Kepler-186 System: A Red Dwarf with a Candidate Earth

The star Kepler-186

An M-type red dwarf

Kepler-186 is a red dwarf of spectral type M, with a mass of about 0.48 solar masses and a surface temperature of 3,500 K (compared to 5,778 K for the Sun). These stars, smaller and cooler than the Sun, represent about 70% of the stars in the Milky Way. They have an extremely long lifespan (hundreds of billions of years), because they burn their fuel (hydrogen) very slowly.

The challenges of red dwarfs for habitability

Although red dwarfs are prime targets for the search for habitable exoplanets (due to their number and their close habitable zone), their stellar activity poses major challenges:

• Stellar flares: Red dwarfs emit powerful flares, capable of stripping a planet of its atmosphere.
• UV radiation: Their light is richer in UV, which can be harmful to surface life.
• Gravitational locking: Close planets (like Kepler-186f) are often in synchronous rotation, with one side always exposed to light and the other in permanent darkness.

Despite these challenges, recent studies suggest that thick atmospheres or oceans could redistribute heat and protect the surface.

The exoplanet Kepler-186f

The first Exo-Earth in the habitable zone

Discovered in April 2014 by a team led by Elisa Quintana (SETI Institute/NASA Ames), Kepler-186f is the first Earth-sized exoplanet (1.11 R⊕) confirmed in the habitable zone of a star other than the Sun. Unlike planets such as Kepler-22b (2.4 R⊕, discovered in 2011), Kepler-186f has a size close to that of Earth, making it a key object of study in the search for extraterrestrial life.

An Earth size, an unknown mass

Kepler-186f has a radius of 1.11 R⊕ (Earth radius), but its mass remains undetermined. Its composition could vary:

• Rocky planet (like Earth, with an iron core and a silicate mantle).
• Ocean planet (covered with a thick layer of water or ice).
• Gaseous world (with a dense atmosphere, like a mini-Neptune).

Without a measurement of its mass (by the radial velocity method or secondary transits), it is impossible to decide.

An atmosphere to characterize

The James Webb Telescope (JWST) could analyze the chemical composition of Kepler-186f's atmosphere, if it has one. Scientists will look for biosignatures such as:

• The presence of water vapor (H₂O).
• Gases such as methane (CH₄) or oxygen (O₂), which could indicate biological activity.
• The absence of carbon monoxide (CO), which would be a sign of chemical equilibrium compatible with life.

Elisa Quintana emphasizes: "The discovery of Kepler-186f is a major step in the quest for planets similar to Earth. It proves that rocky worlds exist in habitable zones, even around stars very different from the Sun."

An orbit of 130 days

Kepler-186f orbits at about 0.4 AU (astronomical unit) from its star, with a period of 130 days. Its position near the outer edge of the habitable zone implies that:

• If its atmosphere is too thin, water could freeze.
• If it is dense enough (thanks to a moderate greenhouse effect), liquid water could exist.

Climate models suggest that Kepler-186f could have a surface temperature between -50°C and 0°C, depending on the composition of its atmosphere.

The inner planets of Kepler-186

Four planets too hot for life

The four other planets in the system (Kepler-186b, c, d, e) orbit very close to their star, with respective periods of:

• Kepler-186b: 4 days.
• Kepler-186c: 7 days.
• Kepler-186d: 13 days.
• Kepler-186e: 22 days.

All these planets receive a stellar flux far greater than that of Earth, making them too hot to harbor liquid water on their surface. Their size varies between 0.9 and 1.4 R⊕, suggesting that they could be rocky, but their proximity to the star exposes them to extreme temperatures (several hundred °C).

Comparison with other exoplanetary systems

Kepler-186 vs TRAPPIST-1

The Kepler-186 system shares similarities with TRAPPIST-1, another red dwarf system discovered in 2017, located 40 light-years from Earth. TRAPPIST-1 hosts seven planets, three of which are in the habitable zone (TRAPPIST-1e, f, g).

Unlike TRAPPIST-1, whose planets are in orbital resonance (their orbital periods are integer multiples of each other), the Kepler-186 system does not exhibit this characteristic. This could indicate a different dynamical history.

Kepler-186 vs Proxima Centauri

Proxima Centauri b, discovered in 2016, is another Earth-sized exoplanet (1.07 R⊕) in the habitable zone, but around the star closest to the Sun (4.24 light-years). Unlike Kepler-186f, Proxima b is much closer to its star (0.05 AU, 11-day orbit), exposing it to intense radiation and probable gravitational locking.

Future observations and scientific challenges

The role of the James Webb Telescope (JWST)

The JWST, launched in 2021, is the most promising tool for studying Kepler-186f. Its instruments, such as NIRISS (Near Infrared Imager and Slitless Spectrograph) and MIRI (Mid-Infrared Instrument), could:

• Detect the presence of an atmosphere via transit spectroscopy.
• Analyze its chemical composition (H₂O, CO₂, CH₄, etc.).
• Measure surface temperature and climate variations.

However, Kepler-186f is at the limit of JWST's capabilities due to its distance (500 light-years) and the low luminosity of its star. Observations could require several hours of telescope time.

Future missions

Several space missions could complement JWST's observations:

• PLATO (ESA): Scheduled for 2026, this European space telescope will search for terrestrial planets around stars similar to the Sun, but could also study red dwarfs like Kepler-186.
• Habitable Worlds Observatory (HWO): NASA project for the 2030s, designed to directly image exoplanets and analyze their atmospheres.
• ELT (Extremely Large Telescope): Giant ground-based telescope (39 m in diameter) under construction in Chile, which could provide complementary data.

These missions will help better understand whether Kepler-186f has a stable atmosphere and conditions conducive to life.

Table: Classification of stars by spectral type