TRAPPIST-1’s Inner Planets Are Likely Bare Rock

Researchers have determined that two Earth-sized planets circling the nearby red dwarf TRAPPIST-1 experience such severe temperature differences between day and night that both are probably exposed rocky bodies.

The finding considerably restricts the locations where astronomers may still expect durable atmospheres around the galaxy’s most plentiful type of star.

Heat without an atmosphere

On TRAPPIST-1’s two innermost planets, one side faces continuous daylight and the opposite hemisphere exists in unending night, producing dramatically separate conditions across each world.

By tracking their thermal output for almost 60 uninterrupted hours, researchers at the University of Geneva (UNIGE) created the first climate maps for these Earth-sized rocky planets.

They observed that nearly all warmth was concentrated on the sides facing the star. The night sides showed almost no measurable heat, producing a difference of more than 500 °C (900 °F) between the two hemispheres.

A contrast this extreme offers little scope for more than a very tenuous atmosphere, while raising the more difficult issue of why these planets lost so much material initially.

Evidence from the dark side

The night side provided the clearest evidence: if a planet has an atmosphere, it should transfer retained heat into its dark hemisphere.

These thermal phase curves - variations in a planet’s heat emission throughout its orbit - indicate whether winds redistribute energy.

In this case, the dark hemispheres remained so dim that almost no heat was being transported, the expected behaviour of an airless surface once daylight ends.

This straightforward signature enabled astronomers to assess atmospheric scenarios directly, rather than infer them from a single observation of a lit hemisphere.

Why the planets are tidally locked

The problem begins with the host star. Nearby red dwarfs can expose their closest planets to destructive radiation for prolonged periods.

Their tight orbits also make these worlds tidally locked: one face always points towards the star, while the other remains permanently unlit.

In the absence of an atmosphere, the dayside continually absorbs incoming energy, whereas the nightside rapidly sheds it as infrared radiation escapes into space.

Such planets may appear temperate in theoretical calculations but remain harsh at the surface, particularly close to the inner edge of a system.

A system worth monitoring

Astronomers have been captivated by the seven-planet TRAPPIST-1 system since 2017, as several of its worlds orbit at distances where liquid water could theoretically be present.

As every planet circles the same cool star, scientists can compare them almost directly and assess how orbital distance alters their fate.

“The TRAPPIST-1 system is incredible! Seven planets, some with masses similar to Earth’s, orbit the same star,” said Emeline Bolmont, associate professor in the Department of Astronomy at UNIGE and director of the Centre for Life in the Universe.

The team began with the nearest pair, TRAPPIST-1b (Planet b) and TRAPPIST-1c (Planet c), where exposure to stellar activity was expected to be greatest.

What TRAPPIST-1 planet b reveals

Planet b produced the most decisive result: its dayside exceeded 200 °C (390 °F), its nightside emitted almost no detectable glow, and there was no evident displacement in the heat signal.

Previous eclipse measurements had already suggested that planet b releases nearly all absorbed starlight again from its dayside.

Models in which heat is efficiently redistributed did not fit the new curve. Airless models, however, matched both its brightness and timing.

Together, these results make a substantial atmosphere on planet b seem very improbable, even before researchers settle questions about its surface composition.

Why TRAPPIST-1 planet c remains uncertain

Planet c likewise displayed a pronounced divide, with a dayside temperature close to 110 °C (210 °F), although its faint signal was insufficient to resolve every question.

Observations from 2023 had already excluded a thick carbon dioxide atmosphere on planet c.

One possibility still under consideration is an extremely thin, oxygen-rich atmosphere, capable of shifting only a limited amount of heat before the world cools once more.

Until more precise measurements are available, both a more reflective bare-rock surface and a tenuous atmosphere remain possible explanations.

The significance of exposed rock

Surface modelling introduced a further complication, as an airless planet’s brightness depends heavily on how its exposed rocks reflect and emit radiation.

A separate modelling study identified ultramafic rock - dark, iron- and magnesium-rich material - as the most likely surface type for planet b.

However, moderate radiation damage could also darken other substances, making that interpretation much less certain.

Consequently, the evidence is stronger regarding the lack of air than it is about the precise nature of the rock.

Priorities in the search for alien life

Questions about life now focus on the system’s outer planets rather than the two worlds nearest their star.

Mercury is a useful comparison, showing that one rocky planet can lose its atmosphere even while nearby planets retain theirs.

Nevertheless, the inner pair demonstrate the effects that intense radiation and tight orbits can have during the early development of a planetary system.

This is relevant to every red dwarf survey, since such stars are widespread and their planets are frequent targets in the search for life.

Webb continues observing TRAPPIST-1

The James Webb Space Telescope has already begun studying planet e, a more distant world located within the system’s habitable zone.

Future data should reveal whether greater distance by itself allows a planet to preserve gases, water and more moderate temperatures.

For now, the inner planets have become a benchmark for interpreting other rocky worlds around dim, active stars.

Every additional measurement will make the distinction clearer between planets that simply share Earth’s size and those able to sustain Earth-like conditions.

What changes now

The revised picture is clear: on the two closest worlds, permanent daylight and permanent darkness are not moderated by a thick atmosphere.

This does not halt the search in this system, but it indicates where prospects diminish most rapidly and where they may still remain.

Image credit: European Space Agency