“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.”
Photosynthesis based on Oxygen
But Rees’ conjecture assumes, one may surmise, carbon-based DNA and conditions similar to that of our pale blue dot. A new analysis of exoplanets suggests that Earth-like conditions may not be common on potentially habitable planets. This research focuses on what conditions are required for oxygen-based photosynthesis to occur on a planet. This would allow complex biospheres similar to those found on Earth.
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. FournierMIT sent 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.
Although the boundaries of the GHZ remain ambiguous, it is clear that the inner regions are rich in metals, but they are dangerous for life. The outer regions of thin disc, however, are more safe, but less likely to have Earth-like planets.
The GHZ is in the middle of all this, which is perfect for life. Our Solar System is located near its center. Our location is ideal for our lives. Our lives are also at an exceptional moment in the Milky Way. 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.
According to The Royal Astronomical Society, there are now thousands of confirmed planets within our Milky Way galaxy. 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.
Only a few of these potentially habitable exoplanets with rocky surfaces are currently 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.
Kepler 442b
Kepler-442b is the closest planet to receive the stellar radiation needed to sustain large biospheres. It’s a rocky planet with a mass about twice that of Earth orbiting a moderately warm star at 1,200 light-years away.
The study examined how much energy a planet receives from its star and whether living organisms could efficiently produce nutrients, as well as molecular oxygen. This is essential for complex life, such that it can be reproduced via normal oxygenic photosynthesis.
Photosynthetically Active Radiation, (PAR).
The team calculated the amount of photosynthetically activate radiation (PAR), that a planet receives by its star. They found that stars at half the temperature of the Sun can’t sustain Earth-like biospheres, as they don’t provide enough energy within the right wavelength range. Although oxygenic photosynthesis is still possible, such planets would not be able to 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 brighter than our Sun are more visible and emit up to ten-times as much radiation in the required range for photosynthesis. However, they do not usually live long enough to support complex life.
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 CovoneUniversity of Naples, the lead author of this 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)Launched later this year, the X-ray Observatory will allow us to view distant worlds from other stars. This will give us a new insight into what it takes for a planet that can support life as we know.
Maxwell Moe, astrophysicist, NASA Einstein FellowUniversity of Arizona via Royal Astronomical Society
Image credit: Panthalassa
Maxwell Moe is an astrophysicist and NASA Einstein Fellow at the University of Arizona. Max can usually be found at Kitt Peak National Observatory on two nights per week exploring the mysteries and wonders of the Universe. Max earned his Ph.D. in Astronomy in 2015 from Harvard University.
