Potentially habitable spheres

MHK
12 min readSep 1, 2023

Every year, scientists make remarkable discoveries in distant star systems, expanding our knowledge of exoplanets and their potential to host life. While we have yet to find an exact twin of Earth, the search continues, and there’s a sense of optimism that a second Earth may one day emerge as a prime candidate in the hunt for extraterrestrial life. Recent developments in exoplanet research have introduced concepts like the Earth Similarity Index (ESI), which measures how similar exoplanets are to our own, based on various factors such as surface temperature, distance from their star, density, and size. In this article, we’ll introduce you to the intriguing exoplanets that rank high on the ESI scale, offering promising possibilities in our quest for another Earth.

Kepler 442b — A Rocky Super-Earth

Kepler 442b

Kepler 442b is an exoplanet that stands out from the crowd. Unlike most other candidates on this list, it doesn’t orbit a red dwarf star. Instead, it gracefully orbits a Type K orange dwarf star, a celestial rarity that presents unique challenges for astronomers due to glare and observational difficulties. However, orange dwarfs are like the goldilocks of stars — active in their youth but settling down into long-lasting stability, with lifespans far surpassing our sun’s. Kepler 442b is over a thousand light-years away, making it a distant neighbor. This super-Earth boasts about 2.3 times the mass of our planet, yet its radius is only 1.34 times that of Earth, suggesting it’s likely a rocky terrestrial world.

While Kepler 442b’s gravity would be approximately 30% stronger than Earth’s, its position in its star system resembles Mercury’s location in our solar system. Remarkably, it doesn’t suffer from tidal locking, allowing for a potential day-night cycle, albeit one that might stretch over months. This exoplanet possibly lacks significant axial tilt, resulting in a lack of distinct seasons.

What makes Kepler 442b truly intriguing is its candidacy for “superhabitability,” an exoplanet even more suitable for life than Earth. Unfortunately, its vast distance poses challenges for further study, with no planned telescopes capable of characterizing its atmosphere in the near future. However, advancements in radio telescope technology hold promise for SETI signal searches from this intriguing world.

Ross 128 B — A Quiet Red Dwarf Neighbor

Ross 128 b — Wikipedia

Ross 128b, situated just 11 light-years away from us, orbits a tranquil red dwarf star — a critical factor for potential habitability. With approximately 35% more mass than Earth and a closer orbit to its star, Ross 128b receives significantly more energy. This surplus energy could facilitate liquid water if it has the right atmosphere. The discovery of Ross 128b highlights a different approach to finding exoplanets. While most are detected via transits (when they pass in front of their stars, causing light dips), not all star systems align with our line of sight. Ross 128b was found using the “radial velocity” method, which detects the gravitational wobble it induces in its parent star. This method promises more exoplanet discoveries in the future.

Ross 128b’s host star closely resembles our Sun chemically, suggesting a similar planetary composition. However, it orbits on the inner edge of its star’s habitable zone, potentially raising questions about its atmospheric composition. Despite these uncertainties, Ross 128b is one of the most Earth-like exoplanets we’ve discovered to date. Unfortunately, it doesn’t transit its star, making it challenging to peer into its atmosphere. Yet, the James Webb Space Telescope and forthcoming giant ground-based telescopes offer hope for future insights into Ross 128b.

GJ 1002 b — The Promising Neighbour

GJ 1002b

GJ 1002b, residing approximately 16 light-years away within its star’s habitable zone, brings us closer to the concept of a “sister Earth.” With a radius similar to Earth’s, about 1.03, it orbits its star swiftly, completing a year in just 10.3 days. While we have limited information about this exoplanet for now, its star offers intriguing possibilities. Red dwarf stars are usually active during their youth, emitting flares that could render their planets inhospitable by stripping away atmospheres and bathing them in radiation. However, GJ 1002b’s star is remarkably calm, making it a potential abode for surface life.

There are two intriguing wildcards with red dwarfs: their age and flare direction. Older red dwarfs might be less active, offering more stable environments for life. Additionally, recent research suggests that red dwarfs’ flares may occur near their poles, leaving their planetary systems relatively unharmed. It raises an intriguing question: If red dwarfs, the most abundant stars, and orange dwarfs, the second most abundant, could support life, why do we orbit the relatively scarce G-type dwarf? The answer remains a mystery, but as our knowledge of exoplanets continues to grow, so does the possibility of finding another Earth among the stars.

