The Real Wolf 359 Revisited – New Planetary Discoveries

For Star Trek fans like myself, the mention of the nearby star, Wolf 359, instantly brings to mind the “Battle of Wolf 359”. Originally seen in the 1990 episode of Star Trek: The Next Generation entitled “The Best of Both Worlds Part II” and again from a different perspective in the opening scene of the premier episode of Star Trek: Deep Space Nine, “The Emissary”, a combined fleet of 40 Federation and Klingon ships is destroyed in a conflict with a single Borg ship intercepted at Wolf 359 on its way to Earth in the year 2367.

While this battle is fictional, Wolf 359 is real star system that has been of interest to astronomers for a century now. And given its comparative closeness, Wolf 359 is a potential target for early interstellar missions. Four years ago, I wrote an essay about what was known at the time about this nearby red dwarf and what searches for exoplanets had not found to that point (see “The Real Wolf 359”). In June of 2019, an international team of astronomers led by Mikko Tuomi (University of Hertfordshire – UK) submitted a paper for peer-reviewed publication with the results of the latest survey for exoplanets orbiting nearby red dwarfs. Combining available data from the world’s major exoplanet surveys, Tuomi et al.  found 118 exoplanet candidates including the first reported detection of exoplanets orbiting Wolf 359. With these new discoveries, it is time to revisit Wolf 359 to see what this system is really like.

Background

Wolf 359 is a red dwarf star with a V-magnitude of 13.5 located in the constellation of Leo. It first came to the attention of astronomers over a century ago because of its relatively high proper motion of 4.7 arc seconds per year first measured in 1917 by German astronomer Max Wolf (1863-1932) of the Heidelberg-Königstuhl State Observatory. It was the 359^th high proper motion star Wolf cataloged as part of his systematic survey of the nighttime sky. Because of brief flares resulting in temporary increases in the brightness first observed in 1969, Wolf 359 received the variable star designation CN Leonis.

A comparison of Wolf 359 with Jupiter and the Sun. (David Jarvis)

Because its high proper motion suggested that it is relatively nearby, the parallax of Wolf 359 was measured for the first time in 1928 revealing it to be one of the closest known stars. The best distance measurement available today for Wolf 359 shows that it is 7.80±0.05 light years away making it the fifth closest star system currently known. Because of its proximity to the Sun, Wolf 359  was included in the first edition of the Gliese Catalogue of Nearby Stars published in 1957 by Wilhelm Gliese (1915-1993) earning it the designation of Gliese 406 (although it is frequently identified as GJ 406 after the initials of Gliese and his colleague, Hartmut Jahreiß, who worked on later editions of the catalog). The best estimates for the properties of Wolf 359 are summarized in the table below.

Properties of Wolf 359

Studies of the spectrum of Wolf 359 show that it is actually cool enough to display molecular absorption features for TiO, VO and even water vapor with a surface temperature of about 2700 K. The spectral type, which has varied from source to source, is considered to be about M6Ve with the “e” indicating the presence of emission lines in its spectrum. Combined with its observed X-ray luminosity, Wolf 359 is the only star of its spectral type observed to display chromospheric and coronal activity. Comparing the properties of Wolf 359 with models of stellar evolution indicates that it is a relatively young 100 to 350 million years old – a mere blink of an eye compared to this star’s estimated lifetime of on the order of trillions of years. The observed activity of this small star would be explained by its relative youth and should decrease quickly as it ages. Interestingly, Wolf 359 displays no excess infrared emissions hinting that it is not surrounded by large amounts of dusty debris left over from the formation of any planets.

The Search for Planets

Like other nearby red dwarf stars such as Proxima Centauri and Barnard’s Star, Wolf 359 has been considered an ideal candidate for the search for small companions like extrasolar planets and has been a target for a variety of surveys for decades (see the Proxima Centauri page and the Barnard’s Star page). Direct imaging searches for faint companions during the 1990s using NASA’s Hubble Space Telescope and ground-based instruments failed to find any evidence for very low mass stellar companions more than about 1 AU from Wolf 359 which corresponds to orbital periods longer than about three years. Given its relative youth, the presence of brown dwarfs, which would still be radiating large amounts of heat from their formation, can also be safely excluded in this region.

