We met Caroline Piaulet, Ph.D. Student at Insтιтut Trottier de recherche sur les exoplanètes (iREx) from Université de Montréal (Canada). 

Team led by Caroline Piaulet, did a detailed study of the planetary system Kepler-138. More precisely, it discovered two “water worlds“: two exoplanets full of water that orbit around the Red Dwarf Star, about 218 light-years away from Earth. A light-year is equivalent to about 9.46 trillion kilometers (5.88 trillion miles).

How was your pᴀssion for exoplanets born? Which exoplanets surprised you the most?

What sparked my pᴀssion for exoplanets was the discovery that we could not only find planets in our solar system, but study their compositions and atmospheres! The fact that we can probe the gases in the atmospheres of distant planets never ceases to amaze me. One of the exoplanets that surprised me the most was for sure WASP-107b, the first planet I studied during my PhD: it’s a Jupiter-size planet that has such a low density that it’s comparable with that of cotton candy – how crazy is that?

How do you manage to study the atmosphere of these very distant planets? Can telescopes only see the shadow of exoplanets?

There are a couple ways one can go about studying exoplanet atmosphere, but the most common is called ‘transmission spectroscopy’ which is similar to the idea of ‘shadow’ you were mentioning. When a planet pᴀsses in front of its star from our point of view, it projects a ‘shadow’ which makes it so that we momentarily see less of the star’s light. 

We call these events ‘transits’, and they enable us to find out about planets we didn’t know existed. When it comes to studying atmospheres, we take advantage of the fact that during a transit, a small portion of the star’s light is filtered through the planet’s atmosphere, and gets imprinted with the signatures of the molecules and atoms present. 

Then ‘transmission spectroscopy’ consists in breaking apart the light we receive from the star during the transit between all its different colors, and identifying the unique fingerprint left in these colors by the molecules in the planet’s atmosphere.

The Université de Montréal team, led by you, discovered two “water worlds”. Two exoplanets filled with water (Kepler-138c and Kepler-138d). What are the characteristics of these two exoplanets?

Our international team discovered two ‘twin’ planets (they have essentially the same size and mᴀss) that are best explained as being water worlds, i.e. having a large fraction of their volume made of water. If you think about Kepler-138d as we understand it now, imagine a large planet (about 1.5 times the size of the Earth) that has about half of its volume made of water in various forms. Starting from the top, you would have to go through a 2000 km deep water layer to reach a rocky interior. 

The water layer would be made of an extended water vapor atmosphere, and as you go deeper where the water is at higher pressures you would reach an ocean of what we believe would be ‘supercritical’, rather than liquid water. Supercritical water is essentially water vapor brought to such high pressure that it reaches a fluid state, but not cold enough to condense out into a liquid water ocean.

How did you discover water? What telescopes did you use?

We used the Hubble and Spitzer space telescopes, and observed 13 new transits of Kepler-138 d. Our method consisted in using the very special setup of the planets in the Kepler-138 system that makes it so that instead of pᴀssing in front of their star at regular intervals (for instance every 5 days for a planet that takes 5 days to go arount its star), the three planets Kepler-138 b, c, and d would sometimes transit the star a little (a few minutes) early, or late. 

This odd behaviour actually originates in the planets regularly close to each other, which perturbs their mutual orbits ever so slightly to produce what we call transit-timing variations (TTVs). Using these TTVs, we are able to measure the planets’ mᴀsses, which enables us to infer their densities. Kepler-138 c and d have densities too low to be made up of only rock similarly to the Earth: although the Earth is covered in oceans, they are very shallow and do not impact its density. 

On the other hand, we demonstrated this 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. A heavier molecule like water or methane is light enough to make for a low planet density, while being much more resistent to being stripped by the star’s energy – enough to explain the low densities of Kepler-138 c and d.

If we could walk on those two exoplanets, what would we see? What is their landscape like?

I belive my answer to question 3 above already gives part of the answer. With so much water, we wouldn’t expect continents or a rocky surface one could walk on. The way I like to imagine these planets is to think of the icy moons of the outer solar system, for which we believe that large water oceans exist below their icy surface. 

For Kepler-138 c and d, we might instead be looking at analogs of our own icy moons, only larger and much closer to the star so that instead of being shielded underneath an ice surface, the water is instead exposed in an extended vapor atmosphere.

We are discovering many exoplanets, and many of them are filled with water and are located in the habitable zone. Do you think the universe is full of life? Is there life everywhere?

From a pure probabilistic standpoint, the ingredients for life to emerge are certainly present around many other stars than our Sun. If the question is ‘do I believe there is life elsewhere in the Universe?’, then yes, I do. 

But the challenging task for us astronomers actually looking for the signatures of extraterrestrial life is to find signs that would unambiguously have to be produced by life and could not be a result of other processes such as chemistry or volcanism. Therefore, being able to answer this question based on scientific evidence for life – or lack thereof – is probably going to have to still wait a few more years.

Source: Nature Astronomy