K2–72e — A Rocky Super-Earth with a Workaround

K2–72e

K2–72e is a confirmed rocky exoplanet that made its debut during the Kepler spacecraft’s second light mission. The primary mission had concluded, and the spacecraft was experiencing technical difficulties, but NASA’s ingenuity prevailed, allowing us to gather more data before Kepler fell silent. Located a distant 217 light-years away, K2–72e is a small super-Earth, just over twice the mass of our home planet. Its host star, once again, is a red dwarf. You might wonder why so many entries on this list revolve around red dwarfs. It’s not because planets don’t orbit other types of stars; it’s because red dwarfs make it easier to detect transits due to their dimness. They represent the low-hanging fruit of exoplanet discovery. Moreover, red dwarfs dominate the majority of stars in the universe. Despite its red dwarf orbit, K2–72e hugs its star closely, receiving about 11% more energy than Earth, a factor known as insolation. This added warmth could pose challenges to habitability, especially when it comes to the potential for liquid water. However, the outcome depends heavily on the planet’s mysterious atmospheric composition. K2–72e likely experiences tidal locking, presenting only one face to its star. The consequences of this phenomenon remain a subject of ongoing study, but it could involve heat circulation that might support life on the dark side of the planet. Beyond these intriguing speculations, we possess scant information about K2–72e, leaving much to the imagination.

Proxima B — Our Closest Exoplanetary Neighbor

Proxima B

Proxima Centauri B stands out because it’s our most likely candidate for close-up photographs in the not-so-distant future, or perhaps even human colonization. It orbits Proxima Centauri, the nearest star to our Sun, a mere 4.2 light-years away. While the system may host other planets, including two disputed candidates, Proxima B is the highlight due to its proximity. Proxima Centauri’s status as a flare star, prone to energetic outbursts, could have stripped these planets of their atmospheres long ago. However, being so close allows us to investigate. Direct observation is one approach, but another exciting possibility is Breakthrough Starshot, which could send tiny light sail-driven probes to Proxima Centauri. In just a few decades, these probes could provide valuable insights into this enigmatic system. In the centuries to come, we may know more about Proxima Centauri than any other star system besides our own, perhaps even contemplating it as a potential second home.

LP 890–9 — A Cool Red Dwarf with Promising Super-Earths

LP 890–9

Situated approximately 105 light-years away, LP 890–9 is a high proper motion red dwarf star, possessing roughly 12% of the Sun’s mass. This unusually cool red dwarf is home to at least two exoplanets, both recent discoveries from 2022, thanks to the TESS spacecraft. Both are likely rocky terrestrial worlds, but they stand out as super-Earths, slightly larger than our own planet.

Among these planets, LP 890–9c holds particular interest. It resides within the star’s habitable zone, making it a candidate for future study by the James Webb Space Telescope. However, it teeters on the inner edge of the habitable zone and is likely tidally locked, meaning it always presents one face to its star. This unique situation creates a wide range of possibilities. Depending on LP 890–9c’s specific conditions, it could resemble Earth or Venus. Fortunately, the James Webb Space Telescope should help us distinguish between these scenarios. Challenges include the planet’s proximity to its star, which could expose it to harmful radiation. Still, there are intriguing hypotheses about how microbial life might harness ultraviolet radiation for energy. While LP 890–9c’s super-Earth status raises questions, it ranks high on the Earth Similarity Index at 0.89, making it a promising candidate in our quest to discover Earth’s cosmic neighbors.

Trappist 1d — A Window to Alien Worlds

TRAPPIST-1d — Wikipedia

The Trappist-1 system is undoubtedly one of the most famous and intriguing exoplanetary systems known to humanity. It’s a star system featuring at least 7 exoplanets, three of which fall within the realm of potential habitability, with one more in contention. Recent observations from the James Webb Space Telescope have begun to shed light on these distant worlds. Trappist 1b, the innermost member of this planetary family, has been found to lack any atmosphere, likely presenting a surface reminiscent of a darker, larger Mercury, adorned with ancient craters. Trappist 1c follows a similar pattern, though with some uncertainty regarding its atmospheric status, potentially oxygen-rich but not due to biological activity. The spotlight truly shines on Trappist 1d, residing on the inner edge of the habitable zone. While its atmosphere remains unconfirmed, if present, it marks the innermost planet in the system where we should search for biosignatures. The possibilities are intriguing: It could resemble Venus, or it might be a tidally locked world with habitable twilight regions. Other candidates in the system, while interesting, rank lower due to various reasons, such as venturing into the realm of icy worlds as you move further away from the habitable zone.

Kepler 1649c — The Rediscovered Earth-Like World

Kepler-1649c — Wikipedia

Kepler 1649c, a rocky exoplanet orbiting the red dwarf Kepler 1649c, takes the third spot on our list. With a mass roughly 1.2 times that of Earth, this tidally locked planet nearly slipped through the cracks of discovery. The algorithm initially flagged it as a false positive, a growing issue in the era of vast astronomical datasets. This discovery underscores the challenges astronomers face in processing immense volumes of data, a dilemma especially relevant to the Search for Extraterrestrial Intelligence (SETI). It’s possible that signals of alien origin remain hidden within datasets, awaiting rediscovery through novel analysis methods.