In addition to direct imaging, searches using precision radial velocity (RV) measurements and astrometry, which measure the reflex motion of a star resulting from an orbiting object, are expected to be promising given the relative closeness of Wolf 359 and its diminutive size. A paper published in 2015 with Cassy Davison (Georgia State University) as the lead author presented the results from the most thorough search for extrasolar planets in this system until that time as part of a larger survey of nearby red dwarf stars. Unlike most other surveys, Davison et al. combined the results of RV and astrometric measurements to search for extrasolar planets.

NASA’s IRTF at the Mauna Kea Observatory in Hawaii used for a recent radial velocity survey of nearby red dwarf stars. (NASA/University of Hawaii Institute for Astronomy)

For the RV measurements, Davison et al. analyzed infrared spectra acquired using the CSHELL cryogenic Echelle spectrograph on NASA’s 3.0-meter IRTF (Infrared Telescope Facility) located at the Mauna Kea Observatory in Hawaii. They used spectra obtained during a dozen observation sessions between May 2009 and March 2011 to derive the RV with a typical measurement accuracy of ±83 meters per second. These data, gathered over 683 days, were sufficient to detect objects with orbital periods less than about 100 days corresponding to a maximum orbital radius of about 0.19 AU.

For the astrometric measurements, Davison et al. employed data acquired using the 0.9-meter telescope at the Cerro Tololo Inter-American Observatory as part of an ongoing program to observe nearby stars. The team used 139 R-band images acquired over the span of 12 years with combined angular measurement errors of 5.2 and 6.6 milliarc seconds in right ascension and declination, respectively. These data were best at detecting objects with orbital periods from 2 to 8 years corresponding to orbital radii in the 0.7 to 1.8 AU range.

These color-coded plots show the fraction of planets that could have been detected orbiting Wolf 359 as a function of orbital period and planet mass. Red indicates a nearly perfect detection rate. Click on image to enlarge. (Davison et al.)

In brief, the analysis of these complimentary data sets by Davison et al. failed to find anything orbiting Wolf 359. By injecting artificial signals into their data representing planets with various masses and orbits, they were able to perform a statistical analysis to place lower limits on what should have been detected if it were present. For planets in orbits with a period of 3 days (corresponding to a distance of 0.018 AU), there was a 90% chance that any planet with a mass as small as 0.5 times that of Jupiter (or M_J) would have been detected. For planets with orbital periods of 10 to 30 days (i.e. 0.04 to 0.08 AU orbits), 1.0 M_J is the 90% detection limit of this study. For planets with periods of 100 days, planets larger than 2.0 M_J are excluded. Detection limits for more distant orbits runs from 7.0 M_J to 3.0 M_J for orbital periods from 3 to 8 years (i.e. 0.9 to 1.8 AU orbits), respectively.

The ESO 3.6m Telescope equipped with the HARPS spectrograph was used to acquire the data used to find the new planet candidates orbiting Wolf 359. (ESO/H.H.Heyer)

While these results seem to eliminate the possibility that Wolf 359 has any Jupiter to super-Jupiter size companions orbiting within a couple of AU, chances are that still smaller planets may be absent as well. As part of an initial systematic survey of nearby red dwarf stars conducted between 2003 and 2009, the European-based HARPS (High Accuracy Radial velocity Planet Search) team made a handful of precision RV measurements of Wolf 359 looking for signs of variations. Any significant variations in the RV of stars in their survey could indicate the presence of extrasolar planets that would then be followed up by a more thorough observation campaign to characterize the system. With the HARPS spectrograph attached to the European Southern Observatory’s 3.6-meter telescope in La Silla, Chile, three measurements found no variation in the RV of Wolf 359 down to the ±5.7 meter per second level, according to results published in 2013. While it is impossible to set any meaningful detection limits for a range of orbital periods with only three data points, their accuracy is over an order of magnitude better then the RV measurements of Davison et al. and suggests that there are probably no planets with masses greater than Neptune (or about 0.05 M_J) in short-period orbits around Wolf 359.

