Since then the Kepler mission, launched in 2009, has shown that that our Sun is not unique in having planets: many 'exoplanets' have been found orbiting other stars in the Milky Way, and the planets in these stellar systems come in an incredible variety of sizes and characters. It seems our solar system and our home planet is just one of a vast multitude of star-planet systems.
Starless planets
As if that is not enough, it seems that a planet does not even need to be orbiting a star - in the past three or four years, several mysterious dim objects, such as the snappily named PSO J318.5-22, have been confirmed as so-called 'rogue' or 'orphan' planets. In other words, unlike the planets we are used to, they are not even part of a stellar system, but floating freely around the galaxy. They are not stars because they are too small to generate their own heat -the ones discovered are approximately the mass of a few Jupiters for example.
Expelled from the family
One theory is that as a planetary system develops over time, the conflicting gravitational effects of the various components may cause the orbit of one or more of them to become unstable, resulting in it being thrown out of the system altogether. In fact, based on computer modelling of our solar system' development, some astronomers argue that it may have once had a fifth giant planet that was kicked out at at early stage.
Another suggestion is that 'rogue planets' are in fact 'failed stars' – clumps of matter that have formed from clouds of dust but that never gathered the amount of mass necessary to trigger nuclear fusion in their cores. For example, recent research has found over a hundred small very dense clouds of gas, dubbed 'globulettes' in the Rosette Nebula, just the right size to end up as planet-sized bodies after collapsing in on themselves due to gravity.
Almost invisible
Exoplanets are most often detected by observing dips in the brightness of the host star as the planet passes in front of it, or slight wobbles in the star's movement due to the planet's graviational pull. But rogue planets have no host star, and they are very dim – so how is it possible to spot them at all?
One way is to observe in infrared, which can pick up very low levels of light; another is by looking out for distorted images of other object – this effect, known as 'microlensing', can be caused by the gravitational pull of a mass of an object that is otherwise too dim to be detected.
Still, it's a real challenge to spot these orphan planets, especially ones smaller that Jupiter. Astronomers suspect that there must be millions or even billions out there, but to date only a handful have been discovered – most of them are just too small and dim!
Nevertheless, the level of detail that can be extracted from successful observation is quite amazing. For example, by carefully recording changes in the brightness of PSO J318.5-22, researchers estimate that this Jupiter-sized rogue planet has several layers of thick and thin cloud, and a temperature of over 800 celsius within the clouds. Not the most hospitable of worlds!
http://phenomena.nationalgeographic.com/2014/03/13/a-guide-to-lonely-planets-in-the-galaxy/
http://www.universetoday.com/104210/rogue-planets-could-form-on-their-own-in-interstellar-space/
http://www.sciencemag.org/news/2015/08/our-early-solar-system-may-have-been-home-fifth-giant-planet
https://astronomynow.com/2015/11/04/distant-worlds-weather-is-mixed-bag-of-hot-dust-and-molten-rain/
1. MPIA/V. Ch. Quetz
2. Mark Booth/Creative Commons
3. ESO/M. Mäkelä