Some of the most promising places to search for life beyond Earth are planets orbiting small, cool stars, where liquid water might exist on a rocky surface. Among the leading candidates are Proxima Centauri b, TRAPPIST-1 e, TOI-700 d, TOI-700 e, LHS 1140 b, and Kepler-186 f. Each has properties that make it scientifically interesting, but none has been confirmed to support life, and scientists do not yet know whether any has all the conditions necessary for habitability.
The challenge is that astronomers can measure some planetary properties, such as a planet’s size, mass, and orbit, more readily than others. A planet may receive an appropriate amount of energy from its star yet lack an atmosphere, have an inhospitable surface, or contain no liquid water. Determining which worlds might be habitable therefore requires more than identifying planets at the right distance from their stars.
What makes an exoplanet potentially habitable?
An exoplanet is a planet orbiting a star other than the Sun. Thousands have been discovered, ranging from small rocky worlds to enormous gas giants. Only a subset have characteristics that make them plausible candidates for environments where life as we know it could exist.
The most important starting point is the possibility of liquid water. On Earth, water provides a solvent in which essential chemical reactions occur, supports the transport of nutrients, and helps sustain the complex chemistry associated with life. Although scientists cannot rule out entirely different forms of life, liquid water is a practical starting point when evaluating potentially habitable worlds.
Astronomers often use the term habitable zone to describe the region around a star where a planet could maintain liquid water on its surface if it has suitable atmospheric conditions. The zone lies closer to a cool, dim star than to a hot, bright one. For a Sun-like star, Earth occupies the habitable zone. Around a small red dwarf, a planet might orbit its star in just days or weeks and still receive a comparable amount of energy.
However, being in the habitable zone does not guarantee a habitable surface. The calculation depends on assumptions about the planet’s atmosphere, reflectivity, and ability to redistribute heat. A thick greenhouse atmosphere could make a planet too hot, while a thin atmosphere could allow water to freeze or escape into space.
Habitability also depends on the availability of chemical elements, a stable energy supply, and conditions that persist long enough for life to emerge and evolve. A planet with liquid water might still lack the chemical environment needed for biology. Conversely, some potentially habitable environments could exist beneath ice or underground, even when surface conditions are hostile.
For these reasons, scientists distinguish between a planet that is potentially habitable and one that is demonstrably inhabited. The first category is based on measurable properties and plausible environmental models. The second would require convincing evidence of life.
Proxima Centauri b: A nearby world with major uncertainties
Proxima Centauri b is one of the most compelling targets in the search for potentially habitable planets because it orbits Proxima Centauri, the nearest star to the Sun. Its proximity makes it an especially attractive candidate for future observations, although distance alone does not make a planet habitable.
The planet is approximately Earth-sized in the broad sense of being a relatively low-mass world, and its orbit places it in the region where liquid water could be possible under suitable conditions. It receives a level of stellar energy that makes it worth investigating as a potential rocky planet with a temperate climate.
Yet Proxima Centauri is a red dwarf, a small star that can produce powerful flares and bursts of energetic radiation. Such activity may damage a planetary atmosphere, especially if the planet lacks an effective magnetic field or other protection. Strong stellar activity can also alter atmospheric chemistry and expose a surface to radiation that would be harmful to many forms of life.
Proxima Centauri b orbits close to its star, completing an orbit in a little over 11 Earth days. It may be tidally locked, meaning that one side always faces the star while the other remains in perpetual darkness. Tidal locking is not automatically fatal to habitability. An atmosphere or ocean could move heat from the dayside to the nightside, reducing extreme temperature differences. Whether that process could sustain a suitable climate depends on the planet’s atmospheric pressure, composition, and circulation.
The central uncertainty is whether Proxima Centauri b has retained an atmosphere and whether its surface conditions permit liquid water. Its mass and orbit make it interesting, but scientists do not yet have a confirmed picture of its atmosphere, climate, or surface.
