The exoplanet Wasp-107b is back in the spotlight with publications in the renowned journal Nature that can be accessed for free on arXiv. Wasp-107b is part of the “cotton candy” planets like Wasp-193b. We have known this for years already using observations made with the Hubble telescope that showed it is one of the exoplanets with the lowest known density. Despite its mass being only 12% of that of Jupiter, its diameter is comparable because it is heavily heated by its host star (slightly cooler and less massive than our Sun), around which it completes its orbit in just six days. With temperatures reaching hundreds of degrees and above, its atmosphere is greatly expanded.
Being only about 200 light-years from the Solar System, its highly expanded atmosphere makes it a prime target for analyzing its composition by measuring the spectrum through the transmission of light from its star once it has passed through this atmosphere (see the video below for more details).
However, if Wasp-107b’s atmosphere is so hot, surprisingly, we cannot explain it with the radiation from its host star. The exoplanet, even orbiting at a seventh of the distance between Mercury and the Sun, does not receive enough energy from Wasp-107 (an orange dwarf cooler than the Sun with a surface temperature of 4,430 kelvins, classifying it as a K6 spectral type) to be so inflated.
There was also another problem challenging the models of gas giant formation.Giant gas planets, but a solution to these puzzles has just been found by two independent teams of researchers.
A planetary atmosphere has a spectral signature that represents its chemical composition, as well as its composition in clouds and “fog.” Through various techniques, it is possible to determine the physico-chemical characteristics of an exoplanet’s atmosphere. Among these techniques are transit spectroscopy, secondary or eclipse transit, direct spectroscopic observation of the planet, and observing the planet at different phases around the star to measure temporal and seasonal variations. Explore exoplanets through our 9-episode web series on our YouTube channel. A playlist offered by CEA and the University Paris-Saclay as part of the European research project H2020 Exoplanets-A. © CEA
### An Atmosphere Probed by Transit Spectroscopy
In a press release from NASA accompanying the announcement of this discovery, astrophysicist Luis Welbanks from Arizona State University (ASU), the lead author of one of the articles published today in Nature, explains that “based on its radius, mass, and age, we thought Wasp-107 b had a very small rocky core surrounded by a huge mass of hydrogen and helium. But it was difficult to understand how such a small core could absorb so much gas and then not fully expand to become a planet the size of Jupiter.” This was not a problem with other “puffy Jupiters” known as most are hotter, more massive, and therefore easier to explain.
David Sing from Johns Hopkins University (JHU), the lead author of a parallel study also published today in Nature, explains that “Wasp-107 b is a very interesting target for the James Webb because its mass is significantly colder and closer to that of Neptune than most other low-density planets, the hot Jupiters, we have studied. As a result, we should be able to detect methane and other…moleculesmolecules capable of providing us with information about its chemistrychemistry and internal dynamics that we cannot obtain from a hotter planet.”
The two teams set out to investigate the composition of the exoplanet’s atmosphere by studying its transmission spectrum, combining observations from not only the NIRCam instruments (Near-Infrared CameraNear-Infrared Camera)) and MiriMiri (Mid-Infrared Instrument) from Webb, but also with the WFC3 (Wide Field Camera 3) from HubbleHubble.
The remarkable precision of the data not only allowed for the detection, but also for the measurement of the abundance of a multitude of molecules. The crucial information that resulted from this is that both spectra show a surprising lack of methane (CH4) in the atmosphere of Wasp-107 b: one-thousandth of the expected amount based on its assumed temperature.
According to David Sing, “this is evidence that the hot gases from the depths of the planet must vigorously mix with the cooler layers higher up. Methane is unstable at high temperatures. The fact that we detected so little of it, even though we detected other carbon molecules, indicates that the interior of the planet must be much hotter than we thought.”
A rocky core twice as massive as previously thought
It now appears that the heatheat contained within the exoplanet is largely due to tidal forces stirring Wasp-107 b because its orbit is slightly elliptical, causing it to periodically approach and then move away from its sun Wasp-107. It is known that tidal forces resulting from this situation can strongly heat a planet, as is precisely what is observed and theorized in the case of Io, Jupiter’s infernal volcanic moon.
This explanation had already been proposed, but proof was lacking. It has now been unexpectedly provided by cosmochemistry.
“If we know the amount of energy present on the planet and what proportion of the planet is made up of heavier elements like carboncarbon, the”nitrogennitrogen, oxygenoxygen, and sulfursulfur, in relation to the amount of hydrogen and helium, we can calculate the mass that the nucleus must contain,” reveals in a NASA press release another member of one of the involved teams, Daniel Thorngren from JHU.
It turns out that the nucleus is at least twice as massive as initially expected, which makes more sense in mattermatter of planet formation, and therefore that “the Webb data tell us that planets like Wasp-107 b must not have formed in a strange way with a very small nucleus and a huge gaseous envelope. Instead, we can consider that we are dealing with an exoplanet somewhat similar to Neptune, with a lot of rocks and not as much gas; just increase the temperature to achieve the appearance it has,” concludes astrophysicist Mike Line, a member of the ASU team that published an article in Nature with his French colleagues Pierre-Olivier Lagage and Achrène Dyrek, members of CEA and CNRS.
A conference from April 2, 2023, “First results from JWST: transiting exoplanets,” by Achrène Dyrek, Astrophysics Department at CEA. © French Physical Society
Researcher Achrène Dyrek is one of the winners of the L’Oréal – Unesco Young Talent for Women in Science 2023 and some time ago, she had given a conference on the first results obtained with the JWSTJWST regarding exoplanet atmospheres. In the case of Wasp-107b, Achrène Dyrek and her colleagues had already conducted work using the low-resolution spectroscope of the Miri instrument on the JWST, an instrument observing in the infraredinfrared and in which CEA was a major contributor.