The number of alien worlds – exoplanets – discovered by astronomers has doubled in the past three years, rocketing from 400 in 2010 to a figure of almost 900 in 2013.
As more and more planets are uncovered in the depths of space, scientists are concluding that our corner of the universe is the antithesis of a vibrant, varied solar system; that is, our Sun’s companions are unremarkable – even boring.
For many people, the structure of planets conforms to two or three basic archetypes: gas giants like Jupiter and Saturn; barren, rocky deserts, similar to Pluto and Mercury, and the habitable garden worlds, akin to Earth and a prehistoric Mars or Venus.
However, astronomers indicate that several common types of planet – possibly, up to nine – are absent from our Solar System, making the Earth and its neighbours something of an oddity in the wider universe. Here is one of those worlds, the enigmatic rogue planets.
- Free-floating Worlds
A rogue planet is a body that, for some reason, decided to go it alone. These giant mavericks, which are not gravitationally attached to any particular star or system, orbit a lonely path around the Milky Way.
Scientists believe that these free-floating planets, which could number in the billions, may have been hauled or pushed out of their solar system – perhaps due to a collision; perhaps in a gravitational tussle with a large gas giant or a passing star.
Alternatively, these hopelessly lost planets may be very small stars, formed much like our Sun, known as sub-brown dwarfs. Note that the boundary between what constitutes a very small star and a gigantic planet is still up for debate.
>> above: this video from Space.com shows a rogue planet travelling through deep space. The video ostensibly depicts CFBDSIR 2149-0403 (below), a planet that appears a deep crimson red in visible light.
Despite the fact that rogue planets are, by definition, not warmed by the heat of a star, life could potentially evolve in such a place.
Amanda Doyle, writing for Astronomy Now (May 2013; 28), suggested that a large body, around 3-4 times larger than Earth, retaining some of the heat from its formation, could be warm enough to keep a liquid ocean from freezing.
The pre-requisites for life, a heat source and liquid water, could also be maintained via a moderate greenhouse effect – possibly ‘topped up’ with volcanic activity – or the action of tidal heating from a moon.
>> above: Jupiter and Io, a moon with the most geologically-active surface in the Solar System, a product of tidal heating. Image from Celestia.
The usefully-named rogue planet, CFBDSIR J214947.2-040308.9 (CFBDSIR 2149-0403 for brevity) is the closest known body of its type to Earth, at 130 light years away. The planet is moving through the galaxy with a family group of 30 stars, known as the AB Doradus Moving Group.
Interestingly, the possibility of a rogue planet settling into a traditional solar system is not beyond the bounds of imagination; in fact, astronomers have suggested that planets orbiting their star backwards, such as WASP-17, could have begun life as a rogue planet, floating aimlessly through deep space.