Earth-like conditions on exoplanets may be rarer than previously believed

Astronomers have finished analyzing known exoplanets and have found that Earth-like conditions on these potentially habitable worlds may be much rarer than previously believed. Researchers on the project focused on conditions required for oxygen-based photosynthesis to develop on the planet. Photosynthesis would enable a complex biosphere's of the type found on Earth.

Scientists continue to discover more exoplanets, with the number of confirmed planets in the Milky Way galaxy confirmed to be into the thousands. Astronomers say that planets that are both Earth-like and in the habitable zone around their host star are much less common. Currently, we are only aware of a handful of rocky and potentially habitable exoplanets in our galaxy. However, the new research indicates that none of those planets has the theoretical conditions to sustain an Earth-like biosphere resulting from oxygenic photosynthesis.

Oxygenic photosynthesis is the mechanism that plants use to convert light and carbon dioxide into oxygen and nutrients here on Earth. The only exoplanet that comes close to receiving the stellar radiation required to sustain a large biosphere is Kepler-442b. That planet is rocky, but about twice the mass of Earth and orbits a moderately hot host star. It's about 1200 light-years away from Earth.

Researchers on the study investigated in detail how much energy is received by a planet from its host star and if living organisms would efficiently produce nutrients and molecular oxygen, which are both essential elements for complex life as we know it. The team discovered that stars around half the Sun's temperature can't sustain Earth-like biospheres because they don't have enough energy in the correct wavelength range.

However, oxygenic photosynthesis would still be possible on planets orbiting those stars, but the planets couldn't sustain a rich biosphere. Planets orbiting a red dwarf star would not receive enough energy to support active photosynthesis. Stars hotter than the sun would emit up to 10 times more radiation than the necessary range for effective photosynthesis but don't live long enough for complex life to evolve. Since red dwarf stars are by far the most common type in the galaxy, the study indicates that Earth-like conditions on other planets may be much less common than previously believed.