On July 24, 2024, an international team of scientists, headed by Dr. Elisabeth Matthews of the Max Plank Institute for Astronomy, announced that they had used the James Webb Space Telescope (JWST) to directly image a temperate super-Jovian exoplanet orbiting the nearby Sun-like star, Epsilon Indi A (or ε Indi A). With its existence suspected for over two decades, this giant exoplanet, designated ε Indi Ab, represents the first time a Jupiter-like exoplanet orbiting an older Sun-like star had ever been imaged. What follows is background information about the ε Indi system and how its “Jupiter analog” was found.
An artist’s rendering of the James Webb Space Telescope (JWST) in action. (NASA)
Background
Known to the ancients, the star ε Indi is a K5V type with a V magnitude of 4.83 located in the southern constellation of Indus – The Indian. Also known by various other catalog designations (e.g. HD 209100 and GJ 845), the common name of ε Indi was established in 1603 by the German celestial cartographer, Johann Bayer (1572-1625), in his famous star catalog called Uranometria. In 1847, German astronomer Henrich Louis d’Arrest (1822-1875) noted that the position of ε Indi had changed when comparing data in star catalogs going back to 1750 indicating that this star has a substantial proper motion. Current measurements indicate a proper motion of 4.7 arc seconds per year – the third fastest moving naked eye star in the sky after the much dimmer Groombridge 1830 and 61 Cygni.
The southern constellation Indus (The Indian) and its surroundings, as drawn in the famous Uranographia published 1801 of German astronomer Johann Elert Bode. Click on image to enlarge. (ESO/Uranographia)
Since high proper motion is suggestive of a nearby star, the parallax of ε Indi was measured for the first time in 1883 by Scottish astronomer David Gill (1843-1914) and American William L. Elkin (1854-1933) while working together at the Royal Observatory at the Cape of Good Hope in South Africa. Their distance measurement of about 15±2 light years was not too far off from today’s best measurement of 11.869±0.004 light years making ε Indi among the closest stars known. Because of its closeness, ε Indi was included in the first edition of the Gliese Catalogue of Nearby Stars in 1957 earning it the designation of GJ 845 after the creator of the catalog, German astronomer Wilhelm Gliese (1915-1993), and his longtime collaborator on later editions, Hartmut Jahreiß.
3D map of stellar systems in the solar neighborhood within a radius of 12.5 light-years. The Sun is at the center and the ε Indi system lies near the bottom. Click on image to enlarge. (ESO/R.D. Scholz et al.)
The best current measurements of the properties of ε Indi give it an effective surface temperature of 4,630 K, a radius 0.73 times that of the Sun, a luminosity of 0.22 times and a mass estimated to be 0.76 times. With a metallicity of about 87% that of the Sun, ε Indi has a slightly lower concentration of elements heavier than helium compared to our system’s central star. Long term photometric observations suggest the period of rotation for ε Indi is about 35 days. Based on a comparison of the star’s properties with models of stellar evolution, it appears that ε Indi is about 3.5 +0.8/-1.3 billion years old, although some work suggests this star may be as old as 5.7 billion years. Overall, ε Indi seems to be a slightly cooler, smaller, dimmer but likely younger version of the Sun.
ε Indi A (the bright star at far right) and its brown dwarf companion ε Indi B (circled). The upper image comes from one of the SuperCOSMOS Sky Surveys (SSS) I-band photographic plates. The lower image is a near-infrared image (Ks-band, 2.1 µm) from the Two Micron All Sky Survey (2MASS). Both images cover roughly 7 by 5 arcmin. Click on image to enlarge. (ESO/R.D.Scholz et al.)
