HR 8799 e
A young, red-hot gas giant with iron and silicate clouds.
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HR 8799 e is a young, hot, and massive gas giant orbiting the star HR 8799, located 129 light-years from Earth. It is the innermost known planet in its system and was the fourth to be discovered. The planet is notable for being the first exoplanet directly observed using optical interferometry, and for its complex atmosphere containing clouds of iron and silicates.
Lore & Background
HR 8799 e was identified from data taken in 2009 and 2010 using the W.M. Keck Observatory in the K and L spectral bands, with findings announced on 22 November 2010 by a team led by Christian Marois. A separate detection, led by Thayne Currie using the Very Large Telescope, was made public six weeks later. The planet orbits at a semi-major axis of 16 AU with an orbital period of 52.37 years, placing it between the orbits of Saturn and Uranus in the Solar System.
In 2013, near infrared spectroscopy from the Palomar Observatory showed evidence of methane and acetylene but no ammonia or carbon dioxide. The strong methane absorption was unexplained, as the other three planets in the system do not show it despite similar atmospheric temperatures.
On 27 March 2019, the Very Large Telescope's GRAVITY instrument produced the first direct observation of any exoplanet using optical interferometry, revealing a spectrum ten times more detailed than earlier observations. The data showed an atmosphere with far more carbon monoxide than methane, attributed to high vertical winds preventing the carbon monoxide from reacting with hydrogen to form methane. The observations confirmed a spectral type of ~L7 and indicated low surface gravity, resolving a previous temperature discrepancy.
In 2021, the detection of water and carbon monoxide in the atmosphere was announced, along with an astrometric measurement of the planet's mass. In 2025, the detection of water and carbon dioxide was announced, marking the first time CO2 was directly detected in an exoplanet.
Reader's Guide
HR 8799 e holds several milestones in exoplanet science. It was the first exoplanet directly observed using optical interferometry, a technique that provided a spectrum ten times more detailed than earlier observations. This revealed a complex atmosphere with clouds of iron and silicates, and an unexpected abundance of carbon monoxide relative to methane, explained by high vertical winds. The planet's low surface gravity, similar to that of young brown dwarfs, confirmed its youth and resolved a prior discrepancy between its spectral type and effective temperature.
The 2025 detection of carbon dioxide via direct imaging was a first for exoplanets, opening a new window for atmospheric characterization. The planet's strong methane absorption, unique among the four known planets in its system, remains unexplained. These findings have advanced understanding of young gas giant atmospheres and the dynamics of planetary systems.
Frequently Asked Questions
What is HR 8799 e?
HR 8799 e is a young, red-hot gas giant that circles the star HR 8799 at a distance of about 129 light-years from Earth. It sits at roughly 16 AU from its host star, making it the innermost confirmed planet in that system.
How was HR 8799 e discovered?
It was identified in 2010 as the fourth planet detected around HR 8799, and it earned a special place in history as the first exoplanet ever directly imaged through optical interferometry.
What is HR 8799 e like physically?
The planet carries about 7.5 Jupiter masses within a radius of roughly 1.13 Jupiter radii, giving it the swollen, dense look typical of very young giants. Its atmosphere is especially exotic, hosting clouds of iron and silicate particles instead of the water or ammonia clouds seen in older gas giants.
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Sources
Compiled from Wikipedia and the sources listed below. Text from Wikipedia is available under CC BY-SA 4.0; this entry is adapted from it.
- Wikipedia: HR 8799 e (CC BY-SA 4.0).
- Word definitions: the Codexery glossary, each quoted from its Wikipedia article.
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