Cross-section of Earth (left) and exoplanet Kepler-138d (right). Like Earth, this exoplanet has an interior made up of metals and rocks (brown part), but Kepler-138 d also has a thick layer of high-pressure water in different forms: supercritical water and potentially liquid at depth of the planet and an extended envelope of water vapor (in blue) above. These layers of water represent more than 50% of its volume, i.e. a depth of about 2000 km. Earth, by comparison, has a negligible fraction of liquid water with an average ocean depth of less than 4 km. Credit: Benoit Gougeon, University of Montreal
A team led by UdeM astronomers has found evidence that two exoplanets orbiting a red dwarf star are “water worlds,” that is, planets where water makes up a large volume fraction. These worlds, located in a planetary system 218 light years away in the constellation Lyra, are unlike any of the planets in our solar system.
The team, led by Caroline Piaulet, a doctoral student at the Trottier Institute for Research on Exoplanets (iREx) at the University of Montreal, published today, December 15, a detailed study of the planetary system called Kepler-138 in the journal Nature Astronomy.
Piaulet, part of Björn Benneke’s research team, observed exoplanets Kepler-138c and Kepler-138d using NASA’s Hubble Space Telescope and the Spitzer Telescope, now retired from the circulation, and discovered that the planets – which are about one and a half times the size of Earth – could be composed largely of water. These planets and a closer planetary companion to the star, Kepler-138b, had previously been discovered by NASA’s Kepler space telescope.
Water was not directly detected, but by comparing the size and mass of the planets to models, the researchers concluded that a significant fraction of their volume – up to half – should be made up of materials more light 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 used to think of planets a bit larger than Earth as big balls of metal and rock, like scaled-down versions of Earth, and that’s why we called them super-Earths,” Mr. Benneke. That’s why we called them super-Earths,” says Benneke. “However, we have just shown that these two planets, Kepler-138c and d, are very different in nature: a large part of their volume is probably made up of water. This is the first time we have observed planets that can be identified with certainty as water worlds, a type of planet whose existence has been theorized by astronomers 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 of the planets barely larger than Earth that have been studied in detail so far all seemed to be rocky worlds like ours. According to the researchers, the closest comparison of the two planets would be some of the outer solar system’s icy moons which are also composed largely of water surrounding a rocky core.
“Imagine larger versions of Europa or Enceladus, the water-rich moons that orbit Jupiter and Saturn, but brought much closer to their star,” Piaulet explained. “Instead of an icy surface, Kepler-138 c and d would harbor large envelopes of water vapor.”
Planets may not have Earth-like oceans directly on the planet’s surface, researchers warn. “The temperature in the atmosphere of Kepler-138c and Kepler-138d is likely above the boiling point of water, and we expect a thick, dense atmosphere made of vapor on these planets. Only under this atmosphere of vapor could there be liquid water at high pressure, or even water in another phase that occurs at high pressure, called supercritical fluid,” Ms. Piaulet.
Recently, another team from the University of Montreal discovered another planet, called TOI-1452 b, which could potentially be covered in an ocean of liquid water, but NASA’s James Webb Space Telescope will be needed 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 discovery was based on a measurable dip in starlight when the planet momentarily passed in front of their star, a transit.
Benneke and his colleague Diana Dragomir from 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 observe further transits of Kepler- 138d, the third planet in the system, and to study its atmosphere.
While earlier observations from NASA’s Kepler Space Telescope only showed the transits of three small planets around Kepler-138, Piaulet and his team were surprised to find that the Hubble and Spitzer observations suggested the presence of a fourth. planet in the system, Kepler-138th.
This newly discovered planet is small and farther from its star than the other three, taking 38 days to complete an 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 to be neither too hot nor too cold to allow for the presence of liquid water.
The nature of this newly discovered additional planet remains an open question, however, as it does not appear to be transiting its host star. Observing the transit of the exoplanet would have allowed astronomers to determine its size.
With Kepler-138e now in the chart, the masses of previously known planets have been measured again via the transit time variation method, which involves tracking small variations in the precise times of planet transits past their star, caused by the gravitational pull of other nearby planets.
The researchers had another surprise: they discovered that the two water worlds Kepler-138c and d are “twin” planets, with almost the same size and mass, whereas they were previously thought to be radically different. The nearest planet, Kepler-138b, on the other hand, is confirmed to be a small Mars-mass planet, one of the smallest exoplanets known to date.
“As our instruments and techniques become sensitive enough to find and study planets further away from their star, we could start finding many more water worlds like Kepler-138 c and d,” Benneke concluded.