exoplanet atmosphere

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The main method is transmission spectroscopy: during transit (the planet passing across the star's disk), some starlight is filtered through the upper layers of the atmosphere. Atoms and molecules absorb radiation at characteristic wavelengths, creating a spectral fingerprint. From this, the presence of water vapor, carbon dioxide, methane, as well as temperature and pressure can be determined.

History

In 2001, astronomers using the Hubble telescope first captured the atmospheric imprint of planet HD 209458 b, seeing traces of sodium in its spectrum.

How it works

Imagine looking at a lightbulb through colored glass: the glass absorbs some light, and you see what color it is. It's similar with a planet's atmosphere: we compare the star's light during the planet's passage and without it, and the difference shows what the gases "drank".

💡 On exoplanet KELT-9b, the dayside is heated to 4600 K — hotter than many stars — and iron and titanium vapors float in its atmosphere.

An exoplanet atmosphere is the outer gaseous envelope of a planet beyond the Solar System. Its study relies mainly on transmission and emission spectroscopy. During transit, starlight passing through the planet's limb is partially absorbed and scattered by the atmosphere, manifesting as an additional eclipse depth that depends on wavelength. Comparing spectra during transit and out of transit yields a transmission spectrum, from which inverse modeling (atmospheric retrieval) reconstructs the vertical temperature profile, chemical composition, and the presence of clouds and hazes. Emission spectroscopy measures the atmosphere's own thermal glow on the dayside, and light curves with phase curves allow construction of temperature maps.

History

The first successful analysis of an exoplanet atmosphere was performed by David Charbonneau's group in 2001 using the STIS spectrograph on the Hubble telescope. Sodium at 589 nm was detected on planet HD 209458 b. Later, the Spitzer infrared telescope and the Kepler space observatory led by William Borucki enabled statistical studies of hundreds of hot Jupiters. With the launch of JWST, the era of detailed chemical analysis of Earth-like planet atmospheres began.

How it works

During transit, the fraction of radiation blocked by the planet is (R_p/R_s)^2, and the thin atmospheric shell adds about 2R_p·h/R_s^2 to the absorption, where h is the effective height of the atmosphere. This height is given by the scale height H = kT/(μg), where k is Boltzmann's constant, T is temperature, μ is mean molecular mass, and g is gravitational acceleration. Absorption at specific wavelengths follows the Bouguer–Lambert–Beer law; the depth of spectral lines reflects the abundance of the corresponding chemical elements. Light curves at different wavelengths are recorded photometrically, and spectral resolution allows identification of molecular bands of H₂O, CO, CO₂, CH₄, etc.

H = \frac{kT}{\mu g}

H — scale height (characteristic thickness of the atmosphere), k — Boltzmann constant (1.380649×10⁻²³ J/K), T — atmospheric temperature (K), μ — mean molecular mass (kg), g — gravitational acceleration (m/s²)

\Delta F \approx \frac{2 R_p h}{R_s^2}

ΔF — excess eclipse depth (relative flux decrease), R_p — planet radius (m), h — effective thickness of the absorbing layer (m), R_s — stellar radius (m)

Open problems & fun facts

Influence of clouds and photochemical hazes masking spectral features; Uncertainty in atmospheric escape rates and loss of volatiles; False-positive biomarkers: abiotic sources of O₂, CH₄; Limited signal-to-noise ratio and model incompleteness in retrieval

💡 The ARIEL mission (Atmospheric Remote-sensing Infrared Exoplanet Large-survey), launching in 2029, plans to conduct a systematic spectroscopic survey of about 1000 exoplanet atmospheres.

The Invisible Conductor of the Super-Puff Planet System

Meticulous 14-year observations of three 'cotton-candy' exoplanets in Kepler-51 yielded an unexpected surprise. The James Webb Space Telescope recorded the outermost known planet crossing the star's disk two hours ahead of schedule. The anomaly was only explained by adding an invisible fourth planet

arXiv:2410.01625 · 2024-10-02

exoplanet atmosphere

History

How it works

History

How it works

History

How it works

Open problems & fun facts

Related articles

The Invisible Conductor of the Super-Puff Planet System