# Mass, Density, and Formation Constraints in the Compact, Sub-Earth Kepler-444 System including Two Mars-Mass Planets

## TL;DR

This study uses transit-timing analysis of Kepler data to determine the masses of five sub-Earth planets in the ancient Kepler-444 system, revealing insights into its formation and orbital evolution.

## Contribution

It provides the first mass measurements of two Mars-mass planets in a compact system, challenging smooth disk migration as the sole formation mechanism.

## Key findings

- Two planets have confidently detected masses near Mars.
- The mass ratio and tidal effects suggest complex formation history.
- The system's architecture likely involved significant post-formation perturbations.

## Abstract

Kepler-444 is a five planet system around a host-star approximately 11 billion years old. The five transiting planets all have sub-Earth radii and are in a compact configuration with orbital periods between 3 and 10 days. Here we present a transit-timing analysis of the system using the full Kepler data set in order to determine the masses of the planets. Two planets, Kepler-444 d ($M_\mathrm{d}=0.036^{+0.065}_{-0.020}M_\oplus$) and Kepler-444 e ($M_\mathrm{e}=0.034^{+0.059}_{-0.019}M_\oplus $), have confidently detected masses due to their proximity to resonance which creates transit timing variations. The mass ratio of these planets combined with the magnitude of possible star-planet tidal effects suggests that smooth disk migration over a significant distance is unlikely to have brought the system to its currently observed orbital architecture without significant post-formation perturbations.