Its proximity gives it a special advantage over more distant candidates: future instruments may have better opportunities to study its environment. For now, Proxima Centauri b is a high-priority target rather than an established habitable world.
TRAPPIST-1 e: A rocky planet in a compact planetary system
The TRAPPIST-1 system contains seven known planets orbiting an ultracool red dwarf. Several receive stellar energy levels that make them interesting for habitability studies, but TRAPPIST-1 e stands out as one of the system’s strongest candidates for Earth-like surface conditions.
TRAPPIST-1 e is a relatively small, rocky planet that receives less stellar energy than Earth. Its position in the system places it within the range where models allow liquid water on the surface, depending on atmospheric composition and other climate factors. Its size and density are also consistent with a rocky composition rather than a gas-rich planet.
The system’s compact arrangement makes it scientifically valuable. Its planets transit in front of their star from our perspective, allowing astronomers to measure their sizes and investigate their atmospheres under favorable observational circumstances. The gravitational interactions among the planets also help researchers estimate their masses and densities.
The main concern is the star itself. Red dwarfs can remain active for long periods, exposing nearby planets to flares and high-energy radiation. During the early stages of a red dwarf’s life, its luminosity can also create conditions that may cause a planet to lose substantial amounts of water. A world that initially possessed oceans might therefore evolve into a dry, airless planet, although the outcome depends on its starting conditions and subsequent evolution.
TRAPPIST-1 e could be tidally locked, like other planets orbiting close to small stars. A sufficiently substantial atmosphere might distribute heat around the planet, but a weak atmosphere could produce much more extreme conditions. The planet’s actual atmospheric pressure, composition, and surface environment remain uncertain.
TRAPPIST-1 e is one of the most interesting candidates because its size, likely rocky nature, and stellar energy level fit several important criteria. Its habitability ultimately depends on questions that orbital measurements alone cannot answer, especially whether it has an atmosphere and whether that atmosphere can support stable surface water.
LHS 1140 b: A potentially water-rich world
LHS 1140 b is another important candidate around a red dwarf. It is larger and more massive than Earth, placing it in the category of super-Earths. That term describes planets more massive than Earth but does not specify whether they resemble Earth in their surface conditions or composition.
Measurements of LHS 1140 b suggest a dense world that may contain a substantial rocky interior. Depending on its internal structure, it could also retain a significant amount of water. This possibility makes it particularly interesting because water-rich planets may provide environments quite different from Earth’s continents and oceans.
However, a planet’s density does not uniquely reveal its surface. Different combinations of rock, iron, water, and atmosphere can produce similar measured masses and radii. A world with abundant water might have a global ocean, a deep high-pressure ice layer, or a thick atmosphere. These possibilities have different implications for habitability.
A global ocean could provide a large environment for life, but deep oceans may also separate surface water from the rocky interior. On Earth, interactions between liquid water and rock help drive chemical reactions that provide energy and cycle essential elements. If a thick layer of high-pressure ice prevents similar interactions, the planet’s chemistry could differ substantially from that of Earth.
LHS 1140 b’s orbit lies in a region where a suitable atmosphere could permit relatively temperate conditions. Its star is also an important part of the assessment: stellar activity and the planet’s atmospheric history influence whether it could retain water and maintain a stable climate.
The planet’s size makes it a less direct Earth analogue than a smaller rocky world, but it may offer a different kind of opportunity. Rather than asking only whether another planet has continents and familiar oceans, researchers can investigate whether water-rich worlds could support life in environments unlike those on Earth.
TOI-700 d and TOI-700 e: Two promising planets around a small star
The TOI-700 system contains multiple small planets, including TOI-700 d and TOI-700 e. Both are interesting because they are relatively close to Earth in size and orbit a cool red dwarf. Their different orbital positions offer researchers an opportunity to examine how relatively small planets may develop under different levels of stellar heating.
TOI-700 d lies within the star’s habitable zone, where suitable atmospheric conditions could allow liquid water to exist on the surface. Its size is consistent with a rocky planet, making it a useful target for comparisons with Earth.
