The Search for Earth 2.0

Worlds Beyond Our Own

For millennia, the points of light in the night sky were regarded as fixed stars or wandering gods. Our solar system, with its familiar collection of planets, was thought to be the sole blueprint for cosmic architecture. That perception shattered in the 1990s with the first confirmed discoveries of planets outside our celestial family. These worlds, orbiting stars other than our Sun, are known as exoplanets, a term derived from “extrasolar planets”. Their discovery transformed what was once the domain of science fiction into a vibrant and rapidly expanding field of astronomy, opening a new frontier in the human quest to understand our place in the universe.

The numbers involved are staggering. To date, astronomers have confirmed the existence of more than 5,800 exoplanets, a figure that grows steadily. Yet this is merely a glimpse of a far grander cosmic reality. These discoveries have been made in a relatively small patch of our own cosmic backyard, within thousands of light-years of Earth. Our Milky Way galaxy is a sprawling spiral of an estimated 400 billion stars. If most of these stars host not just one planet, but entire systems like our own, the number of worlds in our galaxy alone could easily run into the trillions. This vast planetary census makes it statistically probable that somewhere out there, other planets share characteristics with our own.

What has become clear from this ongoing exploration is that the universe is far more creative in building planets than the examples in our solar system might suggest. Astronomers have uncovered a veritable “zoo” of alien worlds, ranging from familiar categories to the ly bizarre. We’ve found gas giants larger than Jupiter, icy worlds akin to Neptune, and rocky, terrestrial planets. But we’ve also found types of planets that have no analog in our system. There are “super-Earths,” rocky worlds significantly more massive than our own, and “mini-Neptunes,” which bridge the gap between terrestrial planets and ice giants. More exotic still are the “hot Jupiters,” massive gas planets that orbit their stars in a matter of days, with scorching surface temperatures that can boil metal. There are lava worlds covered in molten seas, and even “puffy planets” with a density so low it’s comparable to Styrofoam. Some worlds, known as rogue planets, don’t orbit a star at all, wandering alone through the galactic darkness.

This incredible diversity has fundamentally reshaped our understanding of how planetary systems form and evolve. Our solar system, with its orderly arrangement of small, rocky planets in the inner regions and large gas giants farther out, is just one possible outcome of planetary formation. The discovery of hot Jupiters, for instance, was a surprise that challenged early theories. It led to the development of models of planetary migration, where giant planets can form in the cold outer reaches of a system and then spiral inward over millions of years. The sheer variety of worlds being discovered shows that the processes of planet formation are complex and can lead to a vast array of different architectural outcomes. It is within this diverse cosmic landscape that scientists are searching for something much more specific: a world that looks and feels like home.

The Blueprint for an Earth-like Planet

The term “Earth-like” is not a simple checkbox but describes a spectrum of properties. It’s a complex and multifaceted concept that brings together geology, chemistry, and astrophysics to define what makes a planet potentially habitable. At its heart, the search for an Earth-like world is a search for a planet that has the right ingredients and the right conditions to support life as we know it.

The most fundamental requirement is that a planet be terrestrial, meaning it has a solid, rocky surface. This is determined by measuring a planet’s size (radius) and its mass. Together, these two measurements allow scientists to calculate the planet’s bulk density. A high density, similar to that of Earth, Venus, or Mars, suggests a composition dominated by rock and metal. The specific mix of elements like iron, silicon, and magnesium is also important, as it dictates the size of the planet’s metallic core relative to its silicate mantle, a feature that can influence everything from gravity to geological activity. Because planets and their host stars condense from the same primordial cloud of gas and dust, the chemical makeup of a star can offer clues to the raw materials available for its planets. However, recent studies suggest that the relationship isn’t always straightforward, and many rocky exoplanets appear to have compositions close to Earth’s even when their host stars differ.

A rocky composition alone is not enough. A planet’s location is just as important. For a world to be considered potentially habitable, it must orbit its star within the “habitable zone,” a region often nicknamed the “Goldilocks Zone”. This is the orbital band around a star where surface temperatures are “just right”—not too hot and not too cold—for liquid water to exist on a planet’s surface. Liquid water is considered an essential prerequisite for life as we know it. It acts as a universal solvent, a medium in which the complex chemical reactions that underpin biology can take place. The size and position of this habitable zone are not fixed; they depend entirely on the host star. Hotter, more luminous stars have wider habitable zones that lie farther out, while cooler, dimmer stars, such as red dwarfs, have much narrower habitable zones nestled very close to the star.

