15 December 2022.

Excerpt:

>A team led by UdeM astronomers has found evidence that two exoplanets orbiting a red dwarf star are “water worlds,” planets where water makes up a large fraction of the volume.

>The team, led by PhD student Caroline Piaulet of the Trottier Institute for Research on Exoplanets (iREx) at the Université de Montréal, published a detailed study of a planetary system known as Kepler-138 in the journal Nature Astronomy today.

>Water wasn’t directly detected, but by comparing the sizes and masses of the planets [Kepler-138c and d] to models, they conclude that a significant fraction of their volume — up to half of it — should be made of materials that are lighter than rock but heavier than hydrogen or helium (which constitute the bulk of gas giant planets like Jupiter).

>The most common of these candidate materials is water.

> 

>The closest comparison to the two planets, say researchers, would be some of the icy moons in the outer solar system that are also largely composed of water surrounding a rocky core.

>Researchers caution the planets may not have oceans like those on Earth directly at the planet’s surface.

>“The temperature in Kepler-138c's and Kepler-138d’s atmospheres is likely above the boiling point of water, and we expect a thick, dense atmosphere made of steam on these planets. Only under that steam atmosphere there could potentially be liquid water at high pressure, or even water in another phase that occurs at high pressures, called a supercritical fluid," Piaulet said.

>The researchers had another surprise: they found that the two water worlds Kepler-138c and d are “twin” planets, with virtually the same size and mass, while they were previously thought to be drastically different.

Nature Astronomy, 2022. DOI 10.1038/s41550-022-01835-4