The first spark: how a ‘hot Jupiter’ changed everything
On 6 October 1995, a momentous discovery reshaped our view of the cosmos. Michel Mayor and Didier Queloz announced the detection of a planet orbiting a sun-like star outside our Solar System. The star, 51 Pegasi, hosts 51 Pegasi b—a giant gas planet orbiting astonishingly close to its star in about 4.23 days. This was a planet type we had never imagined: a “hot Jupiter.” The technique used, radial velocity or Doppler spectroscopy, measured the star’s tiny wobbles caused by the planet’s gravity, revealing a planetary world in an environment far more brutal than Mercury’s orbit around the Sun.
The technology behind the breakthrough
The instrument at the heart of the discovery was Elodie, a high-precision spectrograph at the Haute-Provence Observatory in France. Elodie dissects starlight into a spectrum, a stellar barcode that exposes chemical fingerprints and motion. When Mayor and Queloz observed the periodic shift in 51 Pegasi’s spectral lines, they inferred a planetary companion tugging on the star. Although the world was unexpected, the signal was confirmed by multiple teams within weeks. The era of exoplanet science had begun.
A revolution that opened the floodgates
Though initially met with skepticism, the 1995 finding proved to be a watershed. The term “hot Jupiter” was coined to describe such close-orbiting giants, and the field exploded. In the 30 years since, more than 6,000 exoplanets or candidates have been catalogued, revealing a staggering diversity—from planets with blistering temperatures to worlds orbiting two stars and even exoplanets with densities rivaling Earth’s yet very different compositions.
From catalogues to cosmic laboratories
Today, researchers use multiple methods to characterize exoplanets. The transit technique, which detects a planet crossing in front of its star, measures a planet’s radius and, when combined with mass estimates from radial velocity, deduces density and composition. Space telescopes such as Kepler and TESS have been pivotal, while ground-based spectrographs like HARPS-N continue refining mass measurements and enabling “mass-radius” relationships for thousands of worlds.
Kepler-78b and the early Earth-like quest
A striking chapter in the Earth-twin story occurred in 2013 with Kepler-78b, a planet whose size closely mirrors Earth. Its mass measurement, however, painted a different portrait: a planet with Earth-like radius but a density suggesting a rocky but extreme lava world, circling in a tight 8.5-hour orbit. The paradox underscored how similar sizes do not guarantee Earth-like habitability and how planetary diversity challenges simple Earth-centric assumptions.
The next frontier: multi-planet systems and the solar-system analogue
Advances in instrumentation and collaborative science revealed complex systems like HIP 41378, a star hosting at least five transiting planets, some of which orbit well beyond Venus’ distance. Each new system is a laboratory for understanding formation, migration, and the potential for Earth-like conditions elsewhere. In these endeavors, the scientific community increasingly collaborates across borders, sharing data and pooling resources to tackle the most challenging signals.
Searching for Earth 2.0: the holy grail remains
Despite the breakthroughs, a true Earth twin—a planet with a mass and radius similar to Earth, and a temperate orbit around a Sun-like star—remains elusive. Radial velocity methods are reaching the limits of current precision, while space-based transit surveys have found thousands of planets but rarely a perfect Earth twin. Yet the quest continues, driven by the tantalizing possibility of discovering a world where life could arise as we know it.
The road ahead
Projects like Harps3, a planned instrument for the Isaac Newton Telescope on La Palma, embody the next leap in precision. Over the next decade, the combination of ultra-stable spectrographs, long-baseline surveys, and international collaboration may finally unveil an Earth-like planet in the habitable zone. If history is a guide, the journey will be as important as the destination—each signal, each data point, brings us closer to answering whether we are alone.
Originally adapted from a piece in The Conversation and republished under a Creative Commons license.