Even a rocky planet in the perfect location needs more to be truly Earth-like. A suitable atmosphere is necessary to exert enough surface pressure to prevent liquid water from boiling away into space. An atmosphere also plays a vital role in regulating a planet’s climate through the greenhouse effect, trapping some of the star’s heat and distributing it around the planet. Protecting that atmosphere over billions of years often requires another feature: a global magnetic field. Generated by the motion of a molten metallic core, a magnetic field acts as a planetary shield, deflecting the charged particles of the stellar wind and protecting the atmosphere from being slowly eroded into space. This is especially important for planets orbiting red dwarf stars, which are known for their violent flares that can unleash torrents of harmful radiation.

The habitable zone is a powerful tool for narrowing down the search, but it’s a simplified guide, not a guarantee of life. True habitability arises from a delicate balance of a planet’s location, its geology, and its atmosphere. Our own solar system provides perfect examples of this complexity. Venus technically orbits within the Sun’s habitable zone, but a runaway greenhouse effect fueled by a thick carbon dioxide atmosphere has turned its surface into a scorching wasteland, far too hot for liquid water. Mars sits on the outer edge of the habitable zone and is now a frozen desert, but compelling evidence suggests it once had a thicker atmosphere and liquid water on its surface, highlighting the importance of retaining an atmosphere over geological time. Conversely, life might exist far outside the traditional habitable zone. Moons like Jupiter’s Europa and Saturn’s Enceladus are frozen on the surface but are thought to harbor vast liquid water oceans beneath their icy shells, kept warm by heat generated from the gravitational push and pull of their giant parent planets. The habitable zone is best understood as a first-pass filter, a way to identify the most promising targets for further investigation.

The type of host star also introduces a fascinating paradox. Red dwarf stars are the most common type of star in the Milky Way, accounting for roughly 75% of the stellar population, and they have incredibly long lifespans, burning for trillions of years—offering a vast, stable timeframe for life to evolve. However, because they are so cool and dim, their habitable zones are located perilously close to the star. Planets in this tight embrace are likely to become “tidally locked,” with one hemisphere perpetually facing the star in constant daylight and the other locked in permanent night. This could create extreme temperature contrasts that would challenge life. Furthermore, many young red dwarfs are “flare stars,” prone to unpredictable and violent eruptions that blast their nearby planets with intense X-ray and ultraviolet radiation, which could strip away atmospheres and sterilize surfaces. This makes the most numerous and long-lived stars in the galaxy potentially some of the most hazardous hosts for life. Sun-like stars (G-type) or the slightly cooler and more stable orange dwarfs (K-type) may offer a more tranquil path to habitability, though they are less common.

The Planet Hunter’s Toolkit

Finding a planet hundreds or thousands of light-years away is an immense technical challenge. These worlds are incredibly faint and are lost in the overwhelming glare of their parent stars. Instead of seeing them directly, astronomers have developed ingenious indirect methods to detect their presence. Two techniques have been responsible for the vast majority of exoplanet discoveries.

Catching a Shadow: The Transit Method

The most productive method for finding exoplanets is the transit method. It relies on a precise alignment of perspective. If a planet’s orbit happens to be edge-on from our point of view, it will periodically pass in front of its star, an event called a transit. This passage blocks a tiny fraction of the starlight, causing a slight, temporary dip in the star’s observed brightness. Astronomers use highly sensitive photometers on telescopes to monitor the brightness of thousands of stars at once, looking for these telltale dips. The data is plotted on a graph called a “light curve,” which shows brightness over time. A single dip could be caused by other phenomena, like a starspot, so astronomers look for repeated, periodic dips to confirm the presence of an orbiting planet. Generally, at least three transits are required for a confident detection.

This method provides a wealth of information. The amount of dimming during a transit, known as the “transit depth,” is directly proportional to the planet’s size relative to its star. Since the star’s size can be estimated from its spectrum, astronomers can calculate the planet’s radius with high precision. The time between consecutive transits gives the planet’s orbital period, or the length of its “year”. Space-based observatories are particularly well-suited for this work because they can stare at star fields continuously without interruption from Earth’s atmosphere or day/night cycle. NASA‘s Kepler Space Telescope, and its successor, the Transiting Exoplanet Survey Satellite (TESS), have used this method to discover thousands of planets, revolutionizing the field.

