The Milky Way may be a much wetter place than we thought.
A new analysis of exoplanets orbiting red dwarf stars suggests that we may have lost a population of “water worlds” – sodden 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 will be bound to hydrated minerals. However, the discovery could have implications for our search for life outside the Solar System.
“It was a surprise to see evidence of so many aquatic worlds orbiting the most common type of star in the galaxy,” says University of Chicago astronomer Rafael Luque.
“It has huge consequences for the search for habitable planets.”
While we can’t see a single red dwarf with the naked eye, these stars are incredibly numerous. Small, cold and dim, red dwarfs are, at most, only about half the mass of the Sun.
Their low melting rate gives them the greatest longevity of all stars; at 13.8 billion years, the Universe is not old enough for a red dwarf star to have lived through its estimated lifespan of 100 billion years.
It is estimated that 73% of the stellar population of the Milky Way is made up of red dwarf stars. Think about it for a moment. When you go out to look at the stars, in a cool field or on a truck bed in the desert on a hot summer night, you can’t even see most of the stars in the sky.
Because they are so dim and red, it’s hard to find exoplanets orbiting red dwarfs. Only a small percentage of the 5,084 exoplanets confirmed at the time of writing have been found around red dwarf stars.
However, our instruments are becoming increasingly sophisticated, so much so that scientists have been able to characterize dozens of small worlds orbiting these small stars.
There are two main signals that scientists look to characterize an exoplanet. The first is a faint regular attenuation of starlight as the orbiting exoplanet passes between us and the star.
The second is a minute lengthening and shortening of the wavelengths of the star’s light, as the orbiting exoplanet exerts a weak gravitational pull.
If you have these measurements and you know how far away the star is (and therefore how much light it emits), you can measure the radius and mass of the exoplanet, two characteristics from which astronomers can derive the density of an exoplanet.
This density can be used to deduce the composition of the exoplanet. Low density is likely to mean an exoplanet with a lot of atmosphere, such as a gas giant. High density is likely to mean a rocky world, such as 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 study on the density of 43 exoplanets orbiting red dwarf stars.
Typically, these exoplanets have been separated into two categories: rocky and gaseous exoplanets with dense atmospheres. But Luque and Pallé saw a curious third category emerge: exoplanets too dense to be gaseous, but not dense enough to be purely rocky.
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 the liquid water on their surface.
If their water were on the surface, it would swell their atmospheres, making them even larger in diameter and lower in density.
“But we don’t see it in the samples,” says Luque. “This suggests that the water is not shaped like a surface ocean.”
Instead, these worlds may resemble another Solar System object: Jupiter’s moon Ganymede, which is roughly half rock and half water, with water hidden under a rocky, icy shell. Or they could be a bit like the Moon (albeit significantly wetter), which has water molecules bound in glass and minerals.
However, these worlds have held their water, if the team’s conclusions are correct, the discovery suggests that these worlds could not have formed where they formed. Instead, they were supposed to form farther away from their stars, rock and ice, and migrate inward to their current locations.
However, without further evidence, it is impossible at this stage to pronounce in favor of this model, one way or another.
“Leaving this possibility aside to discover alien life forms,” writes astronomer Johanna Teske of the Carnegie Institution for Science in a related perspective, “measuring the compositional diversity of the planets around red dwarf stars, the most common type of star in the Milky Way – is important for putting together the complex puzzle of the formation and evolution of small planets. “
The research was published in Science.
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