? Indi A has one known planet, ? Indi Ab, with a mass of 3.3 Jupiter masses in a nearly circular orbit with a period of about 45 years. ? Indi Ab is the closest Jovian exoplanet. The ? Indi system provides a benchmark case for the study of the formation of gas giants and brown dwarfs.
The constellation Indus (the Indian) first appeared in Johann Bayer's celestial atlas Uranometria in 1603. The 1801 star atlas Uranographia, by German astronomer Johann Elert Bode, places ? Indi as one of the arrows being held in the left hand of the Indian.
The star is among five nearby paradigms as K-type stars of a type in a 'sweet spot' between Sun-analog stars and M stars for the likelihood of evolved life, per analysis of Giada Arney from NASA's Goddard Space Flight Center.
? Indi A is a main-sequence star of spectral type K5V. The star has only about three-fourths the mass of the Sun and 71% of the Sun's radius. Its surface gravity is slightly higher than the Sun's. The metallicity of a star is the proportion of elements with higher atomic numbers than helium, being typically represented by the ratio of iron to hydrogen compared to the same ratio for the Sun; ? Indi A is found to have about 87% of the Sun's proportion of iron in its photosphere.
The corona of ? Indi A is similar to the Sun, with an X-ray luminosity of 2×1027 ergs s-1 (2×1020 W) and an estimated coronal temperature of 2×106 K. The stellar wind of this star expands outward, producing a bow shock at a distance of 63 AU. Downstream of the bow, the termination shock reaches as far as 140 AU from the star.
Position of Sun and ? Centauri in Ursa Major as seen from ? Indi
This star has the third highest proper motion of any star visible to the unaided eye, after Groombridge 1830 and 61 Cygni, and the ninth highest overall. This motion will move the star into the constellation Tucana around 2640 AD. ? Indi A has a space velocity relative to the Sun of 86 km/s,[note 1] which is unusually high for what is considered a young star. It is thought to be a member of the ? Indi moving group of at least sixteen population I stars. This is an association of stars that have similar space velocity vectors, and therefore most likely formed at the same time and location. ? Indi will make its closest approach to the Sun in about 17,500 years when it makes perihelion passage at a distance of around 10.58 light-years (3.245 pc).
Artist's conception of the Epsilon Indi system showing Epsilon Indi A and its brown-dwarf binary companions.
In January 2003, astronomers announced the discovery of a brown dwarf with a mass of 40 to 60 Jupiter masses in orbit around ? Indi A at a distance of at least 1,500 AU. In August 2003, astronomers discovered that this brown dwarf was actually a binary brown dwarf, with an apparent separation of 2.1 AU and an orbital period of about 15 years. Both brown dwarfs are of spectral class T; the more massive component, ? Indi Ba, is of spectral type T1-T1.5 and the less massive component, ? Indi Bb, of spectral type T6.
Evolutionary models have been used to estimate the physical properties of these brown dwarfs from spectroscopic and photometric measurements. These yield masses of and times the mass of Jupiter, and radii of and solar radii, for ? Indi Ba and ? Indi Bb, respectively. The effective temperatures are 1300-1340 K and 880-940 K, while the log g (cm s-1) surface gravities are 5.50 and 5.25, and their luminosities are and the luminosity of the Sun. They have an estimated metallicity of [M/H] = -0.2.
Measurements of the radial velocity of Epsilon Indi by Endl et al. (2002) appear to show a trend that indicated a planetary companion with an orbital period of more than 20 years. A substellar object with minimum mass of 1.6 Jupiter masses and orbital separation of roughly 6.5 AU (a Jupiter-analogue) 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 TelescopeNICMOS 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 (to or from Earth) velocity, using the Echelle spectrometer on the HARPS telescope, to follow up on Endl findings, was published in a paper by M. Zechmeister et al. in 2013. The findings confirm that, quoting the paper, "Epsilon Ind A has a steady long-term trend still explained by a planetary companion". This refined the radial-velocity trend observed and indicates a planetary companion with an orbital period of 45 years. A gas giant with a minimum mass of 0.97 Jupiter masses and a minimal orbital separation of roughly 9.0 AU could explain the observed trend.[note 3] 9.0 AU is about the same distance out as Saturn. This does not quite qualify the planet as a true Jupiter analogue because it orbits considerably further out than 5.0 AU. Not only does it orbit further out, but ? Indi A is also dimmer than the Sun, so it would only receive about the same amount of energy per square meter as Uranus does from the Sun. The radial-velocity trend was observed through all the observations so far taken using the HARPS telescope but due to the long time period predicted for just one orbit of the object around ? Indi A, more than 30 years, the astrometric phase coverage is not yet complete.
In March 2018, the existence of the planet was confirmed using radial velocity measurements. At a separation of 3.3 arcseconds from its host star, ? Indi Ab is a candidate for direct imaging by the James Webb Space Telescope.
In October 2019, Feng et al. published an updated orbit for the planet. They show that the orbit is slightly eccentric, with an eccentricity of about 0.26. The mass of the planet is 3.25 Jupiter masses, and it orbits at a distance of 11.6 AU, with a period of 45 years.
^The space velocity components are: U = -77; V = -38, and W = +4. This yields a net space velocity of km/s.
^From ? Indi the Sun would appear on the diametrically opposite side of the sky at the coordinates RA=, Dec=56° 47′ 10″, which is located near Beta Ursae Majoris. The absolute magnitude of the Sun is 4.8, so, at a distance of 3.63 parsecs, the Sun would have an apparent magnitude .
^Kepler's Third Law, assuming a circular orbit gives . The mass and period are known from the paper, so the semimajor axis can be calculated using
^Lachaume, R.; Dominik, C.; Lanz, T.; Habing, H. J. (1999). "Age determinations of main-sequence stars: combining different methods". Astronomy and Astrophysics. 348: 897-909. Bibcode:1999A&A...348..897L. -- This paper gives a median log age = 9.11, with a range of min = 8.91 and max = 9.31. This corresponds to 1.3 Gyr, with an error range of 0.8-2.0 Gyr.
^Smith, Verne V.; Tsuji, Takashi; Hinkle, Kenneth H.; Cunha, Katia; Blum, Robert D.; Valenti, Jeff A.; Ridgway, Stephen T.; Joyce, Richard R.; Bernath, Peter (2003). "High-resolution infrared spectroscopy of the brown dwarf ? Indi Ba". The Astrophysical Journal Letters. 599 (2): L107-L110. doi:10.1086/381248. S2CID117133193.
^ abFeng, 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].
^Callandreau, O. (1886). "Revue des publications astronomiques. Heliometer determinations of Stellar parallax, in the southern hemisphere, by David Gill and W. L. Elkin". Bulletin Astronomique (in French). 2 (1): 42-44. Bibcode:1885BuAsI...2...42C.