What color would plants be if the sun was red? The color of a plant is not arbitrary; it is a direct consequence of the light it receives and how its biological machinery interacts with that light. Understanding this requires examining how plants use sunlight for energy and how different light sources emit distinct colors.
Why Plants Are Green
Earth’s plants appear green because of chlorophyll, a pigment central to photosynthesis. Chlorophyll molecules efficiently absorb light in the red and blue regions of the visible spectrum. This absorption provides the energy needed to convert carbon dioxide and water into sugars, fueling plant growth. However, chlorophyll does not absorb green light effectively.
Instead, chlorophyll reflects green light, which is why our eyes perceive plants as green. Earth’s sun emits a broad spectrum of visible light, including significant red and blue wavelengths that plants readily utilize. The reflection of green light is a characteristic outcome of chlorophyll’s specific absorption properties.
The Nature of a Red Sun
A “red sun” refers to a star like a red dwarf, which is much smaller and cooler than Earth’s sun. These stars have surface temperatures ranging from approximately 2,100 to 3,800 Kelvin. They emit the majority of their radiation in the red and infrared parts of the electromagnetic spectrum.
Compared to our sun, a red dwarf produces very little blue or green light. Its emission peak is heavily shifted towards red wavelengths. This distinct light profile means plant life on an orbiting planet would be exposed to a fundamentally different energy source.
How Plants Absorb Light
Plants do not rely on chlorophyll for light absorption alone; they also possess other photosynthetic pigments. These accessory pigments, such as carotenoids, phycobilins, and anthocyanins, absorb wavelengths of light that chlorophyll cannot. Each pigment has a unique “absorption spectrum,” detailing the specific colors of light it can capture. This diversity allows plants to broaden the range of light they can use for photosynthesis.
Plants evolve to optimize their light absorption based on the available light environment. If a particular wavelength of light is abundant, plants tend to develop pigments that efficiently absorb that light to maximize energy capture. The overall efficiency of photosynthesis is influenced by which wavelengths are available and how well the plant’s pigments can absorb them.
The New Color of Life
If a planet orbited a red sun, the dominant light available for photosynthesis would be red and infrared. In such an environment, plants would likely evolve pigments that are highly effective at absorbing these plentiful red wavelengths. To maximize energy intake from a dimmer star, these plants would need to absorb as much of the available light as possible.
If plants efficiently absorb red light, they would reflect unabsorbed colors, likely from the opposite end of the visible spectrum. This could lead to plants appearing blue, green, or even purple. In scenarios where plants evolve to absorb nearly all available red light, they might appear very dark or even black to our eyes, as this would be the most efficient strategy for energy collection. The specific color would depend on the mix of pigments that evolve to best capture the red sun’s unique light spectrum, leading to a diverse palette of non-green vegetation.