Despite its diminutive size and comparative dimness, Barnard’s Star has played an outsized role in astronomy since its discovery a century ago. Being the second closest known star system, it has been of great interest in exoplanet searches for over half of a century as well as the subject of numerous science fiction stories and even the possible target for our first real-world interstellar voyages. On November 14, 2018, an international collaboration of astronomers announced the first convincing exoplanet detection for Barnard’s Star with hints of more discoveries to come.

Background

Barnard’s Star, whose long-time informal designation was officially adopted by the IAU on February 1, 2017, was named after American astronomer E.E. Barnard (1857-1923). Using the famous 40-inch (one-meter) refractor at the Yerkes Observatory, Barnard made a series of nine position measurements during June and July 1916 of what was later identified as BD+04° 3561 from the Bonner Durchmusterung astrometric star catalog compiled by the Bonn Observatory between 1859 and 1903. Comparing his visual observations with measurements from a photograph he took of the same region of the sky on August 24, 1894 when he was at the Lick Observatory, Barnard discovered that this star was moving at a rate of 10.3 arc seconds per year beating the previous record holder for the highest known proper motion, Kapteyn’s Star, cataloged in 1898 (see “ Habitable Planet Reality Check: Kapteyn b ”). Barnard also reported that a spectrum of the star taken by American astronomer Walter S. Adams (1876-1956) using the 1.5-meter reflecting telescope at the Mt. Wilson Observatory established this star as being a M dwarf. Later parallax measurements from Yerkes and other observatories showed Barnard’s Star to be the second closest star system after α Centauri with the distance currently pegged at 5.95 light years. Because of its closeness, Barnard’s Star was included in the first edition of the Gliese Catalogue of Nearby Stars in 1957 earning its other common designation of Gl 699 after the creator of the catalog, German astronomer Wilhelm Gliese (1915-1993).

Because of its high proper motion, large parallax and the relative ease of observing it from major observatories around the globe due to its location in the constellation of Ophiuchus near the celestial equator, the V magnitude 9.51 Barnard’s Star has been a target of detailed investigation over the last century like no other red dwarf in the sky. Currently, Barnard’s Star is classified as a spectral type M3.5V red dwarf with an effective surface temperature of 3278±51 K, an estimated mass of 0.16±0.02 times that of the Sun and a radius of 0.18±0.01 times or almost twice that of the planet Jupiter (despite having a mass about 170 times that of Jupiter). Combined, these parameters lead to a luminosity of 0.00329±0.00019 times that of the Sun – dim by solar standards but typical for small red dwarfs in our part of the galaxy.

Long term variations in the brightness of Barnard’s Star have suggested a period of rotation of about 130 days while more recent analysis of photometric time series data shows a statistically significant period of 144 days. Analysis of spectroscopic activity indices, which track active regions on the surface of this dwarf as they rotate into and out of view, display periods in the 133 to 143 day range depending on the wavelength. The best guess at this point is a rotational period of ~140 days with the scatter in individual values possibly being due to differential rotation as a function of latitude much as our Sun displays. The fast motion of Barnard’s Star through our part of the galaxy, its comparatively low concentration of “metals” (i.e. what astronomers call elements heavier than helium) and its slow rotation period all suggest that it is an old star with an age of about 7 to 10 billion years. While ancient in comparison to the Sun, Barnard’s Star has completed only a tiny fraction of its lifetime on the main sequence which is estimated to be on the order of a trillion years. Despite its age and low level of chromospheric activity, it has been observed to flare on occasion, but it is considered a BY Draconis type variable whose modest changes in brightness are caused by starspots and other forms of surface activity modulated by the star’s slow rotation.

