Through a detailed study of a planetary system 218 light-years away, a team of international researchers is providing evidence for the existence of a new type of planet called a water world.
The team, led by Caroline Piaulet, Ph.D. student at the Trottier Institute for Research on Exoplanets (iREx) at the University of Montreal, 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 planet, was published today in the journal Nature Astronomy.
Piaulet used data from NASA’s Hubble and Spitzer space telescopes to analyze the composition of the exoplanets Kepler-138 and Kepler-138 and discovered that the planets as a whole must be composed largely of water.
“Previously we 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,” explains Björn Benneke, co-author of the paper . . “But this is 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 Astronomy at the University of New York. . Mexico and co-author of the study. “These planets are different from planets in our Solar System. The closest comparison would be some of the icy moons of the outer Solar System which are also composed of a large fraction of 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,” explains Piaulet. “Instead of an icy surface, they would host 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 given period of time, showed the transits of the planets in front of the star, which are perceived as small periodic dimmings of the star itself.
Benneke and Dragomir came up with the idea of re-observing the planetary system, and proposed and obtained time on the Hubble and Spitzer space telescopes, respectively. Their goal was to capture more transits of Kepler-138 d, the third planet in the system, to study its atmosphere.
Dragomir explains that “Spitzer observations in particular can be notoriously difficult to analyze, and initially it didn’t seem like we could extract much useful information from them. However, I encouraged Caroline to take another stab at analyzing these unique observations. It took a lot of effort, but it was worth it in the end.”
When Piaulet analyzed the complete set of observations from Kepler, Hubble and Spitzer, he 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 found 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 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 small star so that it is neither too hot nor too cold to allow liquid water to exist. The nature of this planet, however, remains an open question because it does not appear to transit its host star. Observing the transit of the exoplanet would have been the method used by astronomers to determine its size.
With Kepler-138 now in the picture, the masses of the previously known planets were remeasured using the Transit Timing Variation (TTV) method, which involves tracking small variations in the precise timing of the planets’ transits in front of their star caused by the gravitational attraction of other nearby planets.
Aquatic worlds
With volumes more than three times that of Earth and masses twice as great, the cid planets have much lower densities than our own planet. This indicates that they cannot be made primarily of metal and rock like Earth. This is surprising because most planets slightly larger than Earth that have been studied in detail so far appear to be large-scale versions of Earth.
By comparing the size and mass 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 hydrogen or helium ( which make up most of the gas giant planets like Jupiter). The most common of these materials is water. Previously, these planets were only hypothesized and astronomers informally called them “water worlds” or “ocean planets”.
“In this case, I don’t really like the term oceanic planet, because the use of the word ocean gives the impression that the planet is covered by large oceans of liquid water”, explains Piaulet. “However, we do not know if this is the case or not 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.”
“If Kepler-138 cid are among a class of planets (water worlds) that are common but hard to find, or really rare, they remind us that exoplanets are often very different from the worlds we know in our Solar System. . These worlds aquatics will continue to stimulate our curiosity and will remain the targets of choice for astronomers’ most powerful telescopes,” concludes Dragomir.
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