The Unseen Dance: The Radial Velocity Method

The first successful technique for discovering exoplanets was the radial velocity method, sometimes called the Doppler wobble method. It works by observing the star, not the planet. A star and its orbiting planet don’t sit perfectly still; they both orbit a common center of mass. Because the star is so much more massive, its orbit is a tiny “wobble” compared to the planet’s wide swing. As the star wobbles, it moves slightly toward and away from Earth. This motion can be detected in the star’s light through the Doppler effect. When the star moves toward us, its light waves are compressed, shifting them toward the blue end of the spectrum (a blueshift). When it moves away, the light waves are stretched, shifting them toward the red end (a redshift).

Astronomers use high-precision instruments called spectrographs to split the starlight into its constituent colors and look for these tiny, periodic shifts in the star’s spectral lines. A regular, repeating pattern of shifting indicates the gravitational tug of an unseen companion. This method allows astronomers to determine the planet’s orbital period and provides an estimate of its mass. It’s only a minimum mass because the technique is only sensitive to the star’s motion along our line of sight. If the planetary system is tilted relative to us, we are only seeing a component of the full wobble, so the calculated mass will be lower than the true mass.

The Power of Synergy: Combining Methods

While each method is powerful on its own, their true strength is realized when they are used together to observe the same planet. The transit method provides the planet’s radius, and the radial velocity method provides its minimum mass. By combining these two fundamental properties, scientists can calculate the planet’s bulk density (mass divided by volume). Density is the crucial piece of the puzzle that allows astronomers to make an educated guess about the planet’s composition. A high density suggests a rocky world made of silicates and iron, like Earth. A low density points to a gas-rich planet like Jupiter or a puffy, ice-and-gas world like Neptune. This synergy is what transforms a simple detection into the first step of true characterization, allowing us to distinguish between giant gaseous worlds and the small, rocky planets that are the primary targets in the search for another Earth.

The history of these detection methods also reveals a critical principle in science: our view of the universe is shaped by the tools we use to observe it. The radial velocity method is most sensitive to the largest stellar wobbles, which are caused by very massive planets orbiting very close to their stars. As a result, the first exoplanets discovered were predominantly hot Jupiters, making this exotic class of planet seem common. This created a biased sample that didn’t reflect the true galactic population. The transit method, especially when employed by a dedicated space telescope like Kepler that could monitor hundreds of thousands of stars for years, was a game-changer. It was sensitive enough to find much smaller planets in longer orbits. The data from Kepler revealed that small, rocky worlds are not rare at all; on the contrary, they are incredibly common. One study based on Kepler data suggested that about half of all Sun-like stars in our galaxy could host a rocky planet in their habitable zone. This dramatic shift in understanding shows how our perception of the cosmos evolves with our technology. We are moving from a biased, limited view to a far more accurate census of the galaxy’s planetary inhabitants.

Reading the Air of Alien Worlds

Discovering a rocky planet in the habitable zone is an exciting first step, but it’s far from the end of the story. The ultimate goal is to search for signs of life, and that requires studying a planet’s atmosphere. Astonishingly, astronomers have developed techniques to do just that, analyzing the chemical makeup of air on worlds light-years away.

An Atmosphere’s Fingerprint: Transmission Spectroscopy

For planets discovered via the transit method, there is a powerful follow-up technique called transmission spectroscopy. When a transiting planet passes in front of its star, a tiny fraction of the starlight shines throughthe upper layers of the planet’s atmosphere before it reaches our telescopes. The atoms and molecules present in that atmosphere will absorb light, but they do so only at very specific wavelengths (or colors) that are unique to each chemical compound. Water vapor, for example, absorbs at different wavelengths than methane or carbon dioxide. By using a spectrograph to split the starlight into a detailed spectrum—like a cosmic rainbow—astronomers can see which wavelengths are missing or diminished. These missing slivers of light act as a chemical “fingerprint” or “barcode,” revealing the composition of the alien atmosphere. This is one of the primary scientific missions for powerful observatories like the Hubble Space Telescope and the even more capable James Webb Space Telescope (JWST), which are designed to capture these faint atmospheric signals.

