In this illustration, exoplanet Kepler-138 d is in the foreground and Kepler-138 ca is to the left. In the background, Kepler 138 b passes in front of its parent star. The low densities of Kepler-138 ci Kepler-138 d indicate that they must be composed largely of water. Credits: NASA, ESA, Leah Hustak (STScI)
A team led by researchers from 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 entire planet. These worlds, located in a planetary system 218 light-years away in the constellation Lyra, are different from the planets found in our solar system.
The team, led by Caroline Piaulet of the University of Montreal’s Institute for Research on Exoplanets (iREx), 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 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, had previously been 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 id, 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, should be made of lighter materials . 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.
“We previously thought that planets that were slightly larger than Earth were big balls of metal and rock, like large-scale versions of Earth, and that’s why we called them super-Earths,” explained Björn Benneke, co-author of the study and teacher. in astrophysics at the University of Montreal. “However, we have now shown that these two planets, Kepler-138c and id, are quite different in nature and that a large fraction of their total volume is likely composed of water. It is the best evidence yet for water worlds, a type of planet that astronomers theorized existed for a long time.”
With volumes more than three times that of Earth and masses twice as great, the cid planets have much lower densities than Earth. This is surprising because most planets slightly larger than Earth that have been studied in detail so 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 that are also largely composed of water surrounding a rocky core.
“Imagine larger versions of Europa or Enceladus, the water-rich moons orbiting Jupiter and Saturn, but much closer to their star,” Piaulet explained. “Instead of an icy surface, they would host large envelopes of water vapor.”
The researchers warn that the planets may not have Earth-like oceans directly on the planet’s surface. “The temperature in Kepler-138d’s atmosphere is likely above the boiling point of water, and we expect a thick, dense atmosphere made of steam on this planet. Only beneath this steam atmosphere could there be to have liquid water at high pressure, or even water in another phase that occurs at high pressures, called a 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 from its star.
This illustration 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 came up with the idea of re-observing the planetary system with the Hubble and Spitzer space telescopes between 2014 and 2016 to catch more transits of Kepler-138d, the third planet of the system, in order to study its atmosphere.
A new exoplanet in the system
The two possible water worlds, Kepler-138c and id, are not in the habitable zone, the area 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 also found evidence of a new planet in the system, Kepler-138e, in the habitable zone.
This newly found planet is small and farther from its star than the other three, taking 38 days to complete one orbit. The nature of this extra planet, however, remains an open question because it does not appear to transit its host star. Observing the transit of the exoplanet would have allowed astronomers to determine its size.
With Kepler-138e now in the picture, the masses of the previously known planets were remeasured using the transit time variation method, which involves tracking small variations in the precise times of the planets’ transits in front of the its star caused by the gravitational attraction of other nearby planets. The researchers had another surprise: they found that the two water worlds Kepler-138c and are “twin” planets, with virtually the same size and mass, whereas they were previously thought to be drastically different. The nearest 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 that are farther from their stars, we could begin to find many more of these water worlds,” Benneke concluded.
More information about the mission
The entire scientific data set 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. Science 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 on NASA’s Spitzer mission, go to:
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