Discovery of a new type of planet composed largely of water: UNM Newsroom

Thanks to a detailed study of a planetary system located 218 light years away, a team of international researchers provides evidence for the existence of a new type of planet called the aquatic world..

The team, led by Caroline Piaulet, a Ph.D. student from the Trottier Institute for Research on Exoplanets (iREx) at the University of Montreal, has published a detailed study of the Kepler-138 planetary system. The study, titled Evidence for the volatile-rich composition of a 1.5 Earth radius planethas been published in the review natural astronomy today.

Piaulet used data from NASA’s Hubble and Spitzer space telescopes to analyze the composition of exoplanets Kepler-138 c and Kepler-138 d and found that the planets as a whole must be composed largely of water.

“We used to think that planets a little bigger than Earth were big balls of metal and rock, like enlarged versions of Earth, and that’s why we called them super-Earths,” says Björn Benneke, co-author of the article. . “But that’s not the case for these two planets, for which we found that up to half of their volume is likely to be water,” adds Diana Dragomir, assistant professor in the Department of Physics and of astronomy from New York University. Mexico and co-author of the study. “These planets are different any planets of our solar system. The closest comparison would be some of the icy moons of the outer solar system that are also largely water, with rock only found in the deep interior.

“Imagine larger versions of Europa or Enceladus, the water-rich moons orbiting Jupiter and Saturn in our own solar system, but much closer to their star,” says Piaulet. “Instead of an icy surface, they would house large envelopes of water vapor.”


A cross-sectional comparison highlights the differences between Earth and Kepler-138 d.

Observing Kepler-138 with Hubble and Spitzer
In 2014, data from the Kepler space telescope allowed astronomers to announce the detection of three planets orbiting Kepler-138, a small star in the constellation Lyra, 218 light-years from Earth. The light curve – the amount of light received from the star over a period of time – showed the transits of the planets in front of the star, which are perceived as small periodic gradations of the star itself.

Benneke and Dragomir came up with the idea of ​​re-observing the planetary system, and they proposed and obtained time on the Hubble and Spitzer space telescopes, respectively. Their goal was to catch more transits of Kepler-138d, the system’s third planet, to study its atmosphere.

Dragomir explains that “Spitzer’s observations in particular can be notoriously difficult to analyze, and initially it didn’t look like we would be able to glean much useful information from them. However, I encouraged Caroline to try again to analyze these unique observations. She put in a lot of effort, but it was worth it in the end.

When Piaulet analyzed all of the observations from Kepler, Hubble, and Spitzer, she found a surprising result: the new observations required the presence of a fourth planet in the system, Kepler-138 e.

A new exoplanet in the system
The newly discovered planet in the system, Kepler-138 e, is a small planet that is farther from its star than the other three and takes 38 days to complete an orbit. The planet is in the “habitable” zone of its star, a temperate region where a planet receives just enough heat from its small star to be neither too hot nor too cold to allow the presence of water liquid. However, the nature of this planet remains an open question because it does not seem to pass through its host star. Observing the transit of the exoplanet would have been the method used by astronomers to determine its size.

With Kepler-138 e now in the picture, the masses of previously known planets were measured again via the Transit Timing Variation (TTV) method, which involves tracking small variations in the precise timings of planet transits past their star. caused by the gravitational pull of other nearby planets.

Aquatic worlds
With volumes more than three times greater than those of Earth and masses twice as large, planets c and d have densities much lower than our own planet. This indicates that they cannot be made mostly of metal and rock like Earth. This is surprising because most of the planets barely larger than Earth that have been studied in detail so far all seem to be scaled-up versions.

By comparing the size and mass of the planets to models, they conclude that a significant fraction of their volume – up to half of it – should consist of materials lighter than rock but heavier than iron. hydrogen or helium (which make up most of the gas giant planets like Jupiter). The most common of these materials is water. These planets were previously only hypotheses and informally referred to as “aquatic worlds” or “ocean planets” by astronomers.

“In this case, I don’t really like the term planet oceanbecause the use of the word ocean gives the impression that the planet is covered in large oceans of liquid water,” Piaulet explains. “However, we don’t know whether or not this is the case for these two planets, given their high temperatures. Instead, below the water vapor atmosphere, we might find liquid water or water in another phase that occurs at high pressures, called supercritical.”

“Whether Kepler-138 c and d are part of a class of planets – water worlds – that are common but hard to find, or truly rare, they remind us that exoplanets are often very different from the worlds we know in our solar system.. These aquatic worlds will continue to stimulate our curiosity and will remain prime targets for astronomers’ most powerful telescopes,” Dragomir concludes.

Leave a Comment