to the oxygen in our atmosphere, it is reasonable to look for aliens on other worlds surrounded by a comparable mix of gases. However, a new study indicates that we will require much patience.
Researchers from Italy’s University of Naples Federico II and INAF Capodimonte Astronomical Observatory examined the light received by ten possibly habitable exoplanets orbiting various types of stars and found no match for Earth’s atmosphere.
Earth is already a member of a relatively elite club, based on our observations of the thousands of planets discovered orbiting other stars. Once multiple gas giants, roasted balls of rock, and frozen super-Earths are ruled out, there are few options with the biochemistry we are familiar with.
Even yet, if even a small proportion of billions of stars has a few massive things circling close enough to allow liquid water to puddle on their surfaces, we could be staring up at hundreds of millions of Edens in our galaxy.
Even the simplest gardens require adequate sunlight to thrive — at least by Earth’s standards.
Additionally, not just any type of solar radiation will suffice. Carbon dioxide and water must be converted into glucose and those oh-so-useful oxygen molecules. This requires a type of light that is powerful enough to initiate reactions without destroying proteins.
Given that exoplanets in habitable zones receive an abundance of sunlight and that oxygenic photosynthesis evolved so early in Earth’s history, it is reasonable to think it is a very widespread phenomenon among the stars.
To test that hypothesis, the researchers measured the amount of light falling on various planetary surfaces and the spread of the wavelengths that make up the radiation, then computed the amount of ‘exergy,’ or work that might be squeezed out of the sunshine.
If only all those stars were as pleasant as ours.
The majority are red dwarfs – temperamental suns capable of blasting their inner planets with gale-force winds that rapidly deplete their atmospheres.
Even if planets were capable of withstanding such outbursts, the researchers discovered that a red dwarf’s colder temperatures would be unlikely to offer an intensity of the wavelengths required to trigger photosynthesis.
“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,” Covone explains.
While brighter stars would be preferable, they are unlikely to survive the billions of years required to pump out the oxygen sophisticated life on Earth requires to begin started.
In sum, a star half the brightness of our Sun might initiate photosynthesis but would have difficulty establishing a sophisticated biosphere.
Among the planets considered as case studies, precisely zero would be capable of powering enough photosynthesis to tip a carbon dioxide-rich atmosphere in an Earth-like direction.
“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 explains.
One planet that we are aware of is relatively near to that sweet spot.
Kepler-442b circles an orange dwarf around 1,200 light years away that is roughly 60% the mass of the Sun. With a mass roughly equal to that of Earth and a rotation that allows it to shed heat, it appears to be a potential paradise thus far.
On Earth, the majority of photosynthesizing processes terminate at wavelengths about 700 nanometers. However, if an alien moss on Kepler-442b gained the ability to absorb slightly longer wavelengths, approximately 800 nanometers, it would gain 20% more photons.
As our library of known worlds grows, it is feasible that we will discover better possibilities for biospheres similar to ours.
On our own planet, evolution continues to astound us; we can only imagine the variety of ecosystems that could exist elsewhere in the galaxy. For all we know, chemosynthetic ice moons may be the majority. Perhaps there are variants of photosynthesis that we are unaware of due to the limitations of our experience on Earth.
Recognizing our uniqueness does not imply that we are inevitably alone. However, given what we are learning, we might take a moment to appreciate how unique our taste of life is.
This study was published in the Royal Astronomical Society’s Monthly Notices.