.png) Epsilon Indi Ab imaged byJWSTMIRI. The star motif marks the position of its host star; whose light is blocked by acoronagraph. | |
| Discovery[1] | |
|---|---|
| Discovered by | Fabo Feng, et al. |
| Discovery date | 21 March 2018 (suspected since 2002) |
| Radial velocity | |
| Orbital characteristics[2] | |
| 29.2+3.3 −3.4 AU | |
| Eccentricity | 0.399+0.059 −0.076 |
| 180+32 −31 years | |
| Inclination | 105.4°+2.5° −2.4° |
| Star | Epsilon Indi A |
| Physical characteristics[3] | |
| 1.08RJup[a] | |
| Mass | 7.29+0.60 −0.61 MJ[2] |
| Temperature | 275 K (2 °C; 35 °F) |
Epsilon Indi Ab is agas giantexoplanet orbiting the starEpsilon Indi A, about 11.9light-years away in theconstellation ofIndus. The planet was confirmed to exist in 2018.[1] It orbits at around 30AU (almost as far asNeptune from the Sun) with a period of around 180 years and a relatively higheccentricity of 0.4, and has a mass around seven times that ofJupiter.[2] It wasdirectly imaged using theJames Webb Space Telescope in 2023[4] and the image was released on 24 July, 2024.[5]
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 imagedbrown dwarf (the exception being the planetary-massWISE 0855−0714).[5] It is predicted, based on evolutionary models, to have aluminosity around to6.31×10−8 L☉.[3]
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]
The first evidence of Epsilon Indi Ab was found in 2002 when measurements of theradial 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 aminimum mass of1.6 MJ and orbital separation of roughly 6.5 AU was within the parameters of the highly approximate data.[6]
A visual search using theESO'sVery Large Telescope found one potential candidate. However, a subsequent examination by theHubble Space TelescopeNICMOS showed that this was a background object.[7] 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.[8]
A longer study of radial velocity, using theHARPS 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".[9] This refined the radial-velocity trend observed and indicated a planetary companion with an orbital period greater than 30 years and a minimum mass of0.97 MJ. 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.[9]
In March 2018, apreprint was posted toarXiv that confirmed the existence of Epsilon Indi Ab usingradial velocity measurements.[1] In December 2019, the confirmation of this planet, along with updated parameters from both radial velocity andastrometry, was published by Fabo Feng et al. inMonthly 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 of3.25 MJ.[10] Updated orbital solutions were published in 2023, finding a higher eccentricity.[11][12]
A direct imaging attempt of this planet using theJames Webb Space Telescope was performed in 2023,[4] 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.[5] The JWST andVLT/VISIR observations imply asuper-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 bymethane,carbon dioxide, andcarbon 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.[13] A second direct imaging attempt on this system to confirm the nature of this planet has been approved.[14] 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 4.1arcsec in JWST MIRI data and 4.8 arcsec in VLT VISIR data. It is undetected in VLT NaCo observations.[3] The temperature of 275 K is slightly warmer or similar to the nearby Y-dwarfWISE J0855−0714 (225 to 260 K or 285 K), making Epsilon Indi Ab likely one of the coldest objects to be directly imaged outside thesolar system. At this temperature, which is warmer thanJupiter (Teff=125 K),[15] but colder than 350 K, it is predicted that such an exoplanet could havewater ice clouds and lower layers ofsulfide clouds.[16]
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.
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.
