When you think about a planet, you would probably imagine a rocky or gassy sphere orbiting around its parent star. For many years, this was the only way planets were thought to exist. After all, they only come into existence from the gas clouds surrounding their parent star due to its immense gravity. For years, it was thought that planets existing away from the influence of a parent star just couldn’t exist.
Orphan planets, or rogue planets, are just that. They exist, free floating in in the darkness of space, without a parent star to orbit around. While it is yet to be elucidated exactly how these form, it thought they are either ejected from a traditional solar system model or that they never orbit stars at all; that they form in a similar way to stars themselves.
One of the main challenges regarding rogue planets continues to be how they are detected. Observation of objects in space require something that makes it distinct from the space around it. Sometimes that is radiation, such as visible light, UV light or indeed anything detectable on the electromagnetic spectrum, and sometimes the object is indirectly detected and in the case of planets this is often through their effect on their parent star. As the planet moves between its star and us, anyone observing the star’s luminosity will see a characteristic dip. Unfortunately, neither of these methods can reliably be used to detect rogue planets, since they are often too small to produce detectable radiation and they of course lack a star to move in front of.
Поташев Роман Евгеньевич, CC BY 3.0 https://creativecommons.org/licenses/by/3.0, via Wikimedia Commons
Despite their lack of warmth failing to really distinguish them from the surrounding space, rogue planets can be detected by direct imaging. The difficulty here is with the detection, but once they have been found they can be tracked and observed well, since there is no glare from a parent star, which normally hampers the process with exoplanet discovery.
Additionally, such planets can be discovered using a process known as gravitational microlensing. This process is characterised by a relatively large object bending the light of a background source. In the context of orphan planets, when one of these objects moves in front of a background star, there is an increase in the observed brightness of that star. This is detected on a graph of magnification against time.
Life On Your Own
We exist in an extremely comfortable corner of space, but it would surely be extremely different on one of these lonely planets? Well, not quite as different as you might expect. Professor David J. Stevenson of the California Institute of Technology believes that these planets are more than just floating balls of ice. He believes that these planets can retain a great deal of their atmospheres, and hence ensure the retention of a degree of heat, by the ‘pressure-induced far-infrared radiation opacity of a thick hydrogen-containing atmosphere’. This wordy process essentially means that planets that are ejected from a developing solar system can retain their hydrogen and helium atmospheres because they are not subject to the same battering by ultraviolet light from the parent star. As such, geothermal energy produced by radio-isotope decay may be sufficient the heat surface temperatures above the melting point of water.
Life? Surely Not….
Whenever liquid water is mentioned, the question of life is put on the table. Since liquid water is thought to exist on some of these planets, it is not unreasonable to suggest that liquid oceans may exist. Since these planets are also thought to remain geologically active for a long time, there is also the potential for a strong magnetosphere and sea floor volcanism. This latter point is thought to be crucial to any potential life on these planets, as volcanoes and liquid water combine to support life at ocean floor vents here on Earth.
Increasingly sensitive detection techniques mean that many more of these planets are likely to be found in coming years. In combination with the current and future analytical technologies we should get to know more about their makeup – the composition of their atmosphere, how well they conserve heat and whether there is any potential for life to exist.
