Introduction: The Groundbreaking Start of Exoplanet Discovery
On October 6, 1995, a watershed moment in astronomy occurred at a scientific meeting in Florence, Italy. Michel Mayor and his Ph.D. student Didier Queloz announced the first planet orbiting a sun-like star outside our solar system: 51 Pegasi b. This discovery proved that planetary systems beyond our own exist and opened a new field of study—exoplanet science—that has transformed our understanding of how planets form and evolve in the cosmos.
What Was Found: A Hot Jupiter in Pegasus
The host star, 51 Pegasi, lies about 50 light-years away in the Pegasus constellation. The companion, 51 Pegasi b, is a gas giant with at least half Jupiter’s mass, completing an orbit in just over four days. It orbits incredibly close to its star, hotter than 1,000°C on the planet itself, and helped redefine what kinds of planets might exist in the universe. The discovery was enabled by the Elodie spectrograph, which detected a rhythmic wobble in the star’s light—an interval of 4.23 days that indicated an unseen companion tugging on the star’s motion.
How We Detect Exoplanets: The Two Primary Techniques
Today, scientists primarily rely on two complementary methods to detect exoplanets: the radial velocity (spectroscopic) method and the transit method. Each technique reveals different planetary properties and together they give a fuller picture of an exoplanet’s mass and size.
Radial Velocity (Doppler) Method
The radial velocity technique measures tiny shifts in a star’s spectrum caused by the gravitational pull of an orbiting planet. As a planet orbits, it tugs the star toward and away from Earth, causing the star’s light to shift slightly in wavelength. Repeated measurements reveal a periodic wobble, from which scientists infer the planet’s mass and orbit. Early detections like 51 Pegasi b demonstrated the power of this method. Modern spectrographs, such as Harps-N and Espresso, push velocity precision to fractions of a centimeter per second, enabling the discovery of smaller, more Earth-like worlds—and still supported by daytime, night-time, and long-term observation campaigns.
Transit Method
The transit method detects a planet when it passes in front of its star from our viewpoint, causing a slight dip in starlight. Measuring the depth and duration of these transits yields the planet’s radius. The method excels for surveying many stars at once, especially with space-based telescopes like Kepler and TESS, and can identify thousands of exoplanets. Transits also allow scientists to combine with radial velocity data to determine a planet’s density and infer its composition—rocky, gaseous, or a mix.
A Quick Timeline: From Skepticism to a Planetary Census
When Mayor and Queloz published their results, the astronomy community questioned whether a planet could exist so close to a sun-like star. Over weeks, independent teams confirmed the signal, and within a few years, the field erupted with hundreds of exoplanet discoveries. By 2019, Mayor and Queloz shared the Nobel Prize for their landmark achievement. Since then, thousands of exoplanets have been cataloged, revealing a breathtaking diversity—from scorching hot Jupiters to compact multi-planet systems and Earth-sized candidates.
What We Learn About Planets—and Why It Matters
Exoplanet research tells us that planetary systems are common, though our own solar system remains unusual in several respects. The discovery of a wide spectrum of worlds—ultra-hot Jupiters, rocky super-Earths, and multi-planet architectures—has reshaped theories of planet formation and migration. Astronomers now seek Earth-like twins in habitable zones, where liquid water could exist. The pursuit blends cutting-edge instrumentation with patient, long-term observation—traits that define modern astronomy.
The Road Ahead: Earth-Like Worlds and Dedicated Campaigns
The holy grail in exoplanet science is finding an Earth twin—a planet with Earth-like mass and radius orbiting a Sun-like star at the right distance. Projects such as Harps3, planned for La Palma, embody the next era of radial velocity campaigns designed to push the limits of detection toward true Earth analogs. The collaboration model, shared data, and multinational teams are essential as we push toward deeper sensitivity and broader exploration of planetary systems.
Conclusion: Our Ongoing Quest to Find Another Earth
Thirty years after the first exoplanet discovery, the search continues with renewed vigor and optimism. Each observation, whether via radial velocity or transit, brings us closer to understanding how common planetary systems are—and whether a true Earth-like world exists among the stars. The journey from 51 Pegasi b to today’s exoplanet landscape reminds us that science thrives on curiosity, collaboration, and the relentless pursuit of answers beyond our solar neighborhood.
