The day the cosmos opened a new chapter
On 6 October 1995, the astronomy world shifted forever. Swiss astronomers Michel Mayor and Didier Queloz announced the first planet found orbiting a Sun-like star: 51 Pegasi b, a gas giant that completed an orbit in just over four days. The discovery, made with the Elodie spectrograph at the Haute-Provence Observatory, revealed a star’s subtle back-and-forth wobble caused by an unseen companion. It wasn’t just a new planet; it was a new way of studying the universe.
The planet’s orbit was so tight that its atmosphere would burn at temperatures around 1,000°C. The concept of a planet so close to its star upended prevailing ideas about how planetary systems could form. The Nature cover that day bore the question, “A planet in Pegasus?”, signaling initial scientific skepticism. Yet within weeks other teams confirmed the signal, and the field of exoplanet science was born.
From hot Jupiters to a crowded zoo of worlds
51 Pegasi b introduced the term hot Jupiter and opened floodgates. In the 30 years since, more than 6,000 exoplanets and candidates have been catalogued. The diversity is staggering: ultra-hot Jupiters, multi-star or circumbinary planets, “super-puffs” with enormous radii and tiny masses, and compact chains of rocky worlds packed near their stars. Yet, despite the explosion of discoveries, a true Earth twin—Earth-like in size, mass, and temperature around a Sun-like star—remains elusive.
Detecting worlds: radial velocity and transit techniques
Our instruments today, including the HARPS-N spectrograph on La Palma, can measure minuscule stellar motions to detect planets a long way away. The radial velocity method watches for a star’s wobble as gravity from an orbiting planet tugoes it, with current precision approaching fractions of a centimeter per second in ideal conditions. The transit method detects when a planet passes in front of its star, dimming its light just enough to reveal the planet’s size. Together, these techniques provide mass and radius, essential for inferring composition.
Kepler’s transit discoveries added speed to the breakthrough, including Kepler-78b, a planet with Earth-like density but an 8.5-hour year, a lava-hot world that challenged our notions of habitability. In 2016, HIP41378 offered a system with at least five transiting planets, some well beyond Venus’s orbit, underscoring how common—and strange—planetary systems can be.
Three decades of looking for Earth’s twin
The Holy Grail for exoplanet researchers remains a true Earth twin: a planet with a similar mass and radius to Earth orbiting a Sun-like star at a comparable distance. While hundreds of alien worlds have been catalogued, none yet matches our solar system’s exact balance. This doesn’t mean Earth is unique; it may reflect current detection limits in a vast cosmos.
To push the boundary, researchers are coordinating global efforts around next-generation instruments. In La Palma, the HARPS-N consortium has continued to refine measurements, and a broader international collaboration is building Harps3 for the Isaac Newton Telescope. The goal is ambitious: a long, dedicated radial-velocity campaign that could reveal Earth-like planets in the next decade.
Why the search matters—and what it might reveal
Beyond sheer curiosity, finding an Earth twin would illuminate how common planetary architectures like ours are, shedding light on planet formation and the potential for life elsewhere. Even when a direct Earth twin remains out of reach, the iterative process of improving sensitivity, combining transit and radial-velocity data, and studying diverse systems brings us closer to understanding our own place in the universe.
As exoplanet hunters like Christopher Watson and Annelies Mortier note, the journey is as important as the destination. The quest keeps scientists peering into the night, using mountains and observatories around the world to measure, model, and dream about the day we finally glimpse another Earth.
Looking ahead
Thirty years after Mayor and Queloz’s Nobel-winning discovery, the field remains a frontier of perseverance and collaboration. The hunt for an Earth-like planet continues with increasing sophistication, shared data, and international partnerships. The question remains open: will we one day confirm an Earth twin, a beacon in the night that may carry life as we know it?