The Telltale Signs of Life: Biosignatures

The ability to analyze an exoplanet’s atmosphere opens the door to the search for biosignatures. A biosignature is any substance, gas, or pattern that provides scientific evidence of past or present life. While life could leave many traces, the most accessible ones for current telescopes are gaseous biosignatures in a planet’s atmosphere.

Several gases are considered strong candidates. On Earth, the vast majority of the free oxygen (O₂) in our atmosphere is a waste product of photosynthesis. Oxygen is a highly reactive gas; without being constantly replenished by life, it would quickly combine with other elements and disappear from the atmosphere. Therefore, detecting a significant amount of oxygen, and its byproduct ozone (O₃), in an exoplanet’s atmosphere would be a very compelling sign of biological activity. Methane (CH₄) is another key gas. While it can be produced by geological processes like volcanism, on Earth, a large amount is produced by microbial life.

Perhaps the most robust evidence for life would not be a single gas, but a combination of gases that shouldn’t exist together. Finding both oxygen and methane in the same atmosphere, for example, would be a discovery. These two gases readily react with and destroy each other. To find them coexisting in large quantities would imply that something is pumping both into the atmosphere at a rate faster than they can be naturally removed. Such a state of chemical disequilibrium would be a powerful hint that two different and widespread biological processes are at work, fundamentally altering the planet’s chemistry.

The Challenge of False Positives

The search for biosignatures is fraught with complexity, and the greatest challenge is the possibility of false positives. Finding a gas like oxygen is not, by itself, definitive proof of life. Nature has non-biological (abiotic) ways of producing gases that can mimic biosignatures. For instance, on a water-rich planet orbiting a star that emits intense ultraviolet radiation, that radiation could break water molecules (H₂O) apart in the upper atmosphere, a process called photolysis. This could lead to a build-up of oxygen in the atmosphere of a completely lifeless world.

This means that confirming the presence of life will require much more than just detecting a single gas. Scientists will need to build a comprehensive case for each candidate planet. This involves not only identifying potential biosignatures but also thoroughly understanding the planet’s context: the type and intensity of radiation from its star, its likely geology, its temperature, and the presence or absence of other gases that could help rule out abiotic scenarios.

This reality has transformed the search for extraterrestrial life from a simple “yes or no” question into a probabilistic science. We will likely never have a single, unambiguous “we found it” moment from a telescope. Instead, it will be a process of slowly accumulating evidence. Scientists will use sophisticated computer models to simulate a planet’s atmosphere under both “life” and “no life” scenarios, and then compare those models to the actual data received from a telescope. Using a statistical approach known as Bayesian analysis, they can then calculate the probability that the observed data is better explained by the presence of life than by any known abiotic process. The first credible clplans of life elsewhere will likely come not as a definitive declaration, but as a statement of high confidence—for example, “there is a 95% probability that the gases we see on this planet are of biological origin.” This methodical, evidence-based approach represents a maturation in how science tackles one of humanity’s oldest and most compelling questions.

A New Era of Discovery: The James Webb Space Telescope

The launch of the James Webb Space Telescope (JWST) in 2021 marked the beginning of a new era in astronomy, and particularly in the study of exoplanets. As the successor to the Hubble Space Telescope, JWST is the most powerful and complex space observatory ever built, and its capabilities are transforming our ability to probe the mysteries of distant worlds.

A Revolutionary Eye on the Cosmos

JWST’s primary advantage is its large, segmented mirror and its optimization for observing in infrared light. While Hubble operates mainly in visible and ultraviolet light, JWST is designed to capture longer wavelengths. This is a crucial advantage for exoplanet science because many of the most important molecules that might be found in an atmosphere—including water (H₂O), methane (CH₄), and carbon dioxide (CO₂)—have their strongest and most distinct absorption features in the infrared part of the spectrum. This makes JWST uniquely suited to perform the kind of detailed atmospheric characterization that was previously impossible.

While telescopes like Kepler were primarily planet finders, designed to detect the presence of planets in great numbers, JWST is a planet characterizer. Its unprecedented sensitivity and spectral resolution allow it to move beyond detection to detailed analysis. For the first time, astronomers can probe the atmospheres of a wide range of planets, from hot gas giants down to smaller, cooler super-Earths and Neptune-sized worlds, with stunning clarity.

