A Roaming Population Of Mysterious Aquatic Worlds May Have Just Been Revealed

A Roaming Population Of Mysterious Aquatic Worlds May Have Just Been Revealed

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

A new analysis of exoplanets orbiting red dwarf stars suggests we may be missing a population of “aquatic worlds” – soggy planets whose composition consists of up to 50% water.

Not all of these worlds will be covered by a global liquid ocean; scientists expect that, for many of them, water is bound to hydrated minerals. However, the discovery may have implications for our search for life outside the solar system.

“It was a surprise to see evidence of so many water worlds orbiting the galaxy’s most common type of star,” said University of Chicago astronomer Rafael Luque.

“This has huge implications 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 dark, red dwarfs are, at most, only about half the mass of the Sun.

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

An estimated 73% of the stellar population in the Milky Way is made up of red dwarf stars. Think about it for a moment. When you go out stargazing, in a cool field or on the bed of a truck in the desert on a hot summer night, 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 increasingly sophisticated – enough that scientists have been able to characterize dozens of small worlds orbiting these tiny stars.

Scientists look at two main signals to characterize an exoplanet. The first is a steady faint attenuation of starlight as the orbiting exoplanet passes between us and the star.

The second is a minute lengthening and shortening of the star’s light wavelengths, as the orbiting exoplanet exerts a weak gravitational pull.

If you have these measurements and know how far away the star is (and therefore how much light it emits), you can measure the exoplanet’s radius and mass – two characteristics from which astronomers can infer the density of an exoplanet.

This density can be used to infer the composition of the exoplanet. Low density probably means an exoplanet with lots of atmosphere, like a gas giant. High density probably means a rocky world, like Earth, Venus, or Mars.

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

Typically, these exoplanets have been separated into two categories: rocky exoplanets and gaseous exoplanets with thick atmospheres. But Luque and Pallé saw the emergence of a curious third category: exoplanets too dense to be gaseous, but not dense enough to be purely rocky either.

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

If their water were on the surface, it would inflate their atmospheres, making them even larger in diameter and lower in density.

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

Instead, these worlds could resemble another solar system object – 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 to glass and minerals.

However, these worlds retained their water, if the team’s conclusions are correct, the discovery suggests that these worlds could not have formed where they did. Instead, they should have formed farther from their stars, rock and ice, and migrated to their current positions.

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

“Leaving aside this possibility of discovering extraterrestrial life forms,” ​​writes astronomer Johanna Teske of the Carnegie Institution for Science from a related perspective, “by measuring the diversity of composition of planets around red dwarf stars – the type most common star pattern in the Milky Way – is important for piecing together the complex puzzle of the formation and evolution of minor planets.”

The research has been published in Science.

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