Rocky exoplanet TRAPPIST-1 b may have a hidden atmosphere

TRAPPIST-1 b, one of seven rocky planets that orbit the star TRAPPIST-1, continues to intrigue astronomers.

Located just 40 light-years away, this planetary system is unique. It offers a rare opportunity to study seven Earth-like planets, three of which reside in the habitable zone – where liquid water could exist on the surface.

Using the James Webb Space Telescope (JWST), researchers have now gathered deeper insights into TRAPPIST-1 b.

Observing TRAPPIST-1 b

A team led by Elsa Ducrot from the Commissariat aux Énergies Atomiques (CEA) in France analyzed thermal infrared measurements captured by JWST‘s Mid-Infrared Imager (MIRI).

The researchers, including experts from the Max Planck Institute for Astronomy (MPIA), published their findings in the journal Nature Astronomy.

The results challenge earlier conclusions and raise questions about the planet’s atmosphere and surface activity.

A geologically active surface?

Last year’s research described TRAPPIST-1 b as a dark, rocky planet without an atmosphere. However, the latest measurements offer new clues.

“The idea of a rocky planet with a heavily weathered surface without an atmosphere is inconsistent with the current measurement,” said Jeroen Bouwman, an astronomer at MPIA. “Therefore, we think the planet is covered with relatively unchanged material.”

Surfaces on rocky planets usually weather due to stellar radiation and meteorite impacts. Yet, TRAPPIST-1 b’s surface appears remarkably young. Researchers estimate the rock on its surface could be only about 1,000 years old – far younger than the planet’s estimated age of several billion years.

Dramatic geological activity on TRAPPIST-1 b

This discovery hints at dramatic geological activity. The planet’s crust could undergo constant changes driven by extreme volcanism or plate tectonics.

Bouwman noted that TRAPPIST-1 b might retain internal heat, much like Earth.

Tidal forces from the host star and neighboring planets could deform its surface, generating internal friction and heat – a phenomenon seen on Jupiter’s volcanic moon Io. Additionally, heating from the star’s magnetic field may contribute.

A hidden atmosphere?

Could TRAPPIST-1 b possess an atmosphere after all?

Thomas Henning, emeritus director at MPIA and co-creator of the MIRI instrument, suggested another possibility.

“Contrary to previous ideas, there are conditions under which the planet could have a thick atmosphere rich in carbon dioxide (CO₂),” said Henning. A crucial factor could be haze – hydrocarbon smog – forming in the upper atmosphere.

The study involved two observational programs that measured the planet’s brightness at 12.8 and 15 micrometers. These wavelengths are sensitive to CO₂ absorption.

Since no dimming occurred, researchers initially concluded that TRAPPIST-1 b lacked an atmosphere. However, new model calculations suggest that haze could alter the scenario.

The effect of haze in planetary atmospheres

Haze absorbs starlight and warms upper atmospheric layers, potentially altering the expected temperature structure.

Instead of a warmer ground-level layer, the upper atmosphere could heat up and emit infrared radiation – a greenhouse effect. Henning compares this to Saturn’s moon Titan, where UV radiation transforms carbon-rich gases into haze.

While plausible, researchers note key differences. Titan’s atmosphere is methane-rich, whereas TRAPPIST-1 b would require CO₂.

Moreover, red dwarf stars like TRAPPIST-1 emit intense radiation and stellar winds, which can strip away planetary atmospheres over billions of years.

Eclipses and occultations: Tools for exploration

The study benefited from the favorable orientation of TRAPPIST-1. From Earth, its seven planets periodically pass in front of the star, dimming its light – a phenomenon known as transit.

These events provide valuable data about planetary size, composition, and atmospheres through a method called transit spectroscopy.

However, transit spectroscopy has limitations for systems like TRAPPIST-1. Cool, red dwarf stars often exhibit starspots and flares that can interfere with measurements.

To address this, researchers observed TRAPPIST-1 b’s thermal infrared radiation. By focusing on the planet’s bright dayside just before and after it vanishes behind the star, scientists gathered detailed information about its surface and atmosphere.

This method, known as secondary eclipse measurement, is time-consuming. The current observations lasted nearly 48 hours but remain insufficient to confirm the presence of an atmosphere.

Lingering questions about TRAPPIST-1 b

NASA recently approved the ambitious “Rocky Worlds” program – 500 hours of JWST observations to study rocky planets around nearby stars. Researchers hope this program will help resolve lingering questions about TRAPPIST-1 b.

One promising method involves monitoring the planet’s entire orbit to create a phase curve, which is a map of its temperature distribution.

This approach detects how heat moves across the planet’s surface. If heat distributes evenly between the dayside and nightside, an atmosphere likely exists. If temperatures shift abruptly, it indicates a lack of atmospheric insulation.

The research team has already begun this phase curve measurement for TRAPPIST-1 b. By analyzing temperature changes, they aim to confirm whether the planet harbors an atmosphere.

“The presence or absence of an atmosphere will significantly impact our understanding of TRAPPIST-1 b and similar exoplanets,” noted Bouwman.

Broader implications of the study

The study highlights the challenges of studying rocky planets, even with advanced tools like the JWST. Compared to gas giants, rocky planets have thinner atmospheres that produce faint signals. Despite the hurdles, TRAPPIST-1 b offers a valuable case study.

The findings also provide insight into exoplanets that orbit red dwarf stars – the most common stars in the galaxy. Understanding their atmospheres, geological activity, and potential habitability will shape future research and exploration.

The study is the result of international collaboration. Researchers from MPIA, CEA, and institutions across Europe contributed to this original work. The MIRI instrument itself is a product of efforts by ESA member states and organizations like the Max Planck Society.

TRAPPIST-1 b remains a puzzle. Is it a geologically active world with a young surface? Could it hide a thick, CO₂-rich atmosphere cloaked in haze?

The answers will require further observations. With JWST’s capabilities and the ongoing Rocky Worlds program, astronomers are closer than ever to solving these mysteries.

The study is published in the journal Nature Astronomy.

Image Credit: JPL-Caltech/NASA

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