Everything we study about our planet, our solar system, and even exoplanets seems to come back to one big question: Are we alone? It’s a huge question, really. Perseverance is looking for evidence that Mars had life. Phosphine was in the news last year because it could have been a biological marker for Venus. Europa Clipper is in the works, with teams meeting this week to discuss the instruments and science objectives, one of which is to look for molecular signs of life in the geysers. And Kepler, wonderful Kepler, its mission was not to hunt for exoplanets. No, its mission was to find Earth-like planets in the habitable zones of their stars.

Today, we have found over 4000 exoplanets. We still haven’t found that Earth 2.0 everyone is looking for, though. Sure, there are planets about the size of Earth. There are planets in the habitable zone. But the stars are too active, the planets too big, or some combination of the two. Nothing seems to fit just right. And it could be that finding that Earth-like planet may be even harder than we originally thought.

In a new study published in the Monthly Notices of the Royal Astronomical Society, scientists examined how much energy planets receive from their host stars and if there was enough energy available for biological life to process using oxygen-based photosynthesis. Some plants and animals here on Earth use the radiation from the Sun to turn carbon dioxide and water into fuel, but to have carbon dioxide, you need oxygen already in place. The results were not encouraging.

Per the press release: Planets around even cooler stars known as red dwarfs, which smolder at roughly a third of our Sun’s temperature, could not receive enough energy to even activate photosynthesis. Stars that are hotter than our Sun are much brighter, and emit up to ten times more radiation in the necessary range for effective photosynthesis than red dwarfs, however generally do not live long enough for complex life to evolve.

A lot of researchers were holding out hope that red dwarf stars, which are the most common type of star in the Milky Way, would be great places to harbor life. This new research actually narrows the exoplanet window we should be looking in. On the one hand, it’s a smaller window. On the other hand, we can focus the search a little better than we could before.

More Information

RAS press release

“Efficiency of the oxygenic photosynthesis on Earth-like planets in the habitable zone,” Giovanni Covone, Riccardo M Ienco, Luca Cacciapuoti, and Laura Inno, 2021 May 19, Monthly Notices of the Royal Astronomical Society