TOI-700 e is also a small planet and orbits closer to its star than TOI-700 d. Its position places it near the inner part of the system’s habitable region, depending on the climate assumptions used. That makes its potential surface conditions particularly sensitive to atmospheric composition and greenhouse warming.
The distinction between the two worlds matters. A planet closer to its star receives more energy, which can increase surface temperatures and potentially push a climate toward a runaway greenhouse state. In such a state, water vapor contributes to further warming, potentially causing extensive water loss. A planet farther out receives less energy and may instead struggle to maintain liquid surface water if its atmosphere is insufficient to trap heat.
These outcomes are not determined by distance alone. Carbon dioxide, water vapor, clouds, atmospheric pressure, and the planet’s reflectivity can all influence its climate. A world near the inner edge of a habitable zone could be temperate with the right atmospheric conditions, while another farther out could remain frozen.
The TOI-700 planets are especially useful for studying the diversity of small planets around red dwarfs. They are plausible candidates for further investigation, but their locations within or near a habitable zone do not establish that either has oceans, a stable atmosphere, or life.
Kepler-186 f: An early example of a potentially habitable rocky world
Kepler-186 f was one of the first Earth-size planets identified in the habitable zone of another star. It orbits a red dwarf and receives less energy from its star than Earth receives from the Sun.
Its relatively small size makes it an important target in the broader search for rocky planets. Unlike much larger planets, it is a plausible candidate for a solid surface rather than a thick envelope of hydrogen and helium. Its orbital position also makes it possible, under suitable atmospheric conditions, for liquid water to exist.
The main difficulty is that the planet receives relatively little stellar energy. If its atmosphere is thin or lacks enough greenhouse gases, surface temperatures could be too low for liquid water to persist. A more substantial atmosphere with effective heat-trapping gases could produce a warmer climate, although the actual outcome depends on atmospheric composition and circulation.
Kepler-186 f is much farther away than Proxima Centauri b and the TRAPPIST-1 planets, which limits the opportunities for detailed atmospheric investigation. Its discovery nevertheless helped establish that small planets in potentially temperate orbits are not confined to our own planetary neighborhood.
The planet remains a useful example of the difference between identifying a potentially habitable orbit and establishing a habitable environment. Its size and position make it worth studying, but the properties that matter most for surface habitability remain poorly constrained.
Why red dwarf stars are both promising and problematic
Many of the leading candidates orbit red dwarfs because these stars are common and relatively faint. Their habitable zones lie much closer to the star than Earth’s orbit lies to the Sun. This makes planets in those regions easier to detect through certain techniques, including the transit method, which measures the small decrease in starlight when a planet passes in front of its star.
Red dwarfs offer several advantages for exoplanet research. Their low mass makes the gravitational effect of an orbiting planet comparatively easier to measure. Their small size also makes a transiting planet block a larger fraction of the star’s light than the same planet would block around a larger star. These effects help astronomers identify small planets and estimate their properties.
But the same proximity that makes these planets detectable creates potential environmental challenges. Red dwarfs can emit flares and energetic ultraviolet and X-ray radiation. Over time, these processes may erode atmospheres or change their chemistry. Planets close to their stars may also experience tidal forces that gradually synchronize their rotation with their orbital motion.
None of these factors automatically rules out habitability. A planet may begin with a substantial atmosphere, replenish gases through volcanic activity, or retain water beneath an atmosphere that limits atmospheric escape. Tidal locking can also be compatible with a temperate climate if winds and oceans redistribute enough heat.
The early history of a star is particularly important. Young red dwarfs can be brighter relative to their later, stable states and may expose nearby planets to prolonged heating. A planet that now receives a moderate amount of stellar energy could have experienced a much harsher climate earlier in its history. Whether it retained its water depends on its initial inventory, atmospheric properties, and the evolution of the star.
For this reason, habitability assessments must consider an entire planetary history rather than just the conditions observed today.
