Earth Twin Search Continues In Space

Cosmic Neighbours: The Relentless Search for Earth’s Twin

Humanity has long gazed at the night sky and pondered a fundamental question: are we alone? For millennia, this inquiry remained in the realm of philosophy and fiction. Our solar system was the only planetary family we knew, a tiny, isolated sample in a vast cosmic ocean. But over the last three decades, a revolution in astronomy has transformed this question into a tangible scientific pursuit. Scientists have now confirmed the existence of thousands of worlds orbiting distant stars, revealing a universe more varied and bizarre than we ever imagined. The monumental quest for a second Earth is well and truly under way.

A Discovery That Changed Everything

A seismic announcement in Florence, Italy, during October 1995 forever changed the field of astronomy. Two Swiss astronomers, Michel Mayor, along with his doctoral candidate Didier Queloz, presented proof of a planet revolving around a star different from our own. This was the moment the theoretical became real. Their research, conducted out of the University of Geneva, had identified a world orbiting the star 51 Pegasi. This sun-like star is situated roughly 50 light-years away from Earth. The planet, named 51 Pegasi b, was nothing like the familiar worlds in our own cosmic neighbourhood. It was a complete departure from established planetary science.

Image  Credit -by Hubble ESA, CC BY 2.0 , via Wikimedia Commons

The First ‘Hot Jupiter’

The newly discovered 51 Pegasi b defied all expectations. This gas giant had a bulk of no less than half of Jupiter's. It completed a full orbit around its parent star in an astonishingly brief four days. Its extreme proximity, significantly inside the orbital path of Mercury, created a furnace-like atmosphere. Scientists calculated that surface temperatures likely exceeded 1,000 degrees Celsius, a truly hellish environment. This finding perplexed the scientific community, which had no working theory to explain how such a massive planet could form or exist in such close proximity to its parent star. It introduced a completely new planetary class: the ‘hot Jupiter’.

The Wobble of a Distant Star

Mayor and Queloz made their historic finding using a highly specialised instrument. The Elodie spectrograph, set up at an observatory located in the south of France, allowed them to analyse starlight with incredible precision. The device splits light into its constituent colours, creating a spectrum that serves as a unique stellar fingerprint. This fingerprint reveals details about a star’s chemical makeup and motion. The astronomers noticed 51 Pegasi’s barcode oscillating with a consistent period of 4.23 days. This rhythmic movement was the unmistakable evidence that the star was being pulled by the gravity of an unseen companion. The star was wobbling.

From Scepticism to Confirmation

Initially, the finding was met with considerable doubt. The idea of a giant planet in an intensely hot orbit seemed impossible. A question mark even appeared on the cover of the journal Nature, reflecting the scientific community’s uncertainty. Other research groups corroborated the 4.23-day signal in a matter of weeks, yet doubts concerning its origin persisted. For nearly three years, alternative explanations were debated and scrutinised. Ultimately, all other possibilities were ruled out. 51 Pegasi b was not just confirmed, but it also opened the floodgates for a new era of exploration.

An Explosion of New Worlds

The confirmation of 51 Pegasi b started a paradigm shift. In the years since, astronomers have catalogued over 6,000 confirmed exoplanets, with thousands more candidates awaiting verification. The sheer variety of these worlds is staggering. Scientists have found ‘ultra-hot Jupiters’ with orbits that take fewer than 24 hours to complete. They have identified planets that circle two stars at once, reminiscent of the fictional planet Tatooine. Other discoveries include strange ‘super-puff’ worlds bigger in size than Jupiter yet possessing only a small amount of its mass, and compact systems of rocky planets huddled close to their stars. The cosmos revealed itself to be a place of far greater variety than previously conceived.

Ancient Echoes of Other Worlds

The concept of worlds existing outside our own solar system is not a recent one. The ancient Greek philosopher Epicurus, writing to Herodotus over two millennia ago, proposed the existence of infinite worlds. He reasoned that if the cosmos consisted of a limitless quantity of atoms, then other planets and solar systems were a logical necessity. His reasoning was philosophical, not observational, but it showed a remarkable conceptual leap. He even speculated that some of these worlds might harbour the seeds of life, while others would be barren. This ancient thought experiment laid a conceptual foundation for modern science.

The Earth-Centred Universe

In stark contrast to Epicurus, his contemporary, Aristotle, was developing a model of the universe that placed Earth at the center. This influential theory positioned a motionless Earth at the absolute middle of everything. The Moon, Sun, and all known planets circled our world. For Aristotle, the planetary system we inhabit constituted the whole of existence. This thinking, which argued that our world was unique, dominated Western thought for nearly 2,000 years. It created a powerful philosophical barrier to the idea of other worlds, a barrier that would take centuries of scientific progress to dismantle completely.

