Sun­shade helps in the search for a sec­ond Earth

How likely are habitable exoplanets, i.e. Earth-like planets outside our solar system? This is the question behind large-scale feasibility study, a project in which Nobel Prize winners Michel Mayor and John Mather were among those involved. Using extensive calculations, an approach was developed to analyse exoplanets for molecules that are essential for life, such as water and oxygen.

The difficulties are numerous: in addition to the great distance of such planets, it is above all the fact that they orbit around their own "sun". "Even the largest telescope is not sufficient for observing Earth-like exoplanets, as they themselves only glow faintly and are always outshone by their parent star," explains Stefan Kimeswenger, astrophysicist at the University of Innsbruck and co-author of the study.

For this reason, the authors of the study propose a combination of telescopes located on the ground with a shadowing satellite orbiting the Earth. Precision is required here: according to the scientists' calculations, this shielding body would have to have a diameter of 99 metres and be placed at a suitable position in the Earth's orbit at an altitude of almost 200,000 kilometres with an accuracy of 2 metres. And: such a "starshade" must have the shape of a flower with around 30 petals (see symbolic image):

Kimeswenger explains: "You can't just use a circular aperture, because then the diffraction of the light at the edge of the circle would be almost as bright as the star that you actually want to darken." With an edge of tapered petals, on the other hand, the diffraction effects are distributed in many directions and largely cancel each other out.

Together with a high-performance telescope, such as the Extremely Large Telescope, which is currently being built in Chile, a starshade formed in this way would then be combined to form the "Hybrid Observatory for Earth-like Exoplanets" (HOEE). According to the modelling calculations of the study authors, this set-up could capture enough light to draw conclusions about the nature of an Earth-like exoplanet.

Detection of water and oxygen theoretically possible

If you want to find out which atoms and molecules are present in the atmosphere of a celestial body, you analyse the spectrum of the light emitted by this celestial body. This is because when light hits certain molecules, its spectrum changes in a characteristic way. This method is also used in the search for Earth-like exoplanets. In the current study, Stefan Kimeswenger from the University of Innsbruck showed that a spectroscopic examination of an exoplanet for water and oxygen would theoretically be possible with the combination of starshade and telescope presented here. The precise characterisation of the Earth's atmosphere is of central importance here. Kimeswenger emphasises:

"The Earth's atmosphere itself contains water and oxygen, so when light enters the Earth's atmosphere, it is altered once again at the exact wavelength at which you are looking for a change."

A few years ago, together with other scientists, he therefore developed a procedure to calculate the effects of the Earth's atmosphere on the radiation coming from an astronomical object.

Roadmap for implementation is being developed

Based on the positive feasibility analysis, the team now wants to develop a strategic roadmap for possible realisation. This would require corresponding funding decisions from international space agencies such as ESA and NASA. In any case, such a project is a long-term endeavour: In view of the usual development and planning periods for large scientific missions, realisation would be conceivable from around 2045 at the earliest. "Projects of this kind require staying power," says Kimeswenger. "Our work shows that the concept is feasible in principle. The next step is to get the international community on board."