ESO telescope observes exoplanet where it rains iron

This illustration shows a night-side view of the exoplanet WASP-76b. The ultra-hot giant exoplanet has a day side where temperatures climb above 2400 degrees Celsius, high enough to vaporise metals. Strong winds carry iron vapour to the cooler night side where it condenses into iron droplets. To the left of the image, we see the evening border of the exoplanet, where it transitions from day to night. Credit ESO/M. Kornmesser
This illustration shows a night-side view of the exoplanet WASP-76b. The ultra-hot giant exoplanet has a day side where temperatures climb above 2400 degrees Celsius, high enough to vaporise metals. Strong winds carry iron vapour to the cooler night side where it condenses into iron droplets. To the left of the image, we see the evening border of the exoplanet, where it transitions from day to night. Credit ESO/M. Kornmesser

Researchers using ESO’s Very Large Telescope (VLT) have observed an extreme planet where they suspect it rains iron. The ultra-hot giant exoplanet has a day side where temperatures climb above 2400 degrees Celsius, high enough to vaporise metals. Strong winds carry iron vapour to the cooler night side where it condenses into iron droplets.

“One could say that this planet gets rainy in the evening, except it rains iron,” says David Ehrenreich, a professor at the University of Geneva in Switzerland. He led a study, published today in the journal Nature, of this exotic exoplanet. Known as WASP-76b, it is located some 640 light-years away in the constellation of Pisces.

This strange phenomenon happens because the ‘iron rain’ planet only ever shows one face, its day side, to its parent star, its cooler night side remaining in perpetual darkness. Like the Moon on its orbit around the Earth, WASP-76b is ‘tidally locked‘: it takes as long to rotate around its axis as it does to go around the star.

On its day side, it receives thousands of times more radiation from its parent star than the Earth does from the Sun. It’s so hot that molecules separate into atoms, and metals like iron evaporate into the atmosphere. The extreme temperature difference between the day and night sides results in vigorous winds that bring the iron vapour from the ultra-hot day side to the cooler night side, where temperatures decrease to around 1500 degrees Celsius.

Not only does WASP-76b have different day-night temperatures, it also has distinct day-night chemistry, according to the new study. Using the new ESPRESSO instrument on ESO’s VLT in the Chilean Atacama Desert, the astronomers identified for the first time chemical variations on an ultra-hot gas giant planet. They detected a strong signature of iron vapour at the evening border that separates the planet’s day side from its night side. “Surprisingly, however, we do not see the iron vapour in the morning,” says Ehrenreich. The reason, he says, is that “it is raining iron on the night side of this extreme exoplanet.”

“The observations show that iron vapour is abundant in the atmosphere of the hot day side of WASP-76b,” adds María Rosa Zapatero Osorio, an astrophysicist at the Centre for Astrobiology in Madrid, Spain, and the chair of the ESPRESSO science team. “A fraction of this iron is injected into the night side owing to the planet’s rotation and atmospheric winds. There, the iron encounters much cooler environments, condenses and rains down.”

This result was obtained from the very first science observations done with ESPRESSO, in September 2018, by the scientific consortium who built the instrument: a team from Portugal, Italy, Switzerland, Spain and ESO.

ESPRESSO — the Echelle SPectrograph for Rocky Exoplanets and Stable Spectroscopic Observations — was originally designed to hunt for Earth-like planets around Sun-like stars. However, it has proven to be much more versatile. “We soon realised that the remarkable collecting power of the VLT and the extreme stability of ESPRESSO made it a prime machine to study exoplanet atmospheres,” says Pedro Figueira, ESPRESSO instrument scientist at ESO in Chile.

“What we have now is a whole new way to trace the climate of the most extreme exoplanets,” concludes Ehrenreich.


Editor’s Note: A previous version of this press release mistakenly indicated the distance to WASP-76b as being 390 light-years, based on a 2016 study. More recent data indicates that the exoplanet is 640 light-years away.

More information

This research was presented in a paper to appear in Nature.