The atmosphere may matter more than the planet’s distance
A planet’s atmosphere determines much of what happens to the energy it receives from its star. Sunlight can warm the surface, while greenhouse gases absorb and re-emit some of the infrared radiation the surface releases. This process helps maintain temperatures suitable for liquid water on Earth.
If a planet has too little atmosphere, heat may escape rapidly, and liquid water may be unstable across much of the surface. If the atmosphere is too thick or contains abundant greenhouse gases, surface temperatures may become dangerously high. The balance depends on the type and abundance of gases, atmospheric pressure, cloud cover, and circulation.
Water also affects climate. Water vapor is a powerful greenhouse gas, and evaporation and condensation move heat through the atmosphere. Clouds can cool a planet by reflecting incoming sunlight or warm it by trapping outgoing infrared radiation. The net effect depends on cloud properties, altitude, and the wavelengths of radiation involved.
Carbon dioxide can help warm a cold planet, while atmospheric pressure influences whether water can remain liquid at the surface. Other gases may contribute to greenhouse warming or alter atmospheric chemistry. The same stellar energy level can therefore produce very different climates on planets with different atmospheres.
Atmospheric loss is another major concern. High-energy radiation can heat the upper atmosphere and allow gas to escape into space. Over long periods, a planet may lose much of its atmosphere and water, particularly if it is close to an active star. Magnetic fields may influence how stellar particles interact with the atmosphere, but their protective role is complex and depends on several factors. A magnetic field alone does not guarantee atmospheric survival.
These mechanisms explain why a planet’s radius, mass, and orbit cannot provide a complete habitability assessment. They identify promising targets, but the atmosphere determines whether those targets may have suitable surface environments.
Could habitable environments exist beneath the surface?
Surface liquid water is a useful criterion, but it is not the only possible environment for life. In our own solar system, icy moons such as Europa and Enceladus provide reasons to consider subsurface oceans as potentially interesting environments. Their existence does not establish that life is present, but it demonstrates that liquid water need not always be exposed to sunlight.
A planet with a frozen surface could potentially contain liquid water beneath an insulating ice layer if internal heat and pressure permit it. That heat could come from radioactive decay inside the planet or from tidal forces, depending on the planet’s structure and orbital history.
Subsurface habitability has important limitations. Without sunlight, ecosystems would need alternative energy sources. On Earth, some microorganisms obtain energy through chemical reactions involving compounds released by rocks, water, and geological processes. Similar processes could, in principle, support life in other worlds if the necessary ingredients and conditions exist.
However, not every water-rich planet would be suitable. Life requires more than water alone. It also needs accessible chemical elements and a usable source of energy. If an ocean is isolated from the rocky interior or lacks the right chemical gradients, it might be much less favorable for life than its water content suggests.
These possibilities complicate comparisons among exoplanets. A world with a frozen surface may be a poor candidate for Earth-like surface life but still have an interior environment worth investigating. Conversely, a planet with a temperate surface may be uninhabitable if its atmosphere, chemistry, or radiation environment is unsuitable.
How scientists evaluate habitability from a distance
Astronomers cannot usually visit exoplanets or directly inspect their surfaces. Instead, they combine several kinds of observations to build models of their physical environments.
The transit method reveals a planet’s size when the star’s radius is known. By measuring how a star moves in response to an orbiting planet, researchers can estimate the planet’s mass. Combining mass and radius gives an average density, which helps distinguish likely rocky planets from worlds with substantial gas or water layers.
These measurements are informative but not definitive. A planet’s average density does not uniquely determine its composition. A dense planet may contain a large iron core, while a lower-density planet may have a thicker atmosphere or more water. Models of internal structure can narrow the possibilities, but different compositions may remain consistent with the same measurements.
Atmospheric studies provide another layer of evidence. When a planet transits its star, some starlight passes through the atmosphere, if one is present. Molecules absorb light at particular wavelengths, potentially leaving detectable patterns in the observed spectrum. Researchers can use these patterns to investigate atmospheric gases and properties.
