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A Rogue Planet

November 28, 2012

Until recently, interstellar planets, planets not associated with any star, were only found in fiction. One of the more famous of these is the planet Mongo from the Flash Gordon serial.[1] Mongo was a rogue planet on a collision course with Earth, but its motions were controlled by its alien emperor, and not by Newtonian gravitation. This serial is notable for the Dr. Zarkov character, a physicist par excellence, played by Frank Shannon.

Just a few decades after Flash Gordon, the movie, When Worlds Collide was made.[2] At that time, it was common knowledge that planets are associated with stars, so it wasn't just a planet on a collision course with Earth, but its associated star as well. The 1933 science fiction novel on which this movie was based had two planets without a companion star wandering into our Solar System, so the screenplay was updated with established scientific facts.

Scientists are the first to admit that so-called "scientific facts" are subject to change, based on better experiments or observations. About a decade ago, planets were discovered that had no obvious stellar host. As I wrote in a previous article (Nomad Planets, March 8, 2012), observations using gravitational microlensing have revealed quite a few free-floating, rogue, or nomad, planets. Computer simulations indicate that there may be thousands of these rogue planets for every star; and, as I wrote in another article (Life from Space, October 12, 2012), interchange of planetary material between stars may be common.

Now, a team of astronomers at the European Southern Observatory has imaged a rouge planet only 65 light-years from Earth.[3-8]. This planet, precisely, but not poetically, named CFBDSIR 2149-0403, was discovered in an infrared survey by the Canada-France-Hawaii Telescope (Mauna Kea, Hawaii).[8] This survey covered a celestial area about a thousand times the solid angle of the Moon, which has an angular diameter of about thirty arc-minutes.[7]

Rogue planet CFBDSIR 2149-0403

Another pale blue dot.

CFBDSIR 2149-0403, the dot that's slightly right of center is quite unlike the Earth. The blue color is from a false-color shading of an infrared image.

(ESO image by P. Delorme, via Wikimedia Commons)

CFBDSIR 2149-0403 is part of a group of very young stars called the AB Doradus Moving Group. It has a mass of 4-7 Jupiter masses, and its temperature is about 430 °C (806 °F).[5] If the planet is as old as the star group, it's about 50-120 million years old, which is quite a bit younger than the Earth. If CFBDSIR 2149-0403 were just a little heavier, at least 13 Jupiter masses, it would have enough mass to achieve nuclear fusion, and it would be a Brown Dwarf and not a planet.

The ESO team was comprised of astronomers from the Institut de Planétologie et d'Astrophysique de Grenoble (Grenoble, France), the Université de Montréal (Montréal, Canada), the Observatoire de Besançon (Besançon, France) and the École Normale Supérieure (Lyon, France). Study collaborator, Etienne Artigau of the University of Montreal, is quoted by the BBC as saying, "We observed hundreds of millions of stars and planets, but we only found one homeless planet in our neighbourhood."[7]

The AB Doradus Moving Group, a group of stars of about the same composition thought to have formed at the same time, is the closest such star group to the Solar System.[4] The team performed a Bayesian statistical analysis of the proper motion of CFBDSIR 2149-0403 which showed an 87% probability that it's associated with the AB Doradus Moving Group. They also determined with a 95% probability that CFBDSIR 2149-0403 is young enough to be a planet, and not a small, failed star.[3-4]

ESO Very Large Telescope

Aerial photograph of the ESO Very Large Telescope array, atop the 2,600 meter Cerro Paranal in the Atacama Desert of Chile.

(ESO photograph by G.Hüdepohl, via Wikimedia Commons)

The relative proximity of CFBDSIR 2149-0403, and the absence of the bright nearby star generally associated with planets, allowed a close study of its atmosphere. The X-shooter spectrograph of the ESO Very Large Telescope showed the presence of methane, which is a component of the atmosphere of our Solar System's gas giants.[4] Says Philippe Delorme, lead author of the study,
"Looking for planets around their stars is akin to studying a firefly sitting one centimeter away from a distant, powerful car headlight... This nearby free-floating object offered the opportunity to study the firefly in detail without the dazzling lights of the car messing everything up."[4]

This year is the fiftieth anniversary of the European Southern Observatory (ESO), which is supported by fifteen countries; i.e., Austria, Belgium, Brazil, the Czech Republic, Denmark, Finland, France, Germany, Italy, the Netherlands, Portugal, Spain, Sweden, Switzerland and the United Kingdom.[4]


  1. Flash Gordon (1936, Frederick Stephani, Director) on the Internet Movie Database.
  2. When Worlds Collide (1951, Rudolph Maté, Director) on the Internet Movie Database.
  3. P. Delorme, J. Gagné, L. Malo, C. Reylé, E. Artigau, L. Albert, T. Forveille, X. Delfosse, F. Allard and D. Homeier, "CFBDSIR2149-0403: a 4–7 Jupiter-mass free-floating planet in the young moving group AB Doradus?" Astronomy & Astrophysics, vol. 548, (December 2012), article no. A26 (14 pages).
  4. Lost in Space: Rogue Planet Spotted? European Southern Observatory Press Release eso1245, November 14, 2012.
  5. Richard Chirgwin, "Rogue planet without a sun spotted in interstellar space," The Register (UK), November 14, 2012.
  6. Astronomers find “homeless” planet wandering through Space, Université de Montréal Press Release, November 14, 2012.
  7. 'Rogue planet' spotted 100 light-years away, BBC, November 14, 2012.
  8. Amina Khan, "Giant rogue planet, without a home star, may roam nearby heavens," Los Angeles Times, November 14, 2012.

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