Piaulet and colleagues observed the exoplanets Kepler-138c and Kepler-138d using NASA’s Hubble Space Telescope and the retired Spitzer Space Telescope and found that the planets may be largely composed 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. A new study has also found evidence for a fourth planet.
Water has not been directly detected on Kepler-138 cad, but by comparing the size and mass of the planets with models, the astronomers concluded that a significant fraction of their volume – up to half – should be made of materials that are lighter than rock but heavier than hydrogen or helium (which makes up most of the gas giant planets like Jupiter). The most common of these candidate materials is water.
“Previously, we thought that planets that are slightly larger than the Earth were large balls of metal and rock, like enlarged versions of the Earth, so we called them super-Earths,” explained Björn Benneke, study co-author and professor. in astrophysics at the University of Montreal. “However, we have now shown that the two planets, Kepler-138 c and d, are completely different in nature and that a large part of their entire volume is likely to be water. It is the best evidence yet for water worlds. The type of planet that astronomers theorized to be has been around for a long time.”
With volumes more than three times that of Earth and masses twice as large, planets c and d have much lower densities than Earth. This is surprising because most planets only slightly larger than Earth that have been studied in detail thus far appear to be rocky worlds like ours. The closest comparison, the researchers say, would be some of the icy moons in the outer solar system, which are also largely composed of water surrounding a rocky core.
“Imagine larger versions of Europa or Enceladus, water-rich moons that orbit Jupiter and Saturn, but are much closer to their star,” Piaulet explained. “Instead of an icy surface, they would be harboring large envelopes of water vapor.” The researchers warn that the planets may not have oceans like those on Earth right on the planet’s surface. “The temperature in Kepler-138 d’s atmosphere is likely above the boiling point of water, and we expect a dense dense vapor atmosphere on this planet. Only below this vapor atmosphere could there potentially be high-pressure liquid water or even water.” in another phase that occurs at high pressures, called 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 the measurable dip in starlight when a planet momentarily passed in front of their star. Benneke and his colleague Diana Dragomir of the University of New Mexico came up with the idea to re-observe the planetary system with the Hubble and Spitzer space telescopes between 2014 and 2016 to catch further transits of Kepler-138 d, the third planet. in the system to study its atmosphere.
A new exoplanet in the system
Two possible water worlds, Kepler-138 cad, 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 additionally found evidence of 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, so it takes 38 days to orbit. However, the nature of this other planet remains an open question, as it does not appear to be transiting its host star. Observing the transit of an 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 transit-variation timing method, which consists of tracking small variations in the exact times the planets transit in front of their star caused by the gravitational pull of other nearby planets.
The researchers had another surprise: they discovered that the two water worlds of Kepler-138 cad are “double” planets with virtually the same size and mass, whereas they were previously thought to be drastically different. On the other hand, a closer planet, Kepler-138b, is confirmed as a Mars-mass minor planet, one of the smallest known exoplanets to date. “As our instruments and techniques become sensitive enough to find and study planets that are farther from their stars, we may begin to find many more of these water worlds,” Benneke concluded.
