These may not be the worlds we should look for Micha Pawlitzki/Getty

Although water worlds are awash with one of the key ingredients for life, surprisingly, they might not be the best places to find it.

Tessa Fisher, a graduate student at Arizona State University in Tempe, and her colleagues presented this counter-intuitive idea last week at the Habitable Worlds conference in Laramie, Wyoming. Her research shows that a planet soaked in oceans could be starved of phosphorus – a major component of DNA and other important molecules.

Unlike other essential nutrients for life, phosphorous is hard to find. It’s mostly locked away in rocks, so it only becomes accessible when rainfall splatters those rocks and flushes phosphorous into water where it can be used by microbes.


Although rainwater is quite efficient at dissolving phosphorus, seawater is not. And that’s a problem for worlds entirely covered by salty seas. Without any exposed land, there will be far less phosphorous available for fledgling life. Fisher and her colleagues have estimated that these worlds will have three to four times less phosphorous in their oceans than seas on Earth.

Not only does Fisher’s work suggest that kick-starting life on such a world would be tricky, it is also possible that should life take hold, astronomers would be hard-pressed to detect it. In fact, Fisher and her colleagues found that even if life such as phytoplankton is present, they would release only one-tenth the amount of oxygen currently in Earth’s atmosphere. That’s far too low to be detectable.

Don’t follow the water

In light of this, astronomers who are striving to find life beyond the solar system might want to point their telescopes toward worlds that aren’t overflowing with water.

The results sent ripples throughout the conference. Shawn Domagal-Goldman at the Goddard Space Flight Center in Greenbelt, Maryland, didn’t even wait for Fisher’s talk to finish before jumping onto his research team’s Slack channel to discuss the results, and even do a little work of his own. “We started modeling what the atmosphere above the ocean would look like in real time because we were that excited by what she was doing,” he says.

Domagal-Goldman hopes to collaborate with Fisher and her colleagues to better assess the chemistry of the atmosphere above the water worlds. Meanwhile, Fisher plans to look at how the rate of available phosphorous varies with differing amounts of water.

While this research shows that there can be too much water for life to flourish, she’s curious to see if there’s a sweet spot where the amount of water is just right.

Life we can find

“What Fisher’s work shows us is that we really have to think beyond whether or not water is plausible when we think about the likelihood of life on these worlds,” Domagal-Goldman says.

“We have to also think about the other things that impact not just whether life could get a foothold but also how productive that life would be. Because if a planet is in a habitable zone, even if it has life, we may not be able to find that life.”

That means astronomers are beginning to shift away from a focus on habitability towards one directed by detectability. “When we observe any kind of atmospheric signature, we’re going to see a tangled mess of everything the planet has to offer,” says Elizabeth Tasker at the Japan Aerospace Exploration Agency in Tokyo.

And untangling what those readings mean for the chances of life is going to take more than just astronomers.

“People who do oceanography and microbial ecology have known for years that you need rainwater to dissolve phosphorus, but none of the astronomers knew that so they didn’t really think about it,” Fisher says. “I think the major breakthrough with this is that someone finally put the astronomers and the oceanographers and the biologists all in one room.”

Read more: Ocean worlds: Searching for life in the solar system’s seas