HR 8799 e

Discovery

A team of researchers led by Christian Marois at the National Research Council's Herzberg Institute of Astrophysics identified the planet from data taken in 2009 and 2010 using the W.M. Keck Observatory in the K and L spectral bands. They announced their findings on 22 November 2010. A separate work reporting the detection of HR 8799 e, led by Thayne Currie and using the Very Large Telescope, was made public six weeks later. Observations obtained since then with the Large Binocular Telescope show that HR 8799 e has a spectrum and temperature similar to HR 8799 c and d.

Observations

In 2013, near infrared spectroscopy from 995 to 1769 nanometers made with the Palomar Observatory showed evidence of methane and acetylene but no signs of ammonia or carbon dioxide gas. There is no explanation as to why the planet shows strong methane absorption, but the other 3 planets in this system do not, despite all 4 planets having similar atmospheric temperatures.

On 27 March 2019, the European Southern Observatory announced the result of their Very Large Telescope astronomical interferometer (VLTI) imaging of HR 8799 e employing the GRAVITY instrument. This was the first direct observation of any exoplanet using optical interferometry. A spectrum ten times more detailed than earlier observations revealed a complex exoplanetary atmosphere with clouds of iron and silicates swirling in a planet-wide storm. Team leader Sylvestre Lacour said:

"Our analysis showed that HR 8799 e has an atmosphere containing far more carbon monoxide than methane — something not expected from equilibrium chemistry. We can best explain this surprising result with high vertical winds within the atmosphere preventing the carbon monoxide from reacting with hydrogen to form methane."

The observations with GRAVITY confirmed the spectral type of ~L7 for the planet HR 8799 e. Previously the spectral type suggested a higher temperature than the measured effective temperature. The GRAVITY observations showed that the planet has a low surface gravity, which solved the discrepancy. A low surface gravity is also observed for many young brown dwarfs and is seen as an indicator of youth.

The detection of water and carbon monoxide in the planetary atmosphere was announced in 2021, as was an astrometric measurement of the planet's mass.

The detection of water and carbon dioxide in the planetary atmosphere was announced in 2025. It was the first time CO₂ was directly (as opposed to by transit spectra) detected in an exoplanet.