The image above (Credit: ESO/L. Calçada) shows CoRoT-7b, an exoplanet located so close to its parent star that the input of radiation causes the surface to melt. It is for this reason that these strange worlds are called "Lava Planets" and they have unique atmospheres that are made up of rocky vapours. PVL PhD student Giang Nguyen has been working on understanding how a similar world, K2-141b, operates in collaboration with Prof. Nick Cowan at McGill University. There will be a paper out soon, but Giang provides a preview of the work below.
by Giang Nguyen
K2-141b belongs to a subset of rocky planets that orbit very closely to their star and are tidally locked. The dayside of the planet is hot enough to not only melt rocks (about half the planet is one giant magma ocean, hence the name lava planet) but to vapourize them as well. This vapourization process ultimately creates a thin atmosphere that may be detectable from hundreds of light year away with the right space telescopes.
For my work, I have been using computer models to simulate the atmosphere of K2-141b. I considered two cases: a sodium atmosphere and a silica atmosphere. Sodium is chosen as it is the most volatile component in minerals while silica is chosen as it is expected to be the most abundant for rocky planets. The atmospheric model is based on the shallow-wave equations with steady-state flow driven mainly by the temperature contrast between the planet’s permanent dayside and nightside hemispheres. As expected, since sodium is much more volatile, the pressure of the sodium atmosphere is more than 50 times that of the silica atmosphere. This also allows the sodium atmosphere to exist beyond the day-night terminator while the silica atmosphere collapses just before this point. However, as sodium has a lower heat capacity than silica, the sodium atmosphere cools off much faster which has implications for observations. In either case, the wind exceeds 1.5 km/s which is useful for high-dispersion spectroscopy.
For my work, I have been using computer models to simulate the atmosphere of K2-141b. I considered two cases: a sodium atmosphere and a silica atmosphere. Sodium is chosen as it is the most volatile component in minerals while silica is chosen as it is expected to be the most abundant for rocky planets. The atmospheric model is based on the shallow-wave equations with steady-state flow driven mainly by the temperature contrast between the planet’s permanent dayside and nightside hemispheres. As expected, since sodium is much more volatile, the pressure of the sodium atmosphere is more than 50 times that of the silica atmosphere. This also allows the sodium atmosphere to exist beyond the day-night terminator while the silica atmosphere collapses just before this point. However, as sodium has a lower heat capacity than silica, the sodium atmosphere cools off much faster which has implications for observations. In either case, the wind exceeds 1.5 km/s which is useful for high-dispersion spectroscopy.
Observing the atmosphere of exoplanet K2-141b will be done mainly through infrared emission measured by the upcoming James Webb Space Telescope and through high-dispersion spectroscopy measured by various ground-based telescopes. Infrared emission is calculated by using the Planck function evaluated at 1 micron and integrated over the planet’s surface. Since the silica atmosphere is much warmer than sodium, the silica atmosphere will give off a stronger signal (more light but weaker absorption features). High-dispersion spectroscopy involves analyzing the doppler shifts of spectra features to observe many things, the most relevant to our case is strong winds. This is often done for transits and mainly for hot Jupiters as the atmospheres of lava planets barely exist past the terminator. However, K2-141b is special among other lava planets as its proximity to its star is close enough that the terminator is extended by 28% around the limb. Another unique property is that during a transit, particularly the ingress and egress, you are not looking at the terminator but significantly off of it. Even though the silica atmosphere collapses before the terminator, we can still see the significant part of the atmosphere during ingress and egress. With significant scale height (silica much larger than sodium), it may be possible to detect doppler shifts in transmission or absorption features due to strong 1.5+ km/s winds.
In terms of mass redistribution, I have found that the silica transportation via the surface is small enough that magma ocean circulation can easily draw silica from the nightside to the dayside. As sodium is more volatile, the evaporation rate is so large that it would be exceptionally difficult to transport back sodium from the nightside to the dayside. This would ultimately lead to the dayside being continually starved of sodium which can change the surface composition over time. This ultimately affects what we see compared to what the model shows.
Overall, the characterization of terrestrial exoplanetary atmospheres and their detection is becoming more and more of a reality. However, we are limited to lava planets for now due to their frequent transits which makes it easy to observe them on a regular basis, with K2-141b the “easiest” terrestrial example to date. I have found that the most volatile components of the atmosphere may not be what we observe and that the silica component of the atmosphere may be easier to detect. While lava planets are a far cry from “Earth 2.0”, the characterization of their atmospheres are a stepping stone to other Earth-like atmospheres. As I wrap up this project, I’ll be revisiting icy-exoplanets and model water-vapour dominant atmospheres. Eventually, I hope to be tasked dealing with more Earth-like exoplanet but until that time, I’ll keep you posted on my progress.
In terms of mass redistribution, I have found that the silica transportation via the surface is small enough that magma ocean circulation can easily draw silica from the nightside to the dayside. As sodium is more volatile, the evaporation rate is so large that it would be exceptionally difficult to transport back sodium from the nightside to the dayside. This would ultimately lead to the dayside being continually starved of sodium which can change the surface composition over time. This ultimately affects what we see compared to what the model shows.
Overall, the characterization of terrestrial exoplanetary atmospheres and their detection is becoming more and more of a reality. However, we are limited to lava planets for now due to their frequent transits which makes it easy to observe them on a regular basis, with K2-141b the “easiest” terrestrial example to date. I have found that the most volatile components of the atmosphere may not be what we observe and that the silica component of the atmosphere may be easier to detect. While lava planets are a far cry from “Earth 2.0”, the characterization of their atmospheres are a stepping stone to other Earth-like atmospheres. As I wrap up this project, I’ll be revisiting icy-exoplanets and model water-vapour dominant atmospheres. Eventually, I hope to be tasked dealing with more Earth-like exoplanet but until that time, I’ll keep you posted on my progress.
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