A Shift in Cosmic Perspective

In the initial decades of the 20th century, scientific thought began to contest the idea of our solar system’s uniqueness. In 1916, the physicist Sir James Jeans proposed that planets formed from gas pulled from stars during rare, close encounters. This tidal hypothesis suggested that the formation of planets was an exceptionally uncommon event. However, this theory did not hold up to scrutiny. Concurrently, the renowned ‘Great Debate’ in 1920 saw a clash between astronomers Harlow Shapley and Heber Curtis regarding the universe's scale. Curtis correctly argued that our Milky Way was merely a single galaxy among billions of others, dramatically expanding our cosmic horizons.

The Modern View Takes Hold

During the 1940s, a pair of developments caused the scientific consensus to shift dramatically. First, prevailing concepts about how planets are created were revised. Scientists began to see planet formation not as an uncommon event but as an organic result of the star-creation process. This raised the exciting prospect that nearly all stars could have planetary companions. Furthermore, reports in 1943 of planets circling nearby stars had a profound impact on scientific thought, even though these claims were later disproven as false positives. The idea of a galaxy filled with countless worlds moved from the realm of speculation into credible scientific inquiry.

Image  Credit - by NASA and ESA, CC BY-SA 4.0, via Wikimedia Commons

Hunting Planets with the Transit Method

While the radial velocity or ‘wobble’ method located the inaugural exoplanet, another technique has proven even more prolific. The transit method watches for the tiny, periodic dimming of a star’s light. This dimming occurs when an orbiting planet crosses the face of its star as viewed from our position, like a moth flying in front of a distant streetlight. The amount of dimming reveals the planet’s size. Missions using space-based telescopes, including NASA's Kepler and TESS, have used this technique to find thousands of worlds. They can monitor the brightness of vast numbers of stars simultaneously, making them incredibly efficient planet-hunting machines.

Refining the Art of Detection

The radial velocity technique remains a crucial tool for exoplanet hunters. It is the only method that can directly measure a planet’s mass by calculating the gravitational pull it exerts on its star. The velocity changes involved are minuscule. The Earth, for example, causes the Sun to wobble at a speed of only 9 centimetres each second—a pace more sluggish than a tortoise. Modern spectrographs like the HARPS-N instrument, located on La Palma, and the ESPRESSO instrument in Chile are engineering marvels. They can detect changes in velocity amounting to mere tenths of a centimetre every second over immense distances spanning trillions of miles.

Direct Imaging: A Glimpse of Another World

A particularly challenging yet rewarding detection method is direct imaging. This technique involves taking an actual photograph of an exoplanet. The difficulty lies in neutralising the intense brightness of the host star, which can be billions of times brighter than the orbiting planet. Astronomers use advanced optical systems called coronagraphs, which work like an artificial eclipse inside the telescope. This method works best for large, young gas giants orbiting far from their stars. Although this method has yielded few discoveries, each image provides an invaluable, direct look at another solar system in the making.

Gravitational Microlensing: Using Gravity as a Lens

Gravitational microlensing is a more unusual technique that uses the principles of Einstein’s theory of general relativity. When a star with a planet moves across the line of sight to a more remote star, its gravity acts like a lens, bending and magnifying the background star’s light. This creates a temporary, predictable brightening event. If the foreground star has a planet, the planet’s own smaller gravity creates a secondary, brief spike in the light curve. This method is singular because it can locate worlds at immense distances from Earth, including free-floating ‘rogue’ planets that drift through the galaxy without a parent star.

The James Webb Space Telescope: A New Era

The launch of the James Webb Space Telescope (JWST) has revolutionised the study of exoplanets. While not primarily a planet-finder, its unparalleled sensitivity allows it to analyse the atmospheres of known worlds in exquisite detail. As a planet transits its star, a tiny fraction of starlight passes through its atmosphere. JWST’s spectrographs can analyse this light to detect the chemical fingerprints of different molecules. This opens up the tantalising possibility of searching for biosignatures—gases like oxygen, methane, and water vapour—that could suggest the existence of biological activity. JWST is our first real tool for atmospheric reconnaissance on other worlds.

