Note: An updated review of Wolf 359 can be found in “The Real Wolf 359 Revisited – New Planetary Discoveries“.
Over the past couple of years I have been rewatching quite a few classic science fiction shows and movies in my spare time some of which I have not seen in decades. One of the gems I recently watched was a 1964 episode of Outer Limits entitled “Wolf 359” where a scientist studies a miniaturized recreation of an environment of a planet found orbiting the nearby star, Wolf 359. Of course my younger readers would recognize this star as the site of the famous “Battle of Wolf 359” originally seen in the episode of Star Trek: The Next Generation entitled “The Best of Both Worlds Part II” which first aired a quarter of a century ago. And we see this famous battle against the Borg from a different perspective in the premier episode of Star Trek: Deep Space Nine, “The Emissary”. A quick search online reveals a smattering of other works of science fiction set in the Wolf 359 system and I am sure that there are many more still that have been published over the decades. Given that it is the setting of these various stories, what is the real Wolf 359 like?
The Star
Wolf 359, also known as GJ 406, is a dim red dwarf star with a V-magnitude of 13.5 located in the constellation of Leo. It first came to the attention of astronomers about 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 359th high proper motion star Wolf cataloged. Because of brief flares resulting in temporary increases in the brightness first observed in 1969, Wolf 359 received the variable star designation CN Leonis.
The dim red star indicated by the arrow in this image is Wolf 359. This view covers an area of 16×12 arc minutes with north up and east to the right. (Klaus Hohmann)
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 7.78 light years away making it the fifth closest star system currently known. Its low apparent magnitude despite its proximity means that Wolf 359 is exceptionally dim with a luminosity of only 0.0009 times that of the Sun. With an estimated radius of only 0.14 times the Sun’s and a mass of just 0.09 times, Wolf 359 is among the smallest known red dwarf stars and just barely above the minimum mass a star can have and still maintain hydrogen fusion in its core.
A comparison of Wolf 359 with Jupiter and the Sun. (David Jarvis)
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 2800 K. The spectral type, which has varied from source to source, is considered to be about M5Ve 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.
An artist’s depiction of an active red dwarf star orbited by a planet. (David A. Aguilar – CfA)
Based on a conservative definition of the habitable zone (HZ) by Kopparapu et al., the HZ of Wolf 359 for an Earth-mass world runs from about 0.031 AU, corresponding to the runaway greenhouse limit, out to about 0.063 AU for the maximum greenhouse limit. These orbits have periods in the 6.6 to 19-day range and assumes that there are no insurmountable impediments for habitability around a star like Wolf 359. Of course, if Wolf 359 does have a planet orbiting in its HZ with the required properties, it would still be in its earliest stages of formation and it would likely be hundreds of millions of years before life might firmly take hold.
Search for Planets
Like other nearby red dwarf stars such as Proxima Centauri and Barnard’s Star, Wolf 359 is 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 Search for Planets Around Proxima Centauri” and “The Search for Planets Around Barnard’s Star”). 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 amount of heat from their formation, can also be safely excluded in this region.
In addition to direct imaging, searches using precision radial velocity 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 recently published paper with Cassy Davison (Georgia State University) as the lead author presents the results from the most thorough search for extrasolar planets in this system as part of a larger survey of nearby M-dwarf stars. Unlike most other surveys, Davison et al. combined the results of radial velocity 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 radial velocity 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 radial velocity with a typical measurement precision 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 taken 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 MJ) 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 MJ is the 90% detection limit of this study. For planets with periods of 100 days, planets larger than 2.0 MJ are excluded. Detection limits for more distant orbits runs from 7.0 MJ to 3.0 MJ for orbital periods from 3 to 8 years (i.e. 0.9 to 1.8 AU orbits), respectively.
While these results seem to eliminate the possibility that Wolf 359 has any Jupiter to super-Jupiter size planets orbiting within a couple of AU, chances are that still smaller planets may be absent as well. As part of their systematic survey of nearby M-dwarf stars conducted between 2003 and 2009, the European-based HARPS (High Accuracy Radial velocity Planet Search) team made a handful of precision radial velocity measurements of Wolf 359 looking for signs of variations. Any significant variations in the radial velocity 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 radial velocity 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, it does suggest that there are probably no planets with masses greater than Neptune (or about 0.05 MJ) in short-period orbits around Wolf 359. At this time it seems unlikely that the HARPS team will be making any follow up observations of Wolf 359 and instead will focus their limited resources on more promising targets.
The ESO 3.6m Telescope equipped with HARPS has made the most accurate published radial velocity measurements of Wolf 359. (ESO/H.H.Heyer)
While there may be some who are alarmed by the lack of any planet detections to date, it is not unexpected given what we have learned about the planetary systems of other M-dwarf stars. A recent statistical analysis of the Kepler database for M-dwarf stars performed by Courtney Dressing and David Charbonneau (Harvard-Smithsonian Center for Astrophysics) has shown that planets with radii greater than about 2.5 times that of the Earth (corresponding to a mass of only 0.014 MJ, assuming a probable Neptune-like density) and orbital periods less than 200 days are rare. Planets larger than Neptune are exceptionally rare in M-dwarf systems. And since the “typical” M-dwarf in the analysis by Dressing and Charbonneau is over five times more massive than the diminutive Wolf 359 (with the corresponding planets also tending to be larger), the lack of any planetary detections to date is even less surprising (see “Occurrence of Potentially Habitable Planets Around Red Dwarfs”). A compact system of Mars- to super-Earth sized planets would easily escape detection by the searches performed to date and will require a new generation of instruments to discover. In the mean time, visions of Wolf 359 and any worlds it may harbor will live on in fiction.
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Related Reading
“Occurrence of Potentially Habitable Planets around Red Dwarfs”, Drew Ex Machina, January 12, 2015 [Post]
“The Search for Planets Around Proxima Centauri”, Drew Ex Machina, February 23, 2015 [Post]
“The Search for Planets Around Barnard’s Star”, Drew Ex Machina, April 23, 2015 [Post]
“The Hubble Space Telescope and the Search for Faint Extrasolar Companions”, SETIQuest, Volume 3, Number 2, pp. 1-9, Second Quarter 1997 [Article]
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
Courtney D. Dressing and David Charbonneau, “The Occurrence of Potentially Habitable Planets Orbiting M Dwarfs Estimated from the Full Kepler Dataset and an Empirical Measurement of the Detection Sensitivity”, The Astrophysical Journal, Vol. 807, No. 1, ID 45, July 2015
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
Ya. V. Pavlenko et al., “Spectral energy distribution for GJ406”, Astronomy and Astrophysics, Vol. 447, No. 2, pp.709-717, February 2006
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
I guess the general focus on finding “habitable” planets would also contribute to there being less interest in surveying Wolf 359: the flares would likely make the conditions rather hostile to maintaining habitable conditions.
Of the red dwarfs within 10 light years, the only one that as far as I’ve managed to find out is not a flare star is Lalande 21185 (despite it being listed as one in SIMBAD). I recall there was an announcement of a giant planet detected by astrometry in the 1990s: it has always surprised me about the apparent lack of any kind of follow-up.