Scientists using NASA’s Hubble and the retired Spitzer space telescopes observed two exoplanets orbiting a red dwarf star. Located nearly 218 light-years away in the constellation Lyra, these planets are unlike any other planet in our solar system.

The detailed observations showed that the planets Kepler-138 c and Kepler-138 d are water worlds. Despite having volumes more than three times that of Earth and masses twice that, planets c and d have much lower densities than Earth.

This is unexpected considering that most of the planets only marginally larger than Earth that have been thoroughly explored to date all appeared to be rocky worlds similar to our own. According to scientists, several of the icy moons in the outer solar system, which also have a rocky core and are mostly water, would be the most similar.

Water was not explicitly found at Kepler-138 c and d. Still, by comparing the planets’ sizes and masses to theoretical predictions, astronomers have concluded that up to half of their contents should consist of substances heavier than hydrogen or helium but lighter than rock (which make up the majority of of gas giant planets such as Jupiter). Water is the most common of these potential materials.

Björn Benneke, co-author of the study and professor of astrophysics at the University of Montreal, said: “We used to think that planets slightly larger than Earth were big balls of metal and rock, like scaled-up versions of Earth, so we called them super-Earths. However, we have now shown that these two planets, Kepler-138 c and d, are very different and that much of their total volume is probably water. It is the best evidence yet for water worlds, a type of planet that astronomers believe has existed for a very long time.”

This is an artist's illustration of a cross-section of the Earth (left) and the exoplanet Kepler-138 d
This is an artist’s illustration showing a cross-section of Earth (left) and the exoplanet Kepler-138 d (right). Like Earth, this exoplanet has an interior composed of metals and rocks (brown part), but Kepler-138d also has a thick layer of high-pressure water in various forms: supercritical and potentially liquid water deep inside the planet and a vast water vapor envelope (shades of blue) above. These water layers make up more than 50% of the volume, or a depth of about 2000 kilometers. In comparison, the Earth has a negligible fraction of liquid water with an average ocean depth of less than 4 kilometers. Credits: Benoit Gougeon (University of Montreal)

Caroline Piaulet of the Trottier Institute for Research on Exoplanets at the University of Montreal said: “The planets may not have oceans like those on Earth right on the surface of the planet. The temperature in the atmosphere of Kepler-138 d is likely above the boiling point of water, and we expect a thick, dense vapor atmosphere on this planet. Only under that steam atmosphere could there potentially be liquid water at high pressure, or even water in another phase that occurs at high pressure, a supercritical fluid.

Kepler-138 c and d, two potential water worlds, are not found in the habitable zone, the region around a star where the temperature allows liquid water on the surface of a rocky planet. However, scientists also discovered evidence of a brand new planet in the system, Kepler-138 e, in the habitable zone in the Hubble and Spitzer data.

This recently discovered planet takes 38 days to complete one orbit and is smaller and farther from its star than the other three. However, since it doesn’t appear to be passing by its host star, the nature of this extra planet is still unknown. Astronomers may have measured the exoplanet’s size by observing its transit.

With Kepler-138 e in view, the masses of the previously identified planets were re-determined using the transit timing-variation method. This technique involves monitoring minute changes in the precise moments of the transits of the planets in front of their star caused by the gravity of other nearby planets.

Another surprise for the scientists was that, contrary to previous beliefs, the two water worlds Kepler-138 c and d are actually “twin planets”, almost identical in size and mass. On the other hand, Kepler-138 b, the closer planet, has been proven to be a small Mars-mass planet and one of the smallest exoplanets discovered to date.

Benneke said, “As our instruments and techniques become sensitive enough to find and study planets farther from their stars, we could begin to find many more of these aquatic worlds.”

Magazine reference:

  1. Piaulet, C., Benneke, B., Almenara, JM et al. Evidence for the volatile composition of a planet of 1.5 Earth radius. Wet Astron (2022). DOI: 10.1038/s41550-022-01835-4