Measuring the spectrum of electromagnetic radiation is known as spectroscopic observation. Being able to interpret the spectrum is important, allowing a wealth of information to be extracted such as the presence and abundance of atoms, molecules, ions, hazes and clouds and the vertical thermal structures. “Such information is needed to test and improve the chemistry and dynamics incorporated in the atmospheric models applied to alien worlds,” says Pierre-Olivier Lagage, based at the Astrophysics Department of the French Alternative Energies and Atomic Energy Commission (CEA) in Saclay, France. Lagage is the principal investigator of the EU-supported ExoplANETS A project which has increased our knowledge of the atmosphere of exoplanets by analysing archived space data with novel tools. “The main challenge in spectroscopic observations of the atmosphere of transiting exoplanets is the characterisation and removal of systematic noise which can be orders of magnitude higher than the signal induced by the exoplanet atmosphere,” explains Lagage.
Unravelling the secrets of distant planets
One of the project’s researchers, Jeroen Bouwman, based at the Max Planck Institute for Astronomy in Heidelberg, Germany, developed a novel method of systematic noise characterisation and removal. In Lagage’s words: “In this method, a data-driven model of the temporal behaviour of the systematics for each pixel of the spectrum can be created using reference pixels at different positions in the spectrum. This is dependent on the underlying causes of the systematics being shared across multiple pixels, which has been the case for the Hubble Space Telescope data we have analysed.” The method has been implemented in the 'Calibration of trAnsit Spectroscopy using CAusal Data' code. Bouwman has applied this approach to all the archived Hubble Space Telescope spectroscopic data on exoplanet atmospheres. “We have analysed about 200 spectroscopic observations resulting in a homogeneous and reliable characterisation of 54 exoplanet atmospheres,” notes Lagage. Under the leadership of astrophysicist Vincent Minier, based at the CEA, and David Barrado, professor at the Spanish Center for Astrobiology, part of the National Institute of Aerospace Technology, the project’s website has been developed to disseminate the science results and provide educational tools.
What the data can tell us
Modelling such systems will enable the exploration of the entire atmospheric area around planets. It will reveal the chemical processes and atmospheric circulation patterns which have no precedent on Earth, or other planets in the solar system. To successfully model the atmosphere of an exoplanet it is necessary to have sound knowledge of the host star. To this end, the project has created a coherent and uniform database of the relevant properties of host stars. This is based on data collected from the European Space Agency archives, combined with data from international space missions and ground-based observatories. These exoplanets and host-star catalogues have been accompanied and interpreted by models to assess the importance of star/planet interactions. So far most of the information on the molecular content of an atmosphere comes from observations with the Hubble Space Telescope, particularly through Wide Field Camera 3. The camera’s wavelength range can probe water vapour which has been detected in several exoplanets’ atmospheres. “The situation will soon change dramatically once the James Webb Telescope is launched,” Lagage explains. “This will provide a large wavelength coverage (0.4 to 28 microns) allowing for the characterisation of the various molecules expected in the atmosphere of the exoplanets such as water, carbon dioxide and ammonia. It will have a large collecting area of 25 square metres which will permit us to characterise previously inaccessible exoplanets.” Lagage notes that this newly acquired knowledge will ultimately help our understanding of our own planet. “The success of the ExoplANETS A project has been down to the collaboration of several leading scientists who have, together, made it possible to peer into the composition of these planets’ atmospheres with more certainty,” he says. “Combine this with the launch of the James Webb Telescope and we will have a strong set of tools with which to interpret the exciting new data that’s coming our way.”
ExoplANETS-A, Hubble Space Telescope, James Webb Telescope, systematic noise, atmosphere, planet, spectroscopic observations