In this illustration super-Earth Kepler-138 d is in the foreground. To the left, the planet … [+]
Not far from our world, at least on the cosmic scale of the universe, a pair of Earth-sized planets orbit the Red Dwarf star Kepler-138. They appear to be true water worlds with unfathomably deep planetary oceans, but not the kind that even Hollywood would be able to imagine humans easily navigating.
NASA’s late, prolific planet-finding Kepler Telescope discovered at least three planets around Kepler-138 some 218 light years from Earth, making Kepler-138 b, c and d among the more nearby known exoplanets.
Last year a team of researchers used data from two other NASA observatories, Spitzer and Hubble, to take a more detailed look at the system, particularly Kepler 138c and d. Their analysis showed that the planets appear to be mostly water.
“We previously thought that planets that were a bit larger than Earth were big balls of metal and rock, like scaled-up versions of Earth, and that’s why we called them super-Earths,” study co-author Björn Benneke from the University of Montreal said in a statement. “However, we have now shown that these two planets, Kepler-138 c and d, are quite different in nature and that a big fraction of their entire volume is likely composed of water. It is the best evidence yet for water worlds, a type of planet that was theorized by astronomers to exist for a long time.”
The study was published in Nature Astronomy in December.
Water world might even be an understatement. The oceans on these planets might be over 500 times deeper than the average ocean depth on Earth.
This is an artist’s illustration showing a cross-section of the Earth (left) and the exoplanet … [+]
But there’s a lot of uncertainty involved at this point with the available data. These planets orbit very close to their home star, outside the habitable zone and with soaring temperatures making for a world that’s more like a steam bath than a pristine pool.
“The temperature in Kepler-138 d’s atmosphere is likely above the boiling point of water, and we expect a thick dense atmosphere made of steam on this planet,” explains lead researcher Caroline Piaulet. “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.”
This world could be far weirder than the Perfect Storm meets Mad Max universe of Kevin Costner’s “Water World.”
While the twin, wet super-Earths aren’t likely to turn up on interstellar travel lists for some time, the team also found a fourth planet, Kepler-138e, which is in the habitable zone.
Very little else can be said about this world at this time, except that it takes about 38 days to orbit. There’s also a good chance that some or all of these planets are tidally locked to their star.
This means that, much the same way the same side of the moon always faces Earth, these planets do not revolve on an axis and instead have a permanent day and night side.
This would mean that one half of the planet is likely overheated while the other is frozen in darkness, pushing any potential for life to the edges where light fades into dark, often referred to as the terminator zone.
Recent research, however, suggests that very wet, tidally-locked worlds are not ideal for hosting a temperate terminator.
So if Kepler-138e is to have the best chance of being livable, it’s better that it not be anywhere near as wet as its sister planets.
