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.

Met with scepticism

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. 

Discovery confirmed

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.

Hot Jupiter

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!). 

According to nomenclature, 51 Pegasi corresponds to the parent star, and the b denotes that it is the first planet discovered in the star system. If more planets are discovered (not the case with 51 Pegasi), they are denoted with c, d, e, and so on. Marcy gave it the name Bellerophon following the convention of naming planets after Greek and Roman mythological figures, while the International Astronomical Union’s NameExoWorlds contest resulted in its other name, Dimidium. 

Wrong assumption

Astronomers who had been hunting for exoplanets had wrongly assumed that Jupiter-like planets couldn’t be so close to their parent star and would take months or years to complete an orbit, thereby not finding them in their data. Once Pegasi 51 b was discovered and confirmed, it was clear that hot Jupiters existed and more were found in existing data. It is believed that these exoplanets form far out and then migrate closer to their parent star over millions of years. 

Before 51 Pegasi b’s discovery, Earth was one of just eight known planets (or nine until 2006, since Pluto’s reclassification as a dwarf planet only took place that year). In the span of 29 years since then, Earth has gone on to become one of over 5,000 known planets. 

There’s reason to believe that every star in the Milky Way hosts at least a planet on average. There are suggestions that one out of every five sun-like stars harbour a potentially habitable, relatively Earth-like planet. This would mean that Earth is one of trillions of planets. And yet, it is the only one (we know of) that harbours life. Try to dwell on that beautiful thought without being overwhelmed.

Techniques employed in planet hunting

There are five techniques currently used to find a planet: radial velocity, transit, direct imaging, gravitational microlensing, and astrometry.

While three planets have been discovered using astrometry, 82 have been discovered using direct imaging and 225 through gravitational microlensing. More than thousand have been discovered by watching out for the wobble using the radial velocity method and over 4,000 by searching for shadows using the transit method. 

In the radial velocity method, also called the Doppler wobble method, exoplanets are detected by measuring the reflex motion of a star. Even as a planet orbits a star due to the latter’s gravity, the planet itself exerts gravitational force on the star. Just as the planet makes a big circle around the star, the star makes a very little circle – both orbiting the centre of mass of the system, called the barycentre. By detecting the slight gravitational wobble a planet causes in its star, exoplanets are discovered.

When a planet passes directly between its star and an observer, the star’s light is dimmed by a measurable amount for the observer. The transit method to find exoplanets is thus similar in theory to the eclipses that we have studied. If the dimming caused is greater, then it implies that the planet has a large diameter. Similarly, if the dimming occurs more frequently, then the faster the planet orbits around its parent star. The Kepler Space Telescope and the Transiting Exoplanet Survey Satellite (TESS) observe thousands of potential transits for astronomers to study by looking at whole expanses of the sky at once, without blinking.

Published - October 06, 2024 12:04 am IST