Solar gravitational telescope puts exoplanets in its sights
The idea is attractive: to use the deflection of light by the Sun to create a telescope of unprecedented power. The challenge? To see details on exoplanets and, perhaps, to detect traces of life... Two American astronomers show that the concept is theoretically possible, even if the technologies to be implemented are still lacking.
The life cycle of an idea until its practical implementation can sometimes extend over several decades. The example of gravitational waves is striking in this respect: predicted in 1916, instruments to detect these tiny bursts of space-time began to be built in the 1980s - at a time when many physicists believed it was out of reach - and it was finally in 2015 that the first gravitational wave emanating from the merger of two black holes was detected.
Turn the Sun into a giant adjustable gravitational telescope!
Will it be the same for the gravitational telescope? For Alexander Madurowicz, of the Kavli Institute for Theoretical Physics in Santa Barbara, California, and lead author of the study to assess the still remote possibility of such a system, there is reason for optimism: "By establishing the theoretical framework today, we hope to show the way for others to continue developing it in the future. We hope that our children and all posterity will benefit.
How would this telescope work? According to Einstein's theory of general relativity, which describes gravitation, masses deflect light that passes near them. Thanks to this property, which has been verified over and over again, we can build a gravitational lens. Astrophysicists routinely use this phenomenon to amplify signals from distant objects. However, all these observations are fortuitous: it is necessary that a massive body - the gravitational lens itself - is aligned between us and the object to be studied for this amplification to occur. The idea studied here consists in placing a spacecraft on a specific orbit around the Sun in order to be able to aim at the targets to be studied. In other words, it is a question of transforming the Sun into an adjustable giant gravitational telescope.
The possibilities of magnification would be increased tenfold. "With the most advanced traditional telescopes, the image of an extrasolar planet is just a dot, like looking at Mars or Jupiter with the naked eye. The enormous magnification of the solar gravitational lens would make it possible to see surface details, such as continents or oceans, of an exoplanet located more than 100 light-years away," says Alexander Madurowicz. The main obstacle - and a big one - is that the instrument itself, which would collect light rays from distant objects deflected by the mass of the Sun, would have to be placed at least 14 times farther from our star than Pluto is, a distance that no human spacecraft has ever traveled.
The ambitious bet of the solar gravity telescope
With current technologies, it would take at least 100 years to reach the lens. By using solar sails, the time could be shortened to 20 or 40 years - which is still considerable. But nothing prevents the development of more efficient propulsion systems. Other challenges include navigational accuracy. In order to maintain alignment between the target exoplanet, the Sun and the telescope, not only is it necessary to know the position of the exoplanet in an unprecedented way, but it would also be necessary to take into account the tiny motions of the Sun due to gravitational interactions with Jupiter and the other planets of the Solar System. However, current observational capabilities on planetary positions are insufficient: "No existing method of characterizing exoplanets has sufficient accuracy to locate the position of the target exoplanet image to a degree comparable to the size of the projected image itself" explains Alexander Madurowicz.
In addition, the scientists behind this study are about to solve an important technical difficulty: the elimination of sunlight. Indeed, the Sun and its corona emit a lot of light, which obscures the signal of the target exoplanet. To reduce these spurious signals, the idea is to separate the foreground and background of the signal using a specially designed instrument called a coronagraph (a technique that hides the spurious sunlight, thus reproducing the phenomenon of total solar eclipses), in combination with a full-field spectrograph (which records the spectrum of radiation from every point in the telescope field). With such a coronagraph and with the help of an algorithm designed by the researchers, by capturing the ring of light around the Sun formed by the exoplanet and reversing the curvature of the gravitational lens, the ring is transformed into a round planet. The sought-after image. An ambitious bet that gives a vision on the very long term.