The Power of Direct Imaging

In addition to studying atmospheres via the transit method, JWST has the remarkable ability to directly image some exoplanets. This is an exceptionally difficult task because a planet is billions of times fainter than its host star, and its light is easily lost in the star’s overwhelming glare. To overcome this, JWST is equipped with instruments called coronagraphs, which are essentially sophisticated masks that can block out the starlight, allowing the faint, reflected light of an orbiting planet to be seen.

In a landmark achievement, JWST captured its first direct-image discovery of a new planet, a world with a mass similar to Saturn’s dubbed TWA 7b. This was the lightest exoplanet ever to be discovered using this technique, demonstrating a powerful new capability for finding planets that may not be detectable by other methods. Direct imaging not only confirms a planet’s existence but also allows for the study of its emitted and reflected light, providing clues about its temperature and atmospheric properties.

Early Science and Future Promise

JWST is already delivering a torrent of groundbreaking data that is reshaping the field. Its observations have confirmed the presence of a wide array of molecules in exoplanet atmospheres, including water, methane, carbon dioxide, and even sulfur dioxide (SO₂), a gas linked to photochemical processes. These detections are providing the first real data to test and refine our theories of planet formation and atmospheric chemistry.

The true power of JWST, however, lies not just in studying individual planets, but in its ability to perform comparative planetology on a galactic scale. For the first time, scientists can analyze the atmospheres of dozens of different worlds in a systematic way, allowing them to search for population-level trends. Early studies are already building frameworks to correlate a planet’s physical properties, like its mass and temperature, with its observed atmospheric chemistry. For example, scientists are observing a strong link between a planet’s equilibrium temperature and the presence of sulfur dioxide, and they are gathering data to test the long-held theory of a mass-metallicity relationship, which suggests that lower-mass planets should be richer in heavy elements. This approach signifies a major leap forward. The field is moving beyond individual “case studies” of peculiar worlds and into a new era of exoplanet climatology and atmospheric science. By observing a diverse sample of planets, we can begin to uncover the general rules that govern how planetary atmospheres form and evolve across the galaxy.

A Gallery of Potentially Habitable Worlds

The search for an Earth-like exoplanet has yielded a fascinating collection of candidates. While no perfect “Earth 2.0” has been found, several worlds stand out for their tantalizing similarities to our own. These planets represent the forefront of the search for habitable environments and are prime targets for further study with the next generation of telescopes. The following table and profiles highlight some of the most notable discoveries to date, offering a glimpse into the variety of potentially life-bearing worlds that may exist in our galaxy.

Notable Earth-like Exoplanet Candidates

The TRAPPIST-1 System: A Miniature Solar System

One of the most extraordinary discoveries in the search for exoplanets is the TRAPPIST-1 system, located approximately 40.7 light-years from Earth. This system is remarkable for its central star, an ultra-cool red dwarf that is only slightly larger than the planet Jupiter, and for the seven rocky, Earth-sized planets that orbit it in a tightly packed formation. The entire planetary system is incredibly compact; all seven worlds orbit their star much closer than Mercury orbits our Sun, creating a miniature analog of our own solar system.

What makes TRAPPIST-1 a prime target in the search for life is that three of its planets—designated TRAPPIST-1e, f, and g—are located squarely within the star’s habitable zone. This is the largest number of potentially habitable, Earth-sized planets ever found in a single system. Studies have revealed that all seven planets have remarkably similar densities, which are about 8% less dense than Earth. This suggests they all share a similar composition that is different from our planet’s, perhaps containing a lower fraction of iron and more lighter elements like water, either as liquid or ice.

Despite the promise, there are significant challenges to habitability in the TRAPPIST-1 system. Because the planets orbit so close to their star, they are almost certainly tidally locked, with one side in perpetual daylight and the other in eternal night. The star itself, while cool, is active and its radiation could pose a threat to any potential life. Observations with the James Webb Space Telescope of the innermost planet, TRAPPIST-1b, have so far found no evidence of a thick, extended atmosphere, suggesting it may be a bare rock.

Among the seven worlds, TRAPPIST-1e is often highlighted as the most promising candidate for habitability. In terms of its size (about 92% of Earth’s radius), mass (about 69% of Earth’s), and the amount of energy it receives from its star, it is the most similar planet to Earth in the system. It completes an orbit in just 6.1 days, a testament to the compact nature of its home system. The TRAPPIST-1 system offers an unparalleled natural laboratory for studying the formation and evolution of Earth-sized planets and for testing the limits of habitability.

