  Venus and Earth provide astonishingly different views of the evolution of a
rocky planet, raising the question of why these two rocky worlds evolved so
differently. The recently discovered transiting Super-Earth LP 890-9c
(TOI-4306c, SPECULOOS-2c) is a key to the question. It circles a nearby M6V
star in 8.46 days. LP890-9c receives similar flux as modern Earth, which puts
it very close to the inner edge of the Habitable Zone (HZ), where models differ
strongly in their prediction of how long rocky planets can hold onto their
water. We model the atmosphere of a hot LP890-9c at the inner edge of the HZ,
where the planet could sustain several very different environments. The
resulting transmission spectra differ considerably between a hot, wet
exo-Earth, a steamy planet caught in a runaway greenhouse, and an exo-Venus.
Distinguishing these scenarios from the planet's spectra will provide critical
new insights into the evolution of hot terrestrial planets into exo-Venus. Our
model and spectra are available online as a tool to plan observations. They
show that observing LP890-9c can provide key insights into the evolution of a
rocky planet at the inner edge of the HZ as well as the long-term future of
Earth.
