Ever since we’ve gazed at the sky with scientific curiosity, a question has haunted us: Is Earth the only world with water in the universe? The answer, until recently, seemed to hide far beyond the reach of our telescopes. Even with advances like Hubble and Spitzer, detecting water on planets light-years away seemed nearly impossible. But… science loves to surprise. And now, a new study has just made global headlines. Using data from two NASA missions, astronomers have found evidence of a type of planet that, until now, existed more theoretically than in practice: worlds covered in water. Most intriguingly, these worlds are unlike anything we have in our cosmic backyard, and the story behind this discovery is worthy of a movie.
Astronomers thought they knew super-Earths — until water turned the rules upside down
For decades, the “super-Earth” category dominated astronomers’ imaginations. After all, they were seen as turbocharged versions of our planet: more mass, more rock, more metal, and no major surprises. But science sometimes needs a push to abandon old certainties. That’s exactly what happened when a group of researchers decided to reanalyze data from a planetary system 218 light-years away, in the constellation Lyra…
The exact location was the Kepler-138 system, and within it, two planets in particular caught their attention: Kepler-138c and Kepler-138d. When comparing size and mass, something didn’t quite match the expected pattern. They were large (three times the volume of Earth), relatively heavy (twice Earth’s mass), but with a density much lower than compact rocks would allow. This means that a significant portion of the planet is made of something lighter than rock and heavier than gas.
Two cosmic twins defy expectations
And then, when redesigning the model of the two planets, scientists realized they were dealing with cosmic twins: virtually identical in size and mass, orbiting the same red dwarf star. It’s as if they were two supersized Europas (Jupiter’s moon), but placed much closer to the local “Sun.” Some of the possible scenarios they came up with were:
- Dense vapor atmosphere: Temperatures above the boiling point would keep water in a gaseous state.
- Liquid water under pressure: Just below this layer, colossal pressures could keep water liquid.
- Supercritical phase: At extreme depths, water could exist in a hybrid state between liquid and gas, something that only occurs under very specific conditions.
The most curious thing is that these two worlds are not in the so-called “habitable zone,” that is, the range in which the temperature would allow oceans on the surface of a rocky planet. Therefore, we are not talking about destinations ripe for life as we know it. It’s worth remembering that it wasn’t only there that scientists found water outside of Earth; recently, another place, not the moon, revealed huge ice oceans.
A hidden planet hints at a universe full of water worlds
Not stopping there, while analyzing data collected by Hubble and Spitzer between 2014 and 2016, researchers found another clue: a fourth planet in the system, dubbed Kepler-138e. This one is in the habitable zone. The problem is that it doesn’t appear to transit in front of the star relative to our line of sight. Without the transit, it’s impossible to accurately measure its diameter, and without it, it’s impossible to say whether it’s rocky, gaseous, or perhaps another water world.
This type of discovery becomes very important because each time we find potentially aquatic worlds, even outside the habitable zone, we learn more about how water and atmosphere interact under different conditions. And, as our instruments advance, we may discover that “ocean worlds” are much more common than we thought. It’s no wonder NASA discovered the first “water world” in history, which is 30% made of oceans.