New Worlds Discovered

While the initial results of precision RV measurements made by HARPS were not promising, additional observations were made not only by the HARPS team but by others as well. All the RV measurements of nearby red dwarfs obtained by various surveys around the world were combined by Tuomi et al. in the latest effort to identify exoplanets orbiting our dim neighbors. For Wolf 359, Tuomi et al. started with an expanded set of 23 HARPS RV measurements and combined them with 41 measurements from HIRES (High Resolution Echelle Spectrometer) on the ten-meter Keck I telescope located on the summit of Mauna Kea which has been used for the long-running Lick-Carnegie Exoplanet Survey. The combined data set of 64 RV measurements spans a period of just over 13 years.

A view of the two ten-meter telescopes of the Keck Observatory atop of Mauna Kea. Keck I is fitted with the HIRES spectrometer used to observe Wolf 359 and other nearby stars. (NASA)

In order to help differentiate an orbiting exoplanet from stellar activity, archived photometric measurements were also analyzed. If signals in the photometry and the RV measurements displayed the same period, it would be taken as evidence that the observed variation in the RV is the result of stellar activity modulated by the star’s rotation and is not an orbiting exoplanet. For Wolf 359, Tuomi et al. used 120 sets of photometric measurements acquired over 44 months by ASAS (All Sky Automated Survey). Started in 1997, the Polish ASAS project has been monitoring the brightnesses of about 20 million stars using small telescopes housed at the Las Campanas Observatory in Chile and the slopes of Haleakala on the island of Maui in Hawaii. In addition to this photometry, stellar activity indicators were also derived from the original HARPS and HIRES spectra to help identify false positives.

In the foreground we see the ASAS (All Sky Automated Survey) telescopes in their open enclosure at the Las Campanas Observatory in Chile. These telescopes provided photometry of Wolf 359 used in the discovery of its new planets. (Krzysztof Ulaczyk)

After a detailed analysis of the available data for Wolf 359, Tuomi et al. found two periodic signals in the combine RV data set. One was a variation with a period of 2938 ±436 days and a semiamplitude of 8.68 +5.09/-4.55 meters per second while the other was a much shorter period signal of 2.68687 +0.00039/-0.00031 days with a semiamplitude of 7.65 ±3.53 meters per second. Analysis of the activity indicators and the photometry did not find any significant periodicities resulting from activity cycles or stellar rotation (in fact the rotation period of Wolf 359 remains unknown currently). The conclusion is that the pair of observed periodic signals in the RV data are the result of orbiting exoplanets. Using the RV analysis results and the estimated properties of the Wolf 359, the properties of the newly found exoplanet candidates can be derived and are listed in the table below. Also included in the table is the effective stellar flux, S_eff, which provides a measure of how much energy the planet receives from its sun.

Wolf 359 Exoplanet Properties

Since the inclination, i, of the orbits of these exoplanets to the plane of the sky cannot be derived directly from RV measurements, only the minimum mass or M_Psini values can be derived with the actual mass, M_P, likely being larger. In any case, the M_Psini values for Wolf 359b and c, as these exoplanets are now known, is well below the upper mass limit derived by Davison et al. four years earlier.

The RV measurements for Wolf 359b plotted over time compared to the model results. Blue data points are from HIRES while those in red are from HARPS. Click on image to enlarge. (Tuomi et al.)

The RV measurements for Wolf 359c plotted as a function of orbital phase compared to the model results. Blue data points are from HIRES while those in red are from HARPS. Click on image to enlarge. (Tuomi et al.)

The larger of the exoplanet candidates identified by Tuomi et al., Wolf 359b, formed far beyond the snowline in this system where water and other volatiles would freeze. The mean stellar flux is equivalent to a distance of about 58 AU from the Sun – a realm where all gases save for hydrogen and helium would freeze solid (barring another source of heat). While the M_Psini value has a large uncertainty (with even more uncertainty in the actual mass due to the unknown value of i), it seems likely that this exoplanet candidate is either an ice giant somewhat larger than Neptune or a small gas giant perhaps a bit smaller than Saturn.

An artist’s depiction of an active red dwarf star orbited by a planet. (David A. Aguilar – CfA)

The smaller exoplanet candidate, Wolf 359c, represents the opposite end of the spectrum. Given its mass, it is likely a volatile-rich mini-Neptune although the possibility that it is a large rocky planet like those found in our inner solar system cannot be excluded. With a mean stellar flux of about three times that of the Earth (and over 60% higher than Venus), there is no likelihood that Wolf 359c is a potentially habitable planet. It is either a super-size, hotter version of Venus or a smaller but much hotter version of Neptune with a deep atmosphere dominated by hydrogen and helium covering exotic phases of water and other volatiles which exist only at high temperatures and pressures. While Wolf 359c is not habitable, there is still plenty of room for a potentially habitable world orbiting farther out from the system’s central star. Given the marginal detection of this exoplanet, a much less massive, Earth-size planet in the habitable zone would have easily escaped detection in the current analysis by Tuomi et al..