Direct imaging offers a different approach by separating a planet’s light from the much brighter light of its star. This technique is difficult for small, Earth-like planets because they are faint and lie close to their stars in the sky. Where observations are possible, measurements of reflected or emitted light can help constrain a planet’s atmosphere, temperature, and cloud properties.
Neither technique makes detecting life straightforward. Many gases that might be associated with biology can also be produced by nonbiological processes. Oxygen, for example, can accumulate through certain chemical processes that do not require living organisms. A convincing assessment would need to consider the planet’s full environment and whether alternative explanations could account for the observations.
Evidence for habitability and evidence for life are therefore different scientific goals. The first involves determining whether a planet could maintain suitable environments. The second requires identifying signs that are difficult to explain without biology.
Which candidates deserve the most attention?
No single ranking can establish which exoplanet is the most habitable, because the available evidence is incomplete and different planets offer different scientific advantages. Still, several candidates stand out for specific reasons.
TRAPPIST-1 e is compelling because it is a relatively small, rocky planet in a well-studied system containing several other terrestrial-size worlds. Comparing the planets may help scientists understand how atmospheric survival and climate depend on orbital position and planetary properties.
Proxima Centauri b is especially attractive because of its proximity to Earth. That advantage makes it an important target for future studies, even though its star’s activity raises serious questions about atmospheric retention and surface conditions.
LHS 1140 b offers the possibility of a water-rich environment, making it useful for investigating whether planets larger than Earth could support habitable conditions. Its internal structure and potential atmosphere, however, remain important uncertainties.
TOI-700 d and TOI-700 e are valuable because they are relatively small planets around the same star but receive different levels of stellar energy. Studying them together could help researchers test how climate and atmospheric evolution vary among planets in neighboring orbits.
Kepler-186 f remains significant as an early example of an Earth-size planet in a potentially temperate orbit. Its distance makes detailed investigation difficult, but it illustrates how the search for potentially habitable planets extends far beyond the nearest stars.
These worlds are not interchangeable. A nearby planet may be easier to investigate but face harsher stellar conditions. A larger planet may retain an atmosphere more effectively yet have a surface very different from Earth’s. A world with abundant water may lack accessible nutrients or a useful energy source. Each candidate must be evaluated according to its own characteristics.
What would it take to identify a truly habitable exoplanet?
A stronger habitability assessment would require more than confirmation that a planet is rocky and receives moderate stellar energy. Scientists would want evidence about its atmosphere, surface temperature, water availability, and long-term environmental stability.
Detecting atmospheric gases would be a major step. Measurements that constrain pressure, composition, and temperature could help determine whether surface liquid water is physically plausible. Additional observations might reveal whether the planet has clouds, whether its atmosphere is being lost, and how its climate responds to stellar energy.
Understanding the host star is equally important. Researchers need to characterize its brightness, age, flaring behavior, and history of energetic radiation. Those properties help establish how much energy the planet has received over time and whether its atmosphere may have survived.
Even with a favorable atmosphere, scientists would still need to investigate whether the planet has the chemical and energetic conditions associated with life. No observation of a potentially temperate climate, by itself, would demonstrate that organisms exist there.
The most convincing evidence for life would involve a combination of observations that is difficult to explain through known nonbiological processes. Such a finding would require careful scrutiny, because remote measurements are limited and planetary chemistry can produce unexpected results.
For now, the best candidates for habitability are worlds that combine a plausible rocky composition, a potentially suitable energy supply, and a realistic prospect of further atmospheric investigation. Proxima Centauri b, TRAPPIST-1 e, LHS 1140 b, TOI-700 d, TOI-700 e, and Kepler-186 f all merit attention, but none can yet be described as a second Earth.
The central question is not simply which planets occupy the right orbits. It is which ones have maintained the atmospheres, water, chemistry, and environmental stability that could allow life to exist. Answering that question will require increasingly precise observations and a willingness to distinguish what astronomers have measured from what they can only infer.
