The discovery of the first planet outside our solar system 20 years ago was a breakthrough in astronomy. today, nearly 2,000 exoplanets are known to exist-- at least one of which is so similar to ours that it could be habitable. thanks to the digital revolution, the number of discoveries is increasing exponentially, and finding alien life, if it exists, is getting closer.
NASA’s announcement in July 2015 of a planet very similar to ours, and therefore potentially habitable, thrust the debate over extra-terrestrial life back to centre stage. “This is a pivotal moment,” says Suzanne Aigrain, professor of astrophysics at the University of Oxford. “We can honestly say that we might discover other life forms in the universe within the next 20 to 30 years if we put our minds to it. There is no technological or scientific limit.”
It all began in 1995 when two astrophysicists from the Geneva Observatory found what only science-fiction writers had until then described: an extrasolar planet. “The discovery opened up incredible opportunities,” recalls Michel Mayor, the University of Geneva astrophysicist who together with his colleague Didier Queloz first identified the exoplanet known as 51 Pegasi b.
Thanks to increasingly effective detection methods, another 2,000 exoplanets have now been identified. The first breakthroughs came after installation of the HARPS telescope at the European Southern Observatory in Chile in 2004. Then came a new phase as scientists began studying exoplanets from satellites on the CoRoT and Kepler missions. Unlike Earth-based observations affected by atmospheric turbulence, satellites can take more accurate measurements and have so far detected nearly 5,000 new celestial bodies, most of which still need to be confirmed as planets. Most important, though, is the digital revolution. “With the increasing power of computers and storage capacity on hard drives, we’ve automated data analysis and archiving, which are two crucial steps in the research process,” explains Aigrain.
Hunting for exoplanets is not just a fancy for astronomers. “In the long term, the knowledge we gain will allow us to better understand our own planet, its formation process and its future evolution,” says Aigrain. But to do that, scientists must first learn more about the size, mass and atmospheric composition of these astronomical objects. That’s where the new European CHEOPS space telescope scheduled for launch in 2017 comes in; it will focus on accurately measuring the characteristics of a limited number of planets.
The biggest challenge is still to observe these faraway objects directly. “Current detection techniques allow us to see exoplanets indirectly,” explains Christophe Lovis of the University of Geneva. “We study the star around which the exoplanets orbit but not the planets themselves.” Only one method can be used to see exoplanets, and that’s direct imaging. In April 2015, a European team applied this technique to detect light reflecting off 51 Pegasi b.
Direct imaging is the only technique that might eventually provide enough information to determine if life does in fact exist elsewhere. But the instruments needed to achieve the next breakthrough have not yet been developed.
FOUR METHODS OF DETECTION
Indirect observations by space telescopes studying light intensity
When a planet passes between Earth and the planet’s host star, the brightness of the star drops. By regularly monitoring the light emitted by a star, astronomers can determine if a planet is orbiting around it. The first exoplanet in transit was discovered using an Earth-based telescope in 1999. Since then, the transit method has been used on the CoRoT and Kepler space missions to detect very low variations in brightness caused by planets as small as Earth and to measure their diameter. By combining that information with the mass determined using radial velocity, astronomers can calculate planet density.
Indirect observations made from Earth by analysing the light spectrum
With their gravitational pull, planets cause the star around which they orbit to “wobble”. This change in radial velocity causes a shift in the star’s spectral lines due to the Doppler effect. The fluctuations observed are so infinitesimal that the method is most effective when massive planets orbit close to their parent star. Radial velocity has become the most commonly used technique since astronomers first imagined it in the mid-20th century. It took nearly 50 years to develop the instruments powerful enough to lead to the discovery of the first exoplanet in 1995.
Indirect observations from Earth by studying light intensity
Two stars and Earth must be perfectly aligned for a microlensing event to occur. The light from the more distant star becomes brighter through a magnification effect as the star in the foreground bends its path. If a planet is revolving around the foreground star, it will cause a noticeable disruption in the otherwise regular magnification pattern. Gravitational microlensing was developed in the 1990s based on Einstein’s Theory of General Relativity. This method can detect small planets orbiting relatively far from their star.
Observations of the light spectrum of planets using devices that block the light from stars
By blocking the light given off by the host star, astronomers can see either the star’s light reflected by the planet or its thermal infrared radiation. The first image of an exoplanet was taken in 2004 using the Very Large Telescope at the European Southern Observatory in Chile. Direct imaging is by far the most valuable technique due to the vast amount of information it provides, including the chemical composition of the planet’s atmosphere and surface. This method, however, is very sensitive to terrestrial atmospheric turbulence.
By Leila Hussein
Illustrations: Sandro Bacco
Read also the second part of this focus on exoplanets: Michel Mayor, the swiss pioneer