The Future

So, what should we expect next? More data will be needed to confirm these new discoveries and get a better handle on their basic properties. Based on their survey work combined with work done with NASA’s Kepler (which looked for exoplanets using the transit method), Tuomi et al. estimate that red dwarf stars are accompanied by about three exoplanets on average which are the about the size of the Earth or larger. Given the spacing between the two exoplanets in this system, there is plenty of room for more exoplanets orbiting Wolf 359 including potentially habitable ones (or, maybe more precisely, exoplanets which could become potentially habitable in a few hundred million years after the planet formation process in this system winds down further and extreme activity in this young red dwarf begins to wane).

While Wolf 359 deserves more attention from ongoing exoplanet surveys to significantly expand the existing RV data set, even a season’s worth of high-cadence RV measurements, as has been done as part of the ongoing Red Dots program (see the Red Dots page), would double the number of precision RV measurements of Wolf 359. Such an expanded data set would help to decrease the uncertainties in the properties of Wolf 359c and could spot other exoplanets in short period orbits, assuming the natural noise or “jitter” from Wolf 359 is not excessive. Wolf 359 would also be an ideal target for newer surveys like CARMENES (Calar Alto high-Resolution search for M dwarfs with Exoearths with Near-infrared and optical Échelle Spectrographs) which is specifically designed to observe cool red dwarfs and has already found its first exoplanets (see “Our New Neighbor Orbiting Barnard’s Star” and “Habitable Planet Reality Check: The Earth-Size Planets of Teegarden’s Star”).

The 3.5-meter telescope at the Calar Alto Observatory in Spain used by CARMENES to make precision radial velocity (RV) measurements of nearby red dwarfs. (Centro Astronómico Hispano-Alemán)

Because of the long, eight-year orbital period of Wolf 359b, it will take years of additional RV data to confirm its planetary nature and help refine its properties. But given its large orbit and the relative proximity of Wolf 359 to the Sun, Wolf 359b would have a maximum apparent separation of 0.8 arc seconds making it a potential target for detection via direct imaging at infrared wavelengths to detect the residual heat from its formation. Such observations would not only independently verify the existence of Wolf 359b but allow key properties to be derived. Even a null result from early observations could set useful upper limits on the actual mass of this exoplanet candidate. Ultimately, given its 7.8 light year distance, the Wolf 359 system would now seem to make a tempting target for early interstellar missions. The months and years ahead promise to reveal much new information about our newly discovered neighbors.

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Related Reading

For a complete collection of articles about our other neighboring star systems and the searches for exoplanets orbiting them, see Drew Ex Machina’s page on Nearby Stars.

General References

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

Cassy L. Davison et al., “A 3D Search for Companions to 12 Nearby M Dwarfs”, The Astronomical Journal, Vol. 149, No. 3, Article ID 106, March 2015

Sergio B. Dieterich et al., “The Solar Neighborhood. XXXII. The Hydrogen Burning Limit”, The Astronomical Journal, Vol. 147, No. 5, Article ID 94, May 2014

Aurora Kesseli et al., “Radii of 88 M Subdwarfs and Updated Radius Relations for Low-metallicity M-dwarf Stars”, The Astronomical Journal, Vol. 157, No. 2, article id. 63, February 2019

Michael Okuda and Denise Okuda, Star Trek Chronology: The History of the Future, Pocket Books, 1993

Daniel J. Schroeder et al., “A Search for Faint Companions to Nearby Stars Using the Wide Field Planetary Camera 2”, The Astronomical Journal, Vol. 119, No. 2, pp. 906-922, February 2000

M. Tuomi et al., “Frequency of planets orbiting M dwarf stars in the Solar neighborhood”, arXiv 1906.04644v3 (revised manuscript submitted for publication in Astrophysical Journal Supplement), July 27, 2019 [Preprint]