The New Promised Lands: Top Candidates for Habitability

Comparison with the habitable planets of the Kepler 11 system — *Visualization of potentially habitable exoplanets around Kepler-11. Image source: NASA (new window)*

What is a habitable planet?

The concept of a habitable planet refers to a celestial body whose physical and chemical conditions allow the emergence, development, and persistence of life as we know it. These conditions mainly include the presence of liquid water, a moderate temperature, a protective atmosphere, and energy sources. In astrophysics, this zone is often called the "habitable zone" (or Goldilocks zone) around a star, that is, the region where a planet can theoretically maintain liquid water on its surface for billions of years.

Physical and chemical criteria for habitability

For a planet to be potentially habitable, several criteria must be met:

Distance to the star: the planet must be in the habitable zone, neither too close (extreme greenhouse effect) nor too far (frozen water).
Radius and mass: a planet that is too small cannot retain an atmosphere; if too massive, it can evolve into a gas giant.
Atmosphere: it must contain gases allowing a moderate greenhouse effect and protect against UV radiation and solar particles.
Presence of water: essential for carbon chemistry, the basis of known life.
Magnetic field: protects the atmosphere from stellar winds and cosmic rays.

Examples of potentially habitable planets

The Kepler, TESS, and now James-Webb missions have identified several rocky exoplanets in the habitable zone of their star. Some of them, such as Proxima b, TRAPPIST-1e, Kepler-442b, or LHS 1140b, are considered serious candidates. However, these observations do not yet allow the direct detection of biosignatures.

Comparison of the main candidates

Comparison of some potentially habitable exoplanets
Exoplanet	Mass (Earth = 1)	Radius (Earth = 1)	Distance (LY)	Habitable zone	Type of star
Proxima Centauri b	1.27	~1.1	4.24	Yes	Red dwarf M5.5
TRAPPIST-1e	0.77	0.92	39.6	Yes	Ultra-cool dwarf
Kepler-442b	2.36	1.34	1,206	Yes	Orange dwarf K
LHS 1140b	6.6	1.7	41	Yes	Red dwarf M4.5
Kepler-22b	~8.7 (estimate)	2.4	620	Yes	Yellow dwarf G5
Kepler-62f	~2.8 (estimate)	1.4	1,200	Yes	K2 dwarf
Kepler-186f	~1.4 (estimate)	1.1	490	Yes	M1 dwarf
Kepler-11f	~2.3	2.6	2,000 (approx.)	Inner limit	Yellow dwarf G
Kepler-11g	~8.0	3.3	2,000 (approx.)	Outer limit	Yellow dwarf G

Life elsewhere: a probability or an exception?

Even if the physical conditions of some exoplanets seem promising, the chemical complexity leading to life remains an open question. The emergence of life depends on many stochastic processes (effect of chance): prebiotic chemistry, self-organization, homochirality, encapsulation, and stability over time. In this context, life could be frequent in the Universe, but complex life probably remains unfindable.

Exo-Earths: These Distant Worlds that Resemble Earth

Habitable planets constitute one of the most dynamic fields of modern astrophysics. They pave the way for a new era of space exploration, focused not only on celestial objects but on the search for another form of life. The development of observatories like JWST or ELT could, in the near future, allow the detection of spectroscopic biosignatures.

References:
• Kaltenegger L., How to Characterize Habitable Worlds and Signs of Life, Annual Review of Astronomy and Astrophysics, 2017.
• NASA Exoplanet Archive: https://exoplanetarchive.ipac.caltech.edu/ (new window)
• Seager S., Exoplanet Habitability, Science, vol. 340, 2013.