As of December 19, 2022, 5,227 extrasolar planets have been confirmed in 3,908 systems, and over 9,000 more await confirmation. While most of these planets are Jupiter- or Neptune-sized gas giants or rocky planets many times the size of Earth (super-Earths), a statistically significant number were planets with water making up a significant portion of their mass fraction – aka. “Water Worlds”. These planets are unlike anything we’ve seen in the solar system and raise several questions about planet formation in our galaxy.
In a recent study, an international team led by researchers from the University of Montreal’s Institute for Research on Exoplanets (iREx) found evidence of two water worlds in a single planetary system located about 218 light-years away in the constellation Lyra. Based on their densities, the team found that these exoplanets (Kepler-138c and Kepler-138d) are lighter than rocky “Earth-like” ones, but heavier than gas-dominated ones. The discovery was made using data from NASA’s now-defunct Spitzer Space Telescope and the venerable Hubble Space Telescope.
The team was led by Caroline Piaulet, a researcher and Ph.D. candidate with the iREx, as part of her Ph.D. Thesis. She was taught by astrophysics professor Björn Benneke, her Ph.D. Advisor at iREx and researcher at the Space Research Institute of the Austrian Academy of Sciences, the Flatiron Institute’s Center for Computational Astrophysics, the NASA Exoplanet Science Institute (NExSci), the NASA Goddard Space Flight Center and several universities. The paper describing their findings was recently published in the journal Nature Astronomy.
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This artist’s rendering shows the planet K2-18b, its parent star, and a companion planet in this system. Photo credit: ESO
During her internship at iREx, Piaulet’s work consisted of using transit and solar eclipse spectroscopy data obtained from Spitzer, Hubble and the James Webb Space Telescope (JWST) to determine the composition of intermediate-sized exoplanets that lie between Earth and Neptune lie greatness. Here are water worlds, planets that are more massive than Earth but many times the volume – meaning they are significantly less dense. As Benneke explained in a recent NASA press release:
“We used to think that planets slightly larger than Earth were big spheres of metal and rock, like enlarged versions of Earth, and that’s why we called them super-Earths. However, we have now shown that these two planets, Kepler-138c and d, are quite different in nature and that much of their total volume is likely to be water. It’s the best evidence yet of water worlds, a type of planet that astronomers suspected had existed for a long time.”
The exoplanets addressed in this study were previously discovered in 2014 by the Kepler space telescope using the transit method, which also discovered a third exoplanet (Kepler-198b) orbiting closer to the star. Between 2014 and 2016, Benneke and his colleague Diana Dragomir (University of New Mexico) came up with the idea of observing the planetary system with Hubble and Spitzer to look for further transits of Kepler-138d to study its atmosphere. In addition to Piaulet, the team was able to constrain the size of Kepler-198c and d based on 13 Hubble and Spitzer transit observations.
These were combined with new radial velocity measurements of the host star made with the high-resolution echelle spectrometer (HIRES) at the WM-Keck Observatory. This allowed the team to constrain the size and mass of Kepler-198c and d, leading them to produce size estimates of roughly two Earth masses, suggesting they were “super-Earths.” These results also indicate that Kepler-198c and d are “twin planets” of virtually the same size and mass – contradicting previous estimates that they were radically different.
The “sea worlds” of the solar system (left to right, top to bottom): Callisto, Europa, Ganymede, Titan, Enceladus, Dione, Triton, and Pluto. Credit: NASA/JPL
However, their estimates also suggest that these exoplanets are about three times the volume of Earth (which means they are less dense). This led to the conclusion that up to half of their volume is made up of volatile elements (most commonly water). These results were surprising since most exoplanets (slightly larger than Earth) studied in detail so far all appeared to be rocky. According to researchers, some of the icy moons in the outer solar system (like Europa, Enceladus, Ganymede, Titan, etc.) would be the closest comparison.
These bodies are composed largely of water and other volatiles surrounding a rocky-metallic core, leading to the nickname “Ocean Worlds”. Similarly, “water worlds” may not have surface oceans like Earth, but interior oceans beneath a layer of surface ice. As Piaulet said:
“Imagine larger versions of Europa or Enceladus, the water-rich moons orbiting Jupiter and Saturn but brought much closer to their star. Instead of an icy surface, they would host large shells of water vapor. The temperature in Kepler-138d’s atmosphere is probably above the boiling point of water, and we expect a thick, dense atmosphere of steam on this planet. Only under this vapor atmosphere could there possibly be liquid water at high pressure, or even water in another phase that occurs at high pressure, a so-called supercritical fluid.”
While neither Kepler-138c nor d are in the habitable zone, the team also found evidence in the Hubble and Spitzer data of a fourth planet that is! This newly discovered planet (Kepler-138e) continues to orbit its parent star, takes 38 days to complete an orbit, and appears to be similar in size to Mars. However, the properties of this planet remain poorly constrained as it does not appear to transit through its host star. But with next-generation telescopes like JWST and more sensitive techniques, astronomers are likely to find more aquatic worlds orbiting farther from their stars.
Artistic representation of a water world. A new study suggests that Earth is in the minority when it comes to planets and that most habitable planets may be covered by very deep oceans. Photo credit: David A. Aguilar (CfA)
Since beginning to observe the cosmos, JWST has demonstrated a key capability, directly imaging exoplanet HIP 65425 b and obtaining spectra from WASP-39b’s atmosphere. In the latter case, the spectra provided the first unequivocal evidence of carbon dioxide in this planet’s atmosphere (considered an important biosignature). During his time at iREx, Piaulet also developed a new method to constrain exoplanet atmospheric temperatures from emission spectra that the JWST will obtain. This method will allow astronomers to directly measure a key indicator of planetary habitability.
Further reading: NASA, natural astronomy