# Tidally-Distorted, Iron-Enhanced Exoplanets Closely Orbiting Their Stars

## TL;DR

This paper models the shape and composition of ultra-short-period exoplanets, revealing that tidally distorted planets like KOI 1843.03 can be highly elongated and iron-rich, with implications for planetary formation.

## Contribution

First three-dimensional interior structure simulations of tidally distorted, iron-enhanced exoplanets, showing extreme elongation and specific composition requirements to avoid tidal disruption.

## Key findings

- KOI 1843.03 may be elongated with an aspect ratio up to 1.79.
- It must be as iron-rich as Mercury to avoid tidal disruption.
- Nearly half of studied ultra-short-period planets are iron-enhanced.

## Abstract

The transiting planet candidate KOI 1843.03 ($0.6 R_\oplus$ radius, 4.245 hour orbital period, $0.46 M_\odot$ host star) has the shortest orbital period of any planet yet discovered. Here we show, using the first three-dimensional interior structure simulations of ultra-short-period tidally distorted rocky exoplanets, that KOI 1843.03 may be shaped like an American football, elongated along the planet-star axis with an aspect ratio of up to 1.79. Furthermore, for KOI 1843.03 to have avoided tidal disruption (wherein the planet is pulled apart by the tidal gravity of its host star) on such a close-in orbit, KOI 1843.03 must be as iron-rich as Mercury (about 66% by mass iron compared to Mercury's 70% by mass iron, Hauck et al. 2013). Of the ultra-short-period ($P_\mathrm{orb} \lesssim 1$ day) planets with physically-meaningful constraints on their densities characterized to date, just under half (4 out of 9) are iron-enhanced. As more are discovered, we will better understand the diversity of rocky planet compositions and the variety of processes that lead to planetary iron enhancement.