by Paul Gilster, writer and author of “Centauri Dreams”.
Edited by Zaira M. Berdiñas
The Red Dots campaign to study Proxima Centauri, Barnard’s Star and Ross 154 gives us a cannily chosen set of targets. All red dwarfs much smaller than the Sun, these stars offer us the opportunity of atmospheric analysis of any planets discovered there by future space-and ground-based instruments because all are close. At 4.2 light years, Proxima Centauri is nearest to the Sun, but Barnard’s Star is a scant 6 light years out, making it the closest known star other than the three Alpha Centauri stars. Ross 154 comes in at just under 10 light years, still very much in the local neighborhood in astronomical terms.
But it is not just their proximity that makes these stars interesting. We’d like to know how stars like this age, considering that young M-dwarfs can show strong flare activity. All three of these stars do, with Proxima Centauri and Ross 154 being catalogued as UV Ceti stars; i.e., stars that produce major flares every few days. Barnard’s Star is of a variable category known as BY Draconis, stars that show starspots, variations in luminosity and other activity.
So consider the spread here. Proxima Centauri is thought to be about 4.85 billion years old, while Barnard’s Star is perhaps twice that. Ross 154, however, shows a high rate of rotation — 3.5 ± 1.5 km/s — that indicates a younger star, perhaps one less than a billion years old. Thus we have three stars and possible planets at markedly different stages of development, giving us the ability to take a deeper look into flare activity on M-dwarfs as they age, and to assess flare effects on planetary habitability, assuming Barnard’s Star and Ross 154 do have planets. We’ll also be investigating the prospects for multiple planets around Proxima Centauri itself.
Barnard’s Star has already produced its own share of notoriety. Working at the Sproul Observatory (Swarthmore College, Pennsylvania), astronomer Peter van de Kamp examined 2,413 photographic plates of the star taken between 1916 and 1962. The astronomer observed what he believed to be a telltale wobble in the motion of Barnard’s Star that fit the profile of a planet about 1.6 times Jupiter’s mass in an orbit at 4.4 AU1. He would later suggest the possibility of two gas giants here2, and by 1973, Oliver Jensen (University of British Columbia) and Tadeusz Ulrych had upped the number to three3.
If confirmed, these would have been the first planets ever detected outside our Solar System, but it was not to be. Follow-up studies by George Gatewood (University of Pittsburgh) and John Hershey (also at the Sproul Observatory) found systematic errors in van de Kamp’s work. The culprit: Lens adjustments to the Swarthmore instrument that were later confirmed by Hershey when he found an identical wobble in the M-dwarf Gliese 7934. Subsequent work by Gatewood and, later, Jieun Choi (UC Berkeley) would be able to detect no planets5,6.
Peter van de Kamp’s tool was astrometry, meaning he used precise measurements in the proper motion of the star to look for the presence of planets, finding minute variations on photographic plates that were consistent with the hypothesis. His observational skills and persistence were rightly praised, but errors in his instrument negated what would have been a major discovery.
So what do we have today? We can rule out gas giants at Barnard’s Star thanks to continuing Doppler monitoring, but we can’t yet rule out small rocky planets of the kind we are now turning up around other M-dwarfs in data from the Kepler mission. Kepler has shown us that planets of a few times Earth-mass are not uncommon, while a 2013 study by Ravi Kopparapu (Pennsylvania State) found that about half of all M-dwarfs should have Earth-size planets in the habitable zone7. What might Red Dots uncover around this tantalizingly close star?
It was Peter van de Kamp’s work that helped the energetic team of starship designers behind the British Interplanetary Society’s Project Daedalus choose Barnard’s Star as their destination. And physicist Robert Forward, no stranger to fiction, would use a planetary system around Barnard’s Star as the setting for his novel Rocheworld (1984). The system is reached by a lightsail beamed by a laser array, a concept not unfamiliar to today’s Breakthrough Starshot (read the article by Avi Loeb), which envisions sending small sails by laser to Proxima Centauri.
How fitting, then, that Red Dots should home in on this interesting system, along with a return to Proxima Centauri and a deep exploration of Ross 154 as well. Red dwarf stars like these account for as much as 80 percent of the stars in our galaxy. The new campaign will let us see, in real time, no less, just how this inspiring search of nearby dwarfs proceeds.
- van de Kamp, P. “Astrometric study of Barnard’s star from plates taken with the 24-inch Sproul refractor”, Astronomical Journal, 68, 515, (1963).
- van de Kamp, P. “Alternate dynamical analysis of Barnard’s star”, Astronomical Journal, 74, 757, (1969).
- Jensen, O. G. & Ulrych, T. “An analysis of the perturbations on Barnard’s Star”, Astronomical Journal, 78, 1104, (1973).
- Hershey, J. L. “Astrometric analysis of the field of AC +65 6955 from plates taken with the Sproul 24-inch refractor”, Astronomical Journal, 78, 421, (1973).
- Gatewood, G. & Eichhorn, H. “An unsuccessful search for a planetary companion of Barnard’s star BD +4 3561”, Astronomical Journal, 78, 769, (1973).
- Choi, J. et al. “Precise Doppler Monitoring of Barnard’s Star”, Astrophysical Journal, 764, 131, (2013).
- Kopparapu, R. K. “A Revised Estimate of the Occurrence Rate of Terrestrial Planets in the Habitable Zones around Kepler M-dwarfs”, Astrophysical Journal, 767, L8, (2013).
About the author
Paul Gilster writes and edits Centauri Dreams, tracking ongoing developments in interstellar research from propulsion to exoplanet studies and SETI. A full time writer for the last thirty-five years, he is the author of the books Centauri Dreams: Imagining and Planning for Interstellar Flight (Copernicus, 2004) and Digital Literacy (John Wiley & Sons, 1997). He is also one of the founders of the Tau Zero Foundation and now serves as its lead journalist. This organization grew out of work begun in NASA’s Breakthrough Propulsion Physics program, and now seeks philanthropic funding to support research into advanced propulsion concepts for interstellar missions. Gilster has contributed to numerous technology and business publications, and has published essays, feature stories, reviews and fiction both in and out of the space and technology arena.