We had the privilege of speaking with Caroline Piaulet, who is a PhD student at the Institut Trottier de recherche sur les exoplanètes (iREx) from Université de Montréal in Canada. Caroline and her team have been conducting a comprehensive study of the planetary system Kepler-138, and their research led to the exciting discovery of two exoplanets that are filled with water. These exoplanets, referred to as “water worlds”, orbit around a Red Dwarf Star located about 218 light-years away from Earth.
When asked about how her passion for exoplanets was born, Caroline shared that she was amazed by the fact that we can not only find planets in our solar system, but also study their compositions and atmospheres. She was particularly surprised by WASP-107b, the first exoplanet she studied during her PhD. WASP-107b is a Jupiter-sized planet with a low density that is similar to that of cotton candy, which she found to be incredibly fascinating.
In terms of studying the atmospheres of distant exoplanets, Caroline explained that one common method is called “transmission spectroscopy”. This method is based on the idea of a “shadow” that is cast when a planet passes in front of its star from our point of view. These events, called transits, allow us to study the atmospheres of exoplanets by analyzing the light that is filtered through the planet’s atmosphere during the transit. This light is imprinted with the signatures of the molecules and atoms present, and through transmission spectroscopy, the light is broken apart into different colors and the unique fingerprint left by the molecules in the planet’s atmosphere is identified.
Caroline and her team discovered two exoplanets, Kepler-138c and Kepler-138d, that are referred to as “water worlds”. These two planets are essentially twins, with similar sizes and masses, and they have a large fraction of their volume made up of water in various forms. Imagine a large planet, about 1.5 times the size of the Earth, with half of its volume being made up of water. The water layer would consist of an extended water vapor atmosphere, and as you go deeper, where the water is at higher pressures, you would reach an ocean of supercritical water, which is essentially water vapor brought to such high pressure that it reaches a fluid state.
The team used the Hubble and Spitzer space telescopes to observe 13 new transits of Kepler-138d and used a unique method to measure the planets’ masses and densities. By using transit-timing variations (TTVs), they were able to determine that Kepler-138c and d have densities too low to be made up of only rock, similar to the Earth. They demonstrated that the low density could not be due to a hydrogen envelope, as hydrogen is very light and can be easily swept away by the star’s irradiation. However, a heavier molecule like water or methane is light enough to make for a low planet density, while being much more resistant to being stripped by the star’s energy, which is enough to explain the low densities of Kepler-138c and d.
If we could walk on these two exoplanets, we wouldn’t expect to see continents or a rocky surface. Instead, the planets would have a landscape that is similar to the icy moons of the outer solar system, where large water oceans are believed to exist.