Early Exoplanet Search Results

One of the best known observers of Barnard’s Star during the 20th century was undoubtedly Dutch astronomer Peter van de Kamp (1901-1995). From 1937 to 1972, van de Kamp was the director of Swarthmore College’s Sproul Observatory in Pennsylvania where he specialized in precision astrometry and became highly respected in the astronomical community for the quality of his work. A firm believer that planetary systems were common, van de Kamp pushed the limits of his equipment and measurement techniques to make not only precision measurements of the parallax and motions of nearby stars like Barnard’s Star, but also to search for the telltale wobble in the motion of a star that would indicate the presence of unseen companions.

In 1963 van de Kamp published the results of his analysis of the motion of Barnard’s Star since 1916 and claimed he found the wobble caused by an orbiting exoplanet with a mass of 1.6 times that of Jupiter or M J and an orbital period of 24 years. This was followed in 1969 when van de Kamp published an updated analysis which now included an extra five years of new data indicating the presence of two planets with periods of 12 and 26 years possessing masses of 0.8 M J and 1.1 M J , respectively. While the discovery prompted much speculation and even led to this system to be selected as the hypothetical target for the Project Daedalus starship studied by the British Interplanetary Society during the mid-1970s, there was genuine skepticism about van de Kamp’s claim. Over the following decades, other observatories failed to find any evidence of these Jupiter-sized exoplanets and it is now believed that subtle instrumental artifacts created a spurious planetary signature (for a detailed account of this and other attempts to search for planets orbiting Barnard’s Star, see “The Search for Planets Around Barnard’s Star”).

With over half a century of published results of searches for planets orbiting Barnard’s Star, by far the most sensitive and comprehensive up until a few years ago was a 2013 paper with Jieun Choi (University of California – Berkeley) as the lead author. For starters, Choi et al. presented an analysis of 248 precision radial velocity (RV) measurements of Barnard’s Star acquired between 1987 and 2012 using equipment at the Lick and Keck Observatories which showed no hints of van de Kamp’s putative exoplanets. Using the more accurate RV measurements taken with HIRES (High Resolution Echelle Spectrometer) at the Keck Observatory after August 2004 along with an independent data set of 226 published RV measurements acquired over six years using ESO’s UVES (Ultraviolet and Visual Echelle Spectrograph), Choi et al. failed to find any hint of orbiting exoplanets with periods out to 5,000 days. Based on a detailed analysis of the data set, Choi et al. found that for orbits with periods less than 10 days, 100 days and two years, planets orbiting Barnard’s Star with M P sini values greater than about 2 Earth masses or M E , 3 M E and 10 M E , respectively, could be excluded. Because the inclination of the potential planets orbit to the plane of the sky, i, cannot be determined directly from RV measurements alone, the M P sini value represents the minimum mass with the actual mass likely being larger.

In addition to these other surveys, the RV of Barnard’s Star has also been monitored by European-based HARPS (High Accuracy Radial velocity Planet Search) team. An analysis of their initial survey results for M dwarfs with team leader Xavier Bonfils (Institut de Planétologie et d’Astrophysique de Grenoble) as the first author was published in 2013. A total of 22 RV measurements of Barnard’s star showed a clear trend over time with a weak periodicity of about 10,000 days and another with a shorter period of 3.1 days. Neither of these signals represented a reliable planetary detection but an unpublished analysis of the historical HARPS, UVES and HIRES radial velocity measurements performed in 2015 suggested the possible presence of a super-Earth size exoplanet in a distant “cold orbit” with an orbital period of perhaps 230 days. More data were obviously needed to verify the presence of any exoplanet orbiting Barnard’s Star.

New Results

Because of the great interest in a possible exoplanet orbiting Barnard’s Star, it was included in the HARPS team’s highly publicized “Red Dots” campaign along with the nearby red dwarfs, Proxima Centauri and Ross 154, which ran from mid-June to early-October 2017 (see “Red Dots: The Search for Nearby Earth Size Exoplanets”). In parallel with this effort, another group of astronomers at the German-Spanish consortium’s Calar Alto Observatory in Spain acquired precision RV measurements of Barnard’s Star using the new CARMENES (Calar Alto high-Resolution search for M dwarfs with Exoearths with Near-infrared and optical Échelle Spectrographs) spectrometer mounted on the observatory’s 3.5-meter telescope which started its survey work at the beginning of 2016.

