# Accounting for Multiplicity in Calculating Eta Earth

## TL;DR

This study refines the estimate of Earth-like planets in the galaxy by incorporating planetary multiplicity and updated data, revealing that a significant fraction of stars may host multiple habitable planets.

## Contribution

It introduces a revised calculation of η⊕ considering planetary multiplicity and updated parameters, providing more accurate habitability estimates.

## Key findings

- Approximately 34% of stars may host Earth-like planets.
- Around 6.4% of GK dwarfs have multiple habitable planets.
- Highly packed habitable systems are rare but possible.

## Abstract

Using the updated exoplanet population parameters of our previous study, which includes the planetary radius updates from Gaia DR2 and an inferred multiplicity distribution, we provide a revised $\eta_{\oplus}$ calculation. This is achieved by sampling planets from our derived population model and determining which planets meet our criterion for habitability. To ensure robust results, we provide probabilities calculated over a range of upper radius limits. Our most optimistic criterion for habitability provides an $\eta_{\oplus}$ value of $0.34\pm 0.02 \frac{\rm planets}{\rm star}$. We also consider the effects of multiplicity and the number of habitable planets each system may contain. Our calculation indicates that $6.4\pm0.5\%$ of GK dwarfs have more than one planet within their habitable zone. This optimistic habitability criterion also suggests that $0.036\pm0.009\%$ of solar-like stars will harbor 5 or more habitable planets. These tightly packed highly habitable system should be extremely rare, but still possible. Even with our most pessimistic criterion we still expect that $1.8\pm0.2\%$ of solar-like stars harbor more than one habitable planet.