The discovery that led to an explosion in planet hunting

On October 6, 1995, astronomers Michel Mayor and Didier Queloz announced the discovery of 51 Pegasi b. This exoplanet, also known by the names Bellerophon and Dimidium, was the first planet discovered orbiting a distant sun-like star. A.S.Ganesh tells you more about this discovery and how it changed in a big way our understanding of our place in the universe…

The 51 Pegasi b is the first exoplanet discovered orbiting a sun-like star. In case you are wondering what exoplanets are, they are planets that orbit a star outside of our own solar system. In the nearly three decades since the discovery of 51 Pegasi b, the number of exoplanets discovered has ballooned to over 5,000, altering our understanding of ourselves. 

But before we get to that, we have to take a closer look at the discovery that led to the birth of the new field of exoplanet science. Notice that when starting out, we added a condition when mentioning about 51 Pegasi b’s discovery. This exoplanet wasn’t the first to be discovered, but it was the first discovered to be orbiting a sun-like star.

The first exoplanets of any type were discovered in 1992. Astronomers Aleksander Wolszczan and Dale Frail detected two planets orbiting a pulsar star. Even though we know, with hindsight, that this is a monumental discovery, it wasn’t received with great enthusiasm during the time for a couple of reasons. One is the fact that pulsars, which are rapidly rotating neutron stars emitting beams of radiation in regular phases, are very unlike stars like our own sun. Second is the fact that much of the scientific community was rather sceptical about the idea that planets could exist around such volatile stars. 

When Swiss astronomers Michel Mayor and Didier Queloz made their stunning announcement of the discovery of an exoplanet orbiting a sun-like star on October 6, 1995, our world wasn’t still ready for it either. In a paper titled “A Jupiter-Mass Companion to a Solar-type Star,” the Swiss duo, who had made their finding using the ELODIE spectrograph at the Observatoire de Haute-Provence, published their findings in the journal Nature. 

It wasn’t until American astronomers Geoff Marcy and Paul Butler quickly provided confirmation – less than a week after Mayor and Queloz had made their announcement – that the initial scepticism was overcome. It was fortunate that Marcy and Butler already had scheduled observation time on a 120-inch telescope at the Lick Observatory, thereby enabling them to confirm the planet orbiting a star. The floodgates for planet hunting were well and truly thrown open after that. 

It’s important to bear in mind that Mayor and Queloz, or Marcy and Butler, didn’t spot Pegasi 51 b directly. Spotting a fly near a beaming floodlight while being seated in a cricket stadium is likely easier than visualising a planet from Earth in the full glare of its parent star. Directly viewing it, basically, is downright impossible. Astronomers, who now employ a variety of techniques for planet hunting, started off by using the radial velocity method that involved the slow process of looking for gravitational wobbles.

As far as 51 Pegasi b is concerned, it is a “hot Jupiter,” meaning it is a gas giant exoplanet. Located 51 light-years from Earth, this exoplanet is 47% less massive than Jupiter, while being 50% larger than Jupiter. It’s a star-hugging planet, meaning it circles just 7 million km away from its host star, taking just four days to complete an orbit (a year for 51 Pegasi b is therefore just four days!).