Scientists determine the “habitability” of exoplanets based on factors like size, mass, temperature, and location within a star’s habitable zone (where liquid water could exist)

. Using the Earth Similarity Index (ESI) as a guide, which ranks planets on a scale of 0 to 1 (with Earth at 1.0), here are some of the most promising potentially habitable exoplanets discovered so far. 

Note that we currently lack the technology to confirm the atmospheric composition or presence of liquid water on these planets, and habitability is not guaranteed. 

Top 10 Potentially Habitable Exoplanets

• Teegarden b: With an ESI of 0.97, this exoplanet is one of two found orbiting the quiet red dwarf star Teegarden’s Star, just 12 light-years away.
• TRAPPIST-1e: Part of a seven-planet system 40 light-years away, TRAPPIST-1ehas an ESI of 0.95 and is considered the most likely in its system to support life. It is similar in size to Earth and likely a rocky planet.
• Gliese 667 Cc: Located 23.6 light-years away, this super-Earth has an ESI of 0.90. It orbits a red dwarf star in its habitable zone, though it may be tidally locked (one side always facing its star).
• Kepler-438b: This planet, which has an ESI of 0.88, is only 12% larger than Earth and orbits a red dwarf star about 640 light-years away. Its star is active, however, which may make the planet less habitable than initially thought.
• Proxima Centauri b: At only 4.24 light-years away, this is the closest known exoplanet to Earth. It has an ESI of 0.87 and orbits a red dwarf star, but it is exposed to extreme ultraviolet radiation due to its close orbit.
• Ross 128 b: Orbiting a quiet red dwarf star 11 light-years away, this planet has an ESI of 0.86. The star’s inactivity makes it a promising candidate for a stable environment.
• Kepler-442b: This rocky planet, located about 1,200 light-years away, has an ESI of 0.84 and is 33% larger than Earth. One study found it might receive enough light to sustain a large biosphere.
• Kepler-452b: Often referred to as Earth’s “older cousin,” this planet is the first near-Earth-sized world found in the habitable zone of a Sun-like star. It has an ESI of 0.84 and takes 385 days to orbit its star.
• Kepler-62e: One of two potentially habitable planets discovered in the Kepler-62 system, this planet has an ESI of 0.83. It is about 60% larger than Earth and likely an ocean planet.
• Gliese 832 c: Located 16 light-years away, this super-Earth has an ESI of 0.81. It receives about the same amount of stellar flux as Earth, but its large mass might give it a dense atmosphere, resembling Venus. 

In the future can we know the presence of atmosphere and liquid water on this habitable planets

Yes, in the future, astronomers will be able to learn much more about the atmospheres of habitable planets and potentially detect the presence of liquid water using advanced and upcoming telescopes. 

Current Capabilities (James Webb Space Telescope)

The James Webb Space Telescope (JWST) is currently leading this effort. It uses a technique called transmission spectroscopy to analyze the light from a star that passes through an exoplanet’s atmosphere when it transits (crosses) its star. This process allows the detection of specific molecules, or “chemical fingerprints,” in the atmosphere, such as water vapor, carbon dioxide, and methane. 

JWST has already provided detailed atmospheric data for gas giants and super-Earths like K2-18b (where it found carbon-bearing molecules and hinted at a “Hycean” world), and has determined that some rocky planets, like TRAPPIST-1b, likely have no significant atmosphere. However, analyzing Earth-sized planets around Sun-like stars is still at the limit of its capabilities. 

Future Telescopes and Methods

The next generation of ground- and space-based observatories will provide even greater capabilities:

• Extremely Large Telescopes (ELTs):Ground-based telescopes currently under development, such as the European Extremely Large Telescope (ELT), will use high-resolution spectroscopy to detect molecules like water vapor, carbon dioxide, and oxygen in the atmospheres of habitable planets.
• Habitable Worlds Observatory (HWO):NASA is in the early stages of planning a purpose-built space telescope for launch potentially in the 2040s. This observatory will be designed to directly image Earth-sized planets around Sun-like stars and search for chemical traces caused by life (biosignatures). 

Detecting Liquid Water

Confirming the presence of liquid water on an exoplanet’s surface remains a challenge with current technology because the signals are very faint. Future missions will employ a few methods: 

• Carbon Dioxide Signature: One promising method involves looking for a depleted amount of carbon dioxide in a planet’s atmosphere compared to other planets in the system. On Earth, oceans and life trap CO2, so a low atmospheric concentration could be a strong indicator of liquid water.
• Specular Reflection (“Glint”): Future telescopes might be sensitive enough to detect the mirror-like reflection (glint) of starlight off large bodies of liquid water, an optical signature unique to liquids.
• Biosignatures: The ultimate goal is to detect biosignatures—gases like oxygen and ozone that, in combination with low CO2, would strongly suggest the presence of a global biosphere that processes vast amounts of carbon, which in turn indicates liquid water. 

Through these advancements, scientists are on a clear path to answering one of humanity’s most profound questions: Are we alone in the universe?

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