A team led by researchers at the University of Montreal has found evidence that two exoplanets orbiting a red dwarf star are “water worlds,” where water makes up a large part of the planet. These worlds, located in a planetary system 218 light-years away in the constellation Lyra, are unlike any planet found in our solar system. The team, led by Caroline Piaulet of the Institute for Research on Exoplanets (iREx) at the University of Montreal, published a detailed study of this planetary system, known as Kepler-138, in the journal Nature Astronomy today. Piaulet and his colleagues observed the exoplanets Kepler-138c and Kepler-138d with NASA’s Hubble and the retired Spitzer space telescopes and discovered that the planets could be composed largely of water. These two planets and a smaller planetary companion closer to the star, Kepler-138b, were previously discovered by NASA’s Kepler space telescope. The new study also found evidence of a fourth planet. Water was not directly detected in Kepler-138c and d, but by comparing the sizes and masses of the planets with models, the astronomers conclude that a significant fraction of their volume – up to half of it – should consist of materials that are lighter than rock, but heavier than hydrogen or helium (which make up most of the gas giant planets like Jupiter). The most common of these candidate materials is water. “Previously we thought that planets slightly 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,” explained Björn Benneke, co-author of the study and professor. in astrophysics at the University of Montreal. “However, we have now shown that these two planets, Kepler-138c and d, are quite different in nature and that a large fraction of their entire volume is likely composed of water. It is the best evidence for water worlds, a type of planet that has been thought by astronomers to exist for a long time.” With volumes more than three times that of Earth and masses twice that, planets c and d are much less dense than Earth. This is surprising because most planets slightly larger than Earth that have been studied in detail so far all seem to be rocky worlds like our own. The closest comparison, the researchers say, would be some of the icy moons in the outer solar system that also consist largely of water surrounding a rocky core. “Imagine larger versions of Europa or Enceladus, the water-rich moons orbiting Jupiter and Saturn, but brought much closer to their star,” Piaulet explained. “Instead of a frozen surface, they would host large envelopes of water vapor.” The researchers warn that the planets may not have oceans like those on Earth directly on the planet’s surface. “The temperature in Kepler-138d’s atmosphere is probably above the boiling point of water, and we expect a thick, dense vapor atmosphere on this planet. Only, beneath this vapor atmosphere could there possibly be liquid water at high pressure, or even water in another phase that occurs at high pressures, called the supercritical fluid,” Piaulet said. In 2014, data from NASA’s Kepler space telescope allowed astronomers to announce the detection of three planets orbiting Kepler-138. This was based on a measurable drop in starlight as the planet momentarily passed its star. This image shows cross-sections of Earth and the exoplanet Kepler-138d. Density measurements of Kepler-138d suggest that it could have a layer of water that makes up more than 50% of its volume, at a depth of about 1,243 miles (2,000 kilometers). Credit: Benoit Gougeon (University of Montreal) Benneke and his colleague Diana Dragomir, of the University of New Mexico, planned to re-observe the planetary system with the Hubble and Spitzer space telescopes between 2014 and 2016 to catch more transits of Kepler-138d, the third planet in the system. in order to study its atmosphere. A new exoplanet in the system The two possible water worlds, Kepler-138c and d, are not in the habitable zone, the region around a star where temperatures would allow liquid water on the surface of a rocky planet. But in the Hubble and Spitzer data, the researchers found additional evidence for a new planet in the system, Kepler-138e, in the habitable zone. This newly discovered planet is small and farther from its star than the other three, taking 38 days to complete one orbit. The nature of this additional planet, however, remains an open question because it does not appear to be transiting its star. Observing the transit of the exoplanet would allow astronomers to determine its size. With Kepler-138e now in the picture, the masses of the previously known planets were measured again using the timing-variation transit method, which consists of watching for small fluctuations in the precise moments of the planets’ transits in front of their star caused by the gravitational pull of other nearby planets. The researchers had another surprise: They found that the two water worlds Kepler-138c and are “twin” planets, with essentially the same size and mass, whereas they were previously thought to be drastically different. The closest planet, Kepler-138b, on the other hand, is confirmed to be a Mars-mass minor planet, one of the smallest exoplanets known to date. “As our instruments and techniques become sensitive enough to find and study planets farther from their stars, we may begin to find many more of these water worlds,” Benneke concluded. More about shipping The entire body of scientific data collected by Spitzer during its lifetime is available to the public through the Spitzer Data Archive, housed in the Infrared Science Archive at IPAC at Caltech in Pasadena, California. NASA’s Jet Propulsion Laboratory, a division of Caltech, managed Spitzer mission operations for the agency’s Science Mission Directorate in Washington. Scientific operations were conducted at the Spitzer Science Center at IPAC at Caltech. Spacecraft operations were based at Lockheed Martin Space in Littleton, Colorado. For more information about NASA’s Spitzer mission, go to: Astrobiology, Avatar, Avatar The Way Of Water
