EarthSky | Are Earth-like Biospheres Rare?


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Artist’s concept of Kepler-442b (left) in contrast to Earth. This potentially habitable rocky exoplanet is about double the mass of Earth. It is the only world found to date that might be able to maintain a life-like surface, atmosphere, and hydrosphere similar to those on Earth. Image via Ph03nix1986 / Wikimedia Commons.

With its incredible diversity of life, Earth is unique in our solar system. But how rare is the Earth’s biosphere – the thin layer of our world that harbors life, stretching high in our atmosphere and deep into our oceans – in our Milky Way galaxy? Astronomers are now discovering exoplanets, or worlds orbiting distant stars, by the thousands. They estimate that there are billions of exomondes in our galaxy alone. The galaxy must surely be teeming with life. Where is it? Researchers at the University of Naples in Italy suggest otherwise. Their new study suggests that Earth-like biospheres on potentially habitable exoplanets may be rare.

The researchers published their peer-reviewed results in May in the Monthly notices from the Royal Astronomical Society.

Earth-like oxygen-based biospheres

The study focuses on Earth-like conditions where oxygen-based photosynthesis can occur. As we know from our own world, these conditions can also allow a more complex life to develop. This process is called oxygenated photosynthesis, where plants on Earth convert light and carbon dioxide into oxygen and nutrients. As stated in the document:

Oxygenic photosynthesis is the most important biochemical process in the terrestrial biosphere and probably very common on other habitable terrestrial planets, given the general availability of its chemical ingredients and light as an energy source. It is therefore important to assess the effective possibility of oxygenated photosynthesis on the planets around the stars as a function of their spectral type and of the planet-star separation.

Many small rocky planets all labeled, including Earth and Mars, with Neptune and Jupiter to scale.
This is the current list of the most probable potentially habitable exoplanets, the catalog of habitable exoplanets (the total of all potentially habitable exoplanets is 60). At present, only Kepler-442b is believed to receive enough energy from its star for an Earth-like biosphere. Image via Planetary Habitability Laboratory / University of Puerto Rico Arecibo.

Not enough stellar radiation

But how easily could this happen on other potentially habitable worlds? It all has to do with the amount of stellar radiation – called photosynthetically active radiation (PAR) – the planet receives from its star. If there is too little radiation, it makes photosynthesis much more difficult to start. Paper :

We aim to estimate the photon flux, exergy and exergy efficiency of radiation in the wavelength range useful for oxygenated photosynthesis as a function of the effective temperature of the host star and the planet-star separation. . We analytically calculate these quantities and compare our results with the estimates for the small sample of known Earth-like planets, and find that exergy is an increasing function of the star’s effective temperature, in the range 2600-7200 K [2300-7000 Celsius or 3700-12,500 Fahrenheit]. It depends on both the star-planet separation and the actual temperature of the star. Biospheres on exoplanets around cold stars could generally be limited by light. So far, we have not observed terrestrial planets comparable to Earth in terms of useful photon flux, exergy and exergy efficiency.

Man standing with one hand on his chin and one on his hip, with table and blackboard behind him.
The new study was led by Giovanni Covone at the University of Naples in Italy. Image via INFN.

The planet must receive enough stellar radiation for living organisms to efficiently produce nutrients and molecular oxygen. Stars that are only about half the temperature of the sun do not produce enough radiation. Photosynthesis on planets orbiting such stars could still occur, but the growth of the biosphere would be limited.

Colder and hotter stars also have problems

The prospects for vibrant biospheres would darken further for red dwarf stars, which are only about a third of our sun’s heat. In this scenario, there wouldn’t be enough stellar radiation for even photosynthesis to start. According to Giovanni Covone of the University of Naples, lead author of the study:

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 hoped.

Stars that are hotter than our sun produce a lot of needed radiation, but don’t live as long as our sun. This therefore limits the time available for life to evolve on all the planets.

Worlds suitable for Earth-like biospheres may be scarce

At present, scientists only know of one potentially habitable exoplanet that is close to receiving enough stellar radiation: Kepler-442b. This planet is approximately double the mass of Earth and is located 1,200 light years away. This is from the known rocky and potentially habitable exoplanets that have been found so far (currently 60). It should be noted, however, that many more rocky worlds like these are expected to be discovered, as technology improves to find them. For the most part, the larger gas giant type planets are still the easiest to detect.

Circular board divided into colorful sections with many designs of various life forms such as insects, plants and fungi.
The Earth has a rich and diverse biosphere, using enough energy from the sun to support photosynthesis and more advanced life processes. Image via Encyclopedia Britannica.

Covone added:

This study places strong constraints on the parameter space for complex life, so it unfortunately seems that the “sweet spot” for hosting a rich Earth-like biosphere is not that large.

Other types of biospheres

The overall results suggest that planets capable of supporting Earth-like biospheres may be scarce. Of course, this is based on life as we know it, what we know about the evolution of life on Earth and the role of photosynthesis. It is possible, however, that there are alien biospheres that do not look like ours. There may be some who use photosynthesis in a way that is not found on our planet, or who do not use photosynthesis at all.

Conclusion: A new study by researchers at the University of Naples in Italy suggests that highly evolved Earth-like biospheres may be rare on exoplanets. Many stars either do not emit enough energy for life to develop beyond the stage of basic photosynthesis or do not live long enough for life to evolve on planets that might otherwise be suitable.

Source: Efficiency of oxygenated photosynthesis on Earth-like planets in the habitable zone

Via the Royal Astronomical Society

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