Lalande 21185

Also known as NSV 18593, Gliese 411, Hip 54035, HD 95735, BD+36 2147, SAO 62377,

LHS 37, LTT 12960, PLX 2576, LFT 756, NLTT 26105, Vys/McC 594, G 119-52, G 147-17, IRAS 11005+3615 and GC 15182

     It is with a sense of relief that I can turn to the next star in our pilgrimage through the solar neighborhood, Lalande 21185. At a distance of “only” 8.29 ly, (not counting "brown stars") only three stellar systems are closer than it; the three suns of the Alpha Centauri system, Barnard’s Star, and Wolf 359. Alpha Centauri is too far south to be observed from Maryland, and I will therefore have little to say about it. Barnard’s Star I will discuss with the stars of summer. And wolf 359 we will encounter very shortly. (Stay tuned!)

     So why the relief? Because finally we come to a star that not only can be easily observed, but to one that is hard to miss! First of all, being three and a half light years closer than DX Cancri doesn’t hurt. But secondly, here we have a star that is intrinsically far brighter than our last target. Whereas DX Cancri possessed only a little more than 1/1000th of the Suns visual luminosity, Lalande 21185 shines out at a mighty 2.5% , or about 40 times that of  DX Cancri. Its surface swelters under a 3,383º Kelvin temperature. (DX Cancri’s surface temperature was but 2,840º). Its mass is 46% that of the Sun, making it for a red dwarf positively obese. All in all, Lalande 21185 stretches the envelope of what it means to be a star of that class. Like all red dwarfs, Lalande 21185 emits most of its radiation in the infrared, but enough is left over in the visible wavelengths for us to capture a few stray photons in our eyepiece with ease. 

     Lalande 21185 would be a fascinating star in its own right, regardless of its distance from the Sun. One of its most remarkable qualities is its motion. The star is part of what is known as the Milky Way’s “Thick Disk”, made up of stars that don’t quite keep to the galactic plane in their movements about the galactic center and typically have highly eccentric orbits. (The sun is a “Thin Disk” star.) These Thick Disk stars are typically older than stars which keep nearer to the galactic plane (Lalande 21185 is a little more than twice as old as the Sun), and generally are what is known as “metal poor”. That is, they possess a lower percentage of elements heavier than hydrogen in their makeup than the norm. Lalande 21185 has a metallicity 63% that of the Sun. This is in keeping with current understanding of how stars acquire heavier elements in their compositions, by forming out of interstellar clouds that have been seeded by previous stellar generations which have since gone supernova. The older the star, the less heavier elements from such seeding.

     From the point of view of Thin Disk stars like our own sun, Thick Disk stars appear to be slicing through the galaxy at tremendous speeds. Lalande 21185, currently 8.29 ly from our solar system, will have nearly halved that distance (to only 4.65 ly) in less than 20,000 years. The distance between the two stars is decreasing by approximately 53 miles (85 kilometers) per second (a bullet typically travels at less than 1/100th of that speed).

But of even more interest than Lalande 21185’s wild careening toward us, is that it is one of those many stars suspected to have a solar system of its own. And therein lies a tale. 

     As far back as 1951, Lalande 21185 became one of the very first stars other than our own Sun for which claims of a planetary system were made. The Dutch astronomer Peter van de Kamp, along with his student assistant Sarah Lippincott, working from photographic plates made using the 24inch Sproul refractor at Swarthmore College, Pennsylvania, concluded that “wobbles” in the star’s motion detected on the plates could only be explained by unseen companions (i.e., planets) gravitationally tugging at the star. (Van de Kamp made similar claims in 1966 for Barnard’s Star, using data from the same plates.)  For a time, these claims were widely accepted within the astronomical community, until a systemic flaw was detected in the plates in 1973, casting doubt on the purported planets’ existence. In the following year, astronomer George Gatewood quite decisively demonstrated that Van de Kamp’s conclusions were unsupportable.

     And there the matter may have lain. But (with considerable irony) none other than Gatewood himself in 1996 announced his own discovery of not one, but two (and possibly even three) planets orbiting Lalande 21185, this time working from more than 50 years of data going back to 1930. Since Gatewood’s announcement, considerable effort has been expended by astronomers in an attempt to confirm the planets’ existence photographically, so far without success. So you are free at this point to believe what you wish.

     But assuming Gatewood is correct (and  admit it, it’s more fun to do so), Lalande 21185 is circled by at least two giant worlds, probably similar to the gas giants we find beyond Mars in our own solar system. At 2.2 AU (Astronomical Units, or the distance from the Earth to the Sun) out, we would come across Lalande 21185b, a planet with 90% of Jupiter’s mass, in a highly circular orbit that it takes 5.8 years to complete. At this distance, Lalande 21185b would be a cold world indeed, considering the feeble light (by our standards) coming to it from its parent star. The temperature at its surface, assuming it has one, would never exceed minus 150º Centigrade. Such a cold, heavy world would be no place to look for life! (Unless we engage in wild speculation, and imagine as yet undiscovered moons about the planet similar to Europa or Enceladus, with subsurface oceans heated by tidal friction and internal processes. You are free to let your imagination run wild.)

     An even harsher environment would be encountered at the second possible world, Lalande 21185c. At 10 AU from the star, this planet would take 30 years to complete one orbit, and would have a surface temperature of minus 220º Centigrade. Lalande 21185c weighs in at 1.6 Jupiters, a mass determined by its gravitational tug upon the star it orbits.

     There is less evidence for a hypothetical third world in the Lalande system, even further out.

     To complete the picture, in 2002, Italian astronomer Christiano Cosmovici of Rome’s Institute for Cosmic and Planetary Science announced that his team of researchers had detected evidence of water vapor in the Lalande 21185 system. Using the 32 meter Medicina radio telescope near Bologna, Cosmovici claimed to have picked up microwave emissions indicating water molecules excited by infrared radiation emitting from the star. But whether the emissions in question originated with the star or from one of the as yet unconfirmed planets is still not clear. One intriguing hypothesis involves the aftereffects of a collision of a large comet with one of the suspected gas giants, similar to the recently observed cometary impacts with Jupiter.