English subtitles for clip: File:Hubblecast 102.webm

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No other topic fascinates both astronomers and the public quite like exoplanets.

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What do they look like? Could we breathe there? Is life possible on them?

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Answering these questions requires us to detect and study the thin atmospheres of these distant objects.

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The atmosphere of an exoplanet can
reveal a wealth of information.

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By determining the composition and
thickness of the atmosphere,

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astronomers can infer many other
characteristics such as the planet's temperature,

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the air pressure, and whether the planet is suitable for life.

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But studying the atmospheres of exoplanets isn't an easy task.

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Planets don’t emit their own light and they are tiny compared to their host stars.

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The only way to study exoplanet atmospheres is by monitoring the host star's light

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as the exoplanet moves between Earth and the parent star — known as a transit.

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During the transit a tiny fraction of the star’s light passes through the atmosphere of the planet

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and interacts with the chemical elements therein.

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Each atom and molecule present in the atmosphere absorbs light at specific wavelengths,

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while allowing other wavelengths to pass.

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By observing the light of a star during a transit

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astronomers can find the fingerprint of the exoplanet's atmosphere in the spectrum of the star.

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Each element creates distinctive dark lines — absorption lines — in the spectrum.

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So these lines act as chemical fingerprints revealing the make-up of the atmosphere.

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Also, the stronger the line, the more of the corresponding element is present in the atmosphere.

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But even the strongest lines of the most abundant elements are incredibly weak and hard to detect:

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only a tiny fraction of the star’s light is interfering with the atmosphere of the exoplanet.

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Hubble is one of the few telescopes powerful enough to perform studies of exoplanet atmospheres.

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It also has instruments to collect spectra ranging from the ultraviolet, through the optical, to the near infrared.

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This is crucial to fully characterise these atmospheres.

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In spite of Hubble’s capacities,

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the analysis of exoplanet atmospheres pushes Hubble’s instrumentation to its limits.

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The telescope can only detect the strongest lines from an atmosphere in a given spectrum.

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It’s enough to give us an idea of the composition of an atmosphere and the appearance of a planet,

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but it is not possible to reveal the fine details.

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While Hubble will continue its studies and will help to advance our understanding of planetary atmospheres,

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astronomers need bigger and even more sensitive instruments

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to detect the weaker signatures in atmospheric spectra:

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the forthcoming NASA/ESA/CSA James Webb Space Telescope will deliver exactly that.

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Transcribed by ESO; Translated by —