Kepler-138 is a red dwarf star located 218 light-years away with three identified planets, Kepler-138 b, c, and d. In this illustration, super-Earth Kepler-138 d is in the foreground, with Kepler-138 c to the left and Kepler-138 b in the background, seen in silhouette transiting its central star. The low density of planets Kepler-138 c and d, which are nearly identical in size, suggests that they must be composed largely of water rather than rock. This conclusion is based on measurements of their mass versus physical diameter. Both planets have twice the mass of Earth but only half its density. Therefore, they are considered a new class of “water planet,” unlike any major planet found in our solar system. Even more remarkable is Kepler-138 b, which is one of the smallest exoplanets known and has both the mass and density of Mars.
A team led by researchers from the University of Montreal has made an incredible discovery. Through careful investigation of a planetary system 218 light-years away in the constellation Lyra, they have found evidence that two exoplanets orbiting a red dwarf star are “water worlds”, meaning that water makes up a large fraction of the entire planet. This finding is remarkable as it is unlike any planet found in our own solar system, and provides us with a deeper insight into the distant universe that surrounds us. It also has potential implications for the search for extraterrestrial life, as many scientists believe that water is essential for it to exist.
Piaulet and colleagues recently observed the exoplanets Kepler-138 c and d with the help of NASA’s Hubble and Spitzer space telescopes. The study found that these two planets, along with the smaller companion planet Kepler-138 b, could be composed largely of water. Furthermore, evidence for a fourth planet was also discovered. Although direct detection of water was not found in Kepler-138 c and d, comparing their size and mass to models suggested that up to half of their volume could be made up of materials like water that are lighter than rock but heavier than hydrogen or helium.
The findings of a study conducted by Björn Benneke, professor of astrophysics at the University of Montreal and co-author of the paper, have overturned traditional beliefs about planets that are a bit larger than Earth. Previous conceptions saw them as big balls of metal and rock, similar to scaled-up versions of our home planet. However, it has now been shown that Kepler-138 c and d are made up of largely water, rather than rock and metal. This discovery provides the strongest evidence yet for the existence of water worlds, something that astronomers have long theorized.
This image shows a cross-section of the Earth (left) and the exoplanet Kepler-138 d (right). Comparatively, Kepler-138 d has an interior composed of metals and rocks (brown portion) with a thick layer of high-pressure water in various forms, including supercritical and potentially liquid water deep inside the planet and an extended water vapor envelope (shades of blue) above it. These water layers make up more than 50% of its volume, or a depth of about 1,243 miles (2,000 kilometers). In comparison, the Earth has a negligible fraction of liquid water with an average ocean depth of less than 2.5 miles (4 kilometers).
Planets C and D are much larger than Earth, with volumes more than three times and masses twice as big, yet they have lower densities than Earth. This is surprising to scientists, who expected them to be similar in composition to the rocky worlds that have already been studied in detail. As an explanation for this unexpected result, researchers suggest that the two planets may be more similar to some of the icy moons found in our outer solar system like Europa and Enceladus. Unlike those icy moons, however, planets C and D would have large water-vapor envelopes instead of icy surfaces due to their proximity to their star. Therefore, these planets can be thought of as larger versions of the icy moons orbiting Jupiter and Saturn.
Researchers caution that planets like Kepler-138 d may not have oceans at the planet’s surface, due to high temperature and an atmosphere of steam. In 2014, a team of astronomers were able to detect the three planets orbiting Kepler-138, based on a dip in starlight as the planet moved in front of its star. Since then, Benneke and Dragomir have proposed re-observing the system with the Hubble and Spitzer space telescopes in order to study the atmosphere of Kepler-138 d more closely. However, it is uncertain whether liquid water could exist on the planet at high pressures or even in a supercritical fluid state. Therefore, researchers are still left with some mystery as to what kinds of atmospheres and oceans may be present on planets far from Earth.
Another new exoplanet in the Kepler-138 system.
The two possible water worlds, Kepler-138 c and d, are not located in the habitable zone, the area around a star where temperatures would allow liquid water on the surface of a rocky planet. However, researchers found evidence for a new planet in the system, Kepler-138 e, in the habitable zone. This new exoplanet is small and orbits its host star in 38 days. Because this new planet does not seem to transit its host star, astronomers could not determine its size. The masses of the previously known planets were measured again through the transit timing-variation method, which tracks variations in the precise moments of the planets’ transits caused by the gravitational pull of the other nearby planets.
The researchers studying exoplanets Kepler-138 c and d were pleasantly surprised to find that the two water worlds were “twin” planets, with almost identical size and mass, despite being believed to have been vastly different. Even more interesting was their finding of the inner planet, Kepler-138 b, which has been confirmed to be one of the smallest exoplanets known to date, with a mass similar to that of Mars. This discovery could indicate that as technology and methods for studying exoplanets become more precise and sensitive, we may find more of these water worlds in our universe. Benneke concluded that this revelation could help us gain a greater understanding of the variety of planets beyond our solar system.
