Cosmic Artist: How Photosynthesis Paints Habitable Worlds ✛ ✦

Imagine an artist whose canvas is an entire planet, and whose palette is molecules drinking starlight. His brush is billions of years of evolution, and his painting is a spectral portrait visible even from another star system. This artist is photosynthesis, and it is his works that astrobiologists hope to find on distant exoplanets.

Earth's biological autograph is well known: biosignatures of oxygenic photosynthesis — a sharp 'red edge' of vegetation at wavelengths of 680–760 nm and the accumulation of oxygen in the atmosphere. This effect occurs when chlorophyll stops absorbing near-infrared light, and leaves begin to reflect it intensely. But nature is not limited to one template. Already known are anoxygenic bacteria working up to 1020 nm, and rhodopsins capable of dominating spectral niches around red dwarfs. If for a planet around a Sun classic chlorophyll a is the perfect choice, then under the dim light of an M-star, pigments absorbing in the near-IR are in demand. This forces us to reconsider the very notion of the photosynthetic 'habitable zone', expanding it into the realm of red suns, whose nature was first revealed by Cecilia Payne-Gaposchkin.

But how to distinguish an original from a fake? Minerals like cinnabar can also create sharp spectral jumps, and only a combination of signs — oxygen, water, context — will point to a living artist.

The key to understanding which palette life wields is given by quantum physics. The energy of a photon, according to Einstein, is determined by the formula E = hν, and only photons with sufficient energy can trigger a chain of phototransformations. This imposes strict limits: for oxygenic photosynthesis, the long-wavelength boundary reaches about 800 nm, and the minimum photon flux density for survival is just 0.04 mmol m⁻² s⁻¹. The evolution of stars sets a variable photon flux: the young Sun shone about 30% fainter, and aging giants flood their planets with different light. However, a planet can hide its habitability for long epochs. Earth mastered oxygenic photosynthesis as early as the Archean, but geochemical sinks devoured oxygen for hundreds of millions of years before it accumulated in the atmosphere. For astronomers peering into the spectra of distant worlds with JWST and future direct-imaging observatories, this is a serious warning: even a living planet may appear lifeless.

Today, astronomy stands on the threshold of an era when telescopes will be able not only to detect planets, as William Borucki did, but also to read their spectral 'canvases'. The planned Habitable Worlds Observatory will search not only for oxygen and water, but also for agnostic autographs — pigment shifts not tied to Earthly biochemistry. Synthetic biology is already attempting to redesign photosystems, predicting which colors evolution might use under stars of a different spectral class. Perhaps in a few decades, on the canvas of another super-Earth, a self-portrait of an unknown phototrophic world will emerge. And then the question 'Are we alone?' will cease to be philosophical, transforming into a story told by light.