The TRAPPIST-1 System at a Glance

(Note: Habitable zone planets are in bold.)

Proxima Centauri b: Our Closest Neighbor

Proxima Centauri b holds a special distinction: it is the closest known exoplanet to Earth, orbiting our nearest stellar neighbor, Proxima Centauri, just 4.2 light-years away. Discovered in 2016 through the radial velocity method, its detection was the result of observing the subtle wobble of its parent star. The planet is classified as a super-Earth, with a minimum mass very similar to our own, at about 1.07 times that of Earth. It orbits its red dwarf star at an extremely close distance of about 0.05 astronomical units (AU), completing a full “year” in just 11.2 Earth days.

This tight orbit places Proxima b squarely within its star’s habitable zone, where temperatures could theoretically allow for liquid water. However, its proximity to its star comes with a severe catch. Proxima Centauri is a flare star, a type of active red dwarf that is prone to violent outbursts. It frequently unleashes powerful flares of extreme ultraviolet and X-ray radiation, bathing its planet in energies hundreds of times greater than what Earth receives from the Sun. Over billions of years, this intense stellar weather could have easily stripped away any atmosphere the planet might have possessed, likely rendering its surface sterile and uninhabitable. Despite this harsh reality, its status as our closest exoplanetary neighbor makes Proxima b an irresistible target for study and a benchmark for understanding the challenges of habitability around red dwarf stars.

Kepler-452b: Earth’s Older, Bigger Cousin

Discovered by NASA‘s Kepler Space Telescope, Kepler-452b is a particularly compelling exoplanet because it orbits a star very much like our own Sun. Located approximately 1,400 light-years away, it was the first near-Earth-sized planet found in the habitable zone of a G-type star. This has earned it the nickname “Earth’s older, bigger cousin”.

Kepler-452b is a super-Earth, with a diameter about 60% larger than Earth’s. Its orbital period is 385 days, remarkably similar to our 365-day year. While its exact mass is not known, estimates place it at around five times the mass of Earth, which strongly suggests it has a rocky composition. A planet this massive would likely have a dense interior and could experience significant volcanic activity.

What makes Kepler-452b particularly intriguing is the age of its star. At 6 billion years old, its host star is 1.5 billion years older than our Sun and has begun to evolve, becoming slightly larger, hotter, and about 20% more luminous. As a result, Kepler-452b receives about 10% more energy from its star than Earth currently does from the Sun. This has led scientists to speculate that the planet might be undergoing a “runaway greenhouse effect,” where its oceans, if they ever existed, could be boiling away. In this sense, Kepler-452b may offer us a sobering glimpse into the distant future of our own planet, as our Sun continues its own stellar evolution over the next billion years.

Kepler-186f: A Pioneer Earth-Sized World

The discovery of Kepler-186f in 2014 was a landmark moment in the search for other Earths. Located about 580 light-years away, it was the first validated planet with a radius similar to Earth’s to be found orbiting within the habitable zone of another star. This discovery confirmed that Earth-sized worlds in potentially habitable regions are not just a theoretical possibility but a reality in our galaxy.

Kepler-186f is only slightly larger than our planet, with a radius about 1.17 times that of Earth, making it very likely to be a rocky world. It orbits its host star, a cool red dwarf, every 130 days. Its orbit places it near the outer edge of its star’s habitable zone, meaning it receives significantly less energy than Earth—only about one-third of the sunlight we get. To put this in perspective, the brightness of its star at high noon on Kepler-186f would be comparable to the brightness of our Sun about an hour before sunset.

Because it is on the colder edge of the habitable zone, Kepler-186f’s potential for hosting liquid water depends heavily on its atmosphere. To stay warm enough, it would need to have a substantial atmosphere capable of trapping heat through a strong greenhouse effect, likely requiring a much higher concentration of gases like carbon dioxide than is found on Earth. While many questions about its composition and atmosphere remain, Kepler-186f’s Earth-like size and location make it a pioneering discovery and a cornerstone in the catalog of potentially habitable worlds.

