The climate of a terrestrial exoplanet is controlled by the type of host star, the orbital configuration and the characteristics of the atmosphere and the surface. Many rocky exoplanets have higher eccentricities than those in the Solar System, and about 18% of planets with masses < 10 M⊕ have ? > 0.1. Underexplored are the implications of such high eccentricities on the atmosphere, climate, and potential habitability on such planets. We use WACCM6, a state-of-the-art fully-coupled Earth-system model, to simulate the climates of two Earth-like planets; one in a circular orbit (? = 0), and one in an eccentric orbit (? = 0.4). We quantify the effects of eccentricity on the atmospheric water abundance and loss given the importance of liquid water for habitability. The asymmetric temperature response in the eccentric orbit results in a water vapour mixing ratio in the stratosphere (> 20 ppmv) that is approximately five times greater than that for circular orbit (∼ 4 ppmv). This leads to a ∼ 3 time increase in the atmospheric hydrogen loss rate and a corresponding ∼ 3 times decrease in the ocean loss timescale. Thus, highly-eccentric Earth-like exoplanets can still retain their oceans over the lifetime of the system. Using the Planetary Spectrum Generator, we simulate the idealised transmission spectra for both cases. We find that the water absorption features are stronger at all wavelengths for the ? = 0.4 spectrum than for the circular case. Hence, highly-eccentric Earth-like exoplanets may be prime targets for future transmission spectroscopy observations to confirm, or otherwise, the presence of atmospheric water vapour.

The netCDF (.nc files) data were produced from simulations using the WACCM6 (CESM2.1.3) model.

See https://www2.acom.ucar.edu/gcm/waccm and https://doi.org/10.1029/2019JD030943 for more information.

These simulations were performed on the ARC4 HPC at the University of Leeds.

The output variables from the simulations are in terms of monthly mean files (h0.files), and they are grouped in two folders for the circular and the eccentric cases.

The 1-day instantaneous 'snapshots' (cam.h1 files) are used to generate synetic transit spectra using the Planetary Spectrum Generator (PSG; https://psg.gsfc.nasa.gov/) The .txt files are produced by converting the WACCM6 output (.nc files) to a binary file format (.gcm files) which can be processed through the PSG web interface. All PSG related files are grouped into two folders for the circular and eccentric cases.

To produce the figures in the manuscript, Python programming language was used.

Python programming language (e.g. xarray) was used to open and manipulate the NetCDF files.