The team is composed of David Ehrenreich (Observatoire astronomique de l’Université de Genève, Geneva, Switzerland [UNIGE]), Christophe Lovis (UNIGE), Romain Allart (UNIGE), María Rosa Zapatero Osorio (Centro de Astrobiología, Madrid, Spain [CSIC-INTA]), Francesco Pepe (UNIGE), Stefano Cristiani (INAF Osservatorio Astronomico di Trieste, Italy [INAF Trieste]), Rafael Rebolo (Instituto de Astrofísica de Canarias, Tenerife, Spain [IAC]), Nuno C. Santos (Instituto de Astrofísica e Ciências do Espaço, Universidade do Porto, Portugal [IA/UPorto] & Departamento de Física e Astronomia, Faculdade de Ciências, Universidade do Porto, Portugal [FCUP]), Francesco Borsa (INAF Osservatorio Astronomico di Brera, Merate, Italy [INAF Brera]), Olivier Demangeon (IA/UPorto), Xavier Dumusque (UNIGE), Jonay I. González Hernández (IAC), Núria Casasayas-Barris (IAC), Damien Ségransan (UNIGE), Sérgio Sousa (IA/UPorto), Manuel Abreu (Instituto de Astrofísica e Ciências do Espaço, Universidade de Lisboa, Portugal [IA/FCUL] & Departamento de Física da Faculdade de Ciências da Universidade de Lisboa, Portugal [FCUL], Vardan Adibekyan [IA/UPorto], Michael Affolter (Physikalisches Institut & Center for Space and Habitability, Universität Bern, Switzerland [Bern]), Carlos Allende Prieto (IAC), Yann Alibert (Bern), Matteo Aliverti (INAF Brera), David Alves (IA/FCUL & FCUL), Manuel Amate (IA/UPorto), Gerardo Avila (European Southern Observatory, Garching bei München, Germany [ESO]), Veronica Baldini (INAF Trieste), Timothy Bandy (Bern), Willy Benz (Bern), Andrea Bianco (INAF Brera), Émeline Bolmont (UNIGE), François Bouchy (UNIGE), Vincent Bourrier (UNIGE), Christopher Broeg (Bern), Alexandre Cabral (IA/FCUL & FCUL), Giorgio Calderone (INAF Trieste), Enric Pallé (IAC), H. M. Cegla (UNIGE), Roberto Cirami (INAF Trieste), João M. P. Coelho (IA/FCUL & FCUL), Paolo Conconi (INAF Brera), Igor Coretti (INAF Trieste), Claudio Cumani (ESO), Guido Cupani (INAF Trieste), Hans Dekker (ESO), Bernard Delabre (ESO), Sebastian Deiries (ESO), Valentina D’Odorico (INAF Trieste & Scuola Normale Superiore, Pisa, Italy), Paolo Di Marcantonio (INAF Trieste), Pedro Figueira (European Southern Observatory, Santiago de Chile, Chile [ESO Chile] & IA/UPorto), Ana Fragoso (IAC), Ludovic Genolet (UNIGE), Matteo Genoni (INAF Brera), Ricardo Génova Santos (IAC), Nathan Hara (UNIGE), Ian Hughes (UNIGE), Olaf Iwert (ESO), Florian Kerber (ESO), Jens Knudstrup (ESO), Marco Landoni (INAF Brera), Baptiste Lavie (UNIGE), Jean-Louis Lizon (ESO), Monika Lendl (UNIGE & Space Research Institute, Austrian Academy of Sciences, Graz, Austria), Gaspare Lo Curto (ESO Chile), Charles Maire (UNIGE), Antonio Manescau (ESO), C. J. A. P. Martins (IA/UPorto & Centro de Astrofísica da Universidade do Porto, Portugal), Denis Mégevand (UNIGE), Andrea Mehner (ESO Chile), Giusi Micela (INAF Osservatorio Astronomico di Palermo, Italy), Andrea Modigliani (ESO), Paolo Molaro (INAF Trieste & Institute for Fundamental Physics of the Universe, Trieste, Italy), Manuel Monteiro (IA/UPorto), Mario Monteiro (IA/UPorto & FCUP), Manuele Moschetti (INAF Brera), Eric Müller (ESO), Nelson Nunes (IA), Luca Oggioni (INAF Brera), António Oliveira (IA/FCUL & FCUL), Giorgio Pariani (INAF Brera), Luca Pasquini (ESO), Ennio Poretti (INAF Brera & Fundación Galileo Galilei, INAF, Breña Baja, Spain), José Luis Rasilla (IAC), Edoardo Redaelli (INAF Brera), Marco Riva (INAF Brera), Samuel Santana Tschudi (ESO Chile), Paolo Santin (INAF Trieste), Pedro Santos (IA/FCUL & FCUL), Alex Segovia Milla (UNIGE), Julia V. Seidel (UNIGE), Danuta Sosnowska (UNIGE), Alessandro Sozzetti (INAF Osservatorio Astrofisico di Torino, Pino Torinese, Italy), Paolo Spanò (INAF Brera), Alejandro Suárez Mascareño (IAC), Hugo Tabernero (CSIC-INTA & IA/UPorto), Fabio Tenegi (IAC), Stéphane Udry (UNIGE), Alessio Zanutta (INAF Brera), Filippo Zerbi (INAF Brera).

