Our New Neighbor Orbiting Lalande 21185

Before the flood of discoveries resulting from NASA’s Kepler mission, the majority of extrasolar planets were found by analyzing precision radial velocity measurements of stars. Periodic variations in a star’s radial velocity could be the result of reflex motion caused by an orbiting exoplanet. One of the longest running surveys of this type is the Lick-Carnegie Exoplanet Survey (LCES) which has been collecting data since 1994. On February 12, 2017 the LCES team including famed exoplanet hunters Paul Butler (Carnegie Institution for Science), Steven Vogt (UCO/Lick Observatory) and Gregory Laughlin (Yale University), among others, submitted a paper for publication in the peer-reviewed Astrophysical Journal with the results of their 20-year (and still counting) radial velocity survey. In addition, they have also made their data available to the public.

Since its inception, the LCES has monitored the radial velocities of 1,624 nearby F, G, K and M type stars within about 100 parsecs of the Sun generating a total of 60,949 precision measurements in the process. In the new paper by Butler et al., a total of 357 significant periodic signals of constant period and phase were tabulated which were obviously not associated with stellar activity. Out of these, 225 have already been published by LCES team members and/or others as exoplanetary discoveries while 54 are unpublished but classified as “significant signals that require confirmation by additional data before rising to classification as planet candidates.” Another 60 are previously unpublished exoplanetary candidates awaiting follow up observations to verify their planetary nature.

An H-R diagram of all the stars within 100 parsecs of the Sun with those that are part of the LCES shown in red. Click on image to enlarge. (Butler et al.)

Out of these five dozen new exoplanet candidates is a “super Earth” size world found orbiting the nearby star commonly known by the name Lalande 21185. After the exoplanet found orbiting Proxima Centauri b in 2016, this new find is the second closest exoplanet currently known (see “Habitable Planet Reality Check: Proxima Centauri b”). But this is not the first claim of a planet found orbiting this nearby star – there have actually been several claims made since the middle of the 20th century, although it appears that this is the best one made to date.

 

Background on Lalande 21185

Lalande 21185 (also known as Gliese 411, HD 95735 and BD+36° 2147, among others) is a V magnitude 7.52 red dwarf located in the circumpolar constellation of Ursa Major. This spectral type M2V star was first found listed in Histoire Céleste Française published in 1801 and prepared by the French astronomer Jérôme Lalande (1732-1807) of the Paris Observatory. With its number, “21185”, assigned by English astronomer Francis Baily (1774-1844) in his 1847-edition of this catalog of 47,390 stars, this star is one of only a handful still commonly known by its Lalande designation.

Engraving of French astronomer Jérôme Lalande who was the first to catalog Lalande 21185 in 1801.

German astronomer Friedrich Wilhelm Argelander (1799-1875) was the first to note the relatively fast 4.80 arc second per year proper motion of Lalande 21185 in May 1857 earning it the informal name of “Argelander’s Second Star” in some European astronomical circles (“Argelander’s First Star” is another nearby high proper motion star better known as Groombridge 1830). Since high proper motion suggests that a star is nearby, German astronomer Friedrich August Theodor Winnecke (1835-1897) proceeded to take on the task of visually measuring the parallax of Lalande 21185 for the first time. His results, published in 1858, showed the star to be the second closest to the Sun then known. Today’s best parallax measurements shows that Lalande 21185 is 8.29 light years away making it the fourth closest main sequence star system known today after α Centauri, Barnard’s Star and Wolf 359 (see this site’s Alpha Centauri page , “The Search for Planets around Barnard’s Star” and “The Real Wolf 359” for more on these stars).

Because it is the brightest red dwarf visible from the northern hemisphere, Lalande 21185 has been an obvious target of study over the decades by those interested in this intrinsically dim class of stars. Direct measurements of the angular diameter by Lane et al. using the Palomar Testbed Interferometer in 1999 and 2000 combined with the best distance measurements indicates that Lalande 21185 has a radius 0.393 times that of the Sun. Modelling suggests that the star has a mass 0.46 times that of the Sun making Lalande 21185 rather large among red dwarfs. With a surface temperature of 3828 K, Lalande 21185 has a luminosity that is 0.030 times that of the Sun (calculated assuming a solar surface temperature of 5780 K).

Lalande 21185 is also classified as a BY Draconis variable star with the designation of NSV 18593 in the General Catalogue of Variable Stars. Typically dim M and K type stars, the variations in the brightness of BY Draconis stars is caused by starspots and other forms of moderate chromospheric activity modulated by the stars’ rotation. Lalande 21185 appears to be a relatively quiet star although it has been erroneously listed as a flare star in some catalogs (a claim that is not supported by the primary literature supporting these catalogs). The low level of activity, its orbit through the galaxy and other properties suggest that Lalande 21185 is older than the Sun with an age somewhere in the 5 to 10 billion year range. It is difficult to pin down the age any more precisely since stars of this type evolve so slowly over lifetimes that are on the order of hundreds of billions of years.

