Exoplanets. Exoplanets everywhere. Since the 1990s, we have discovered over 4,000 exoplanets. Hot Jupiters – big, puffy gas giants orbiting close to their parent stars. Mini-Neptunes, super-Earths, and everything in between, or so it seems. We’re only just starting to dig into the compositions and atmospheres of these worlds, but there have been exoplanets where diamonds likely rain down, where magma flows from one half to the other, tidally locked worlds, worlds around white dwarfs, destroyed exoplanets.

None of these sound particularly habitable for life like us. And that’s the big question. Where are the worlds with liquid water where we, or life as we know it, could exist?

To answer this question, scientists have focused on figuring out how the Earth came by its store of water. One of the prevailing hypotheses is that water arrived by chance, through the strike of large asteroids that brought their ice with them. We mentioned that earlier in the show. Now, a team at the University of Copenhagen has put forth a different hypothesis, and in a new study published by Science Advances, they calculate that water was present during planetary formation.

According to lead author Anders Johansen: All our data suggest that water was part of Earth’s building blocks, right from the beginning. And because the water molecule is frequently occurring, there is a reasonable probability that it applies to all planets in the Milky Way. The decisive point for whether liquid water is present is the distance of the planet from its star.

Basically, Earth collected the first one percent of its current mass by capturing millimeter-sized dust particles that contained ice and carbon. These ingredients are common around young stars, so it makes sense that they were present around our Sun, too. Then the planet’s growth sped up, collecting more and more material. After about five million years, Earth had its current size. And Anders further explains: …the temperature on the surface rose sharply, causing the ice in the pebbles to evaporate on the way down to the surface so that, today, only 0.1 percent of the planet is made up of water, even though 70 percent of Earth’s surface is covered by water.

This new theory increases the probability that there are plenty of worlds out there that have liquid water because they all came out of essentially the same material, and that material included water ice from the start. No more chance acquisition. And, as co-author Martin Bizzarro notes: With our model, all planets get the same amount of water, and this suggests that other planets may have not just the same amount of water and oceans, but also the same amount of continents as here on Earth. It provides good opportunities for the emergence of life.

Now, all we have to do is find those planets. But at least we know they’re likely out there to find.

More Information

University of Copenhagen press release

“A pebble accretion model for the formation of the terrestrial planets in the Solar System,” Anders Johansen et al., 2021 February 17, Science Advances