ESO is the foremost intergovernmental astronomy organisation in Europe and the world’s most productive ground-based astronomical observatory by far. It has 16 Member States: Austria, Belgium, the Czech Republic, Denmark, France, Finland, Germany, Ireland, Italy, the Netherlands, Poland, Portugal, Spain, Sweden, Switzerland and the United Kingdom, along with the host state of Chile and with AustralIA/FCULas a Strategic Partner. ESO carries out an ambitious programme focused on the design, construction and operation of powerful ground-based observing facilities enabling astronomers to make important scientific discoveries. ESO also plays a leading role in promoting and organising cooperation in astronomical research. ESO operates three unique world-class observing sites in Chile: La Silla, Paranal and Chajnantor. At Paranal, ESO operates the Very Large Telescope and its world-leading Very Large Telescope Interferometer as well as two survey telescopes, VISTA working in the infrared and the visible-light VLT Survey Telescope. Also at Paranal ESO will host and operate the Cherenkov Telescope Array South, the world’s largest and most sensitive gamma-ray observatory. ESO is also a major partner in two facilities on Chajnantor, APEX and ALMA, the largest astronomical project in existence. And on Cerro Armazones, close to Paranal, ESO is building the 39-metre Extremely Large Telescope, the ELT, which will become “the world’s biggest eye on the sky”.


Research paper –

Photos of ESPRESSO –

Photos of the VLT –

More about ESPRESSO and how it finds exoplanets –

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Harry Passfield
March 13, 2020 2:11 am

You can see where Bkb Dylan got his inspiration: A Hard Rain’s Gonna Fall’

March 13, 2020 3:31 am

So there is this continuous accumulation of iron on the night side of the planet close to evening. Doesn’t sound sustainable.

Björn Eriksson
March 13, 2020 3:45 am

No, I commented on that. There is probably rotation (driven by) revapourising the deposits. But it rotates so slow that astronomers cant detect it. Common sense has a place in science still.

Jeremiah Puckett
Reply to  Björn Eriksson
March 13, 2020 6:39 am

Well, if that were true, rotational speed would eventually increase to that of average wind speeds. Also, this would create a wobble that would likely destroy the planet. Another thing, it wouldn’t just be iron. Silica’s melting point is far below the 2400 C they are proposing as a high temperature. I love science that’s out there, but I hate the unobservable and unproveable ideas they publish.

Reply to  Jeremiah Puckett
March 13, 2020 2:22 pm

night side, where temperatures decrease to around 1500 degrees Celsius.

That is just about the melting point of iron. Though they seem to skip making this link. Are they perhaps hoping it will flow back as a liquid to avoid the sustainability problem?

I have an even better headline grabber. After while the build up of weight on the night side builds up enough mass to break it out of tidal lock and it swings around to lock onto the other now heavier face.

Tom Johnson
March 13, 2020 6:13 am

“Doesn’t sound sustainable.”

That, in combination with it’s short “year” (less than 2 earth days), high temperatures, and high temperature gradients, says to me that it’s “days are numbered” (in quotes because, as tidally locked, its own days are actually endless).

Rainer Bensch
March 13, 2020 6:16 am

Why not? It recycles through the center of the planet.

Crispin in Waterloo
Reply to  Rainer Bensch
March 13, 2020 12:24 pm


That was my assumption too. If metal accumulates on one side, the centre of gravity moves what was the middle closer to the sun. So it recycles the entire volume of material in the planet. It might have an average temperature well below that of the Earth unless it takes hundreds of millions of years to be recycled because the core would be exposed and lose its primordial and nuclear heat.

It sounds like a Green Dream Planet where everything, one hundred per cent, even the rock, is recycled.

Greg Cavanagh
Reply to  Crispin in Waterloo
March 13, 2020 1:43 pm

I get the impression that the entire planet is molten. In which case it makes more sense, except for the tidally locked part. I’d like to see a model of just how this could work as it raises more questions than answers.

