By Marcus Woo
On a winter’s night in January 1995, rain fell on the Haute-Provence Observatory in southern France. Didier Queloz, an astronomy graduate student, was supposed to be observing. But since the weather was not cooperating, he sat in the library, writing computer code to make sense of the data he had so far.
The data suggested a bright star called 51 Pegasi was wobbling ever so slightly. It was the kind of wobble Queloz was looking for – a stellar jostling that could mean the presence of a planet.
The discovery of a planet around another star would be one of the most profound in human history. It would prove the Solar System was not alone. It would propel us closer to knowing if other life was out there, and redefine our place in the Universe.
That made history. It shook the entire field.
So Queloz had to be sure. Eventually, after more data and analysis, he realised the wobble was indeed real, caused when an orbiting planet’s gravity tugs at the star. “At this time, I was the only one in the world who knew I had found a planet,” Queloz says. But he also knew he could have made a mistake, and the stakes were high. “I was really scared, I can tell you.”
He turned out to be right. “That made history,” says Steve Vogt, an astronomer at the University of California at Santa Cruz, US. “It shook the entire field.”
Today, thanks to the Kepler space telescope, astronomers have discovered thousands of planets. According to estimates, our galaxy teems with hundreds of billions of worlds – including Earth-sized ones that could harbour liquid water, teasing us with the potential for life. In August 2016, astronomers reported that the nearest star, Proxima Centauri, is orbited by one of these Earth-sized planets.
But no one could have predicted these developments 20 years ago, when Queloz and his advisor, Michel Mayor of the University of Geneva, Switzerland, announced their discovery of the planet known as 51 Peg b. It was an extraordinary claim, and, as in science, the discovery was met not with fanfare but with scepticism.
To be clear, Mayor and Queloz did not actually discover the first planet outside the Solar System, known as an exoplanet.
If you [went] to an astronomy meeting and people asked what you do, you couldn’t say you were looking for planets
In 1992, American astronomers Alex Wolszczan and Dale Frail found two planets. But instead of orbiting a normal star like the Sun, the planets orbited a dead one: a rapidly-spinning stellar corpse called a pulsar, which blasts powerful beams of radiation into space.
It was a bizarre planetary system that could not be hospitable to life. “Those didn’t feel like planets because of the hellacious conditions,” Vogt says.
Astronomers viewed these pulsar planets as a cosmic anomaly: intriguing but not necessarily groundbreaking. They have since only found three more planets around pulsars.
Finding planets around normal stars like the Sun just seemed too difficult. “If you [went] to an astronomy meeting and people asked what you do, you couldn’t say you were looking for planets. They would move away from you like you smelled bad,” says Paul Butler, an astronomer at the Carnegie Institution of Science. “You might as well be talking about little green men.”
Our goal all along was to find planets
He should know. With Geoff Marcy, both at San Francisco State University in the US at the time, Butler had embarked on the search for exoplanets in 1986.
Using Vogt’s state-of-the-art instruments, they were trying to detect the same kind of stellar wobbles that Queloz saw. For example, Jupiter – the most massive planet in the Solar System – causes the Sun to move at roughly 35km/h. Detecting those relatively slow velocities from trillions of kilometres away is not easy. Until the 1980s, Butler says, no one could measure any speeds slower than about 1,000km/h.
To detect these stellar wobbles, they needed to measure the star’s spectrum; how the starlight splits into its constituent wavelengths. When the star moves towards or away from you, the wavelength of its light shortens or lengthens, respectively. This tiny change is called a Doppler shift. The problem was that vibrations in the instrument, fluctuating temperatures and other factors overwhelmed it.