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What If The Earth Orbited TRAPPIST-1 Instead of the Sun?

Presentation #629.07 in the session Habitability.

Published onApr 03, 2024
What If The Earth Orbited TRAPPIST-1 Instead of the Sun?

The origin of oxygenic photosynthesis is perhaps the most significant event in the history of life on Earth after the origin of life itself. Oxygenic photosynthesis generally requires light in the 400-700 nm range (Photosynthetically Active Radiation or PAR); Near-IR photons carry too little energy. TRAPPIST-1 has 7 Earth-size planets, 3 of which are in the habitable zone. The star is an ultra-cool M dwarf with a temperature of only 2560 K. Consider a hypothetical Earth at the same location as TRAPPIST-1 e. The star’s extremely red spectrum would have a profound effect on the evolution of life because the number of PAR photons is only 0.9% of what the Earth received from the Sun. On Earth it took ~700 million years between the origin of oxygenic photosynthesis and the rise of O2 in our atmosphere (the Great Oxygenation Event or GOE). If we assume the rate of oxygen production is linearly proportional to the number of PAR photons, the time to reach a GOE-like event on this hypothetical Earth is ~77 Gyrs.However, the rate of O2 production is highly non-linear due to an effect called photoinhibition. At high, or even moderate light levels, O2 production saturates and then declines. This drops the timescales to ~21-34 Gyrs. The Achaean Sun was 25% fainter, and this increases the timescales modestly. Tidal locking however doubles the number of day-side photons and reduces the timescales by a factor of 2. Extending the PAR out to 750 nm also doubles the number of photons because the stellar spectrum rises so steeply in the NIR. These lead to a GOE timescale of ~4-8 Gyrs; a Cambrian explosion would take 15-26 Gyrs.Oxygenic photosynthesis evolved only once on Earth - it was a singular event. Given numerous cases of convergent evolution, this uniqueness suggests the mechanism is particularly challenging to arise. By contrast, non-oxygenic photosynthesis is present in many forms, and it can use lower-energy NIR photons. On a planet orbiting TRAPPIST-1, non-oxygenic photosynthesis would have a huge advantage - and likely dominate. In this scenario, O2 would never reach significant levels in the atmosphere and a GOE, let alone a Cambrian explosion, would not occur. Thus one should not expect complex macrocellular animal life.

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