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The majority of habitable zone planets around red dwarf stars may be 'water worlds'

When it comes to the hunt for alien life beyond our planet, we’re really having to take a hard look at our assumptions on where E.T. could be hiding. Finding a plausibly habitable planet in the Goldilocks zone around Proxima Centauri B, along with one around the even smaller red dwarf Trappist-1, were wake-up calls. They showed everyone we’d been paying less attention to red dwarf stars than we probably should, because red dwarf stars could play host to many more habitable-class planets than we knew. Now, the University of Bern has put forth new research suggesting that most of the hab-zone planets around red dwarf stars could be of Earth-like mass and size — but that they may tend to be water worlds.

Red dwarf stars are pretty common around the Local Group. To study the way planets form around such stars, the researchers sought to answer a few questions: What size do the planets tend to be that form around red dwarf stars? What do they end up being made of? How far from their stars do they tend to orbit? For their starting conditions, the authors used the parameters of a series of “identical, low-mass stars” typical of known red dwarf stars like Proxima B, with an associated protoplanetary disk of dust and gas. Finally, they assumed 10 planetary embryos about the size of our own Moon, randomly scattered at various orbital distances around each star. What the model returned shows us something about our position on some important bell curves.

For all that our planet’s surface is more than two-thirds water, the actual water mass fraction of our planet is 0.02%. Remember when it wasn’t certain whether we’d find water on other planets at all? Remember when finding water in Martian soil was huge news? 0.02% is like a puddle on the roof of a skyscraper compared with the 10% water or more that could be the norm on planets orbiting red dwarfs. With a water mass fraction that high, these planets would be different than anything we’ve seen. We think the Mariana trench is deep. Imagine an ocean so fathomless that it went beyond the crushing depth of the abyssal zone into pressures great enough to freeze the water solid regardless of temperature. There might not even be rocky continents to speak of on such a planet, or land as we know it, because how could such an unevenly composed planet be symmetric enough not to fly apart?

The reason for the high water content, the authors conclude, is that “planets have more time to migrate.” The typical protoplanetary disk has a snow line, outside of which the temperature is low enough that water can freeze. Higher-mass stars have higher-mass disks of debris, and the disks get hotter, so the snow line is further out, which means they tend to form more dry planets even though they form more planets total. When a planet forms beyond the snow line around such a star, it tends to be closer to Earth-like mass, and also tends to take a couple million years to migrate inward to the habitable zone. The further out a planet embryo is beyond the snow line, the longer it has to migrate inward, and the more icy planetesimals it can aggregate before things get too dry.

Clearly water is pretty abundant indeed. But planet size is also important. Most of the planets that formed in this study were of about one earth mass. Planets that didn’t have water were all smaller than ours. Small planets also tend to be tucked in close to their stars. “Planets of at least a fraction of an Earth mass,” the authors said in a statement, “must therefore have migrated from further out in the disk.” That means that around “typical” red dwarf stars, Earth-sized planets in the habitable zone will tend to have at least a little water.

“While liquid water is generally thought to be an essential ingredient, too much of a good thing may be bad,” said coauthor Willy Benz.  The layer of ice at the bottom of the ocean could disrupt the CO2 and carbonate-silicate cycles which stabilize and regulate climate over long timescales.

“Our models succeed in reproducing planets that are similar in terms of mass and period to the ones observed recently,” said coauthor Yann Alibert. “Interestingly, we find that planets in close-in orbits around these type of stars are of small sizes. Typically, they range between 0.5 and 1.5 Earth radii with a peak at about 1.0 Earth radius. Future discoveries will tell if we are correct.”

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