Reply to  Greg Cavanagh
March 13, 2020 2:26 pm

The idea of tidal locking requires an axially asymmetric distribution of mass. I don’t see that working on a liquid planet.

It sounds to me like they have one observation ( more iron signal on the evening side than dawn side ) and running off with wild untested hypotheses in way typical on archeology.

This seems now to be fashionable in astronomy too.

Björn Eriksson
March 13, 2020 3:34 am

It probably rotates but very slowly, driven by the vapourised metal deposits.

March 13, 2020 3:44 am

Based on a 2016 study being 390 light-years, more recent data indicates that the exoplanet is now 640 light-years away.
This planet is racing trough Galaxy at the speed Einstein could only dream of /sarc

Jeff Alberts
Reply to  Vuk
March 13, 2020 10:21 am

Yeah, they got the distance way off, but they’re sure it rains iron on a planet they can’t even see, but can only infer its existence.

March 13, 2020 3:59 am

Has Greta already: (i) blamed this on CO2; (ii) predicted the same is gonna happen to our planet if we do not act fast enough?

Jeff Alberts
Reply to  Ric
March 13, 2020 10:20 am

No, that would be the “astrophysicist” Hansen, who thinks Earth could become like Venus just because of a little more CO2.

March 13, 2020 4:04 am

We’ve had a lot of heavy rain in the UK lately.

Reply to  Dennisa
March 13, 2020 4:28 am

It’s the corona virus rain, don’t you know.
Stay indoors, the BoJo’s orders.

March 13, 2020 5:08 am

At one parsec, a distance of one astronomical unit (AU) subtends about 1 second of arc. One AU is about 93,000,000 miles. This planet has a diameter of 165,000 miles. link It’s about 200 parsecs away. If I got my numbers correct, it subtends an angle of 1/400,000,000 of a degree. I’d be really surprised if they could directly observe that the planet is tidally locked, or directly observe the sunrise and sunset.

Are there other explanations for the data? Sure. How about there’s a giant ball of iron vapor orbiting the star at around the same radius as the planet.

Reply to  commieBob
March 13, 2020 9:57 am

This has to be a case of WAG since nothing can be deduced directly about an exotic exoplanet hundreds of light years away. Only an educated opinion and speculation. It is too bad this kind of Pop Sci isn’t identified as such in the opening paragraph of their summary and that these are the best informed information and opinions that will of course be subject to change as more information becomes available.

This has what has happened with climate science..that WAGS have now replaced opinion and speculation as fact but is also really another form of Pop Sci. Which is leading to disastrous public policy, but is disguised as that for much more nefarious political control and taxation. But if scientists want to speculate on what they think might be happening on any distant exotic exoplanet, fly at it, but preface it first that is just an educated opinion.

March 13, 2020 5:50 am

VLT is polite for BFT

Jeremiah Puckett
March 13, 2020 6:35 am

Wouldn’t such a reality cause just about everything in the planet to boil and completely destroy the planet?

Don K
Reply to  Jeremiah Puckett
March 13, 2020 7:47 am

Maybe not. There are a few substances — Tungsten, Carbon,Tantalum — with melting points above 2400C. And maybe some chemical compounds are stable at those temperatures. So simplistically, there could be a tide-locked planet with a (small?) core of high melting point/high decomposition temperatures material, oceans of liquid metals boiling on the sunny side, and an atmosphere of low boiling point stuff like, for example Helium,Mercury,Lead.

Does that work? Damned if I know. Maybe the IPCC would like to model it.

Don K
Reply to  Don K
March 13, 2020 3:45 pm

Oh yeah, and the molecules in the atmosphere (if any) have to be massive enough and/or the planet itself massive enough that molecules on the hot “day” side don’t have so much velocity that an appreciable number fly off into space. Probably no Helium in that atmosphere unless the planet is really huge?

Reply to  Don K
March 14, 2020 3:28 am

It is described as a “giant exoplanet” That, at least generally, seems to mean “Jupiter like” which is a large planet composed of a great deal of relatively light materials. Heavier elements are supposedly products of super nova, meaning their abudance is relatively small at the current stage of evolution in this galaxy, no? How much of the planet could be iron?

March 13, 2020 6:44 am

Weird and cool!

David Chappell
March 13, 2020 7:23 am

Not good for a holiday destination I guess.

BTW Are all the team counted as authors of the paper? If so that must be a record.

