Artist’s impression of the deep blue planet HD 189733b This illustration shows HD 189733b, a huge gas giant that orbits very close to its host star HD 189733. The planet’s atmosphere is scorching with a temperature of over 1000 degrees Celsius, and it possibly rains glass, sideways, in howling 7000 kilometre-per-hour winds. At a distance of 63 light-years from us, this turbulent alien world is one of the nearest exoplanets to Earth that can be seen crossing the face of its star. By observing this planet before, during, and after it disappeared behind its host star during orbit, astronomers were able to deduce that HD 189733b is a deep, azure blue — reminiscent of Earth’s colour as seen from space. © NASA, ESA, M. Kornmesser
Probing the atmosphere of exoplanets Exoplanets orbit a star other than our own sun, and over recent decades more than 4,000 have been detected across many different planetary systems. As Principal Investigator of the ExoplANETS A project, Dr Pierre-Olivier Lagage is working to characterise the atmosphere of these exoplanets that lie far beyond our own solar system. Over 4,000 exoplanets
have been detected over the last few decades, including some within a few parsecs (around 3.26 light years) of the Earth, while others are much further away. As the Principal Investigator of the ExoplANETS A project, Dr Pierre-Olivier Lagage is now looking to gain deeper insights into the atmosphere around exoplanets. “We are interested in characterising the atmosphere of exoplanets, which is a new avenue of research,” he says. An exoplanet can be initially detected using a variety of methods, one of which is called exoplanet transit. “When a planet passes between us and the star we are looking at, there is a faint decrease in the brightness that we can see from Earth. This is because the planet is preventing some of the light of the star from
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coming to us; this is what we call transit,” explains Dr Lagage. “This is one way to detect an exoplanet, and if that exoplanet has an atmosphere, the molecules in the atmosphere can also absorb some of the light coming from the star in specific wavelength bands, especially in the infrared.”
Spectroscopic observations By performing spectroscopic observations in the infrared at this point, researchers can see some of the features of these molecules in the atmosphere, as they stop part of the light emanating from the star. This represents one way to observe the atmosphere of an exoplanet, while Dr Lagage says it’s also possible to gain further insights at the point when an exoplanet passes behind its
star. “When the exoplanet is behind the star, we only see the starlight, and we can draw comparisons between before and after to learn about the emission from the exoplanet,” he explains. Of the more than 4,000 exoplanets that have been detected, spectroscopic information is available on around 100, but with only a limited wavelength coverage. Characterising the atmosphere of an exoplanet is a major challenge. “It’s much more difficult to characterise the atmosphere of an exoplanet than it is to detect it in the first place,” stresses Dr Lagage. This is an issue at the core of the project’s research, in which the key objective is to develop novel methods to analyse and interpret the available data. Indeed, one major challenge
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