Two Earth-Size Worlds in TRAPPIST-1 System Reveal Stark Divide Between Day and Night

New Webb observations of two exoplanets TRAPPIST-1b and TRAPPIST-1c show scorching days and freezing nights, offering the first detailed climate maps of rocky exoplanets and dimming hopes of habitability.

This artist’s impression shows an imaginary view from the surface of one of three exoplanets orbiting the ultra-cool dwarf star TRAPPIST-1, 38.8 light-years from Earth. These alien worlds have sizes and temperatures similar to those of Venus and Earth. In this view, one of the inner planets is seen transiting through TRAPPIST-1’s disk. Image credit: M. Kornmesser / ESO.

Red dwarfs – smaller and cooler than our Sun – make up more than three-quarters of the stars in the Milky Way.

Astronomers have discovered that Earth-sized planets are particularly common around these dim stars, raising a pressing question: Could life arise on worlds so different from our own?

One of these systems, TRAPPIST-1, became the focal point of this research.

Discovered in 2017, the system is located 38.8 light years away in the constellation Aquarius.

It consists of seven transiting planets: TRAPPIST-1b, c, d, e, f, g and h.

All of these worlds are similar in size to Earth and Venus, or slightly smaller, and have very short orbital periods.

“The TRAPPIST-1 system is incredible! Seven planets, some with masses similar to that of Earth, orbit the same star,” said Emeline Bolmont, astronomer at UNIGE.

“At least three planets are located in the star’s habitable zone, where surface temperatures would allow the presence of liquid water.”

“This is the ideal playground for comparative planetology, unraveling the mysteries of this type of planet and testing our hypotheses about the development of life around these stars.”

Dr. Bolmont and his colleagues used the NASA/ESA/CSA James Webb Space Telescope to observe two innermost planets in the system: TRAPPIST-1b and TRAPPIST-1c.

“Although red dwarf stars and their planets are common in our Galaxy, their habitability is not necessarily guaranteed,” they said.

“First, these stars are very active and bombard their planets with intense ultraviolet radiation and energetic particle streams, which could erode their atmospheres and eradicate any life that might exist.”

“Second, the planets are in the habitable zone of a red dwarf orbiting very close to their star, and tidal forces synchronize their rotation with their orbital period, much like the Moon does with the Earth.”

“These planets therefore rotate on their axis at the same time as they rotate around their star. This results in permanent day on one side and permanent night on the other.”

This artist’s impression shows TRAPPIST-1 and its planets reflected in a surface. Image credit: NASA / R. Hurt / T. Pyle.

By measuring the light flux of the star and the two planets, astronomers were able to determine the surface temperatures of TRAPPIST-1b and TRAPPIST-1c with high precision, both on the day and night sides.

During the day, the surface temperatures of both planets exceed 200 degrees Celsius and nearly 100 degrees Celsius, respectively, while their nights are plunged into freezing temperatures below minus 200 degrees Celsius.

This enormous contrast suggests a lack of energy redistribution between the two faces of the planets, and therefore the absence of atmospheres.

If both planets had atmospheres during their formation, these were completely stripped by the extreme conditions imposed by their star.

According to the team, the absence of dense atmosphere on the two inner planets of the TRAPPIST-1 system supports the hypothesis that intense radiation and energetic ejections from red dwarfs play an important role in the evolution of planets around this type of star.

“TRAPPIST-1 serves as a reference system. Our theoretical models show that the outermost planets in the TRAPPIST-1 system can have an atmosphere despite the absence of an atmosphere on the two inner planets,” said Dr. Bolmont.

“This is similar to Mercury, the closest planet to our Sun, which has no atmosphere, while Venus and Earth have retained theirs.”

“We look forward to continuing the exploration of the TRAPPIST-1 system.”

The results were published April 3 in the journal Natural astronomy.

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Mr. Gillon and others. No thick atmosphere around TRAPPIST-1 b and c according to JWST thermal phase curves. Nat Astronpublished online April 3, 2026; doi: 10.1038/s41550-026-02806-9