Exoplanet discovered with molten iron rain

• The newly-discovered exoplanet WASP-76b rains molten iron.
• The planet is tidally locked with its star, creating an enormous difference in temperatures between its faces.
• The ESPRESSO instrument on ESO's Very Large Telescope allowed astronomers to explore the distant planet's climate.

If the world’s astronomers ever came together and held a competition to name the most metal planet ever discovered, chances are they’d only be one winner: WASP-76b – a planet with an atmosphere so extreme it rains molten iron.

The gas giant exoplanet is located 390 light-years away from the Earth in the constellation Pisces and was observed using a new instrument dubbed ESPRESSO built by a team at the University of Geneva and fitted to the Very Large Telescope (VLT) based at the European Southern Observatory (ESO) in Chile’s Atacama Desert.

WASP-76b is tidally locked, meaning it takes as long to rotate around its axis as it does to go around its parent star, this results in it only ever showing one of its sides to the star, leaving its colder night side in perpetual darkness – much like the Moon does as it orbits the Earth.

This gives it an ultra-hot day side with tempera­tures regularly reaching 2,400°C or more – high enough to vaporise metals. Strong winds that rage across the planet’s surface then carry the iron vapour over to much-cooler night side where it condenses into iron droplets and falls as rain.

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“We thought very early on that we could use the instrument not only to discover new planets, but also to characterise those that are already known. However, until 2018, we didn’t realise how powerful ESPRESSO really was in this field,” said Francesco Pepe, team member and professor of astronomy at the University of Geneva.

ESPRESSO – the Echelle Spectrograph for Rocky Exoplanets and Stable Spectroscopic Observations – was built by the Astronomy Department of the University of Geneva with help from teams in Portugal, Italy, Switzerland, and Spain. It was originally designed to hunt Earth-like planets around Sun-like stars, but has proven to be much more versatile than first anticipated.

“Thanks to this technology, we now have a completely new way of tracing the climate of the most extreme exoplanets”, said David Ehrenreich, a professor in the Department of Astronomy in the Faculty of Science at UNIGE and the study’s leader. “One could say this planet gets rainy in the eve­ning, except that it rains iron.”

Five more bizarre exoplanets

WASP-76b isn’t the only exoplanet we have discovered that looks and acts nothing like the eight planets found in our Solar System. Here are five more impressively bizarre extrasolar objects:

If WASP-76b is the most metal planet, then TrES-2b must surely be the most goth. It was identified in 2011 by the Transatlantic Exoplanet Survey Satellite and is the darkest exoplanet ever discovered, reflecting less than 1 per cent of all light that hits it – that’s darker than coal. It is as yet unknown what makes the planet so dark.

The top prize for the exoplanet with the most bling goes to 55 Canceri e, a planet about twice the size of Earth, located around 40 light-years away in the constellation Cancer.

It is thought that roughly one third of the planet’s mass is made up of carbon. Combine this with current estimations of the temperature and pressure in 55 Canceri e’s interior, and researchers think it is likely that much of the carbon contained within it is in the form of diamond. Fancy.

And now for something truly weird: this exoplanet, located just 30 light-years away in the constellation Leo, is thought to be covered in ‘burning ice’. Even though its surface temperature clocks in at a blistering 440 degrees, powerful gravitational forces squeeze the surface material so tightly that it remains solid despite the extreme heat.

Nicknamed Methuselah, at 12.7 billion years old, PSR B1620-26b is one of the oldest known extrasolar planets discovered to date. It was first identified back in 1993 and is located 12,400 light-years from the Earth in the constellation Scorpius. For context the Universe is thought to be 13.7 billion years old, and the Solar System about 4.6 billion years old.

At nearly more than 250,000km across, or 1.8 times the diameter of Jupiter, TrES-4b is one of the largest exoplanets discovered so far. It was spotted in 1,400 light-years away in the constellation Hercules by the Transatlantic Exoplanet Survey in 2006.