Researchers have identified an iron absorption line on the ultra-hot exoplanet, a planet outside our solar system, called Wasp-76b.

Published in the Nature journal, the findings suggest it could rain iron on the nightside of the planet 390 light years away.

Some days its surface temperature exceeds 2400C, hot enough to evaporate metals.

But at night the cooler temperatures mean the vapour turns into drops of iron.

The discovery is the first result for the high resolution Echelle Spectrograph for Rocky Exoplanets and Stable Spectroscopic Observations, known as Espresso.

Astrophysicists have identified chemical variations between day and night on the planet.

They detected the trace of iron vapour just at the division between the daytime and the night-time sector of the planet.

"However, surprisingly we do not see this iron vapour at dawn," said David Ehrenreich, a researcher at the University of Geneva and the first author of the article.

"The only explanation possible for this phenomenon is that it rains iron on the dark side of this exoplanet with extreme conditions."

Jonay I Gonzalez Hernandez, a researcher at Instituto de Astrofisica de Canarias, Spain, said: "Just like the moon around the Earth, this planet always keeps the same face towards its star as it rotates around it, which causes this extreme difference in temperature between day and night on the planet."

Australian Associated Press