How the first exoplanet was found
On October 6, 1995, a pivotal discovery reshaped our view of the cosmos. Swiss astronomers Michel Mayor and his Ph.D. student Didier Queloz announced the detection of a planet orbiting a star beyond our Sun. The star was 51 Pegasi, about 50 light years away in the Pegasus constellation, and the planet, 51 Pegasi b, entered the record books as the first confirmed exoplanet circling a sun-like star.
This was no ordinary planet. A gas giant with at least half Jupiter’s mass, it completed an orbit in a remarkably short 4.23 days, skimming extremely close to its star. At such proximity, the planet’s atmosphere would be a furnace, with temperatures soaring past 1,000°C. The discovery shattered preconceptions about planetary systems and introduced a new class of worlds—“hot Jupiters.”
The instrument behind the breakthrough was Elodie, a spectrograph at the Haute-Provence Observatory in southern France. Elodie breaks starlight into a spectrum, revealing a stellar barcode of chemical lines. Mayor and Queloz watched this barcode drift back and forth in rhythm with the star’s motion, caused by the gravitational tug of an unseen companion every 4.23 days. This radial velocity signal indicated a planet in a close orbit, setting the stage for a revolution in astronomy.
Their claim, published with the front-page abbreviation “A planet in Pegasus?”, faced initial skepticism. Yet the signal held up under scrutiny and was quickly confirmed by independent teams. Over the next two to three years, scientists settled on a robust conclusion: a giant planet exists around a sun-like star far beyond our solar system.
The birth of a new field: exoplanets explode into view
Around the same era, the field blossomed with other detection methods, notably transits. In a transit, a planet passes in front of its star from our viewpoint, causing a tiny dip in starlight. This method not only confirms a planet’s presence but also lets scientists measure its radius by observing the depth of the dip. The second major milestone came in 1999 when a known exoplanet, HD209458b, was observed transiting, confirming the power of transit observations to characterize distant worlds.
Today, two primary techniques drive exoplanet science: radial velocity (the original method used to discover 51 Pegasi b) and transits (which has flourished with space telescopes like Kepler and TESS). Radial velocity measures the star’s subtle wobble due to gravitational tugs, yielding a planet’s minimum mass. Transits provide planetary radii and, when combined with mass data, reveal density and composition. The synergy of these approaches has allowed astronomers to map the diversity of exoplanets—hot Jupiters, water-rich super-Earths, multi-planet systems, and even lava worlds with days-long years like Kepler-78b.
A legacy of curiosity and collaboration
Three decades of exploration have broadened our view of planetary systems. From the discovery of dozens of hot Jupiters to the identification of rocky worlds and multi-planet families, the tally of confirmed exoplanets now surpasses 6,000. Yet, Earth-like twins remain the Holy Grail. The quest continues with ever more precise instruments and ambitious collaborations.
Researchers on La Palma, part of the Harps-N consortium, have become frontline explorers in the hunt for an Earth twin. Harps-N and its successor Harps3 aim to push radial velocity measurements—the backbone of planet detection—to new levels of precision. The ultimate goal is to detect a true Earth analog around a sun-like star, a discovery that would hint at habitable conditions beyond our solar system.
The road ahead: will we find another Earth?
Despite vast progress, many planetary systems reveal architectures quite unlike our own. Yet every new detection brings clues about how planets form and evolve. By combining radial velocity with transit measurements, astronomers can estimate a planet’s mass and radius, constraining its possible composition and internal structure. The search for an Earth twin continues, driven by a simple question: could life exist somewhere else among the stars?
As explorers of the heavens, we look to future campaigns and instruments—the kind that might finally reveal a world with a similar size, mass, and temperature to Earth. Until then, the story of exoplanets remains a testament to curiosity, collaboration, and the enduring human desire to understand our place in the cosmos.
