Evrim Yazgin
Cosmos science journalist
We’ve all looked at the weather forecast only to be disappointed that the promise of a beach-worthy Saturday has been spoiled by a thunderstorm – especially if, like me, you live in Melbourne. No disrespect to meteorologists, the weather on Earth is a fickle beast.
But telescopes have gotten so good now that astronomers can tell what the weather is like on planets hundreds of light-years away.
Earlier this month, astronomers reported in a Nature paper that a giant gas exoplanet in the YSES-1 system has gigantic clouds which rain not water, but coarse silicate-rich sand.
This system is about 300 light-years from Earth.
To demystify the science behind exoplanet weather, Cosmos spoke with Devika Kamath, a stellar astrophysicist and senior lecturer in Astronomy & Astrophysics at Australia’s Macquarie University.
Extreme weather comes when still young and hot
“Mineral-rich clouds on exoplanets are different to clouds that we see,” Kamath notes. “They’re not made of water, but condensed minerals – so solid particles which contain compounds like silicates, iron in some cases, aluminium oxides, titanium oxide.”
Kamath says that these mineral-rich clouds aren’t found on older exoplanets which have had time to cool. They are typically found around hot exoplanets which orbit young stars early in the formation of the star’s planetary system – these planets are often called “hot Jupiters”.
“These minerals condense out of very hot atmospheres. Typically, you find these hot atmospheres around giant exoplanets which are still very young and hot, so they haven’t had the chance to evolve as much.
“Just like water vapor condenses into rain on Earth, in these exoplanets what you will find is all of these vaporised minerals and rocks eventually condense into some sort of ‘rain’, except that it’s not going to be made out of water, but things like silicates and iron,” she adds.
Variety is the spice of exoplanet life
“The extreme weather is iron rain, storms of mineral dust and extreme turbulence in the upper atmospheres. So, it’s unlike what you would find, thankfully, on Earth,” Kamath jokes.
Kamath says that other exoplanets have been found to have all sorts of minerals in their rainstorms including glass and corundum – a crystalline form of aluminium oxide found in rubies and sapphires.
Exoplanet HAT-P-7 b, 1,040 light-years from Earth, is believed to have gemstone rain of rubies and sapphires.
“I think it’s quite interesting to see how different exoplanets have different sorts of minerals in their clouds,” Kamath says. “Even though these are all young, hot Jupiters, there is a diversity in the chemistry that you see in these clouds, which means that it’s really about how these planets are being formed … how the parent star is being formed.”
“Together with these mineral rich clouds, you get very, very high winds,” says Kamath. “You’re talking about supersonic winds of up to 70,000 kilometres per hour. This has also been detected on quite a lot of young, hot exoplanets.”
Kamath notes that even in the YSES-1 system there is variety. She was not involved in the Nature study.
“In this system, one of the exoplanets has a mineral cloud. The other one – which is similar in age and size – doesn’t have a cloud, but it has a small circumplanary disc. So, it’s quite a diverse system.
“If you draw the system, you have a star, a big disc, and then 2 planets and a cloud on top of one planet, and a disc around another planet. It’s quite a lot of dynamics going on in that system, which I think is pretty interesting.”
Tell-tale chemical signatures
But how do astronomers look at specks in the night sky and determine the kind of mineral rain on exoplanets hundreds of light-years away?
Kamath explains that it’s a mix of technology and chemistry.
Light from distant stars reaches telescopes on Earth or in space. The colour of the light contains information about which chemicals are in that system. These colour maps are called absorption spectra.
“Absorption spectra are common to all chemical elements because they absorb light in different ways,” Kamath explains. “Every chemical, whether it’s an elemental transition like pure calcium or sulphur, zinc, etc., or if it’s a molecule like, say, titanium oxide, aluminum oxide, every one of these will have a wavelength of light they tend to appear in. If it’s a molecule, it’s normally not a single spectral line, but a band in the absorption spectrum.”
“For example, the silicate absorption feature is a broad absorption feature, starting at 9 microns and ending at about 11 microns.”
Kamath’s own research centres on the origin of chemical elements which are formed in star cores before those stars die and can end up in new stellar systems in the discs that form planets.
Eyes in the sky – for a short time
“We’ve got a jackpot with the James Webb Space Telescope (JWST),” Kamath says.
The YSES-1 system was studied using data from JWST.
“Every telescope operates in different parts of the electromagnetic (light) spectrum. You can think of our eyes as optical detectors seeing anything that emits energy in optical wavelengths. It’s a range of colours,” Kamath explains.
“But if you want to look at dust, you cannot see through dust in the optical spectrum. You have go to infrared. When you study in infrared, you’re looking at light coming from a star or young planet, then it gets absorbed by the dust and then scatters from the dust. The light going into dust comes to you with energy that has signatures of the chemistry of the dust. The JWST has the resolution and instruments that peak in far to near infrared wavelengths to do this.”
“We don’t have a lot of instruments that cover that range of wavelengths to the resolution that JWST does, so this kind of discovery would not have been possible 5 years ago,” Kamath adds.
Kamath notes that it is a painstaking process to get time to use the JWST to research particular objects. But the rewards are worth it.
“JWST is very expensive. We’d been trying to get time on it for almost 4 years. In the fourth year we got some time. Having said that, we had 5 proposals go in, and only 1 got time. So, it’s quite competitive,” she says.