The Wandering Planet

Rogue planets wander about in interstellar space, detached from their stars and dislodged from their orbits to float freely into the desolate void of the cosmos! A mechanism which presents a very intriguing phenomenon for astronomers. Although such planets are quite elusively hard to detect owing to their relatively faint nature, a number have been discovered. Estimates put the number of these lonesome worlds at 400 billion, adrift in our galaxy alone. It is quite possible indeed that our own solar system commenced with more than the number of the current planets that we know, which they were consequently flung out into cosmic desolation to become these rogue planetary wanderers.

However, are rogue planets really planets and should they be accorded such a name, in line with the naming standards of the International Astronomical Union? In order to answer that question, the mechanism by which they formed first needs to be ascertained. To call a rogue planet a planet means to presume that it was birthed from a proto-planetary disk around a parent star and was then jettisoned outwards as a result of gravitational interactions with other planets. But, perhaps, it wasn’t bound to any star and formed independently like a brown dwarf as a collapsing entity of interstellar dust. As the mechanism by which these objects formed remains at present unresolved, I shall stretch a point and proceed to call them rogue planets, for the purposes of this article.

A number of rogue planets have been evidenced to exist. Rogue planets are faint and dark objects that emit no light and therefore are hard to detect, as established by their lack of an illuminating star.  But, astronomers can make use of gravitational microlensing techniques to detect those dark planets. Gravitational microlensing is based on the fact that gravity affects light. Therefore, massive objects should cause light to bend. Any invisible rogue planets would impose gravitational effects on the light of stars they pass in front of. The brightness of these stars would be consequently enhanced. Thus, an observer in the focal line of the background star would see the light as bent and an increase in luminosity would arise. The rogue planet is thus employed as an interrupting lens that deflects light due to general-relativistic effects and as a result, allows the observer to collect more light. For this to work, the observer, the lens (rogue planet), and the source (background) have to be positioned in perfect alignment.

The closest rogue planetary mass object ever to be discovered (WISE 0855–0714) is at 7 light years away and is about 3 – 10 times that the mass of Jupiter. Recently also, using the Canada-France-Hawaii Telescope and the Very Large Telescope, astronomers discovered CFBDSIR2149, a gas giant believed to be about 4 – 7 times the mass of Jupiter, and positioned 100 light-years away. It was found emitting a faint infrared glow and seems to be associated with the young AB Doradus stellar group. In 2013, astronomers discovered the free-floating PSO J318.5-22 with the lowest mass as that of 6 Jupiters, at 80 light years away.

PSO-J318.5-22 Rogue Planet. Image credit: Pan-STARRS1 Science Consortium


Such rogue planets have also countenanced the possibility of alien life. Suggestions have been made that such planets might be potential extraterrestrial life abodes, given the possibility that they might possess subterranean oceans powered by geothermal energy, as reasoned by their jettisoning outside their parent solar system quite early. Evolution would have to be creative, given that photosynthesis would not be possible, owing to the lack of a light source. Any such life on these planets would therefore remotely resemble life on Earth.

The free-floating planet CFBDSIR J214947.2-040308.9 (annotated)
The closest free-floating planet CFBDSIR J214947.2-040308.9 as captured by the SOFI instrument on the European Southern Observatory’s New Technology Telescope in infrared light. As it does not orbit an illuminating parent star, it only emits faint infrared glow. Image credit: ESO

There could be perhaps as many rogue planets as there are stars. Who knows if life is scattered throughout these fascinating cosmic wanderers? However unlikely, we still have to entertain the possibility!



Han C, Chung SJ, Kim D, et al. 2004. Gravitational microlensing: a tool for detecting and characterising free-floating planets. ApJ 96: 372-378.

T. Sumi et al. 2011. Unbound or distant planetary mass population detected by gravitational microlensing. Nature 473: 349–52.

Featured image source: European Southern Observatory. Artist’s impression of CFBDSIR2149.



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