A stray population of mysterious aquatic worlds may have just been revealed

The Milky Way could be a much wetter place than we thought.

A new analysis of exoplanets orbiting red dwarf stars suggests we may have missed a population of “water worlds” — waterlogged planets composed of up to 50 percent water.

Not all of these worlds will be covered by a global liquid ocean; Scientists expect that for many of them the water will be bound in hydrated minerals. However, the find could have implications for our search for life outside the solar system.

“It was a surprise to see evidence that so many aquatic worlds orbit the most common type of star in the galaxy,” says University of Chicago astronomer Rafael Luque.

“This has enormous consequences for the search for habitable planets.”

Although we cannot see a single red dwarf with the naked eye, these stars are incredibly numerous. Small, cool, and faint red dwarfs are at most about half the mass of the Sun.

Their low fusion rate gives them the greatest longevity of any star; At 13.8 billion years old, the Universe is not old enough for a red dwarf star to survive its entire estimated 100 billion year lifespan.

An estimated 73 percent of the Milky Way’s stellar population consists of red dwarf stars. Just think about it for a moment. If you’re stargazing on a warm summer night in a cool field or on the bed of a truck in the desert, you can’t even see most of the stars in the sky.

Because they are so dark and red, it is difficult to find exoplanets orbiting red dwarfs. Only a small percentage of the 5,084 confirmed exoplanets at the time of writing have been found around red dwarf stars.

However, our instruments are becoming more sophisticated – enough that scientists have been able to characterize dozens of small worlds orbiting these tiny stars.

There are two main signals that scientists look for to characterize an exoplanet. The first is a regular dimming of starlight as the orbiting exoplanet passes between us and the star.

The second is a tiny lengthening and contraction of the star’s light wavelength as the orbiting exoplanet exerts a weak gravitational pull.

Once you have these measurements, and know how far away the star is (and how much light it is emitting), you can measure the exoplanet’s radius and mass — two properties that astronomers can use to derive an exoplanet’s density.

From this density, conclusions can be drawn about the composition of the exoplanet. A low density likely means an exoplanet with a lot of atmosphere, like a gas giant. A high density likely means a rocky world like Earth, Venus, or Mars.

Luque and his colleague, astronomer Enric Pallé from the Institute of Astrophysics of the Canary Islands and La Laguna University in Spain, conducted a density study of 43 exoplanets orbiting red dwarf stars.

Typically, these exoplanets have been divided into two categories: rocky exoplanets and dense-atmospheric gaseous ones. But Luque and Pallé saw a curious third category emerge: exoplanets too dense to be gaseous but not quite dense enough to be purely rocky either.

Their conclusion was that the rock composition of these mid-range exoplanets was mixed with something lighter…like water, maybe. But while it’s tempting to imagine a world of stormy seas, these planets are too close to their stars for liquid water on their surfaces.

If their water were at the surface, it would swell their atmosphere, making it even larger in diameter and less dense.

“But we don’t see that in rehearsals,” says Luque. “This suggests that the water is not in the form of a surface ocean.”

Instead, these worlds could look like another object in the solar system – Jupiter’s moon Ganymede, which is roughly half rock and half water, with the water hidden under a rocky, icy shell. Or they could be a bit like the moon (though significantly wetter), which has water molecules bound in glass and minerals.

However, these worlds have been holding back their waters, if the team’s conclusions are correct, the discovery suggests these worlds could not have formed where they did. Instead, they should have moved farther from their stars, formed out of rock and ice, and migrated inward to their current positions.

However, without further evidence, at this point it is impossible to make a decision in favor of this model one way or another.

“Apart from this possibility of detecting extraterrestrial life forms,” ​​writes astronomer Johanna Teske of the Carnegie Institution for Science in a related perspective, “measuring the compositional diversity of planets around red dwarf stars – the most common type of star in the Milky Way – is important to measure to piece together the complex puzzle of the formation and evolution of small planets.”

The research was published in Science.