It was long thought that ε Indi was a single star similar to the Sun-like nearby stars ε Eridani and τ Ceti (see “Habitable Planet Reality Check: Tau Ceti”) making it a target for the search of a solar system analog which might include a Earth-size planets orbiting in its habitable zone (HZ) as well as a frequent SETI target. But in January 2003, Scholz et al. announced the discovery of a brown dwarf moving with ε Indi at a projected distance of 1,459 AU. With the orbital period probably on the order of a couple of tens to a couple of hundreds of thousands of years, it will be some time before any orbital motion would become apparent. Subsequent high-resolution infrared imagery acquired by McCaughrean et al. later that year using the NAOS/CONICA adaptive optics imaging system (or NACO, for short) on the European Southern Observatory’s (ESO’s) 8.2-meter VLT in Cerro Paranal, Chile revealed that ε Indi B, as it was now called, was actually a pair of brown dwarfs with an apparent separation of just 0.732 arc seconds making this the closest brown dwarf binary then known (surpassed in 2013 by the discovery of Luhman 16 just 6.5 light years away). The discovery also made ε Indi the third closest triple star system known after α Centauri (see the α Centauri page) and Luyten 789-6.
Artist conception of the ε Indi B binary brown dwarf. Click on image to enlarge. (Gemini Observatory)
The pair of brown dwarfs distantly orbiting the Sun-like ε Indi A have been designated ε Indi Ba and Bb. The brighter component, ε Indi Ba, is a type T1-1.5 brown dwarf with a temperature in the 1,352 to 1,385 K range. The dimmer ε Indi Bb is spectral type T6 with a cooler temperature in the 976 to 1,011 K range. Using nine years of observations by NACO of the motion of the components of ε Indi B from 2004 to 2013, Chen et al. were able to determine in 2022 that these brown dwarfs are locked in a moderately eccentric 1.11 by 3.71 AU orbit with a period of 11.0 years. The dynamical masses of ε Indi Ba and Bb were determined to be 66.9±0.4 times that of Jupiter (or MJ) and 53.3±0.3 MJ, respectively. Continued observations of the components of ε Indi B promise to provide much new information on the properties and early evolution of brown dwarfs.
Initial Exoplanet Search Results
Because of the Sun-like nature of ε Indi and its relative closeness, it has been a high-priority target for southern hemisphere exoplanet surveys. The best published survey results as of the turn of the century were by Endl et al. in 2002. This international collaboration observed ε Indi as part of the CES Survey of 37 late-type, nearby star which ran from November 1992 to April 1998. Using the Coude Echelle Spectrometer and Long Camera (CES LC) on ESO’s 1.4-meter Coude Auxiliary Telescope (CAT) in La Silla, Chile, Endl et al. obtained a long series of precision radial velocity (RV) measurements in the hopes of detecting exoplanets orbiting their nearby targets.
ESO’s 1.4-meter Coude Auxiliary Telescope (CAT) was used between November 1992 and May 1998 to gather precision RV measurements of ε Indi and other stars as part of the CES Survey. (ESO)
In the case of ε Indi (which was still thought to be a single star at the time), Endl et al. secured 73 RV measurements over a span of 5.2 years. While the team failed to find any statistically significant periodic signals in the data indicative of an exoplanet in a short-period orbit, they did note a long term trend in the RV amounting to an increase of 21 meters per second during their survey. The measurements the increase in RV was 4.4 meters per second per year with an RMS scatter of 11.6 meters per second in their data set. This trend could be explained by the presence of a companion in an orbit with a period greater than 20 years. Endl et al. suggested that the trend could be caused by an exoplanet with a semimajor axis of ~6.5 AU (equivalent to an orbital period of ~19 years) with an MPsini of 1.6 MJ. Because the inclination, i, of an exoplanet’s orbit to the plane of the sky cannot be determined directly from RV measurements alone, only the minimum mass or MPsini can be determined. The actual mass would almost surely be higher. When the ε Indi B brown dwarf binary was found in 2003, it was quickly realized that they could not be responsible for the observed RV trend bolstering the case for a Jupiter-like gas giant distantly orbiting ε Indi A.
The RV measurements from the original CES Survey showing the clear increase over time. Click on image to enlarge. (Endl et al.)
As the ESO team continued to gather precision RV data to provide a definitive detection, others employed ever improving direct imaging techniques to spot this possible exoplanet. The most sensitive such search to date was made by Janson et al. who made observations of ε Indi A on the nights of July 3, October 31 and November 2, 2008 using ESO’s NAOS/VLT operating at the infrared wavelength of about 4 μm where a young, massive gas giant would be expected to be comparatively bright. Janson et al. failed to find anything near ε Indi A in their images placing strict upper limits on the mass of the suspected exoplanet. Combined with the trend derived from the latest RV measurements (which was now pegged at 2.6 meters per second per year) and considering the projected separation could be smaller than the semimajor axis at the time of their observations, Janson et al. concluded that the possible companion of ε Indi A must have a mass in the 5 to 20 MJ range (depending on the assumptions made about the age of the system), a semimajor axis somewhere in the 10 to 20 AU range and an inclination, i, of greater than 20°.
