Escaping magnesium reveals an extended upper atmosphere on planet WASP-76 b

Observations of the ultra-hot Jupiter WASP-76 b show evidence of magnesium escaping from its upper atmosphere. By analyzing near-ultraviolet light, researchers identified high-altitude gas extending beyond the planet's gravitational influence. This finding, consistent with hydrodynamic escape, indicates that the planet is losing mass without requiring unusual chemical compositions.

Star system: WASP-76 | Distance from the Sun: 637 light years

The ultra-hot Jupiter exoplanet WASP-76 b is losing magnesium from its upper atmosphere, according to recent near-ultraviolet observations. This gas exists at high altitudes, far exceeding the typical radius of the planet. These findings indicate that the atmosphere is highly extended and actively escaping into space, a process driven by the intense heat and radiation environment of this planet.

To measure this atmospheric loss, the Hubble Space Telescope used transmission spectroscopy. This technique involves analyzing light as it passes through the planet's atmosphere during a transit, when the planet moves in front of its host star. By focusing on specific wavelengths, specifically the magnesium doublet, researchers detected excess absorption in the blue wing of the spectrum. This signal reveals gas moving at high velocities, confirming that the atmosphere is not static but flows outward.

The data suggests that this magnesium escape is consistent with hydrodynamic escape, where the atmosphere acts like a fluid driven away by stellar radiation. The observations indicate that the presence of magnesium does not require extreme chemical enrichment, aligning with findings from previous studies that showed the planet has a near-solar chemical makeup. The escaping gas extends beyond the Roche-lobe, which is the region where the planet's gravity dominates over the star's pull, further confirming the gas is leaving the planet.

Key findings
Magnesium II doublet absorption reveals gas escaping the atmosphere of WASP-76 b.
The detected gas extends well beyond the planet's optical radius and Roche-lobe.
The calculated mass-loss rate for the planet is approximately 10 to the power of 12.14 grams per second.
The chemical composition of the upper atmosphere is consistent with near-solar metallicity.
The escape of magnesium is likely driven by hydrodynamic processes rather than extreme chemical enrichment.

Scientific study

Detection of escaping magnesium in the upper atmosphere of WASP-76b (external link)