James Webb Space Telescope Delivers New Insights on Exoplanet Atmospheres, Surfaces, and Formation

A series of new studies using the James Webb Space Telescope (JWST) has yielded a wealth of data on exoplanets, from the formation of distant gas giants to the stark, airless surfaces of rocky worlds.

HR 8799 System: Gas Giant Formation

A research team led by the University of California San Diego used spectral data from JWST to investigate the formation mechanism of gas giants in the HR 8799 star system. The findings were published in Nature Astronomy.

• The HR 8799 system is located approximately 133 light-years away in the constellation Pegasus.
• It hosts four gas giant planets, each with a mass between 5 and 10 times that of Jupiter.
• These planets orbit their star, HR 8799, at distances ranging from 15 to 70 astronomical units (AU).
• The system is estimated to be about 30 million years old.

Two primary theories explain gas giant formation:

• Core Accretion: Solid cores grow within a protoplanetary disk by accumulating material, eventually attracting surrounding gas.
• Gravitational Instability: A cloud of gas within the disk rapidly collapses into a massive object.

Astronomers analyzed the atmospheres of the system's three innermost planets. By focusing on refractory elements such as sulfur, which exist as solids within a planet-forming disk, the team aimed to distinguish between the two formation mechanisms.

• JWST's high-resolution spectrograph allowed for the detection of distinct molecular features in the planets' atmospheres.
• The team confirmed the presence of hydrogen sulfide in the atmosphere of HR 8799 c. Researchers inferred the likely presence of sulfur in all three inner planets.
• The inner planets also exhibited higher concentrations of heavy elements such as carbon and oxygen compared to their host star.
• The detection of sulfur provides evidence for formation via core accretion, as the element would have been incorporated from solid materials within the disk.

Co-author Jean-Baptiste Ruffio noted that the findings suggest the HR 8799 planets likely formed through a process similar to Jupiter, despite their significantly larger masses. Researchers stated that the results support newer models of core accretion, which can operate at greater masses and orbital distances than previously thought. The work received support from the National Aeronautics and Space Administration (NASA).

"The detection of sulfur provides evidence for formation via core accretion."

TRAPPIST-1 System: Climate Mapping and Atmospheric Absence

An international team of researchers, including scientists from the University of Bern (UNIBE) and the University of Geneva (UNIGE), used JWST to map the climate of two rocky exoplanets in the TRAPPIST-1 system. The findings were published in Nature Astronomy.

• The TRAPPIST-1 system, discovered ten years ago, contains seven planets, some with masses similar to Earth.
• The system's star is a red dwarf, which is cooler and smaller than the Sun. Red dwarfs constitute over 75% of stars in the galaxy.
• The study focused on the two innermost planets: TRAPPIST-1b and TRAPPIST-1c.
• Researchers conducted 60 hours of continuous infrared observations of the planets over a full orbit.

• Measurements of the light flux allowed for precise determination of surface temperatures.
  • TRAPPIST-1b's daytime surface temperature exceeds 200°C; its nighttime temperature is below -200°C.
  • TRAPPIST-1c's daytime surface temperature is nearly 100°C; its nighttime temperature is below -200°C.
• The temperature difference between the day and night sides of both planets exceeds 500 degrees Celsius.
• This significant temperature contrast suggests a lack of energy redistribution between the hemispheres.
• Researchers concluded that this indicates the absence of substantial atmospheres on TRAPPIST-1b and TRAPPIST-1c.

The findings support the hypothesis that intense radiation and energetic particle fluxes from red dwarf stars can erode planetary atmospheres. Co-author Emeline Bolmont stated the system serves as a laboratory for comparative planetology. Co-author Brice-Oliver Demory noted that the presence of an atmosphere on tidally locked planets could allow for energy transfer and more moderate temperatures. Researchers theorize that the outermost planets in the TRAPPIST-1 system, including TRAPPIST-1e which lies within the star's habitable zone, might still possess atmospheres. The JWST is currently observing TRAPPIST-1e.

"The temperature difference between the day and night sides of both planets exceeds 500 degrees Celsius."

LHS 3844 b: Surface Composition

Using JWST's Mid-Infrared Instrument (MIRI), a team led by Sebastian Zieba and Laura Kreidberg analyzed the surface composition of the rocky exoplanet LHS 3844 b. The results were published in Nature Astronomy.

• LHS 3844 b is a rocky planet approximately 30% larger than Earth.
• It orbits a cool red dwarf star every 11 hours and is tidally locked, with a permanent dayside and nightside.
• The dayside temperature is about 725°C (1000 K).
• The planet is located 48.5 light-years from Earth.

• JWST's MIRI detected infrared radiation from the planet's dayside at wavelengths of 5–12 micrometers.
• The spectrum was compared to models and mineral libraries from Earth, the Moon, and Mars.
• The surface composition was found to be inconsistent with Earth's silicate-rich crust but consistent with dark basaltic or magmatic rock, rich in magnesium and iron.
• No sulfur dioxide (SO₂) was detected, which researchers cited as evidence against recent volcanic activity.

The data is consistent with two potential scenarios:

1. Fresh dark solid rock, which could suggest recent geological activity.
2. Darkened regolith (fine powder) from prolonged space weathering, indicating geological inactivity.

Researchers concluded the findings favor the weathered scenario, suggesting the planet may lack an atmosphere and have a surface similar to the Moon or Mercury. Further JWST observations are planned to distinguish between these scenarios.

29 Cygni b: Determining a Formation Mechanism

An international team led by William Balmer of Johns Hopkins University and the Space Telescope Science Institute used JWST to directly image the exoplanet 29 Cygni b. A paper describing the findings was published in The Astrophysical Journal Letters.

• 29 Cygni b has a mass approximately 15 times that of Jupiter.
• It orbits its host star at an average distance of 1.5 billion miles (2.4 billion kilometers).
• The planet's mass places it at the theoretical boundary between objects thought to form via accretion and those possibly formed via disk fragmentation.
• The host star has a composition similar to the Sun.

The team used JWST's Near-Infrared Camera (NIRCam) in coronagraphic mode for direct imaging. They analyzed light absorption to detect carbon dioxide and carbon monoxide in the planet's atmosphere.

Key findings include:

• The planet is enriched in heavy elements (metals) relative to its host star. The estimated metal content is equivalent to approximately 150 Earth masses.
• Observations from the CHARA ground-based telescope array confirmed the planet's orbital plane is aligned with the spin axis of its host star.

Two primary formation mechanisms were considered:

• Accretion: A bottom-up process where material in a protoplanetary disk accumulates.
• Disk Fragmentation: A top-down process where a portion of the disk collapses under its own gravity.

Lead author William Balmer stated that in computer models, disk fragmentation can produce objects with masses much higher than 29 Cygni b, while accretion has an upper mass limit around this object's mass. The research team concluded that the evidence, including the metal enrichment and orbital alignment, suggests 29 Cygni b formed via rapid accretion of metal-rich material within a protoplanetary disk. Co-author Ash Messier noted that the observed orbital alignment is similar to that of planets in our solar system. The team plans to gather data on three additional targets in their program to further investigate formation mechanisms.

Additional Observation: TOI-2031Ab

Astrophysicist Paul Smith and collaborators used JWST to observe exoplanet TOI-2031Ab, located 901 light-years from Earth. The planet has a circumference 25% larger than Jupiter but 20% less mass and orbits its star every 6 Earth days. Using near-infrared spectrographic sensors, the team detected water and carbon dioxide in the planet's atmosphere, which is primarily composed of hydrogen and helium.