A planet with iron rain has been discovered

Located 390 light-years away from Earth is WASP-76b, a new giant exoplanet with a little less mass than Jupiter but puffier and spongy and with about 1.9 times the size of the fifth planet in the solar system, which registers during the day temperatures in excess of 2400 ºC, more than sufficient heat to allow the precipitation of metals such as iron: metals evaporate at such high temperatures.

Metal vapour

An international team of scientists with the participation of the Higher Council for Scientific Research (CSIC) has located it in the constellation of Pisces. The funny thing is that their faces are fixed because it is blocked by the tide to their star. It takes as much to revolve around its axis as it does to revolve around the star in a vertiginous orbit of just 1.8 days. With respect to their fixed faces: the daytime side has temperatures of 2,400ºC and at night, the strong winds carry the iron steam until it drops.

It’s during the day, that the planet receives thousands of times more radiation from its parent star than the Earth of the Sun. It is so hot that molecules are separated into atoms and metals as iron evaporates into the atmosphere. The difference in extreme temperature is palpable: at night they decrease to about 1,500ºC, the cold side stays in perpetual darkness.

WASP-76b, which has 1.5 times the mass of the Sun, not only has different day-night temperatures, but also has a different chemistry by day and night.

"You could say that on this planet it rains at night, except it rains iron," explained David Ehrenreich, an astronomer at the University of Geneva and leader of the work published by Nature magazine.

Ultra-hot gas giant

These kinds of worlds are "freaks" of the cosmos. They are expelled with so much radiation from their suns that their daytime temperatures rival some stars. Meanwhile, the night side of ultra-hot gas giants tend to be much softer.

The team used the Very Large Telescope in Chile to examine the filtering of starlight through the atmosphere of WASP 76b when the exoplanet passed in front of its sun during two orbits in 2018. These observations revealed chemical components from different regions of the atmosphere. While the atmosphere showed traces of iron gas where the planet was in transition to night (in the night terminator), no iron was detected in the transition from night to day (in the morning terminator), suggesting the condensation of iron into liquid raindrops. So, considering that this is a gaseous planet, there is no land for it to form puddles of iron, so researchers suspect that these drops of iron rain reach depths so high that they vaporize again in iron gas.

WASP 76b is not the hottest exoplanet we’ve ever discovered. It holds the KELT-9B record, a world so hot that... it’s literally evaporating.

We will have to see what other unusual worlds we will encounter when the long-awaited James Webb space telescope is up and running. Its launch is scheduled for 2021.

Reference: David Ehrenreich et al. Nightside condensation of iron in an ultrahot giant exoplanet, Nature (2020). DOI: 10.1038/s41586-020-2107-1 , https://www.nature.com/articles/s41586-020-2107-1