LHS 1140 b: The Water World Candidate

LHS 1140 b is a massive super-Earth located about 49 light-years away that has emerged as one of the most exciting candidates for habitability. It has a mass approximately 5.6 times that of Earth and a radius about 1.7 times larger, and it orbits its quiet, inactive red dwarf star every 25 days.

Initially, its properties suggested it was an incredibly dense rocky planet with a massive iron core. However, more recent and precise measurements of its mass and radius have revised its density downward. This new data points to a fascinating possibility: LHS 1140 b is likely not a pure rock-and-iron world, but a “water world” with a substantial portion of its mass—estimated to be between 9% and 19%—composed of water. This is a tremendous amount of water, potentially enough to cover the entire planet in a deep, global ocean.

The planet orbits in the cooler, outer region of its star’s habitable zone, meaning its surface could be quite cold without a significant atmosphere. However, recent observations with the James Webb Space Telescope have found tentative hints of a nitrogen-rich atmosphere, much like Earth’s. Models suggest that if the planet is tidally locked, it could maintain a large, liquid water ocean on its star-facing side, with temperatures warm enough for life, creating a so-called “eyeball” planet configuration. The combination of its large size, which helps it retain an atmosphere, its potentially water-rich composition, and its quiet host star makes LHS 1140 b a top-tier target for future atmospheric studies and the search for life.

Teegarden’s Star b: A High-Fidelity Earth Analog

Just 12.5 light-years from Earth lies Teegarden’s Star b, one of the most Earth-like exoplanets discovered to date. Found in 2019 using the radial velocity method, this planet orbits a very cool and old red dwarf star. What makes it so compelling is its remarkable similarity to our own world in key aspects. Its minimum mass is almost identical to Earth’s, at just over 1.05 times that of our planet, which strongly indicates it is a rocky world. It zips around its star in a very tight orbit, with a year lasting only 4.9 Earth days.

This close orbit places Teegarden’s Star b squarely within its star’s conservative habitable zone. In fact, it has one of the highest Earth Similarity Index (ESI) scores ever calculated, a measure that compares a planet’s physical properties to Earth’s. A crucial factor for its potential habitability is the nature of its host star. Teegarden’s Star is unusually quiet and inactive for a red dwarf, with far fewer violent flares than stars like Proxima Centauri. This tranquil environment means that Teegarden’s Star b has a much better chance of having retained its atmosphere over billions of years. Models suggest there is a high probability that the planet could host temperate surface conditions suitable for liquid water, making it one of the most promising and accessible targets in our cosmic neighborhood for the search for life.

Summary

The exploration of worlds beyond our solar system has fundamentally altered our perspective on the cosmos. What began as the detection of a few strange and unexpected planets has blossomed into a galactic census, revealing that planets are a common feature of the universe. The initial discoveries, biased by the limits of our technology, painted a picture of a galaxy filled with “hot Jupiters.” But as our tools have become more sensitive, a new picture has emerged: one in which smaller, rocky worlds like our own are abundant, with trillions potentially scattered across the Milky Way.

The quest for a second Earth has been refined into a scientific discipline, guided by a clear blueprint for habitability. This search focuses on terrestrial planets orbiting within their star’s “Goldilocks Zone,” where temperatures might allow for liquid water. But we now understand that location is just the beginning. True habitability is a complex interplay of factors, including the presence of a protective atmosphere and a magnetic field, the stability of the host star, and the planet’s own geological life.

We have entered a new era of characterization, moving beyond simply finding planets to analyzing them in detail. Transformative technologies, most notably the James Webb Space Telescope, are providing us with the unprecedented ability to read the chemical composition of alien atmospheres. This allows us to hunt for the first tentative clues of life—biosignatures like oxygen and methane—and to begin the careful, methodical process of distinguishing between biological processes and geological false positives.

While a true Earth twin remains elusive, the gallery of known exoplanets is filled with remarkable and diverse worlds that are pushing the boundaries of our knowledge. From the tantalizing but hazardous Proxima Centauri b next door, to the miniature solar system of TRAPPIST-1 with its trio of habitable-zone worlds, to the massive water world candidate LHS 1140 b, each discovery adds a new piece to the puzzle of planetary formation and evolution. We have not yet found another Earth, but in the process of searching, we have discovered that the galaxy is filled with an astonishing variety of planetary cousins, each one teaching us more about what makes a world habitable and, in turn, what makes our own planet so special.