# The secondary transit of the hot Jupiter WASP-121b at 2 $\mu$m

## TL;DR

This study reports ground-based 2 μm secondary transit observations of hot Jupiter WASP-121b, revealing atmospheric features and a low orbital eccentricity, supporting models with inefficient heat redistribution.

## Contribution

First ground-based 2 μm secondary transit detection of an exoplanet using a 1m-class telescope, providing new atmospheric and orbital insights for WASP-121b.

## Key findings

- Detected a 0.228% transit depth at 2 μm.
- Supported complex atmospheric models with emission/absorption features.
- Confirmed low eccentricity consistent with previous estimates.

## Abstract

Ground-based observations of the secondary transit in the 2MASS K band are presented for the hot Jupiter WASP-121b. These are the first occultation observations of an extrasolar planet carried out with an instrument attached to a 1m-class telescope (SMARTS' 1.3 m). We find a highly significant transit depth of (0.228 +/- 0.023)%. Together with the Hubble Space Telescope near infrared emission spectrum, current data support more involved atmosphere models with species producing emission/absorption features, rather than simple smooth black body emission. Analysis of the time difference between the primary and secondary transits and the durations of these events yield an eccentricity of e=0.0207 +/- 0.0153, which is consistent with the earlier estimates of low/zero eccentricity, but with a smaller error. Together with the existing K-band data on other systems, WASP-121b lends further support to the lack of efficient heat transport between the day and night sides for nearly all Hot Jupiters.