Kepler 1649c is of particular interest because it receives about 75% of the solar radiation that Earth receives from the Sun. Although its host star might be a flare star, an observation that hasn’t occurred yet, this exoplanet could potentially possess an atmosphere conducive to liquid water. Interestingly, this system also contains a Venus analog, a planet similar in size and existing under similar conditions. Whether it’s a Venus-like hot house or a more temperate world remains unknown. Additionally, the presence of this Venus analog might affect Kepler 1649c’s orbit, raising questions about its true Earth-likeness.

TOI 700d — A Promising Super-Earth

An artist’s impression of TOI 700d

The TOI 700 star system, located just over 100 light-years away from Earth, boasts multiple planets, with the outermost, TOI 700d, taking the spotlight. This super-Earth, roughly 1.7 times the mass of our planet, falls within the potential habitability range, with a mass limit generally considered to be two Earth masses or lower. TOI 700d orbits a red dwarf star, representing about 40% of the Sun’s mass. Despite this star’s lower mass, the planet receives a substantial 86% of Earth’s solar radiation. The outcome depends heavily on its atmosphere; it could be relatively cold or potentially subject to a runaway greenhouse effect, akin to Venus. The star’s apparent low activity, combined with similar solar wind exposure to Earth, suggests that TOI 700d could indeed have an atmosphere, making it an enticing candidate for further exploration. On the Earth Similarity Index, TOI 700d ranks second, with a score of 0.93, indicating remarkable potential for Earth-like conditions and the possibility of hosting life.

Teegarden’s Star — The Cosmic Surprise

Teegarden’s Star

Teegarden’s Star serves as a reminder of the vast cosmic frontier we have yet to explore fully. Our search for exoplanets demands scrutinizing millions of red and orange dwarf star systems, many of which remain undiscovered, even in our cosmic backyard. Teegarden’s Star, a diminutive red dwarf, remained hidden from our knowledge until 2003 when it was revealed to be just 12.5 light-years away from Earth. This small star, only about 9% the mass of the Sun, teeters on the edge of being a star rather than a brown dwarf. What elevates this system to the pinnacle of the Earth Similarity Index is not just one, but two potential Earth-like worlds nestled within the star’s habitable zone. Teegarden’s Star B and C, with B being the more promising, orbit within the star’s optimistic habitable zone. B could be likened to a place between Earth and Venus in our solar system but much closer in. It currently holds the record as the most Earth-like exoplanet we’ve encountered, scoring a remarkable 0.95 on the scale. On the other hand, Teegarden’s Star C resembles Mars, residing near the outer edge of the habitable zone. While less optimal, if it boasts a thick, heat-retaining atmosphere, it too could favor life, albeit with a score of 0.68 on the scale, which is still promising. What sets this system apart is Teegarden’s Star’s striking inactivity for a red dwarf, despite the potential for these stars to be turbulent flare stars. This serenity enhances the prospects for these planets to retain their atmospheres and support liquid water on their surfaces, especially for Teegarden’s Star B, which could have up to a 60% chance of hosting liquid water. However, there remains considerable debate about the nature of these planets. They are likely tidally locked, which could impact their atmospheric retention. Moreover, it’s possible that these worlds are entirely oceanic with no land, making them vastly different from Earth, despite their high similarity index.

Conclusion

In the vast cosmic theater, we find ourselves on an exhilarating quest to uncover Earth’s cosmic neighbors. The journey takes us to distant stars, many of which were hidden until just a few years ago and reveals a captivating array of exoplanetary worlds. From the stunning Trappist-1 system, with its tantalizing trio of potentially habitable planets, to the rediscovered Kepler 1649c, a rocky world tucked away in a vast dataset, our understanding of distant exoplanets grows with each discovery. We venture further to TOI 700d, a promising super-Earth that tantalizingly flirts with the boundaries of habitability, and then to Teegarden’s Star, an unassuming red dwarf hiding a cosmic surprise. Two of its planets, Teegarden’s Star B and C, emerge as Earth-like contenders, inviting us to imagine their potential as havens for life. As we explore these distant realms, we’re reminded of the mysteries that await us. Are these planets truly Earth twins, or do they harbor their own unique ecosystems? Can they support life, and if so, what forms might it take? Our cosmic odyssey continues, fueled by technology, curiosity, and the desire to answer these profound questions. With each discovery, we draw closer to unveiling the secrets of distant worlds, expanding our horizons, and pondering the possibilities of interstellar real estate. In the grand tapestry of the universe, the search for exoplanets remains one of humanity’s most awe-inspiring adventures, offering glimpses of the remarkable diversity of celestial neighbors and the tantalizing prospect of finding life beyond our pale blue dot.

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