Two 'water worlds' observed 218 light-years from Earth

Artistic representation of the planetary system of the star Kepler-138. We see Kepler-138 d in the foreground and closer to the star Kepler-138 c. It is these two planets which are undoubtedly composed mainly of water. The small planet Kepler-138 b is seen transiting in front of the star. A fourth planet, Kepler-138 e, is further away and is not visible in the image.

Not all “super-Earth” exoplanets are completely rocky. Doctoral student Caroline Piaulet and her colleagues at the University of Montreal's Institute for Research on Exoplanets (iREx) have found evidence that two exoplanets orbiting a star 218 light-years from Earth are “water worlds”.

Observations by the iREx team, made with NASA's Hubble and Spitzer space telescopes, suggest that these rocky exoplanets, which are about one and a half times the size of Earth, are largely made up of 'water.

The scenario we put forward is that they are composed, approximately 10% of their masses and 50% of their volumes, of a material which would be water or another molecule of a similar density, explains Caroline Piaulet.

Cross-section of Earth and exoplanet Kepler-138 d. Like Earth, Kepler-138 d has an interior composed of metals and rocks, but it also has a thick layer of high-pressure water in various forms at its depths (in dark blue) and a thick envelope of vapor water above. These layers of water represent more than 50% of its volume, i.e. a depth of about 2000 km.

These planets orbit the red dwarf star Kepler-138 located in the constellation of Lyra.

Named Kepler-138 c and Kepler-138 d, these planets are different from those of our solar system, since they are larger than the rocky planets of Mercury, Venus, Earth and Mars, but smaller than the gas planets of Jupiter, Saturn, Neptune and Uranus.

Until now, astronomers thought that planets slightly larger than Earth (called super-Earths) were big balls of metal and rock.

This study tends to show that this is not always the case, and seems to confirm the existence of an unknown type of rocky planets which contain a lot of water, even if this has not been directly detected.< /p>

“Once we know the mass and size of a planet, we can estimate its density and probable composition. These two planets have masses far too low to be composed only of rock or too heavy to be made of hydrogen or helium.

— Caroline Piaulet, iREx

• As many as 5227 exoplanets have been officially detected in over 3908 planetary systems.
• Over 9151 additional exoplanets are currently awaiting confirmation.
• According to the most recent estimates, there are as many as 700 million billion rocky planets in the observable Universe alone.

Planets Kepler-138 c and Kepler-138 d thus have much lower densities than that of Earth.

They could be compared to certain icy moons of our solar system which are also composed a rocky core and a large part of water.

“These are planets that have the same amounts of water as ice moons like Europa or Ganymede, but much bigger and closer to their star. Thus, instead of having a layer of ice, they would have extended atmospheres of water. They represent a new type of exoplanets. »

— Caroline Piaulet, iREx

The frozen surface of Enceladus, a moon of Saturn, as seen by the Cassini spacecraft.

However, instead of an icy surface, Kepler-138c and Kepler-138 d would harbor vast envelopes of water vapour.

The researcher doubts that these planets have oceans directly on their surface, as it does on Earth, since the temperature is probably higher than the boiling point of water in their thick, dense, and vaporous atmospheres.

“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.

— Caroline Piaulet, iREx

Kepler-138 c and Kepler-138 d aren't the only planets orbiting the star. A companion, Kepler-138 b, is in orbit much closer to the star.

But recent observations by Caroline Piaulet and her colleagues have established that there may be a fourth planet in the system.

“Knowing the number of planets orbiting a star, we are able to predict when they will pass in front of the star. But our observations were not consistent with the presence of only three planets. »

— Caroline Piaulet, iREx

The doctoral student puts forward the scenario of the existence of Kepler-138 e, a small planet farther from its star than the other three, which takes 38 days to travel one orbit.

It would even be in the habitable zone around its star, explains Caroline Piaulet. This region corresponds to the place in an orbit where a planet receives just the right amount of energy so that it is neither too hot nor too cold and can thus allow the presence of liquid water necessary for the presence of life. as we know it.

It was the presence of Kepler-138e that led the team to reevaluate the masses of other planets using the technique of change in transit time. This method consists in following the small variations of the precise moments of the transits of the planets in front of their star, caused by the gravitational attraction of the close planets.

This work allowed astronomers to establish that the two water worlds Kepler-138c and Kepler-138d are strikingly similar, twin planets, of the same size and mass.

For its part, the planet Kepler-138 b, whose orbit is closest to the star, would be a small planet of the mass of Mars, one of the smallest exoplanets known to date .

Last August, another team from the University of Montreal discovered a planet called TOI-1452b, which could potentially be covered in an ocean of liquid water. However, NASA's James Webb Space Telescope will need to be used to study its atmosphere and confirm the presence of this ocean.

Details of this work are published in the journal Nature Astronomy (in English).