By Emily Levesque, University of Washington
By the early 1990s, planet researchers were left with something of a conundrum. An extremely unlikely multi-planet system, and the first evidence of planets forming around pulsars, had been found. And yet, what about planets a bit more like our earth? What about an exoplanet orbiting stars like our Sun? That discovery, it turned out, was just a couple more years down the road, the discovery of the exoplanet 51 Pegasi b. Read on more to find out.
A Quest for Exoplanets
The search for exoplanets around pulsars was on. In the fall of 1994, astronomers Michel Mayor and Didier Queloz were observing a set of bright sun like stars at the Haute-Provence Observatory in France. They were specifically searching for any evidence of planets orbiting these stars. The method that they were using was one in which they were observing the spectrum of each star, over and over again. This was in order to try and spot the telltale red-or-blue shift in light caused by a nearby planet.
Mayor was a professor at the University of Geneva in Switzerland, and Queloz was his student. Together they had designed a brand-new spectrograph with the specific aim of spotting small planets around Sun-like stars. Combining a star as small as our sun, with a small planet, meant that any corresponding orbital shifts in the star’s spectrum would also be small. This meant that in order to spot it, they would need an incredibly sensitive spectrograph capable of splitting a star’s light very finely into different wavelengths. This would enable them to spot even a tiny redward or blueward shift.
Almost as soon as Mayor and Queloz started using their new spectrograph, they spotted shifts in a star named 51 Pegasi, or 51 Peg for short.
51 Peg
The 51 Peg seemed to rock towards the Earth and then away from it again, over and over, completing one cycle every four days. This suggested a nearby planet was orbiting 51 Peg once every four days, which immediately struck Mayor and Queloz as odd.
They knew from the size of the spectral shift that the planet causing it must be pretty massive, but the timing of the shift told them that the planet must be shockingly close to its star. The data was telling them they’d found a star like the Sun, with a planet bigger than Saturn following an orbit far closer than Mercury’s.
At the time, most astronomers believed that such a system shouldn’t exist. They saw a clear difference between the smaller rocky planets close to our Sun, like Mercury, Venus, Earth, and Mars, and the larger gas giants like Jupiter, Saturn, Uranus, and Neptune orbiting at greater distances. Theory had traditionally suggested, that, large gas giants should only be able to form further out in their planetary systems, where both rocks and ice could be used as building blocks.
This article comes directly from content in the video series Great Heroes and Discoveries of Astronomy. Watch it now, on Wondrium.
51 Peg b
A planet, like the one apparently orbiting 51 Peg, was referred to as a ‘hot Jupiter’—too massive to form without using ice as an ingredient, but too close to its hot, host star to be in a region where ice would exist. Finding such a massive planet in such a small orbit seemed like it had to be a mistake. However, other groups soon also observed evidence of the planet, and Mayor and Queloz carefully tested and ruled out alternative hypotheses.
The planet around 51 Peg was real and soon nicknamed 51 Peg b, following the convention of naming exoplanets after their host stars with a lowercase letter indicating the order of their discovery. The b marks this as the first exoplanet discovered around 51 Peg; if others were discovered, they would be designated as 51 Peg c, d, and so on, but, so far, 51 Peg b is the only exoplanet we’ve found in this system.
Today, we think that 51 Peg b must have formed far away from its host star and then migrated inwards, spiraling towards the star on a decaying orbit that ultimately brought it much closer to the center of the planetary system than it had been at birth.
A Groundbreaking Achievement
The discovery marked the first observations of an exoplanet around a Sun-like star, hailed as a groundbreaking achievement and valuable evidence of planetary systems like our own waiting to be discovered elsewhere in the universe. Mayor and Queloz ultimately shared the 2019 Nobel Prize, in Physics, for the discovery. In the immediate aftermath of their 1995 publication announcing their discovery, the study of exoplanets exploded.
The 51 Peg b and the three planets, orbiting Alex Wolszczan and Dale Frail’s pulsar, comprised the first handful of exoplanets to be discovered.
Today we use many methods to study exoplanets, including finding planets through transits, using gravitational lensing, and capturing an actual picture of one. Astronomers can now count and study thousands of planets orbiting other stars, building a detailed exoplanet library that’s rapidly expanding our understanding of how planets form and evolve and just how many are still waiting to be discovered.
Common Questions about Michel Mayor and Didier Queloz’s Discovery of Exoplanet 51 Pegasi b
Michel Mayor and Didier Queloz were observing a set of bright sun like stars at the Haute-Provence Observatory, in France. They were specifically searching for any evidence of planets orbiting these stars.
The planet around 51 Peg was nicknamed 51 Peg b, following the convention of naming exoplanets after their host stars with a lowercase letter indicating the order of their discovery. The b marks this as the first exoplanet discovered around 51 Peg.
Today, we think that 51 Peg b must have formed far away from its host star and then migrated inwards, spiraling towards the star on a decaying orbit that ultimately brought it much closer to the center of the planetary system than it had been at birth.
