I love the concept of a ‘puffy’ planet! The exoplanets discovered that fall into this category are typically the same size of Jupiter but 1/10th the mass! They tend to orbit their host star at close in orbits and are hot but one has been found that is different from the normal. This Neptune-mass exoplanet has been thought to be cooler but still have a lower density. The James Webb Space Telescope (JWST) has recently discovered that tidal energy from its elliptical orbit keeps its interior churning and puffs it out.
WASP-107b is more than three quarters the volume of Jupiter but, like most fluffy planets, is one-tenth the mass making it one of the least dense planets known. Its unusual property however is that whilst most puffy planets are hot, WASP-107b is relatively cool. This goes against initial observations which had also suggested, due to its mass, radius and age it was thought to have a small rock core with a hydrogen and helium rich atmosphere.
Recent observations of this exoplanet by the JWST revealed far less methane in the atmosphere than expected. The orientation of the orbit making it edge on to us means we can study the planet’s atmosphere by examining the light from the star as it passes through the gas. This technique known as transmission spectroscopy can be used to identify the signatures of gasses in the star’s spectrum. Using JWSTs Near-Infrared Camera and Mid-Infrared Instrument and data from Hubble’s Wide Field Camera 3, the abundances of methane, water vapour, carbon dioxide, carbon monoxide, sulphur dioxide and ammonia could be revealed.
Not only did this reveal the lack of methane but also provided evidence that hot gas from lower altitudes was mixing with cooler gas layers from higher up. One of the properties of methane is that it is unstable at high temperatures and, beyond 1200 degrees the bonds between hydrogen and carbon breakdown. This is not the case with other carbon based molecules suggesting the higher temperature. It suggests that the interior of the planet must be hotter than thought with a more massive core than expected. It’s thanks to JWST’s higher level of sensitivity that the mystery looks like it may finally have been solved.
The team, led by Luis Welbanks from Arizona State University (ASU) explored a number of possibilities. First that it had more mass in its core than first expected. If this was true then the atmosphere is likely to have contracted as the planet cooled. In time and, without a source of heat to give the atmosphere energy and cause it to expand, the planet should be much smaller than observed. Even though the planet orbits the star at a distance of of just over 8 million kilometres it still does not get enough energy to drive the inflation of the atmosphere.
One theory is that the higher internal temperatures are generated by tidal heating. In just the same way that the gravitational force of Jupiter causes tidal heating on Io, the highly elliptical orbit of WASP-107b could be the answer. As the planet swings by the host star in its non-circular orbit it is squished and squashed providing a source of heat.
Understanding the source of heat on WASP-107b has helped the team learn more about the properties and processes. Knowing how much energy is there helps to determine the proportions of other elements like carbon, nitrogen, oxygen and sulphur. Calculating this helps to determine the mass of the core which, according to the recent studies reveal is twice as massive as originally estimated.