The upper limits of the masses found by Janson et al. for three different presumed ages for any exoplanet orbiting ε Indi A as a function of projected separation (which can be smaller than the actual semimajor axis). The solid line are the values suggested by the RV trend with i=0° while the dashed line represent i=60°. Click on image to enlarge. (Janson et al.)
A More Detailed Search
By 2013, the ESO team performing RV surveys searching for exoplanets had a substantially expanded data set to characterize the possible substellar companion of ε Indi A. In the best published results at this time, Zechmeister et al. presented a new analysis for ε Indi A based on not only the original 78 CES Survey RV measurements but new ones with significantly improved measurement accuracy. Between November 1999 and May 2006, Zechmeister et al. secured 54 new measurements taken over 5.8 years using CES with the VLC (Very Long Camera) now on ESO’s 3.6-meter telescope at La Silla. From November 2003 to December 2009, an additional 457 measurements were acquired over a period of 5.9 years using the then-new HARPS (High Accuracy Radial velocity Planet Search) spectrograph also on the 3.6-meter telescope which has been used to spot so many nearby exoplanets in recent years (see the HARPS page).
The ESO 3.6m telescope equipped first with CES VLC and then the HARPS spectrograph was used to gather precision RV measurements of ε Indi A. (ESO/H.H. Heyer)
As before, Zechmeister et al. found no evidence for any statistically significant periodic signals in their precision RV data set that would suggest the presence of exoplanets in short period orbits. They did, however, confirm the 2.4 meter per second per year increase in RV noted earlier in the combined CES LC and VLC data set. This was much larger than the 0.009 meter per second per year rate expected from the reflex motion from the ε Indi B brown dwarf binary. The data suggested an orbital period greater than 30 years corresponding to a semimajor axis in excess of 9 AU for a planet with a MPsini of at least 0.97 MJ. These new findings further confirmed the limits based on the work by Janson et al..
The CES LC, CES VLC and HARPS data sets analyzed by Zechmeister et al. showing the clear increase in the RV of ε Indi A over time. Click on image to enlarge. (Zechmeister et al.)
RV analysis result shared in 2018 from Feng et al. started with the data set used by Zechmeister et al. and added 4,149 new RV measurements derived from ESO’s archived HARPS spectra including 3,636 high cadence measurements taken over two weeks as part of a campaign to monitor oscillations in ε Indi A. Feng et al. also analyzed various subsets of these newer data: 518 measurements which excluded the high cadence measurements with a low signal-to-noise ratio and a more conservative set of 465 point which excluded HARPS RV measurements made after 2015, which have an as yet imprecisely characterized offset after an upgrade to the instrument’s optical fiber feed. Using the HARPS spectra, Feng et al. also calculated a series of standard stellar activity indicators to see how they may correlate with periodic signals in the RV measurements. A strong correlation would favor a non-planetary explanation for any observed RV variations.
A detailed analysis of the various data sets (and subsets) did indeed reveal three signals with periods of 11, 17.8 and 278 days. Unfortunately these periods seem to be the result of stellar activity and how it is modulated by the 35-day rotation period of ε Indi A. There is also a period of 2500 days noted in the measures of stellar activity corresponding to long term cycles in magnetic activity (shorter than the Sun’s activity cycle of about 4,000 days). The results suggested that there are no exoplanets present around ε Indi A which would produce short-period RV variations with a semiamplitude greater than about 1 meter per second. For the habitable zone (HZ) of ε Indi A which is conservatively defined by Kopparapu et al. (2013, 2014) to range from 0.47 to 0.87 AU (corresponding to orbital periods of ~136 to ~339 days), this result eliminated the possibility of any exoplanets with an MPsini of about 7 to 9 times the mass of the Earth. While this effectively excludes the possible presence of Neptune-mass exoplanets or larger orbiting inside of the HZ, Earth-size exoplanets could still be present and remain undetected by the surveys to date.
