Since the first observation of an extrasolar planet, or exoplanet, almost 4000 planets have been identified orbiting other stars in the Milky Way. With these new discoveries, scientists are now increasingly investigating their atmospheres, the composition and structure of gas hull. Determining the substances and elements that make up these atmospheres is particularly difficult: until now, only a good handful of light elements such as hydrogen, oxygen, carbon as well as sodium and potassium have been detected. Now, with the detection of titanium oxide, German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR) researcher Elyar Sedaghati has, for the first time, been able to identify a heavier molecule in the atmosphere of a so-called 'hot Jupiter', and publish this result in the prestigious scientific journal Nature.
Sedaghati carried out his observations of the exoplanet – WASP-19b – together with a team of nine astronomers using the European Southern Observatory (ESO) Very Large Telescope (VLT) in Chile. WASP-19b was discovered and investigated with what is known as the 'transit method': When the planet passes in front of its star (known as a transit) as viewed by the observer, the telescope's sensor records a drop in brightness. If this dimming, typically between 0.01 and 1 percent, is periodic, the observer can infer that a planet is orbiting the star. If the planet has an atmosphere, then as the light from the star passes through it, a small, but characteristically different, change in the stellar spectrum can be detected. As these changes are tiny they can only be detected with highly sensitive measurements and sophisticated evaluation methods.
Planet with stratification and temperature inversion
"This phase, when the planet crosses the star's disc, is crucial for investigating its atmosphere," explains Sedaghati. "The tiny variations in the wavelengths and intensity of the light from the star enable the determination of some properties of the atmosphere and its composition by comparing them with atmospheric models. We have repeatedly carried out these measurements for two years." The titanium oxide (chemical: TiO) in the atmosphere of WASP-19b, a Jupiter-sized planet, was revealed in the highly scattering haze.
But it is not only the discovery of the comparatively heavy molecule titanium oxide and water in the gaseous envelope of an extrasolar planet that is of scientific importance. "It is also the first time that, in investigating the structure of the atmosphere of a hot giant planet orbiting another star, we discover that this planet may have stratification, certainly a stratosphere," explains Heike Rauer of the To the Institute's website who has contributed to the study and supervised Sedaghatis's doctoral thesis. You could in some way compare the titanium oxide haze to Earth's ozone layer: In general, an atmosphere gets colder upwards, but due to such a starlight-absorbing inversion layer it will become suddenly warmer on the day side. "This observation opens the door for the characterisation of the atmospheric chemistry of extrasolar planets," adds Rauer.
The team of scientists around Sedaghati observed WASP-19b with the VLT FORS2 instrument and collected millions of individual spectra across the entire optical spectral range. In order to probe the chemical composition of the atmosphere, special algorithms were applied that also take into account different temperatures and the varying properties of clouds and haze layers. In addition to titanium oxide, the astronomers also found evidence of water and sodium. WASP-19b was discovered in 2009. It is a heavily 'bloated' giant gas planet similar to Jupiter with approximately 11 percent more mass, and a radius approximately 40 percent larger than that of Jupiter's. The temperature of its atmosphere is estimated to be approximately 2000 degrees Celsius. This is a consequence of its proximity to its host – it orbits the star WASP-19 in the Vela constellation every 19 hours at a distance of just 2.4 million kilometres. With an age of 11.5 billion years, WASP-19 is more than twice as old as the Sun, and has a diameter of 1.4 million kilometres and a surface temperature of more than 5000 degrees Celsius.
Atmospheric determination of earth-like exoplanets in future
The measurement methods and instruments as described in Sedaghati's article will provide the key to detecting rocky planets located in the star's habitable zone. Such a 'second Earth' has yet to be found. The European Space Agency's (ESA) PLATO mission planned for 2026, in which DLR is involved, is designed to find exactly such planets. It is, of course, very important that appropriate measuring methods and instruments have been developed in order to provide reliable detection of molecules in the atmosphere of an Earth-like planet.
Earth in itself shows how difficult this undertaking is: its atmosphere envelopes the planet, which has a diameter of over 13,000 kilometres, like a thin peach skin. From the point of view of a hypothetical observer located many light years away, Earth's atmosphere is but a faint ring around the planet. The starlight that passes through this thin ring is altered by the molecules in the atmosphere. If there is a stratosphere – i.e. an air layer with an inverse thermal profile – then certain molecules emit radiation, which can be detected with appropriately sensitive devices and analytical methods. Rauer sees this work as a step towards a great future objective: "We are trying to reach the goal step by step: the discovery of rocky planets similar to Earth that exhibit temperatures at which liquid water is present and are enveloped by atmospheres that could allow the existence of life – an Earth 2.0, as it were."