 

The Search for Planets

Being a relatively bright nearby star, Lalande 21185 has been the target of many searches for substellar companions (i.e. brown dwarfs and exoplanets) over the last two thirds of a century. The first attempts concentrated on using the technique known as astrometry – the astronomical discipline which measures the positions of objects in the sky and how they change over time. While an orbiting object might not be readily visible in a telescope, the small periodic wobble caused by the reflex motion as it orbits its parent star could still reveal its presence. It was this technique which was used in 1844 by German astronomer Friedrich Bessel (1784-1846) to discover the dim white dwarf companion of Sirius some 18 years before it was observed visually for the first time (see “Sirius: The Search for Companions Continues“). The relative closeness and low mass of Lalande 21185 makes it an ideal target for astrometric searches for small companions just like many other nearby red dwarfs.

The first published claim of a planetary detection for Lalande 21185 came in 1951 in a paper by Dutch-born American astronomer Peter van de Kamp (1901-1995), who was well known for his precision astrometric work at Swathmore College’s Sproul Observatory, and his long-time protégé, Sarah Lee Lippincott. Based on measurements from 679 photographic plates acquired on 188 nights between 1937 and 1950, van de Kamp and Lippincott found a wobble with a semimajor axis of 29 milliarc seconds and a period of 1.14 years suggesting the presence of a substellar companion with a mass on the order of a couple of tens times that of Jupiter (or MJ) in an eccentric orbit.

A plot showing the individual measurements of the position of Lalande 21185 in right ascension and declination made by Lippincott after the star’s proper motion is subtracted. The curves show the results for Lippincott’s orbital solution. Click on image to enlarge. (Lippincott)

In 1960, Lippincott published the results of her new analysis of the motion of Lalande 21185 based on an expanded set of data from 955 photographic plates taken over 315 nights between 1913 and 1959 using Sproul’s 61-centimeter refractor. She now found a wobble with a semimajor axis of 33.6 milliarc seconds but a period of 8.0 years. The result suggested the presence of a 10 MJ exoplanet or brown dwarf in an orbit around Lalande 21185 with an eccentricity of 0.3. Lippincott continued to stand by her claim in the following years despite doubts expressed by other astronomers. Finally in 1974, George Gatewood of the University of Pittsburg’s Allegheny Observatory published the results of his analysis of 143 exposures of Lalande 21185 on 56 plates acquired between 1934 and 1972 using the 76-centimeter Thaw photographic refractor. Gatewood found no evidence for Lippincott’s planet down to the few milliarc second level. As had been the case with the purported planets of Barnard’s Star, it was concluded that subtle artifacts in the Sproul Observatory data had lead to a spurious detection (see “The Search for Planets around Barnard’s Star”).

While Gatewood’s work was instrumental in disproving the existence of a 10 MJ planet orbiting Lalande 21185, his later work would spark another controversial planetary detection claim in 1996 (see “Extrasolar Planet Update“). Using 8.5 years of data from the University of Pittsburg’s Multichannel Astrometric Photometer (MAP), Gatewood claimed to have found a wobble with a 2.2 milliarc second amplitude and a period of 5.8 years indicating the presence of a 0.9 MJ planet in a low eccentricity orbit. When combining these new data with the earlier series of photographic plates from the Allegheny Observatory, the presence of a planetary body with an orbital period of around 30 years was suggested. Despite the fact that the exoplanet candidate in the 5.8-year orbit with a semimajor axis of about 2.5 AU is predicted to appear as far as 0.8 arc seconds from Lalande 21185, repeated attempts to image this candidate or otherwise confirm its presence have all failed casting doubt on Gatewood’s two-decade old claim.

 

New LCES Results

While the claims made over the decades for the presence of planets orbiting Lalande 21185 have been based on astrometric measurements to detect the reflex motion of the star in the plane of the sky, there is another technique currently available to measure the perpendicular component of that motion which has yielded results for other stars over the last two decades: precision radial velocity measurements. Like so many other nearby red dwarfs, Lalande 21185 has been the target of radial velocity surveys over the last two decades. Unfortunately, as the measurement accuracy of the instruments have improved, Lalande 21185 continued to show a remarkably constant radial velocity.

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 support the ongoing LCES. (NASA)

One of the longest running precision radial velocity surveys has been the ongoing Lick-Carnegie Exoplanet Survey (LCES) which has been using the HIRES spectrometer on the ten-meter Keck I telescope located on the summit of Mauna Kea in Hawaii. While the limited availability of Keck I results in data sampling which makes it difficult to detect exoplanets with periods less than one day or near one month (as a result of Keck I being available for exoplanet searches during the brightest phases of the Moon which interferes with other work), the LCES team has been able to build up enough data of sufficient accuracy to make a planetary detection with a high degree of certainty.

A plot of probability density as a function of period which clearly shows the 9.87-day period of the new planet candidate orbiting Lalande 21185. Click on image to enlarge. (Butler et al.)