The analysis of the RV data for ε Indi A combining the various data set over the decades showing that the increase in RV is beginning to flatten out. The data are fitted to an orbit with a period of 52.62 years. Click on image to enlarge. (Feng et al. 2018)
The results of the analysis by Feng et al. also suggested that the long term RV trend appeared to be flattening out and possibly even reversing. The analysis of the combined long term RV data sets spanning a quarter of a century by Feng et al. confirmed that ε Indi A is apparently orbited by a gas giant in a distant orbit. Their best fit to the available data suggested an essentially circular orbit with a period of 52.62 +27.70/-4.12 years corresponding to a semimajor axis of 12.82 +4.18/-0.71 AU – broadly consistent with the earlier limits by Janson et al.. The semiamplitude of 24.67 +14.28/-3.50 meters per second suggested an exoplanet with a MPsini of 2.71 +2.19/-0.44 MJ. Although not strictly considered a “Jupiter analog” because of its comparatively large orbit, this represents the closest known Jovian exoplanet.
Building the Case for ε Indi Ab
In 2019, Feng et al. published yet another analysis that included not only the RV dataset from their earlier work, but astrometric measurements from Hipparcos and the DR2 (Data Release 2) from Gaia to further constrain the orbital properties and the actual mass of ε Indi Ab. This new analysis found that the orbital period of this exoplanet was 45.20 +5.74/-4.77 years with a semimajor axis of 11.55 +0.98/-0.86 AU. This more refined orbit determination found the eccentricity to be 0.26 +0.07/-0.02 indicating that this exoplanet’s orbit around ε Indi A ranges from about 8.5 to 15 AU. Thanks to the astrometric measurements, the inclination of the exoplanet’s orbit to the plane of the sky, i, was determined to be about 64° +14°/-6°. Combined with the MPsini value calculated from the RV data analysis, the mass of ε Indi Ab was determined to be about 3.3 +0.4/-0.7 MJ. This newly derived orbit led Feng et al. to predict that ε Indi Ab would increase its apparent distance from its sun from 1.1 arc seconds in 2020 to 3.3 arc seconds in 2030 making it an ideal target for direct imaging by JWST.
The orbit derived by Feng et al. (2019) of the orbit of ε Indi Ab relative to is host star projected onto the plane of the sky. Click on image to enlarge. (Feng et al. 2019)
In February 2023, Philipot et al. published a new analysis of the properties of ε Indi Ab based on an expanded set of RV and astrometric measurements. In addition to the RV measurements used earlier by Zechmeister et al. and Feng et al., Philipot et al. used an expanded set of 4,278 RV measurements from HARPS covering from 2003 to 2016 as well as 163 UVES RV measurements from 1996 to 2017. They combined this expanded RV data set with astrometric measurements from Hipparcos and the EDR3 (Early Data Release 3) from Gaia, whose measurements were of higher quality than DR2. Philipot et al. found that the orbit of ε Indi Ab has a semimajor axis of 8.8 +0.2/-0.1 AU (corresponding to a period of about 30 years) and an eccentricity of 0.48±0.1 so that the exoplanet ranges from about 4.6 to 13 AU – slightly smaller and more eccentric than the orbit found by Feng et al. in 2019. With the orbital inclination now pegged at 91° +4°/-5°, the estimated mass of ε Indi Ab came in at 3.0±0.1 MJ which was still consistent to within the uncertainties of the earlier result.
Orbital fits for ε Indi Ab. Top: fit of the Epsilon Indi A RV data. Bottom: fit of the ε Indi A astrometric acceleration in right ascension (left) and declination (right). The black points correspond to the measurements obtained Hipparcos and Gaia EDR3. In each plot, the black curve shows the best fit; The color bar indicates the log likelihood of the different fits plotted. Click on image to enlarge. (Philipot et al.)
