Epsilon Indi Ab

Epsilon Indi Ab is a gas giant exoplanet orbiting the star Epsilon Indi A, about 11.9 light-years away in the constellation of Indus. The planet was confirmed to exist in 2018. It orbits at around 8.8 AU with a period of around 29.9 years and a relatively high eccentricity of 0.48, and has a mass around 3 times that of Jupiter. Direct imaging of this planet using the James Webb Space Telescope is planned.

The Epsilon Indi system also contains a pair of brown dwarfs, Epsilon Indi Ba and Bb, at a wide separation from the primary star. As such, this system provides a benchmark case for the study of the formation of gas giants and brown dwarfs.

Discovery
The first evidence of Epsilon Indi Ab was found in 2002 when measurements of the radial velocity of Epsilon Indi by Endl et al. appeared to show a trend that indicated a planetary companion with an orbital period of more than 20 years.

A substellar object with a minimum mass of and orbital separation of roughly 6.5 AU was within the parameters of the highly approximate data.

A longer study of radial velocity, using the HARPS echelle spectrometer, to follow up on Endl's findings, was published in a paper by M. Zechmeister et al. in 2013. The findings confirm that, quoting the paper, "ε Ind A has a steady long-term trend still explained by a planetary companion".

This refined the radial-velocity trend observed and indicated a planetary companion with an orbital period greater than 30 years and a minimum mass of. The radial-velocity trend was observed through all the observations taken using the HARPS spectrometer, but due to the long period predicted for just one orbit of the object around ε Indi A, more than 30 years, the phase coverage was not yet complete.

In March 2018, a preprint was posted to arXiv that confirmed the existence of Epsilon Indi Ab using radial velocity measurements. In December 2019, the confirmation of this planet, along with updated parameters from both radial velocity and astrometry, was published by Fabo Feng et al. in Monthly Notices of the Royal Astronomical Society. This study found a semi-major axis of about 11.6 AU, an orbital period of about 45 years, an eccentricity of about 0.26, and a mass of. The orbit was revised in 2023, finding a shorter period and higher eccentricity.

A direct imaging attempt of this planet using the James Webb Space Telescope was performed, detecting a candidate object with color and brightness consistent with an approximately 10 Jupiter masses planet. Having different position angle and mass from expected values, this suggests that the system may be orbited by two giant planets. The second direct imaging attempt on this system to confirm the nature of this planet has been approved.