Cross-section of the Earth (left) and the exoplanet Kepler-138 d (right). Like the Earth, this exoplanet has actually an interior composed of metals and rocks (brown part), however Kepler-138 d also has a thick layer of high-pressure water in numerous forms: possibly liquid and supercritical water deep inside the planet and a prolonged water vapour envelope (tones of blue) above it. These water layers make up more than 50% of its volume, or a depth of about 2000 km. The Earth, in contrast, has a minimal portion of liquid water with a typical ocean depth of less than 4km. Credit: Benoit Gougeon, Université de Montréal
Team of astronomers led by the University of Montreal (UdeM) find that 2 exoplanets might be mostly water.
Astronomers have discovered evidence that 2 exoplanets orbiting a red dwarf star are “water worlds,” worlds where water makes up a large fraction of the volume. Found in a planetary system 218 light-years away in the constellation Lyra, these worlds differ from any planets found in our planetary system.
A comprehensive study of a planetary system understood as Kepler-138 will be released today (December 15) in the journal Nature Astronomy. The group was led by PhD trainee Caroline Piaulet of the Trottier Institute for Research on Exoplanets (iREx) at the Université de Montréal.
Like the Earth, this exoplanet has an interior made up of metals and rocks (brown part), however Kepler-138 d also has a thick layer of high-pressure water in numerous forms: supercritical and potentially liquid water deep inside the world and an extended water vapour envelope (tones of blue) above it. By comparing the sizes and masses of the planets to models, they conclude that a considerable portion of their volume– up to half of it– should be made of materials that are lighter than rock however much heavier than hydrogen or helium (which constitute the bulk of gas giant worlds like Jupiter). It is the very first time we observe worlds that can be confidently identified as water worlds, a type of planet that was thought by astronomers to exist for a long time.”
It is the very first time we observe planets that can be confidently recognized as water worlds, a type of planet that was thought by astronomers to exist for a long time.”
According to the researchers, the worlds might not have oceans straight at the worlds surface like those we have on Earth.
Piaulet, who is part of Björn Bennekes research study group, observed exoplanets Kepler-138c and Kepler-138d with NASAs Hubble and the retired Spitzer space telescopes. She found that the planets– which are around one and a half times the size of the Earth– might be composed mostly of water. These planets and a planetary buddy better to the star, Kepler-138b, had been found previously by NASAs Kepler Space Telescope.
Water wasnt straight detected. By comparing the sizes and masses of the worlds to models, they conclude that a significant fraction of their volume– up to half of it– should be made of products that are lighter than rock however much heavier than hydrogen or helium (which make up the bulk of gas giant planets like Jupiter). Water is the most typical candidate material that fits these specs.
” We previously believed that worlds that were a bit larger than Earth were big balls of metal and rock, like scaled-up variations of Earth, and thats why we called them super-Earths,” described Benneke. “However, we have actually now revealed that these two worlds, Kepler-138c and d, are rather various in nature: a big portion of their entire volume is likely composed of water. It is the first time we observe planets that can be with confidence determined as water worlds, a type of planet that was thought by astronomers to exist for a long time.”
” We formerly thought that worlds that were a bit bigger than Earth were big balls of metal and rock, like scaled-up variations of Earth, and thats why we called them super-Earths,” discussed Benneke. “However, we have actually now revealed that these 2 planets, Kepler-138c and d, are rather different in nature: a huge fraction of their entire volume is likely composed of water. It is the very first time we observe worlds that can be confidently determined as water worlds, a type of world that was thought by astronomers to exist for a long time.”
With volumes more than three times that of Earth and masses twice as big, planets c and d have much lower densities than Earth. This is surprising due to the fact that the majority of the planets just slightly larger than Earth that have been studied in information up until now all appeared to be rocky worlds like ours. The closest contrast to the 2 worlds, say scientists, would be a few of the icy moons in the external solar system that are likewise largely made up of water surrounding a rocky core.
” Imagine larger variations of Europa or Enceladus, the water-rich moons orbiting Jupiter and Saturn, however brought much closer to their star,” described Piaulet. “Instead of an icy surface, Kepler-138 c and d would harbor big water-vapor envelopes.”
According to the researchers, the planets might not have oceans straight at the planets surface like those we have on Earth. “The temperature level in Kepler-138cs and Kepler-138ds environments is likely above the boiling point of water, and we expect a thick, thick atmosphere made of steam on these planets. Just under that steam atmosphere there could possibly be liquid water at high pressure, and even water in another phase that occurs at high pressures, called a supercritical fluid,” Piaulet said.
Recently, another group at the University of Montreal discovered another world, called TOI-1452 b, that could potentially be covered with a liquid-water ocean, however NASAs James Webb Space Telescope will be required to study its environment and verify the existence of the ocean.
A new exoplanet in the system
In 2014, astronomers revealed the detection of 3 planets orbiting Kepler-138, a red dwarf star in the constellation Lyra, utilizing information from NASAs Kepler Space Telescope. This was based upon a transit– the quantifiable dip in starlight as the planet for a moment passed in front of its star.
Benneke and his colleague Diana Dragomir, from the University of New Mexico, created the concept of re-observing the planetary system with the Hubble and Spitzer area telescopes in between 2014 and 2016 to capture more transits of Kepler-138d, the third world in the system, in order to study its environment.
While earlier NASA Kepler area telescope observations only revealed transits of three small worlds around Kepler-138, Piaulet and her team were shocked to find that the Hubble and Spitzer observations recommended the existence of a 4th world in the system, Kepler-138e..
This newly found planet is small and farther from its star than the 3 others, taking 38 days to complete an orbit. The planet remains in the habitable zone of its star, a temperate region where a planet gets simply the correct amount of heat from its cool star to be neither too hot nor too cold to permit the existence of liquid water.
The nature of this additional, freshly found planet, however, stays an open question since it does not appear to transit its host star. Observing the exoplanets transit would have enabled astronomers to determine its size.
With Kepler-138e now in the photo, the masses of the formerly known planets were measured once again through the transit timing-variation technique, which includes tracking small variations in the exact minutes of the planets transits in front of their star brought on by the gravitational pull of other close-by worlds.
The scientists had another surprise: they found that the two water worlds Kepler-138c and d are “twin” worlds, with essentially the same size and mass, while they were formerly thought to be dramatically various. The closer-in planet, Kepler-138b, on the other hand, is confirmed to be a small Mars-mass planet, one of the tiniest exoplanets understood to date.
” As our strategies and instruments end up being delicate enough to find and study planets that are further from their stars, we may start discovering a lot more water worlds like Kepler-138 c and d,” Benneke concluded.
Reference: “Evidence for the volatile-rich structure of a 1.5-Earth-radius planet” 15 December 2022, Nature Astronomy.DOI: 10.1038/ s41550-022-01835-4.
The short article “Evidence for the volatile-rich composition of a 1.5-Earth-radius planet” will be published today (December 15, 2022) in the journal Nature Astronomy. In addition to Caroline Piaulet and Björn Benneke (iREx, UdeM, Canada) and Diana Dragomir (University of New Mexico), the group includes co-authors from France, USA, and Austria.
