As of 2024[update], Epsilon Indi Ab is the nearest exoplanet to be directly imaged, and with a temperature of about 275 K (2 °C; 35 °F), is also the coolest exoplanet to be directly imaged, and cooler than all but one imaged brown dwarf (the exception being the planetary-mass WISE 0855−0714).[4] It is predicted, based on evolutionary models, to have a luminosity around to 6.31×10−8L☉.[2]
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.[1]
History of observations
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 1.6MJ and orbital separation of roughly 6.5 AU was within the parameters of the highly approximate data.[5]
A visual search using the ESO's Very Large Telescope found one potential candidate. However, a subsequent examination by the Hubble Space TelescopeNICMOS showed that this was a background object.[6] As of 2009, a search for an unseen companion at 4 μm failed to detect an orbiting object. These observations further constrained the hypothetical object to be 5–20 times the mass of Jupiter, orbiting between 10 and 20 AU and have an inclination of more than 20°. Alternatively, it may be an exotic stellar remnant.[7]
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".[8] 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 0.97MJ. 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.[8]
In March 2018, a preprint was posted to arXiv that confirmed the existence of Epsilon Indi Ab using radial velocity measurements.[1] 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 3.25MJ.[9] Updated orbital solutions were published in 2023, finding a higher eccentricity.[10][11]
A direct imaging attempt of this planet using the James Webb Space Telescope was performed in 2023,[3] and the image was released in 2024. The detected planet's mass and orbit are different from what was predicted based on radial velocity and astrometry observations.[4] The JWST and VLT/VISIR observations imply a super-Jupiter with a mass of about 6 Jupiter masses. The object is fainter than expected in the shorter wavelengths, which might be due to absorption by methane, carbon dioxide, and carbon monoxide commonly found in giant planets. This might be confirmed in the future with a spectrum. Alternatively this could be explained with a cloudy atmosphere.[12] A second direct imaging attempt on this system to confirm the nature of this planet has been approved.[13] The new orbital parameters were calculated with archived radial velocity data, the Hipparcos-Gaia astrometry of the host star and the position of the planet from the images. The planet has a semi-major axis of around 30 AU, an eccentricity of 0.4 and an inclination of 104°. The planet-star separation is 4.1 arcsec in JWST MIRI data and 4.8 arcsec in VLT VISIR data. It is undetected in VLT NaCo observations.[2] The temperature of 275 K is slightly warmer or similar to the nearby Y-dwarf WISE J0855−0714 (225 to 260 K or 285 K), making Epsilon Indi Ab likely one of the coldest objects to be directly imaged outside the solar system. At this temperature, which is warmer than Jupiter (Teff=125 K),[14] but colder than 350 K, it is predicted that such an exoplanet could have water ice clouds and lower layers of sulfide clouds.[15]
^While more complete orbital solutions have been published in earlier work, they are significantly discrepant from the most recent orbital solution including imaging data, so only parameters from the most recent solution are included here.
^Calculated using given a semi-major axis of 28.4 AU and a host star mass of 0.76 M☉
^ abcdFeng, Fabo; Tuomi, Mikko; Jones, Hugh R. A. (23 March 2018). "Detection of the closest Jovian exoplanet in the Epsilon Indi triple system". arXiv:1803.08163 [astro-ph.EP].
^ ab"A direct detection of the closest Jupiter analog with JWST/MIRI". stsci.edu. STScI. Retrieved 31 July 2022. We will collect the first direct images of a radial velocity planet, by targeting Eps Indi Ab with JWST/MIRI. [...] Our simulations confirm that we will detect Eps Indi Ab's thermal emission at high confidence, regardless of its cloud properties or thermal evolution.
^Geißler, K.; Kellner, S.; Brandner, W.; Masciadri, E.; Hartung, M.; Henning, T.; Lenzen, R.; Close, L.; Endl, M.; Kürster, M. (2007). "A direct and differential imaging search for sub-stellar companions to epsilon Indi A". Astronomy and Astrophysics. 461 (2): 665–668. arXiv:astro-ph/0611336. Bibcode:2007A&A...461..665G. doi:10.1051/0004-6361:20065843. S2CID119442720.
^"Confirmation of the closest directly detected exoplanet: a super-Jupiter orbiting Eps Ind A". stsci.edu. STScI. Retrieved 1 March 2024. JWST Cycle 1 images of Eps Ind A reveal a candidate companion that is consistent in color and magnitude with a massive (~10Mjup) planet. However, the position angle and mass of the candidate is different than expected from RV/astrometric models of the companion orbit - perhaps suggesting there are two giant planets in this system, with only one of these detected in the MIRI images.
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