The universe beyond our solar system lay largely unimagined until a pivotal scientific revelation in 1995. It was on October 6th, at a conference in Florence, Italy, that two Swiss astronomers, Michel Mayor and his PhD student Didier Queloz from the University of Geneva, unveiled an astounding discovery: a planet orbiting a star other than our Sun. This momentous announcement would irrevocably alter humanity's understanding of celestial bodies.
The target of their breakthrough was 51 Pegasi, a star approximately 50 light-years away within the constellation Pegasus. Its newly identified companion, named 51 Pegasi b, defied all prevailing planetary theories. This colossal gas giant, at least half the mass of Jupiter, completed an orbit around its star in a mere four days. Its extreme proximity to 51 Pegasi—a mere 1/20th of Earth's distance from the Sun, well within Mercury's orbit—meant its atmosphere would incinerate at temperatures exceeding 1,000°C (1,830°F).
Their groundbreaking detection was made possible by Elodie, a sophisticated spectrograph installed two years prior at the Haute-Provence observatory in southern France. Developed by a Franco-Swiss team, Elodie functioned by splitting starlight into a spectrum of colours, revealing a unique 'stellar barcode' of dark lines that offered chemical insights into distant stars. Mayor and Queloz observed 51 Pegasi's barcode exhibiting a rhythmic, 4.23-day oscillation, a clear indicator that the star was being gravitationally tugged by an unseen companion. Despite initial scientific skepticism, highlighted by the question mark in Nature journal's headline 'A planet in Pegasus?', the signal was rapidly confirmed. 51 Pegasi b not only marked the first exoplanet discovered orbiting a solar-like star, but it also introduced a new class of celestial body, later termed 'hot Jupiters', signifying a planet type previously considered impossible to form so close to a star.
This singular discovery cracked open the door to a new era of cosmic exploration. In the ensuing three decades, humanity has catalogued over 6,000 exoplanets and candidates, revealing a breathtaking array of worlds. From ultra-hot Jupiters with sub-day orbits to planets circling two suns like Tatooine, strange 'super-puff' gas giants, and compact systems of rocky worlds, the diversity is staggering. This revolution earned Mayor and Queloz the Nobel Prize in 2019, confirming that most stars likely host planetary systems, yet a perfect analogue to our own Solar System or an identical 'Earth twin' remains elusive.
The ongoing quest for an Earth twin—a planet matching our home in size, mass, and temperature—continues to inspire modern-day astronomers like Christopher Watson and Annelies Mortier. Their 'expeditions' take them to remote, often mountainous observatories globally, where they operate instruments like the Harps-N spectrograph on the Telescopio Nazionale de Galileo in La Palma, Canary Islands. This device enables them to intercept starlight, searching for minuscule clues that could lead to another habitable world.
For millennia, our Solar System was the only known example of planetary arrangement. Early philosophical thought varied wildly, with Epicurus (341-270BC) speculating on 'an infinite number of worlds,' while Aristotle (384-322BC) championed a singular, Earth-centric universe. This view of planets as rare persisted for 2,000 years. Even in the early 20th century, astronomer Sir James Jeans' tidal hypothesis suggested planets were exceedingly uncommon. However, a scientific paradigm shift in the 1940s, driven by the understanding of star formation as a natural byproduct of planet formation, and a growing appreciation for the immense scale of the universe, dramatically altered this perspective. By 1943, influential American astronomer Henry Norris Russell's article, 'Anthropocentrism's Demise,' predicted 'thousands of inhabited planets in our galaxy.' The question then became: where were they?
Modern exoplanet detection relies primarily on two ingenious techniques: radial velocity and transit. The radial velocity method, pioneered by Mayor and Queloz, involves meticulously measuring minute 'wobbles' in a star's movement caused by the gravitational pull of orbiting planets. For instance, Earth makes the Sun wobble at a mere 9 cm per second. State-of-the-art spectrographs like Harps-N and Espresso can detect velocity shifts as tiny as tenths of a centimetre per second, though still not sensitive enough for a true Earth twin. The transit method, first demonstrated in 1999 by David Charbonneau with the hot Jupiter HD209458b, detects planets by observing a momentary dimming of a star's light as a planet passes in front of it. Space telescopes like Kepler and TESS have utilized this method to discover thousands of exoplanets, as stellar brightness can be measured more accurately and for many stars simultaneously from space. Combining both techniques allows scientists to determine both a planet's mass and radius, providing crucial data for inferring its composition.
Astronomers can model possible compositions by assuming small planets possess an iron-rich core, a rocky mantle, surface water, and an atmosphere. The universe continually surprises us, revealing rocky worlds being torn apart and strange planetary arrangements hinting at violent pasts. From the distant Sweeps-11b, nearly 28,000 light-years away, to those orbiting our nearest star, Proxima Centauri, planets are abundant.
A significant milestone occurred in 2013 when a team including Christopher Watson, using Harps-N, focused on Kepler-78b, a planet identified by the Kepler space telescope with a radius similar to Earth's. Their radial velocity measurements confirmed Kepler-78b's mass to be 1.86 times that of Earth, resulting in a density nearly identical to our planet's. However, the similarities ended there; Kepler-78b's orbital period is a scorching 8.5 hours, rendering it a hellish lava world despite its Earth-like size and density.
Another exciting find came in 2016 from the Kepler telescope: a system with at least five transiting planets around the star HIP41378 in the Cancer constellation. Notably, some of these planets orbit much further from their star than typical transiting exoplanets, resembling the distances within our own Solar System. This discovery spurred a global collaboration to gather years of data, slowly unveiling a system that, for the first time, begins to mimic our own. While its planets are larger and more massive than our rocky worlds, their orbital distances offer invaluable insights into planetary system formation.
After three decades of observation, the initial 'hot Jupiters' found to be rare, have given way to a new class of 'super-Earths' and 'mini-Neptunes.' Yet, despite thousands of discoveries, a system truly resembling our Solar System, or a planet genuinely identical to Earth, remains undiscovered. While it's tempting to conclude Earth is unique, a more reasonable explanation points to the current limitations of our technology in a universe of unimaginable vastness. The 'holy grail' for exoplanet explorers is still to find a true Earth twin—a planet with similar mass, radius, and orbital characteristics to Earth, circling a Sun-like star. This would be the prime candidate in the search for extraterrestrial life as we know it.
Today, the radial velocity method, which kickstarted this journey, remains the most promising path. Didier Queloz is now spearheading a new international collaboration building Harps3, a dedicated instrument to be installed at the Isaac Newton Telescope on La Palma. With its advanced capabilities, scientists believe a decade of data from Harps3 could finally unveil our first true Earth twin… unless, of course, we are unique after all.
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Originally published at: https://www.bbc.com/future/article/20251003-the-epic-hunt-for-a-planet-just-like-earth
