Introduction: a quest sparked by a tiny wobble
The search for a planet that mirrors Earth in size, warmth, and potential for life has been a long, winding journey. It began with a radical breakthrough in 1995 when Michel Mayor and Didier Queloz announced the discovery of a planet orbiting a Sun-like star outside our Solar System. That first exoplanet, 51 Pegasi b, was a hot Jupiter—an alien world unlike anything in our own neighborhood—and it reshaped our understanding of how planetary systems form and evolve.
The landmark discovery that upended planetary science
51 Pegasi b was detected using the Elodie spectrograph, which listened to the star’s “stellar barcode” and noticed a regular back-and-forth shift every 4.23 days. The conclusion: a gas giant orbiting perilously close to its star. The discovery, published in Nature with the headline “A planet in Pegasus?”, startled scientists and marked the birth of exoplanet science. It also introduced the term hot Jupiter, a class of planets now known to be rarer than initially thought, but instrumental in propelling the field forward.
From dramatic surprises to patient, precise measurements
Two decades of progress followed as technology improved. The radial velocity technique—measuring the star’s tiny wobbles caused by orbiting planets—remains foundational. Meanwhile, the transit method, which observes a planet blocking starlight as it passes in front of its star, allowed scientists to infer planet sizes and, with combined data, their densities. The Kepler mission, and later TESS, revealed thousands of exoplanets, including several “super-Earths” and mini-Neptunes, transforming our view of planetary diversity.
Significant milestones include Kepler-78b, a world with Earth-like density yet an 8.5-hour year, a lava-scorched edge case that underscored how extreme exoplanets can be. These discoveries sharpened the question at the heart of modern exoplanet science: could there be a true Earth twin—an Earth-sized planet with a similar mass and temperate zone around a sun-like star?
The Harps-N era: precision science from La Palma
Today, scientists like Christopher Watson and Annelies Mortier chase this holy grail using Harps-N, a high-precision spectrograph on the Telescopio Nazionale Galileo in La Palma, Canary Islands. Harps-N, and related instruments such as Espresso, are engineered to detect minute velocity shifts in stars—down to tenths of a centimetre per second. While such sensitivity doesn’t yet guarantee an Earth twin, it brings us closer by enabling mass measurements for small exoplanets detected by transit surveys.
Why the Earth twin remains elusive
Despite thousands of exoplanets catalogued, a perfect Earth twin—similar mass and radius, temperate climate, and a host star analogous to the Sun—has not yet been found. Detection methods have biases: radial velocity excels for larger planets, while transit surveys favor close-in worlds. Kepler-78b taught us that Earth-like densities can exist in the wrong places, and many systems so far lack the familiar architecture of our own.
A collaborative path forward: new instruments and decades of data
The field is moving toward longer, more ambitious campaigns. The planned Harps3 instrument at the Isaac Newton Telescope on La Palma represents a major international effort to accumulate years of data with enough precision to reveal an Earth-like mass in Earth-like orbit. If successful, a dedicated, long-term radial velocity campaign could deliver the world we’ve been seeking for thirty years: a planet that resembles Earth in mass, radius, and orbital distance.
What would an Earth twin mean for science and life?
Finding an Earth twin would not only reshape our understanding of planetary formation but also sharpen the search for life beyond the Solar System. It would provide a natural laboratory for studying planetary atmospheres, geologic activity, and climate stability, potentially guiding future observations aimed at detecting biosignatures. Until then, every new exoplanet—whether a hot Jupiter or a distant, Earth-analog candidate—adds a piece to the puzzle of our place in the cosmos.
Conclusion: the journey continues
The hunt for an Earth-like planet is a story of breakthroughs and perseverance. From the first detection in 1995 to today’s global collaborations, scientists remain guided by a simple, profound question: are we alone? The next decade of data, improved instrumentation, and coordinated international effort may finally reveal our true Earth twin.
