A team led by UdeM astronomers has found evidence that two exoplanets orbiting a red dwarf star are “water worlds”, planets where water makes up a large part of the volume. 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 doctoral student Caroline Piaulet of the Trottier Institute for Research on Exoplanets (iREx) at the University of Montreal, published a detailed study of a planetary system known as Kepler-138 in the journal Nature Astronomy today.
Piaulet, who is part of Björn Benneke’s research team, observed the exoplanets Kepler-138c and Kepler-138d with NASA’s Hubble and retired Spitzer space telescopes and discovered that the planets, which are about approximately one and a half times the Earth, they could be composed mostly of water. These planets and a planetary companion closer to the star, Kepler-138b, had previously been discovered by NASA’s Kepler space telescope.
Water was not directly detected, but by comparing the sizes and masses of the planets with models, they conclude that a significant fraction of their volume, up to half, should be made of materials lighter than rock but heavier than the 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,” Benneke explained. “However, we have now shown that these two planets, Kepler-138c and id, are quite different in nature: a large fraction of their entire volume is likely composed of water. This is the first time we have observed planets that can be identified with confidence as water worlds, a type of planet that astronomers theorized existed for a long time.”
“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,” Benneke explained. “However, we have now shown that these two planets, Kepler-138c and id, are quite different in nature: a large fraction of their entire volume is likely composed of water. This is the first time we have observed planets that can be identified with confidence as 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 to the two planets, the researchers say, would be some of the icy moons of 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, Kepler-138 cid would harbor 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 the atmospheres of Kepler-138c and Kepler-138d is probably above the boiling point of water, and we expect a thick, dense atmosphere made of steam on these planets. Only under this steam atmosphere could to have liquid water at high pressure, or even water in another phase that occurs at high pressures, called a supercritical fluid,” Piaulet said.
Recently, another team from the University of Montreal found another planet, called TOI-1452 b, that may be covered with an ocean of liquid water, but it will take NASA’s James Webb Space Telescope to study its atmosphere and confirm the presence of the ocean.
A new exoplanet in the system
In 2014, data from NASA’s Kepler space telescope allowed astronomers to announce the detection of three planets orbiting Kepler-138, a red dwarf star in the constellation Lyra. This was based on a measurable drop in starlight as the planet momentarily passed from its star, a transit.
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.
While previous observations by NASA’s Kepler space telescope only showed transits of three minor planets around Kepler-138, Piaulet and his team were surprised to find that Hubble and Spitzer observations suggested the presence of a fourth planet in the system, Kepler-138e.
This newly found planet is small and farther from its star than the other three, taking 38 days to complete one orbit. The planet is in its star’s habitable zone, a temperate region where a planet receives just the right amount of heat from its cool star so that it is neither too hot nor too cold to allow liquid water to exist.
The nature of this newly found 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 id 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 water worlds like Kepler-138 cid,” Benneke concluded.
About this study
The article “Evidence for the volatile-rich composition of a 1.5-Earth-radius planet” was published in Nature Astronomy on 15 December 2022. In addition to Caroline Piaulet and Björn Benneke (iREx, UdeM, Canada) and Diana Dragomir (University). of New Mexico), the team includes co-authors from France, USA and Austria.
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