The CARMENES team, led by Ignasi Ribas (Institut de Ciències de l’Espai), combined their precision RV measurements from 2016 and 2017 with archived data from six other exoplanet survey programs including HIRES, UVES, HARPS (pre- and post-upgrade) and HARPS-N (HARPS-North) as well as the PFS (Carnegie’s Planet Finder Spectrograph in Chile) and APF (Lick Observatory’s Automated Planet Finder). A total of 771 RV epochs or nightly averages with a precision of 0.9 to 1.8 meters per second spanning over two decades were analyzed and the results published in the peer-reviewed science journal, Nature, on November 15, 2018. This paper, Ribas et al., has 62 coauthors including the leads of contributing exoplanet survey programs from around the globe.

Using models which made different assumptions about the nature of the noise in the data, Ribas et al. found a statistically significant periodicity of 232.8±0.4 days with a semiamplitude of just 1.20±0.12 meters per second. Since this period was not found in any of the stellar activity indicators or photometry for Barnard’s Star, it is likely that the observed variation in RV represents the reflex motion of an orbiting exoplanet to better than 99% certainty and it is not some form of stellar activity creating a spurious signal. The best fit for the observations is for an exoplanet with an M P sini or minimum mass of 3.2±0.4 M E in an orbit with a semimajor axis of 0.404±0.018 AU and a moderate eccentricity of 0.32 +0.10/-0.15. This mass is comfortably below the detection threshold of the earlier work by Choi et al.. With a current mean stellar flux of only 0.020±0.002 times that of the Earth, this newly discovered exoplanet formed at what is known as the “snow line” where the temperatures in the protoplanetary nebula out of which it grew were low enough for water and other volatiles to freeze solid – the perfect location to form a massive exoplanet. This important detection, which remains to be confirmed independently, of an exoplanet in a distant orbit around a red dwarf will have important implications for the formation of planets around this class of stars.

Given its low temperature at formation and that the possible mass values for this new exoplanet, called Barnard’s Star b, straddle the mass range where the population of known exoplanets transitions from being predominantly rocky planets like the Earth to volatile-rich “mini-Neptunes”, our new neighbor is likely to be the latter – a cold exoplanet with a deep atmosphere rich in hydrogen and helium over layers of exotic phases of water-rich ices which only exist at high pressures and temperatures. There is no possibility that an exoplanet of this sort could support life as we know it, but Barnard’s Star b could harbor icy moons with biocompatible environments like Europa or Enceladus may have.

The current analysis of the RV data suggests that Barnard’s Star b might not be alone. The continued long-term variation in the RV measurements superimposed on the 233-day variations in RV caused by Barnard’s Star b suggest the presence of another exoplanet in a long-period orbit. Although the data are currently insufficient to make a definitive detection or pin down its properties, the observed residual RV variation is broadly consistent with an exoplanet with a mass >15 M E in a ~4 AU orbit. Only more data will allow astronomers to determine if this large, possible distantly orbiting exoplanet is real.

There are additional marginally significant periodic signals in the RV data such as at 81 days (corresponding a semimajor axis of ~0.2 AU) but more data will again be required to determine if this is the result of an actual exoplanet or a subtle artifact of noise, natural or otherwise. Ribas et al., however, have been able to place more stringent upper limits for the size of any exoplanets orbiting within the habitable zone of Barnard’s Star based on their data set. Ribas et al. found that exoplanets with orbital periods of 10 and 40 days (corresponding to the optimistic inner and outer edges of the habitable zone at about 0.05 and 0.12 AU) would have been detectable in their data if they had M P sini values larger than 0.7 M E and 1.2 M E , respectively. Given a randomly oriented orbit with respect to the plane of the sky, exoplanets with masses greater than ~2.4 M E and ~3.8 M E , respectively, have been excluded from the habitable zone to ~95% certainty. Potentially habitable exoplanets about the mass of the Earth or smaller could exist and still evade detection in the available RV data.

