Title: Unveiling the Interior Structure and Thermal Evolution of Super-Earth GJ 486b

Authors: Chandan K. Sahu, Liton Majumdar, Sudipta Mridha, and Harshit Krishna

First Author’s Institution: National Institute of Science Education and Research, Jatni 752050, Odisha, India

Status: published in The Astrophysical Journal [open access]

Planets between the size of Neptune and Earth, known as Super-Earths, are the most frequently found planets in the galaxy. With no analog in our Solar System, these worlds can offer a unique look at planetary formation, evolution and potentially life in the Universe. To truly understand a planet’s environment and assess the potential for its habitability, we must look at every part of it. This includes not just the atmosphere, but also the interior. Rocky planets, such as the inner Solar System planets, typically don’t have enough gravitational pull to retain their primary atmospheres. Thus, only their secondary atmospheres remain. These secondary atmospheres are directly linked to the interior of the planet and are usually formed by volcanic outgassing, material from impacts on the planet’s surface, and other interior processes. By observing a rocky planet’s atmosphere and determining its composition, we can learn something about the interior of the planet as well.

The subject of today’s paper, GJ 486b is a warm super-Earth orbiting a M-dwarf star every 1.5 days. The transmission spectrum of the planet was recently observed by JWST and when the data was analyzed, a potential water-rich atmosphere was discovered! Today’s authors take these recent discoveries of GJ 486b’s atmosphere and use them to model the interior of the planet.

One Model to Rule Them All

The interiors of rocky planets are not very well known. This is because the best way to understand what’s inside a rocky planet is through seismology. In order to do this, one must put a seismograph on a planet’s surface and measure how seismic waves move through the material inside a planet. Only three bodies have had seismographs landed on them: the Earth, moon and Mars. Rocky exoplanet interiors are even harder to understand when the only observable properties we have are the atmospheres, mass and radius. To understand exoplanet interiors, we have to use modeling to infer properties of the inside of the planet.

The authors of today’s paper use the 1-D modeling package SERPINT to model GJ 486b’s interior based upon JWST observations of its atmosphere and assumptions made about rocky planets. Their model is broken down into four different parts, as illustrated in Figure 1.

In the structure module, the authors model the layers in the planet. They assume an Earth-like interior structure with an iron core, an inner and outer mantle. The crust is not modeled as it’s considered a very complex structure and outside the scope of the paper. The thermal module takes into account how the host star’s light changes over time and the radioactive decay of elements in the planet’s interior. The volatile module models the water and oxygen in the planet and how they are exchanged between the interior, surface and atmosphere. Finally, the escape module looks at how the atmospheric escape of elements such as hydrogen and oxygen affect the atmosphere (and therefore the interior) over time.

Taking all of these factors into account is important when creating a comprehensive picture of a planet. Each module models different processes that influence the planet’s interior, surface and atmosphere, making sure that we’re taking all of these things into account when trying to understand the planet. It truly is one model to rule them all.

Outgassing and Photolysis and Escape, Oh My!

They start the model for the planet and star together and allow them both to evolve for 10 billion years. The planet starts with a mantle temperature of 5000 K which ensures the surface is molten, creating a magma ocean. This is typically how we think all rocky planets started their formation, including the Earth! Figure 2 illustrates how the mantle solidifies over time, influencing the overall structure of GJ 486b. Both layers of the mantle start to cool after the start of the simulation, and the lower mantle cools and solidifies first followed by the upper mantle. This is due to the different composition in each layer and thus, a different temperature at which they will solidify. Cooling is delayed for a while due to internal heating from radioactive decay of the elements. This can be seen in Figure 2b with the shallow slope of the orange line. The orange line represents the mantle’s equilibrium temperature and the yellow shaded region is the time in which the mantle has not yet solidified. As the mantle cools, it turns into a thick mush and then eventually into solid rock (as shown in Figure 2d). This indicates the mantle has solidified, and we move from the yellow shaded region on the graphs into the orange shaded region.

They start the simulation with 10 Earth-sized oceans of water on the planet. This water is initially inside the magma ocean but as the mantle cools and solidifies, some of the water becomes solid while the rest of it is transferred to the atmosphere in the form of water vapor. Water vapor outgassing from the interior of the planet stops when the mantle becomes solid. At this same time, the luminosity of the host star shrinks, (as shown in Figure 2a) causing the atmospheric escape of hydrogen and oxygen to stop and for atmospheric water vapor to stop being broken down by radiation from the host star (known as photolysis). This results in a build-up of oxygen and water over time. This final atmospheric composition matches the transmission spectroscopy data from JWST. This exchange between outgassing, the breaking down of atmospheric water vapor and hydrogen/oxygen escape impacts the planet’s sources of water (shown in Figure 3) and influences the atmosphere we see on GJ 486b today.

Today’s paper summarizes the possible interior structure of the super-Earth GJ 486b. While the modeling is able to reproduce the atmospheric data obtained with JWST, there are many limitations to the model. It first assumes a simplified version of Earth’s interior structure, when in reality, Earth’s structure has a much more complex composition and these minerals are distributed non-uniformly across the Earth’s layers. This can result in different temperatures and pressures for each layer, something that was not modeled in this paper. It’s challenging to determine the composition of rocky exoplanets and because super-Earths are much larger than Earth, the pressures inside of the planet will be more extreme, leading to unknown effects on the materials inside. Furthermore, GJ 486b is expected to be tidally locked, presenting a whole new set of assumptions and challenges to modeling its interior. And lastly, even though there is evidence to suggest that GJ 486b could have a water-rich atmosphere, the JWST transmission spectrum could also be attributed to starspots on the host star, making the assumption of a water-rich atmosphere wrong. While this is a good first step, more data and new ways to understand the interiors of exoplanets will be needed to truly understand what is going on inside GJ 486b.

Astrobite edited by Tori Bonidie

Featured image credit: NASA, ESA, CSA, Joseph Olmsted (STScI), Leah Hustak (STScI)