# Proxima Centauri b is not a transiting exoplanet

## TL;DR

This study used Spitzer Space Telescope data to conclusively rule out transits of Proxima Centauri b at the 4.5 micron wavelength, refining its size constraints and suggesting focus on other observational methods.

## Contribution

First infrared transit observations of Proxima Centauri b that set stringent upper limits on its radius and dismissed previous transit claims.

## Key findings

- No transits detected at 4.5 microns at 200 ppm level
- Proxima b's radius upper limit is 0.4 Earth radii
- Stellar activity is lower in infrared wavelengths

## Abstract

We report Spitzer Space Telescope observations during predicted transits of the exoplanet Proxima Centauri b. As the nearest terrestrial habitable-zone planet we will ever discover, any potential transit of Proxima b would place strong constraints on its radius, bulk density, and atmosphere. Subsequent transmission spectroscopy and secondary-eclipse measurements could then probe the atmospheric chemistry, physical processes, and orbit, including a search for biosignatures. However, our photometric results rule out planetary transits at the 200~ppm level at 4.5$~{\mu}m$, yielding a 3$\sigma$ upper radius limit of 0.4~$R_\rm{\oplus}$ (Earth radii). Previous claims of possible transits from optical ground- and space-based photometry were likely correlated noise in the data from Proxima Centauri's frequent flaring. Follow-up observations should focus on planetary radio emission, phase curves, and direct imaging. Our study indicates dramatically reduced stellar activity at near-to-mid infrared wavelengths, compared to the optical. Proxima b is an ideal target for space-based infrared telescopes, if their instruments can be configured to handle Proxima's brightness.