The Future

As mentioned earlier, more data will be needed to provide a definitive detection of additional exoplanets orbiting Barnard’s Star as well as provide more accurate values for the properties of Barnard’s Star b. Without a doubt, more data will be gathered by CARMENES over the years to come and possibly other instruments like HARPS. Future instruments capable of providing even more precise RV data for red dwarfs will also be of great help.

Because of the proximity and relative brightness of Barnard’s Star, any exoplanets in orbit could be detected by other means providing independent confirmation of the existence of these worlds as well as vital new information. Barnard’s Star b would be expected to produce a reflex motion induced wobble of at least 0.013 milliarc seconds in its primary star’s movement across the sky – the component perpendicular to that being measured by precision RV measurements. Astrometric measurements from ESA’s ongoing Gaia mission might be able to detect this motion allowing the orbit inclination, i, to be determined or, at very least, constrained along with the actual mass of this new find. The more distant exoplanet suggested by the RV data, which would be expected to produce a 3 milliarc second wobble over five years, could potentially be detected by Gaia as well.

Given the closeness of Barnard’s Star and the size of its new exoplanet’s orbit, Barnard’s Star b could be detectable by direct imaging. While at maximum elongation (i.e. the greatest separation between a star and its orbiting planet) Barnard’s Star b would be about 220 milliarc seconds from its primary, the estimated contrast ratio of ~10-9 (i.e. the ratio between the exoplanet and star brightness) is three orders of magnitude too small to be detected with current instruments. However, Barnard’s Star b is expected to be within reach of the next generation of instruments in the next decade or so. Such observations would provide not only another independent means of verifying its existence, but also vital data about its optical properties and composition. Such data will be vital to our understanding of the origins and evolution of exoplanets orbiting red dwarfs. After such a long wait, at least we now know that an interstellar mission to Barnard’s Star will have at least one interesting target.

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Related Reading

“The Search for Planets Around Barnard’s Star”, Drew Ex Machina, April 23, 2015 [Post]

“Red Dots: The Search for Nearby Earth Size Exoplanets”, Drew Ex Machina, July 3, 2017 [Post]

General References

E.E. Barnard, “A Small Star with Large Proper-Motion”, The Astronomical Journal, Vol. 29, No. 23, pp. 181-183, September 15, 1916

A. Bond and A.R. Martin, “Project Daedalus: the mission profile”, Journal of the British Interplanetary Society, Vol. 29, pp. 101-112, 1976

X. Bonfils et al., “The HARPS search for southern extra-solar planets XXXI. The M-dwarf sample”, Astronomy & Astrophysics, Vol. 549, ID A8, January 2013

Jieun Choi et al., “Precise Doppler Monitoring of Barnard’s Star”, The Astrophysical Journal, Vol. 764, No. 2, Article id. 131, February 2013

Peter van de Kamp, “Astrometric Study of Barnard’s Star”, Astronomical Journal, Vol. 68, p. 295, No. 5, June 1963

Peter van de Kamp, “Parallax, Proper Motion, Acceleration, and Orbital Motion of Barnard’s Star”, Astronomical Journal, Vol. 74, No. 2, pp. 238-240, March 1969

Peter van de Kamp, “Alternate Dynamical Analysis of Barnard’s Star”, Astronomical Journal, Vol. 74, No. 6, pp. 757-759, August 1969

Peter van de Kamp, “Dark companions of stars – Astrometric commentary on the lower end of the Main Sequence”, Space Science Reviews, Vol. 43, pp. 211-327, April 1986

I. Ribas et al., “A super-Earth planet candidate orbiting at the snow-line of Barnard’s star”, Nature, Vol. 563, No. 7731, pp. 365-368, November 15, 2018

Super-Earth Orbiting Barnard’s Star, ESO Science Release 1837, November 14, 2018, [Science Release]