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What Can Make a Planet Purple?

Published in Astrobiology 2 mins read

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:

  1. Early Earth's Biology: It may help explain why early life developed as it did.
  2. 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.