A planet can potentially appear purple if its early life forms utilized retinal-based photosynthesis rather than chlorophyll.
The Purple Earth Hypothesis
The idea that a planet could be purple stems from the Purple Earth Hypothesis (PEH). This hypothesis, proposed by molecular biologist Shiladitya DasSarma in 2007, suggests that:
- Early Earth's Photosynthesis: The first photosynthetic organisms on Earth might have used retinal, a simpler molecule, for photosynthesis.
- Retinal vs. Chlorophyll: Instead of the more complex porphyrin-based chlorophyll used by most plants today, these early life forms could have relied on retinal.
- Purple Hue: Retinal-based photosynthesis would absorb green light and reflect purple light, giving the planet a distinctive purple appearance.
Why Purple?
Here's why a planet with retinal-based life could be purple:
- Light Absorption: Retinal absorbs green light most efficiently.
- Color Reflection: When a substance absorbs a certain color of light, it reflects the remaining colors. Therefore, if retinal absorbs green, it will reflect mostly purple light.
- Planetary Appearance: If enough of the surface is covered in retinal-based photosynthetic life, the planet could appear purple from space.
Implications
The Purple Earth Hypothesis offers a possible explanation for:
- Early Earth's Biology: It may help explain why early life developed as it did.
- Exoplanet Discoveries: It could help in the search for life on other planets, as we may see purple hues on exoplanets harboring life.
Therefore, the key to making a planet purple is the dominance of organisms that use retinal for photosynthesis.