Epsilon Indi Ab is a

gas giant A gas giant is a giant planet composed mainly of hydrogen and helium. Jupiter and Saturn are the gas giants of the Solar System. The term "gas giant" was originally synonymous with "giant planet". However, in the 1990s, it became known that Uranu ...

exoplanet An exoplanet or extrasolar planet is a planet outside the Solar System. The first confirmed detection of an exoplanet was in 1992 around a pulsar, and the first detection around a main-sequence star was in 1995. A different planet, first det ...

orbiting the star Epsilon Indi A, about 11.9

light-year A light-year, alternatively spelled light year (ly or lyr), is a unit of length used to express astronomical distances and is equal to exactly , which is approximately 9.46 trillion km or 5.88 trillion mi. As defined by the International Astr ...

s away in the

constellation A constellation is an area on the celestial sphere in which a group of visible stars forms Asterism (astronomy), a perceived pattern or outline, typically representing an animal, mythological subject, or inanimate object. The first constellati ...

of Indus. The planet was confirmed to exist in 2018. It orbits at around 30 AU (almost as far as

Neptune Neptune is the eighth and farthest known planet from the Sun. It is the List of Solar System objects by size, fourth-largest planet in the Solar System by diameter, the third-most-massive planet, and the densest giant planet. It is 17 t ...

from the Sun) with a period of around 180 years and a relatively high eccentricity of 0.4, and has a mass around seven times that of

Jupiter Jupiter is the fifth planet from the Sun and the List of Solar System objects by size, largest in the Solar System. It is a gas giant with a Jupiter mass, mass more than 2.5 times that of all the other planets in the Solar System combined a ...

. It was directly imaged using the James Webb Space Telescope in 2023 and the image was released on 24 July, 2024. , Epsilon Indi Ab is the nearest exoplanet to be directly imaged, and with a temperature of about , 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). It is predicted, based on evolutionary models, to have a

luminosity Luminosity is an absolute measure of radiated electromagnetic radiation, electromagnetic energy per unit time, and is synonymous with the radiant power emitted by a light-emitting object. In astronomy, luminosity is the total amount of electroma ...

around to , which together with its effective temperature of 275 K implies a radius of . 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.

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 and orbital separation of roughly 6.5 AU was within the parameters of the highly approximate data. A visual search using the ESO's Very Large Telescope found one potential candidate. However, a subsequent examination by the

Hubble Space Telescope The Hubble Space Telescope (HST or Hubble) is a space telescope that was launched into low Earth orbit in 1990 and remains in operation. It was not the Orbiting Solar Observatory, first space telescope, but it is one of the largest and most ...

NICMOS showed that this was a background object. 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. A longer study of radial velocity, using the

HARPS The High Accuracy Radial Velocity Planet Searcher (HARPS) is a high-precision Echelle grating, echelle planet-finding spectrograph installed in 2002 on the ESO 3.6 m Telescope, ESO's 3.6m telescope at La Silla Observatory in Chile. The First l ...

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 arXiv (pronounced as "archive"—the X represents the Chi (letter), Greek letter chi ⟨χ⟩) is an open-access repository of electronic preprints and postprints (known as e-prints) approved for posting after moderation, but not Scholarly pee ...

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 ''Monthly Notices of the Royal Astronomical Society'' (MNRAS) is a peer-reviewed scientific journal in astronomy, astrophysics and related fields. It publishes original research in two formats: papers (of any length) and letters (limited to ...

''. 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 . Updated orbital solutions were published in 2023, finding a higher eccentricity. A direct imaging attempt of this planet using the James Webb Space Telescope was performed in 2023, 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. The JWST and VLT/VISIR observations imply a

super-Jupiter A super-Jupiter is a gas giant exoplanet that is more massive than the planet Jupiter. For example, substellar companion, companions at the planet–brown dwarf borderline have been called super-Jupiters, such as around the star Kappa Andromedae. ...

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 Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The abundance of methane on Earth makes ...

carbon dioxide Carbon dioxide is a chemical compound with the chemical formula . It is made up of molecules that each have one carbon atom covalent bond, covalently double bonded to two oxygen atoms. It is found in a gas state at room temperature and at norma ...

, and

carbon monoxide Carbon monoxide (chemical formula CO) is a poisonous, flammable gas that is colorless, odorless, tasteless, and slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom connected by a triple bond. It is the si ...

commonly found in giant planets. This might be confirmed in the future with a spectrum. Alternatively this could be explained with a cloudy atmosphere. A second direct imaging attempt on this system to confirm the nature of this planet has been approved. 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. 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 The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Sola ...

. At this temperature, which is warmer than

(T_eff=125 K), but colder than 350 K, it is predicted that such an exoplanet could have water ice clouds and lower layers of

sulfide Sulfide (also sulphide in British English) is an inorganic anion of sulfur with the chemical formula S2− or a compound containing one or more S2− ions. Solutions of sulfide salts are corrosive. ''Sulfide'' also refers to large families o ...

clouds.

Notes

References

{{Indus (constellation) Exoplanets discovered in 2018 Exoplanets detected by radial velocity Exoplanets detected by astrometry Exoplanets detected by direct imaging Giant planets Indus (constellation)

History of observations

See also

Notes

References