Using light from the orbiting star, astronomers scanned the atmosphere of an exoplanet 850 light-years away. Not just any exoplanet, but one of the hottest we’ve ever found – and now at least seven metals have been identified floating in its atmosphere as gas.
The exoplanet is WASP-121b, a type of planet we call hot Jupiter. This is because it is a gas giant so close to its star that its temperature is equal to that of the stars themselves; fantastic stars, sure, but stars nonetheless.
WASP-121b is quite famous, as far as exoplanets are concerned. It was first discovered in 2015, an exoplanet about 1.18 times the mass and 1.81 times the size of Jupiter, on a close orbit of only 1.27 days. Two years later, it became the first exoplanet in whose stratosphere water was found, although given the planet̵
Now astronomers have taken a closer look at the exoplanet’s atmosphere and what they found has surprised them.
At temperatures between 2,500 and 3,000 degrees Celsius (around 4,500 and 5,500 degrees Fahrenheit), it’s not the hottest of these exoplanets we’ve seen.
But it’s so hot that its atmosphere should be much simpler than what astronomers have observed in previous studies – complex molecules shouldn’t be able to form at such high temperatures.
These previous studies have suggested that molecules containing the rare metal vanadium and a lack of titanium could explain the spectrum in previous observations of the atmosphere of WASP-121b.
“Previous studies have tried to explain these complex observations with theories that did not seem plausible to me,” said astronomer Jens Hoeijmakers of the Universities of Bern and Geneva in Switzerland.
“But it turned out they were right. To my surprise, we actually found strong vanadium signatures in the observations.”
Peering into the atmospheres of exoplanets is not an easy thing to do. First, you need the exoplanet to pass between us and the star. This is actually a good way to find exoplanets in the first place – you look for very faint and regular dips in starlight to tell you that something large is orbiting the star.
To study the atmosphere, you need even weaker signals.
As the exoplanet passes in front of the star, some of the star’s light passes through the atmosphere. Depending on the elements present in the atmosphere, some wavelengths of light will be absorbed and enhanced. If you can take a full spectrum of wavelengths, these will appear as absorption and emission lines.
As you can imagine, the signal is not very strong and there is a lot of noise. So, for starters, you need good noise reduction tools that don’t destroy the data you need.
The signal can also be amplified and clarified by taking multiple transit spectra and stacking them, so exoplanets with short orbital periods that allow us to acquire more transit spectra will be easier to analyze. An exoplanet on a 12-year orbit like Jupiter’s would not be an ideal candidate, for example. But the narrow orbit of WASP-121b works well.
To obtain a strong spectrum for WASP-121b, Hoeijmakers and his team used three previously observed transits using the HARPS spectrograph instrument on the European Southern Observatory’s La Silla 3.6m telescope and reprocessed the data.
And they found an interesting metallic cocktail in the exoplanet’s atmosphere. Of course there was the aforementioned vanadium. Additionally, the team identified the spectral signatures of iron, chromium, calcium, sodium, magnesium, and nickel. In particular, there is no titanium, consistent with previous results.
“All the metals evaporated due to the high temperatures prevailing on WASP-121b, thus ensuring that the air on the exoplanet is composed of evaporated metals, among other things,” explained Hoeijmakers.
Hot Jupiter are very mysterious planets and such analyzes of their atmospheres can help us understand them. We don’t know why or how they are so close to their stars, and knowing what’s in their atmosphere can help us understand whether they formed there or migrated inward from a more distant orbit.
But these studies are also helping to develop the toolkit for probing planets for alien life. What we use today to identify iron and sodium could, with more sensitive equipment, one day help find the molecules produced and used by living organisms, such as oxygen and methane.
“After years of cataloging what’s out there, we’re not just taking measures anymore,” said Hoeijmakers.
“We are really beginning to understand what the data from the instruments show us. How planets look alike and differ from each other. In the same way, perhaps, that Charles Darwin began developing the theory of evolution after characterizing countless species of animals, we are starting to understand more about how these exoplanets formed and how they work. “
The research was published in Astronomy and astrophysics.