Finally, in October 2023, Feng et al. published an updated orbital analysis of their own. They used the same expanded data set as Philipot et al., but employed both DR2 and DR3 (which was functionally equivalent to EDR3) from Gaia to further constrain the orbital elements of ε Indi Ab. The team found that ε Indi Ab had a mass of 2.96 +0.41/-0.38 MJ, an orbital period of 42.92 +6.38/-4.09 years, and an eccentricity of 0.42±0.04. These parameters implied an orbit which ranged from about 6.5 to 16 AU. While consistent with their earlier results from 2019, Feng et al. found a significantly longer orbital period than Philipot et al.. Feng et al. attributed the mismatch to the different data analysis techniques they employed, how noise was modeled, and the inclusion of the Gaia DR2 data. Despite the somewhat disparate results, there was high confidence that a gas giant orbited ε Indi A and that JWST, which became operational in early 2022, would be capable of imaging this world providing an independent means of verifying its existence.
JWST Observations
With their proposal to use JWST to observe ε Indi Ab, Matthews et al. were able to secure 23.9 hours of observation time using MIRI (Mid-Infrared Instrument) during Cycle 1. MIRI, which can acquire images in the mid to long infrared wavelength band from 5 to 28 µm, is equipped with a coronagraph to block out the light from the bright ε Indi A in order to reveal dimmer objects in its vicinity after the images have been properly processed to remove residual artifacts. MIRI’s image scale of 0.11 arc seconds/pixel translates to 60 million kilometers per pixel at the distance of ε Indi A. This is too coarse to resolve the disk of ε Indi Ab (or reveal any nearby orbiting exomoons it might have), so this exoplanet would appear as only a point of light in the images.
The pair of insets in this composite illustration show the JWST/MIRI images of ε Indi Ab at wavelengths of 10.65 µm and 15.55 µm. The position of the host star, ε Indi A, is indicated by the star symbol. Click on image to enlarge. (T. Müller – MPIA/HdA, E. Matthews – MPIA)
The images released by Matthews et al. on July 24, 2024 readily revealed a bright object near ε Indi A in the data acquired at wavelengths of 10.65 µm and 15.55 µm. As a check, Matthews et al. searched archived images from ground-based infrared instruments to see if the same object was present. They found a faint source right where it was expected in reprocessed images acquired in 2019 by ESO’s VLT spectrometer and imager for the mid-infrared (VISIR) instrument lending further weight to their JWST observations. The object was not found in archived images acquired by ground-based telescopes in the 3.5 to 5 µm wavelength range. Old archived imagery from NASA’s Spitzer Space Telescope taken at wavelengths of 8 µm and 24 µm, when the proper motion of ε Indi A had the system in a slightly different part of the sky, failed to reveal any distant background object in the position of the source seen by JWST further bolstering the case of a planetary detection.
Here is a view of VLT spectrometer and imager for the mid–infrared (VISIR) located at the Cassegrain focus of one of ESO’s VLT Telescopes. Archived VISIR images of ε Indi A from 2019 helped to verify the existence of the object found by Matthews et al. using JWST. (ESO)
While an exciting result, the object detected by Matthews et al. was in a significantly different position relative to ε Indi A than had been predicted earlier by the analyses of precision RV and astrometric data. Based on their observations, Matthews et al. estimate that the observed exoplanet has a mass of about 6 MJ and is in a much more distant 20 by 40 AU orbit with a period of about two centuries. While the presence of a second gas giant was initially suspected to explain the discrepancy, that possibility has been eliminated based on more detailed follow on analysis of the available data. Despite the differences, Matthews et al. have designated their new find as ε Indi Ab.
Based on the available data, Matthews et al. have been able to infer some important properties for the super-Jovian ε Indi Ab. Its infrared brightness suggests an effective temperature of about 275 K. While about 100 K warmer than the gas giants in our Solar System, this temperature appears roughly consistent with existing models for how large gas giants cool over time. The lack of any detections in the 3.5 to 5 µm wavelength range suggests the exoplanet’s atmosphere is opaque in this band. This might be the result of clouds in the atmosphere or suggests the presence of gases like methane, carbon dioxide, and carbon monoxide hinting at a high metallicity exoplanet with a larger carbon-to-oxygen ratio than expected.