Butler et al. found a clear periodicity in the precision radial velocity measurements of Lalande 21185 with an amplitude of 1.90±0.31 meters per second and a period of 9.8693±0.0016 days. Combined with the mass estimate of Lalande 21185, the semimajor axis of this new find is 0.070 AU and Butler et al. calculate an MPsini of 3.8 ME (or Earth masses) – far smaller than any previous exoplanetary claim made for this star. Since the inclination, i, of the planet’s orbit with respect to the plane of the sky is not currently known, this represents the minimum mass of this exoplanet with its actual mass likely being higher. Even though the detection of what will be called Lalande 21185b (or GJ 411b, after its designation in the Gliese Catalog of Nearby Stars) is fairly robust, it will require additional observations to verify its planetary nature. And while the issue was not specifically addressed by Butler et al., there is no hint of Gatewood’s claimed 0.9 MJ exoplanet with a period of 5.8 years (although a dedicated analysis would be required to set any meaningful detection limits).

A plot of the radial velocity measurements of Lalande 21185 obtained by the LCES as a function of orbital phase. The gray data points are individual velocity measurements while the red data points are the average of the individual measurements divided over 30 bins. Click on image to enlarge. (Butler et al.)

So given what little we know about this new world (and assuming that it is real), what kind of place is it? Butler et al. calculate that the effective stellar flux (i.e. the amount of energy this exoplanet receives from its sun) is roughly 5.3 times greater than the Earth’s. With an effective stellar flux more than twice as great as that of Venus and higher than even the most optimistic definitions of the habitable zone, this new planetary candidate has no prospects of being habitable. However, given the nature of other planetary systems found associated with red dwarfs, it is likely that Lalande 21185 has several other planets orbiting it which have escaped detection so far (see “Architecture of M-Dwarf Planetary Systems”). This includes the possibility of planets orbiting farther away from Lalande 21185 inside its habitable zone.

An estimation of the bulk composition of this new find will require a determination of the inclination, i, of this planet’s orbit as well as its radius. With a projected separation of less than about 30 milliarc seconds, direct imaging of this new find will require a large space-based telescope specifically designed for the task – something that probably will not be available for a minimum of another decade or more. In the mean time, there might be another way to get the required data. The probability the orbit of Lalande 21185b is by chance oriented to produce transits observable from the Earth is estimated to be 2.6%. Such transits would allow the inclination and radius of this planet to be determined as well as independently verify its existence. Given the brightness of Lalande 21185 and its circumpolar location, it is an excellent candidate for a photometric monitoring campaign to look for transits with a depth of 0.1% or greater.

Artist’s conception of the probable planet orbiting a star called Lalande 21185 or GJ 411. (Ricardo Ramirez)

While we wait for more definitive information on the purported Lalande 21185b, we can look at other exoplanets of similar size to estimate the probability that it is a rocky world or a volatile-rich mini-Neptune. Work by Rogers suggests that overall population of planets transition from being primarily rocky to volatile-rich at a radius of no greater than 1.6 times that of the Earth which corresponds to a mass of 6 ME, assuming an Earth-like composition (see “Habitable Planet Reality Check: Terrestrial Planet Size Limits”). Given an unconstrained orbit orientation, there is only about one chance in four that the actual mass of Lalande 21185b exceeds this threshold. However, more recent work by Chen and Kipping suggests that the gradual transition of the exoplanetary population from rocky planets starts at about 2 ME significantly increasing the chances that this is a mini-Neptune.

So at this time it is a toss up as to whether Lalande 21185b is a super-Venus with a deep hot atmosphere dominated by carbon dioxide or a mini-Neptune with an even deeper hotter atmosphere dominated by hydrogen and steam overlaying thick layers of exotic high-pressure/temperature phases of ice. No matter how it turns out, Lalande 21185b and any sister planets waiting discovery will be a worthwhile targets for study in the decades to come.

 

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Related Reading

For a complete collection of articles about our other neighboring star systems, see Drew Ex Machina’s page on Nearby Stars.

 

General References

R. Paul Butler et al., “The LCES HIRES/Keck Precision Radial Velocity Exoplanet Survey”, arXiv 1702.03571 (accepted for publication in The Astronomical Journal), Submitted February 12, 2017 [Preprint]

Jingjing Chen and David Kipping, “Probabilistic Forecasting of the Masses and Radii of Other Worlds”, The Astrophysical Journal, Vol. 834, No. 1, Article id. 17, January 2017

George Gatewood, “An astrometric study of Lalande 21185”, The Astronomical Journal, Vol. 79, No. 1, pp. 52-53, January 1974

G. Gatewood, “Lalande 21185”, Bulletin of the American Astronomical Society, Vol. 28, p. 885, May 1996

B.F. Lane, A.F. Boden and S.R. Kulkarni, “Interferometric Measurement of the Angular Sizes of Dwarf Stars in the Spectral Range K3-M4”, The Astrophysical Journal, Vol. 551, No. 1, pp. L81-L83, April 10, 2001

Sarah Lee Lippincott, “Astrometric Analysis of Lalande 21185”, The Astronomical Journal, Vol. 65, No. 7, pp. 445-448, September 1960

Leslie A. Rogers, “Most 1.6 Earth-Radius Planets are not Rocky”, The Astrophysical Journal, Vol. 801, No. 1, Article id. 41, March 2015

Peter van de Kamp and Sarah Lee Lippincott, “Astrometric Study of Lalande 21185”, The Astronomical Journal, Vol. 56, pp. 49-50, April 1951