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.
Discoverer
University of Geneva astrophysicist Michel Mayor identified the first known exoplanet, baptized 51 Pegasi b.
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
Transit
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.
Radial velocity
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.
Gravitational microlensing
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.
Direct imaging
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.
The Swiss Pioneer
Along with his colleague Didier Queloz, University of Geneva astrophysicist Michel Mayor identified the first known exoplanet, baptized 51 Pegasi b. He recalls his discovery and analyzes the development of exoplanet research.
Technologist: How did you become interested in astronomy?
Michel Mayor: To be honest, it was a happy coincidence. As a teenager I loved science, but had no particular preference. First, I studied physics at the University of Lausanne. Then after I finished my Master’s degree, a position opened for a doctorate in astronomy at the Geneva Observatory. So I accepted without really thinking about it. In 1970, I had a surprise encounter with the British astronomer Roger Griffin from the Cambridge Observatory. He had developed a new spectrograph to measure radial velocity. After talking with him, I was sure I could achieve greater precision and efficiency. So I took the plunge, even though many people were amused to see a theoretician like myself start developing instruments.
T. What was your reaction when you discovered the very first exoplanet, 51 Pegasi b?
M. M. I was overcome with doubt. It’s always hard to interpret the phenomena observed and be 100% sure that you’re dealing with an exoplanet. Astronomy didn’t have a good reputation in the 1990s. A lot of people had already been wrong about discovering exoplanets. The scientific community was considerably distrustful. We had to be especially careful and analytical. That’s why Didier Queloz and I decided to wait for the following season to observe 51 Pegasi b again before announcing it officially.
T. Why does this quest fascinate scientists so much?
M. M. It’s just human curiosity – the need to position human life within the rest of the universe. Even back in ancient Greece, they studied the multiplicity of worlds. We confirmed 20 years ago that extrasolar planets exist. By studying their characteristics, we can better understand how planetary systems are formed, and more specifically our solar system.
T. What about extra-terrestrial life?
M. M. That’s definitely one reason everyone is so excited about exoplanets. But to search for extra-terrestrial life we have to be able to analyse the chemical composition of rocky planets and look for spectral signatures that point to the development of life. It’s a fantastic challenge, but we shouldn’t underestimate how much work still needs to be done. I think it’ll take at least instruments that can answer that question.
T. What’s the future of exoplanet research?
M. M. Extrasolar bodies will continue to arouse interest in the scientific community. In fact, there have never been so many new recruits. We used to be a small club of researchers who all knew each other. Now, conferences and other meetings are crawling with people. But it’s a good thing, because there’s so much that still needs to be studied – the internal structure of planets, how they form, how they evolve.
T. Where does your career stand today?
M. M. As a professor emeritus, I can carry on with my research. I’m currently studying the statistical properties of planets. These data are useful for understanding the physics of planetary formation. The rest of the time, I travel abroad to attend scientific meetings and give lectures. I recently went to Japan where I received the Kyoto Prize (an international award honouring individuals who have made a significant contribution to science). As you can see, I’m having a hard time getting off the train.