An artist rendition of a cold gas giant. (T. Müller – MPIA/HdA)
While the detection of ε Indi Ab is an important milestone in the study of nearby exoplanets, it represents only an initial step. Matthews et al. have already had their proposal approved for 12.4 hours of observing time using JWST/MIRI during the current Cycle 3. This second set of observations are required to confirm that the purported ε Indi Ab shares its proper motion with ε Indi A in order to confirm its association with this star (instead of the unlikely case that it is an unrelated background object). Further multiband photometry and spectrometry using JWST will also allow the exoplanet’s atmosphere and temperature to be studied in more detail. Combining JWST’s astrometric measurements with earlier astrometric and RV data should also help to sort out the discrepancies between the actual and earlier predicted positions of this gas giant. The addition of new data from ongoing RV programs and the expected release of Gaia’s DR4 in mid-2026 should help resolve the situation further and derive a more accurate orbit and mass for this world giving astronomers vital new information about the nature and evolution of gas giant exoplanets.
Related Reading
“Webb’s First Glimpse of Jupiter, Its Moons & Rings”, Drew Ex Machina, July 13, 2022 [Post]
“Epsilon Indi’s Jovian Exoplanet”, Drew Ex Machina, March 28, 2018 [Post]
General References
Minghan Chen et al., “Precise Dynamical Masses of ε Indi Ba and Bb: Evidence of Slowed Cooling at the L/T Transition”, The Astronomical Journal, Vol. 163, No. 6, pp. 288-303, June 2022
M Endl et al.,” The planet search program at the ESO Coudé Echelle spectrometer. III. The complete Long Camera survey results”, Astronomy and Astrophysics, Vol.392, pp. 671-690, September 2002
F. Feng, M. Tuomi and H.R.A Jones, “Detection of the closest Jovian exoplanet in the Epsilon Indi triple system”, arXiv 1803.08163, March 21, 2018
Fabo Feng et al., “Detection of the nearest Jupiter analogue in radial velocity and astrometry data”, Monthly Notices of the Royal Astronomical Society, Vol. 490, No. 4, pp. 5002–5016, December 2019
F. Feng et al., “Revised orbits of the two nearest Jupiters”, Monthly Notices of the Royal Astronomical Society, Vol. 525, No. 1, pp. 607-619, October 2023
David Gill, “Nouvelles Recherches dur les Distances des Etoiles”, L’Astronomie, Vol. 3, pp. 456-459, December 1884
M. Janson et al., “Imaging search for the unseen companion to ɛ Ind A – improving the detection limits with 4 μm observations”, Monthly Notices of the Royal Astronomical Society, Vol. 399, No. 1, pp. 377-384, October 2009
R.K. Kopparapu et al., “Habitable zones around main-sequence stars: new estimates”, The Astrophysical Journal, Vol. 765, No. 2, Article ID. 131, March 10, 2013
Ravi Kumar Kopparapu et al., “Habitable zones around main-sequence stars: dependence on planetary mass”, The Astrophysical Journal Letters, Vol. 787, No. 2, Article ID. L29, June 1, 2014
E.C. Matthews et al., “A temperate super-Jupiter imaged with JWST in the mid-infrared”, Nature, doi.org/10.1038/s41586-024-07837-8, July 24, 2024
MJ McCaughrean et al., “ε Indi Ba, Bb: The nearest binary dwarf”, Astronomy & Astrophysics, Vol. 413, pp. 1029-1036, January 2004
F. Philipot et al., “Updated characterization of long-period single companion by combining radial velocity, relative astrometry, and absolute astrometry”, Astronomy & Astrophysics, Vol. 670, id. A65, February 2023
R.D. Scholz et al., “ε Indi B: A new benchmark T dwarf”, Astronomy & Astrophysics, Vol. 398, pp. L29-33, January 2003
M. Zechmeister et al., “The planet search programme at the ESO CES and HARPS. IV. The search for Jupiter analogues around solar-like stars”, Astronomy & Astrophysics, Vol. 552, Article ID A78, April 2013
“NASA’s Webb Images Cold Exoplanet 12 Light-Years Away”, JPL, July 24, 2024 [Press Release]
“Webb images nearest super-Jupiter, opening a new window to exoplanet research”, Max Plank Institute for Astronomy, July 24, 2024 [Press Release]