“The volume of space-time within range of our telescopes—what astronomers have traditionally called ‘the universe’—is only a tiny fraction of the aftermath of the big bang,” says astrophysicist Martin Rees. “We’d expect far more galaxies located beyond the horizon, unobservable, each of which –along with any intelligences it hosts– will evolve rather like our own.”
But Rees’ conjecture assumes, one may surmise, carbon-based DNA and conditions similar to that of our pale blue dot. However, 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 is published today in Monthly Notices of the Royal Astronomical Society.
Carbon –Not a Good Indicator of a Biosphere
“Life on Earth is carbon-based, in that all complex biomolecules use carbon as their scaffold,” geobiologist Gregory P. Fournier at MIT wrote in an email to The Daily Galaxy. “However,” he adds, “the carbon chemistry involved is extremely diverse, and works in concert with many other elements as well. Carbon is uniquely suited to this role among all the elements, and I don’t think there is any basis for speculating that non-carbon based biology is a possibility.
“That being said,” Fournier concludes, “the overall abundance of carbon on a planet may not be a good indicator of the plausibility of a biosphere being present; only a very small fraction of Earth’s carbon, or any other element, is tied up in living systems.”
Galactic Habitable Zone
In 2019, Astronomers modeled the evolution of the Milky Way Galaxy over the past two decades to trace the distribution in space and time of four prerequisites for complex life, what they coined the “Galactic Habitable Zone” (GHZ): the presence of a host star, enough heavy elements to form terrestrial planets similar to Earth, sufficient time for biological evolution, and an environment free of life-extinguishing supernovae or gamma ray bursts.
While the exact boundaries of the GHZ are not clear, the inner regions of the Milky Way have plenty of metals but are hazardous for life, while the outer regions of the thin disc are safer, but metal-poor and less likely to contain Earth-like planets.
In between, there is the GHZ, that is just right for life, with our Solar System sitting near its center. We live in a particularly favorable location in the Milky Way for life. We also live at a special moment in the Milky Way for life. Until about 5 billion years ago, even apart from the shortage of metals needed for life’s origin, the activity of star birth and the supernovas would have made life hazardous.
The number of confirmed planets in our own Milky Way galaxy now numbers into the thousands, reports The Royal Astronomical Society. 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.
Photosynthetically Active Radiation (PAR)
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 smoulder 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.
The “Red Dwarf” Equation
“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 astrophysicist Giovanni Covone of the University of Naples, lead author of the paper. “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.
Image Credit: Panthalassa
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