Ron in Austin
March 13, 2020 7:31 am

I’ll never complain about rainy weather again.

March 13, 2020 7:35 am

Absolute speculative bullshit article is absolute speculative bullshit.

I bet dog-men live on that planet. You can’t say they don’t, you’ve not been there and brought back any bones to prove me wrong.

Dan Hawkins
Reply to  Prjindigo
March 13, 2020 9:23 am

Right on, Prjindigo. The first line of the press release contains the first lie, “Researchers…have observed…” They have observed nothing. All this fairy tale is built on INFERENCE which, being a human mentation, is subjective. Entirely too much certainty is expressed in such lurid descriptions of exoplanets, in my humble opinion.

Jeff Alberts
Reply to  Dan Hawkins
March 13, 2020 10:18 am

Yup. I posted my comment below before seeing yours.

March 13, 2020 7:46 am

Does this planet have irony seasons?

March 13, 2020 7:50 am

Cool. A planetary iron smelter. Seed the atmosphere w/some carbon, nickel & chromium, and it rains stainless steel.

March 13, 2020 8:49 am

And you thought climate change research was a huge waste of money…..

Reply to  Sheri
March 13, 2020 12:22 pm

Throw the researchers in irons.


PS: When I read the headline, I had a cartoon visual of clothes irons falling from the sky.

March 13, 2020 9:46 am

Doesn’t quite fit the situation, but I’ll quote it anyway:

There are more things in heaven and earth, Horatio,
Than are dreamt of in your philosophy.
– Hamlet (1.5.167-8), Hamlet to Horatio

March 13, 2020 10:13 am

I think there’s a quote attributed to Mark Twain that covers this subject but the exact quote eludes me, at the moment. Anyway, it would appear that an exoplanet has been detected, it’s REAL close to it’s central star, and it is assumed that the planet is tidally locked. Also, gaseous iron has been detected…I’m guessing through spectral analysis? That about covers the ‘known’ for this discovery, as far as I can tell. Pretty boring. No wonder the press release punched things up a bit to garner more interest from the general public.



Philip Mulholland
Reply to  Max
March 13, 2020 6:23 pm

Here it is:

“There is something fascinating about science. One gets such wholesale returns of conjecture out of such a trifling investment of fact.”

Jeff Alberts
March 13, 2020 10:16 am

If they “observed” it, why aren’t there pictures? Sounds more like they inferred it.

Then we have the headline “ESO telescope observes exoplanet where it rains iron”

Then in the first sentence “Researchers using ESO’s Very Large Telescope (VLT) have observed an extreme planet where they suspect it rains iron.”

These are very different statements.

Joel O'Bryan
March 13, 2020 12:09 pm

I wonder if someone has done orbital decay modeling of this exoplanet? Seems to me it would make for a good show as that exoplanet descends into a superhot corona around its stellar blast furnace.

Mental picture: An entry tail of million degree plasma (stripped from the planet) many millions of miles long behind it as it makes a death dive. And that dense plasma tail connecting to magnetic loops of in situ plasma in the lower coronasphere might cause massive reconnection events, releasing a massive string-like chain of exploding CMEs in its wake.

March 13, 2020 12:21 pm

By my rough count, there are 95 members of the team – if these are all co-authors it must be an all-time record!

March 13, 2020 7:52 pm

I will go on record for posterity saying that much of this Exoplanet Science is total speculation. It is a fun game for astronomers and astrophysicists sitting around the cafeteria or out at the pub after (before?) work to make up “Just-So” stories about the “exoplanets” they have discovered.

The amount of solid scientific evidence they have for any of this is vanishingly small — the amount of true “signal” (real information) that can be conceived to have traveled safely and without compromise across 640 lights-years of space and captured by planet Earth surface-based telescope (no matter how large) is even smaller.

Gotta love their imaginations….and they will probably never be proven wrong — they can speculate and invent interesting planets all they want — and we’ll never know the truth (certainly now in our own lifetimes, maybe never…)

Even with near-lightspeed space travel, humanity will never see these distant places, if they exist.

I’d love to be alive when wormhole travel or other super-lightspeed travel is possible — but I grew up on science fiction — and this whole exoplanet movement/fad is just more sci-fi as far as I am concerned.

Jeff Alberts
Reply to  Kip Hansen
March 16, 2020 2:03 pm

I’m not even sure it rises to the level of speculation. Fantasy might be a better word.

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