What was it like when the first living worlds formed? | by Ethan Siegel | Starts With A Bang! | Jan, 2024

Artist’s conception of the exoplanet Kepler-186f, which may exhibit Earth-like (or early, life-free Earth-like) properties. As imagination-sparking as illustrations like this are, they’re mere speculations, and the incoming data won’t provide any views akin to this at all. Kepler 186f, like many known (r so-called) Earth-like worlds, isn’t orbiting a Sun-like star, but that might not necessarily mean that life on this world is impossible. Early exoplanets, formed long before Earth, may have been excellent candidates for life to first arise in the Universe. (Credit: NASA Ames/SETI Institute/JPL-Caltech/T. Pyle)

Life became a possibility in the Universe as soon as the raw ingredients were present. But living, inhabited worlds required a bit more.

It’s almost unfathomable how far humanity has come in just the past few decades in terms of discovering potentially habitable worlds. As recently as 1990, there were no known, confirmed planets beyond the ones here in our Solar System; as of today, there are more than 5000 confirmed exoplanets, from super-Jupiters all the way down to sub-Earth sized worlds. Many of the smaller ones are around stable, Sun-like stars; many of them are thought to have thin atmospheres; many likely possess the heavy elements needed for life processes; many are part of multi-planet systems, with the potential for Earth-like temperatures and pressures at their surface.

While Earth might be the template for what we think of as an ideal, habitable world, we can still envision a wide variety of circumstances that are very different from our own that might also support life on a long-term basis. It took more than 9 billion years of cosmic evolution before Earth formed, however. It’s wildly unreasonable to assume that the Universe required all of that time to create the necessary conditions for habitability.

When we look at the minimum recipe for habitability, with the fewest assumptions, those conditions could have originated far earlier. The raw, chemical ingredients for life are a part of that puzzle, but can’t be the entire story. But in order to form a habitable planet, we have to look more deeply. Here’s what we need to know about the earliest habitable worlds.

With recent discoveries, we have a tremendous amount of knowledge about the number of planets out there, including what stars they’re around and what sizes and distances from their star they possess. But when it comes to the question of whether they’re inhabited, we have no information at all. (Credit: Lucianomendez)

The first thing you need, if you want to have an inhabited world, is the right type of star for it to orbit around. There could be all sorts of scenarios where life on a planet can survive even around an active, violent star, and remain habitable despite the hostility.

The lowest mass stars, the red dwarf stars like Proxima Centauri, emit flares and high-energy radiation in bursts. Any potentially habitable…

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