Rocky exoplanets can vary in composition based primarily on the compositions of their stellar hosts. This is because the stars and planets form from the same clouds of dust and gas. One of the key observables in a star is its metallicity (or the amount of iron and other metals in the star). The metallicity of a planet does not always match with the star's, however. Why is that?
To address this question and many others, I have been working on a team based at Arizona State University called Tracing Rocky Exoplanet Compositions (TREC). I work alongside Prof. Steve Desch and undergraduate Jason Lee using a code called kyushu to study how ultra-short period exoplanets (USPs) can exist in stable configurations. USPs are exoplanets that orbit their stars in hours or days. To do this, these planets must be incredibly close to their host stars, but being so close, they should be ripped apart by the star's gravitational forces. That these planets exist at all has to be due to what they are made out of. The models we run approximate how much of these planets are pure iron metallic core versus a rocky mantle. Once we have a baseline, we then change the composition of the mantle to include iron and change the composition of the core to include lighter elements like silicon. We do this to find the limits of what a USP could be made out of and what it means for exoplanets in general.
We also can learn about Mercury and how it might have formed from studying USPs. Mercury has a very high core-mass fraction of ~70%, meaning that 70% of its total mass is concentrated in the core. This is twice as high as the core-mass fractions of every other rocky planet in our Solar System. Planetary scientists think that Mercury must have had its mantle stripped by a giant collision that removed most of the outermost rocky layer. By doing our work on USPs, we are testing the hypothesis that Mercury could have formed with a high core-mass fraction instead of having survived a massive impact event.
This work is current as of February 2025.
To address this question and many others, I have been working on a team based at Arizona State University called Tracing Rocky Exoplanet Compositions (TREC). I work alongside Prof. Steve Desch and undergraduate Jason Lee using a code called kyushu to study how ultra-short period exoplanets (USPs) can exist in stable configurations. USPs are exoplanets that orbit their stars in hours or days. To do this, these planets must be incredibly close to their host stars, but being so close, they should be ripped apart by the star's gravitational forces. That these planets exist at all has to be due to what they are made out of. The models we run approximate how much of these planets are pure iron metallic core versus a rocky mantle. Once we have a baseline, we then change the composition of the mantle to include iron and change the composition of the core to include lighter elements like silicon. We do this to find the limits of what a USP could be made out of and what it means for exoplanets in general.
We also can learn about Mercury and how it might have formed from studying USPs. Mercury has a very high core-mass fraction of ~70%, meaning that 70% of its total mass is concentrated in the core. This is twice as high as the core-mass fractions of every other rocky planet in our Solar System. Planetary scientists think that Mercury must have had its mantle stripped by a giant collision that removed most of the outermost rocky layer. By doing our work on USPs, we are testing the hypothesis that Mercury could have formed with a high core-mass fraction instead of having survived a massive impact event.
This work is current as of February 2025.