An international team led by astronomers at Queen’s University Belfast has detected a new “chemical signature” in the atmosphere of a planet that orbits a star other than our Sun. It may ultimately lead to an answer to the question: “Are we alone?”
The discovery by researchers from QUB; the Astrobiology Center (ABC) of the National Institutes of Natural Sciences in Tokyo and Trinity College Dublin helps pave the way to probing atmospheres of smaller rocky planets resembling the Earth.
Exploration of the atmospheres of these “extrasolar” planets assists in the search for Earth-like exoplanets and may provide an indication of the likelihood of life on other planets.
The hydroxyl radical – known as OH – was found on the dayside of the exoplanet WASP-33b. This planet is known as an “ultra-hot Jupiter’’; a gas-giant planet orbiting its host star much closer than Mercury orbits the Sun, and therefore reaching atmospheric temperatures of more than 2,500 degrees; hot enough to melt most metals.
The lead researcher based at QUB and ABC Dr Stevanus Nugroho, said: "This is the first direct evidence of OH in the atmosphere of a planet beyond the Solar System. It shows not only that astronomers can detect this molecule in exoplanet atmospheres, but also that they can begin to understand the detailed chemistry of this planetary population."
‘Atmospheric detergent’
In the Earth’s atmosphere, OH is mainly produced by reaction of water vapour with atomic oxygen. It is referred to as an “atmospheric detergent” and plays a crucial role in the Earth’s atmosphere to purge pollutant gasses that can be dangerous to life, such as methane and carbon monoxide.
In a much hotter and bigger planet like WASP-33b, where astronomers have previously detected signs of iron and titanium oxide gas, OH plays a key role in determining the chemistry of the atmosphere through interactions with water vapour and carbon monoxide, he explained.
Most of the OH in the atmosphere of WASP-33b is thought to have been produced by destruction of water vapour due to extremely high temperatures.
“We see only a tentative and weak signal from water vapour in our data, which would support the idea that water is being destroyed to form hydroxyl in this extreme environment,” explained Dr Ernst de Mooij of QUB, a co-author of the study published in Astrophysical Journal Letters.
The team used the InfraRed Doppler (IRD) instrument at the 8.2-meter diameter Subaru telescope located in the summit area of Maunakea in Hawaii – about 4,200 m above sea level. It can detect atoms and molecules through their "spectral fingerprints"; unique sets of dark absorption features superimposed on the rainbow of colours (or spectrum) that are emitted by stars and planets.
As the planet orbits its host star, its velocity relative to the Earth changes with time. Just like the siren of an ambulance or the roar of a racing car’s engine changes pitch while speeding past us, the frequencies of light – eg colour – of these spectral fingerprints change with the planet’s velocity. “This allows us to separate the planet’s signal from its bright host star, which normally overwhelms such observations, despite modern telescopes being nowhere near powerful enough to take direct images of such ‘hot Jupiter’ exoplanets,” he added.
Tiny signal
Dr Neale Gibson of TCD who was also involved in the research said: "The science of extrasolar planets is relatively new, and a key goal of modern astronomy is to explore these planets' atmospheres in detail and eventually to search for 'Earth-like' exoplanets – planets like our own. Every new atmospheric species discovered further improves our understanding of exoplanets and the techniques required to study their atmospheres, and takes us closer to this goal."
The unique capabilities of IRD, enabled detection of the tiny signal from hydroxyl in the planet’s atmosphere.
"These techniques for atmospheric characterisation of exoplanets are still only applicable to very hot planets, but we would like to further develop instruments and techniques that enable us to apply these methods to cooler planets, and ultimately, to a second Earth," said Dr Hajime Kawahara of the University of Tokyo.
Another co-author based at QUB Prof Chris Watson added: "While WASP-33b may be a giant planet, these observations are the testbed for the next-generation facilities like the 30-Meter Telescope and the European Extremely Large Telescope in searching for biosignatures on smaller and potentially rocky worlds, which might provide hints to one of the oldest questions of humankind: 'Are we alone?'"
