Once planets orbiting other stars were unknown as they could not be directly observed. Several decades ago one astronomer was certain he had found the planets of a nearby star and his work seemed so painstaking that the astronomical community accepted their existence. Today they are forgotten. Whatever happened to the planets of Barnard’s Star?
EE Barnard (1857-1923) was a skilful astronomical observer who made several important discoveries. Barnard found both the fifth Jovian satellite to be discovered, known today as Amalthea, and a vast faint ring of nebulosity in Orion, known as Barnard’s Loop. Yet his best known discovery is a feeble little star far too dim to be seen with the naked eye. This is Barnard’s Star.
Just under six light years (1.8 parsecs) away, Barnard’s Star is the fourth closest star to the Sun. An M class red dwarf in the constellation Ophiuchus, Barnard is an ancient Population 2 star perhaps twice as old as the Sun. Old stars are misers, spending their energies slowly, so Barnard is only about a two thousandth as luminous as our Sun. It is tiny too, perhaps about twice as wide as the planet Jupiter. One of Barnard’s Star’s statistics is large though, it has among the highest known proper motions.
Proper motion is a star’s actual movement across the sky (more accurately, its angular change in position over time with respect to the Solar System’s centre of mass). The proper motion of Barnard’s Star corresponds to a relative lateral speed (“sideways” across our line of sight to the Sun) of 90 km/s. Basically in 175 years, Barnard’s Star will have moved roughly the angular diameter of the full Moon. Remarkable as this speedy motion is, Barnard’s Star once had a more startling claim to fame. In the 1960s astronomer Peter van de Kamp announced the discovery of two planets in orbit about Barnard’s Star!
Van de Kamp (1901-95), was director of the Sproule Observatory near Philadelphia in the period 1937-72. One of his specialities was astrometry, making precise measurements of the positions of stars. His extensive observations to refine the proper motions and locations of stars (including Barnard’s Star) led him to his dramatic conclusion. His unmatched set of more than 2400 carefully made photographic images of the star, made over decades, showed it to wobble with respect to the background stars as it sped across the celestial sphere. Why should this be? Stars are much larger than any planets locked in their gravitational embrace yet a planet’s gravity should still cause a small shift in its star’s position as it goes around it. Imagine an eager toddler tugging a parent’s hand as they walk together along a street, the adult may walk in a straight line but the child’s yanks will still cause occasional variations in the parent’s path. Extra-solar planets are small and dim, and outshone by their star, so at the time it was impossible to observe them directly, so this technique was the only promising way to find planets beyond our Solar System. Large planets orbiting a small star (like Barnard) ought to be relatively easy to find this way.
Saying this was “relatively easy” is grossly unfair to van de Kamp and his collaborators. Making these astrometric measurements was extremely difficult. The astronomers at Sproule were looking for variations of about a thousandth of a millimetre in the position of the star on a photographic plate. In other words, they were looking for a fuzzy little blob to have shifted a hundredth of the width of a human hair. To reduce error each measurement was made by ten people and averaged, so they were confident in their method’s accuracy. In the original 1962 paper, Barnard’s Star was reported to have a single giant planet in a highly elliptical 24 year orbit, but by 1969 this had been refined. According to van de Kamp’s later work, his measurements showed that Barnard’s Star was circled by B1, slightly more massive than Jupiter with an orbital period of 26 years, and B2 a little less massive than Jupiter with an orbital period of 12 years. This was just what an alien star system was supposed to look like and van de Kamp was widely acclaimed. Suddenly the existence of other solar systems was confirmed with all the implications for life elsewhere that followed from that.
In the mid-1970s, artist’s impressions of the Barnard system appeared in coffee table astronomy books and a mission there was even planned. This was when the British Interplanetary Society chose Barnard’s Star as the destination for their celebrated Daedalus interstellar probe based on de Kamp’s work. Van de Kamp later claimed to have found planetary systems around the nearby stars Lallande 21185, Epsilon Eridanii, 61 Cygni and many others. Extra-solar planets were common said van de Kamp, and so it seemed.
Alas, sceptical voices were raised. In 1973, measurements using a new automated technique to eliminate observer bias on a much smaller sample of photographic plates from other observatories failed to confirm the planets’ existence. Then another researcher used the same plates as van de Kamp to study another tiny M class red dwarf in Draco called Gliese 793 and found that it and Barnard’s Star appeared to wobble in synch. Astronomers began to smell a rat. Eventually it was discovered that all the stars in Sproule’s astrometric archive displayed similar movements, worse still, the movements could be linked precisely to maintenance and repair periods for the telescope used. Van de Kamp had attempted to compensate for these but not well enough. Although works published as late as the 1980s still considered the planets of Barnard’s Star to possibly exist, they gradually joined planet Vulcan and the canals of Mars in whatever shadowy Valhalla discredited astronomical bodies go to.
Van de Kamp refused to accept that his lifetime’s work was in error, defending his research but eventually he started from scratch to repeat his measurements. Sadly, few were interested in this work.
Yet in a way van de Kamp was right. Extrasolar planets are common. In the past fifteen years or so astronomers have discovered more than 500 planets orbiting other stars, many found by modern refinements of the “look for the wobble” technique of van de Kamp. The vast majority of these exoplanets so far discovered are huge gas giant planets, most falling into the previous unsuspected class of worlds known as “Hot Jupiters”. Such worlds are giant planets wheeling around their stars in astonishingly close orbits, and easier to find than planets in outlying orbits. As technology improves we ought to find worlds more like Earth. Perhaps one day soon we will find the real planets of Barnard’s Star.