Analysing Alien Atmospheres

JWST has already delivered groundbreaking results. It has confirmed that water vapour is present, along with clouds and haze, in the atmosphere of a hot gas giant over 1,000 light-years away. More recently, it has studied the atmospheres of rocky planets, including those in the famous TRAPPIST-1 system. In one fascinating case, the telescope detected carbon dioxide and methane in the atmosphere of K2-18 b, a ‘sub-Neptune’ world with a potential liquid water ocean. While not proof of life, these detections demonstrate a technological capability that was pure science fiction just a decade ago. Each observation enhances our comprehension of the conditions on these far-off worlds.

A Gallery of Cosmic Oddities

The exoplanet discoveries have revealed a veritable zoo of strange worlds. One of the most extreme is Kepler-78b. This rocky planet is nearly a perfect match for Earth regarding its size and density. However, its similarities end there. It orbits its star in a mere 8.5 hours, meaning its surface is a hellish ocean of molten lava. Its scorching temperature would ensure any rock on the surface is in a molten state. This world highlights how factors like orbital distance are just as crucial as size and mass in determining a planet's characteristics and its capacity to support life.

The TRAPPIST-1 System: A Prime Target

A particularly thrilling discovery to date is the TRAPPIST-1 system, located just 40 light-years away. This system features seven rocky, Earth-sized planets orbiting a small, cool star. No fewer than three of these worlds are situated within the star's habitable zone—the region where temperatures could allow liquid water to exist on the surface. Because the star is so dim, these planets huddle much closer than Mercury does to our sun. The TRAPPIST-1 system is a perfect natural laboratory for studying the formation of rocky planets and is a top priority for atmospheric analysis by the James Webb Space Telescope.

The Search for the Habitable Zone

The ultimate prize for many astronomers is finding a genuine terrestrial counterpart. This means locating a planet of similar size and mass to our own, revolving around a star comparable to our own Sun from a similar range. This specific orbital distance is often called the ‘habitable zone’ or ‘Goldilocks zone’, where conditions are not too hot and not too cold for liquid water. While we have found many rocky planets, and some in the habitable zones of their stars, we have not yet found a system that truly mirrors our own. This is likely a limitation of our current technology rather than an indication of our uniqueness.

The Instruments of the Hunt

The quest for other worlds is driven by incredibly sophisticated technology. International collaborations have built and operate instruments like the HARPS-N spectrograph in La Palma. This machine is affixed to the Telescopio Nazionale Galileo and is designed specifically to detect the minute oscillations that result from small, rocky worlds. Its precision allows scientists to rudely interrupt the journey of starlight that has been travelling unimpeded for decades or even millennia. Each new signal gathered by these instruments could possibly move us closer to understanding how common planetary systems like our own may be.

Building for the Future

The technological race continues. A significant global partnership is currently constructing a next-generation instrument, HARPS3, which will be fitted to the Isaac Newton Telescope, another facility on La Palma. This dedicated project, with pioneers like Didier Queloz at the helm, represents the next major advance for the radial velocity method. Its enhanced capabilities are specifically designed to detect the faint signal from a terrestrial-type planet revolving around a star like our Sun. The team believes that ten years' worth of data from this new instrument will likely be sufficient to at last identify our first genuine terrestrial counterpart.

Challenges on the Cosmic Frontier

The hunt for exoplanets is fraught with challenges. The signals astronomers are searching for are incredibly faint and can easily be mistaken for stellar activity, such as starspots or flares. Confirming a planet, especially a small one with a long orbit, requires years of patient observation and data gathering from multiple observatories. The sheer distances involved are also mind-boggling. Even Proxima Centauri, the star system closest to our own, is over four light-years away. A signal from its planets takes over four years to reach us, and any physical journey there with current technology would require many thousands of years.

The Allure of Proxima Centauri

Proxima Centauri, the star closest to us, offers a tantalising glimpse of what might lie nearby. This small red dwarf star is host to at least two confirmed planets. One of these, Proxima Centauri b, is a rocky world possessing slightly more mass than our own world. It orbits within the star’s habitable zone, completing a revolution every 11 days. However, red dwarf stars are known for their violent flares of radiation, which could make the surface of any nearby planet inhospitable. Nevertheless, its proximity makes it a compelling target for future study and perhaps, one day, for interstellar probes.

Why the Search Matters

The endeavor to locate another Earth is about more than just cataloguing distant worlds. It is a search for context and for our own place within the cosmos. Locating a genuine terrestrial counterpart is the initial move toward resolving whether life exists elsewhere. It would force us to confront the possibility that the biological processes that led to us are not unique. Even if we find that such planets are barren, that discovery would be equally profound. It would underscore the preciousness and fragility of our own living world, a rare jewel within the immense cosmic dark. The search continues.