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‘Water worlds’ observed orbiting red dwarf star

The third and fourth planets orbiting Kepler-138 may have temperatures closer to Earth's than most exoplanets — though "closer" is a relative term.

(CN) — Most exoplanets observed so far with a size one and half times the size of Earth are likely rocky worlds. But Caroline Piaulet and her colleagues at the Université de Montréal discovered something different about two exoplanets orbiting a red dwarf star 218 light-years from Earth.

Piaulet and her team used models she created that reproduce planetary structures, including "high-pressure material deep in their interior and their extended gaseous atmospheres." They discovered the two planets are likely what they called "water worlds.

They published their findings Thursday in Nature Astronomy

“The planets have masses too low to be composed of rock alone,” Piaulet said in an email. “When it comes to hydrogen, we found that if it was the main component of the atmosphere, it would only make up 0.01% of the total planet mass and therefore could not be retained due to the star’s intense irradiation.”

Piaulet and her team therefore concluded that the most likely candidate for material heavier than hydrogen but lighter than rock is water.

For years, astronomers and researchers theorized the existence of “water worlds” due to the abundance of ice around stars during planet formation. Piaulet said her study offers “the first observational evidence for this new type of planet, and also demonstrates that planets slightly larger than the Earth exhibit much more diversity in their composition than previously thought.”

One planet orbiting the red dwarf star, Kepler-138e, is significant for what Piaulet called “unlocking the compositions of the other planets in this system."

"It enabled us to discover that Kepler-138c and d are likely water-rich twins. Beyond this, it completes a ‘near-resonant chain’ of planets already formed by the other three known planets: it takes nearly exactly the same time for Kepler-138d to complete five orbits around the star as it takes Kepler-138e to complete three," Piaulet wrote, adding, "These ‘water worlds’ could form much further from their host star, where water is abundant in ice form, and then become “locked” in such a chain there before migrating together closer to the star at later times.”

However, the astronomers say Kepler-138c and d may not have oceans like Earth’s on their surfaces. In fact, Piaulet said the planets are likely hotter than Earth.

"The temperature in the atmospheres of Kepler-138c and d are regulated by how much energy they receive from their star, and by how much of this energy they are able to absorb. Assuming they can absorb as much of the stellar light as the Earth’s atmosphere can, the bulk of the atmospheres will be at temperatures of about 70 to 130 degrees Celsius, or 158 to 266 Fahrenheit, near the boiling point of water," Piaulet said.

Piaulet said the findings pave the way for study of similar exoplanets in other star systems.

“The next step would be to unambiguously identify the composition of Kepler-138c and d by looking for the fingerprints of molecules making up their atmospheres,” Piaulet said in the email. “While it was relatively easier to study planets that are hot and close to their star, we are only now starting to reveal the compositions of small planets that have temperatures closer to that of the Earth. We expect that we may find other planets like Kepler-138 c and d as we explore more of these small temperate planets.”

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