Interstellar space, this vast region of space that lies between stars, is a very empty place with an average density of only about one atom per cubic centimeter. However, even though it is empty, interstellar space is not completely devoid of matter. It mainly contains a variety of gases, dust, and radiation.
The most abundant gas in interstellar space is hydrogen, followed by helium. These gases are mostly in the form of atomic hydrogen and helium, but they can also be found in molecular form. Other gases that are found in interstellar space include carbon monoxide, nitrogen, oxygen, and neon.
Dust is also a major component of interstellar space. Dust particles are made up of small grains of solid material, and they can be found in a variety of sizes. The smallest dust particles are about the size of molecules, while the largest dust particles can be as large as sand grains. Radiation is another important component of interstellar space. The most common form of radiation in interstellar space is ultraviolet radiation, which is emitted by stars. Ultraviolet radiation can interact with the gases and dust in interstellar space, and it can also damage the DNA of living organisms.
The combination of gases, dust, and radiation in interstellar space creates a complex and dynamic environment. The gases and dust can clump together to form clouds, and these clouds can eventually collapse to form stars and planets. Radiation can also interact with the gases and dust to create new molecules, and it can also help to drive the formation of stars and planets. The study of interstellar space is a fascinating and challenging field of astronomy.
However, a lesser-known aspect is that interstellar space is also home to rogue planetary bodies. These objects vary in size, ranging from small comets to gas giant planets, and some are most likely even accompanied by a satellite or ring system. Ejected from their original location due to the complexities inherent to the dynamical evolution of a Solar System - as hypothesized by the Nice/Grand Tack models - these wandering bodies remain far too distant and diminutive in size to be observed, studied, and comprehensively understood. Whether we will ever be able to discover rogue bodies near our solar system in the future remains to be seen. For several years now, astronomers have been diligently searching for Planet Nine, a theoretical planet initially suggested in 2014 by astronomers Mike Brown and Konstantin Batygin. This elusive planet is believed to reside ‘only’ at 450 AU from the Sun, and should it truly exist, it would not be surprising for its discovery to require many more years of dedicated effort, given the difficulties of such a task.
So, aside from the exoplanets orbiting distant stars, numerous other planetary objects wander among the interstellar spaces, destined to remain forever concealed. From what we know about planetary science, it is not impossible for some of these rogue planets to harbour icy moons, similar to Europa or Enceladus, where enough tidal energy allows liquid water to be present. Perhaps, there is a chance that the conditions are favorable for life to have emerged on these worlds. In such a scenario, these life forms would truly become wanderers, forever drifting across the vast expanses of our galaxy.
So, next time you gaze up at the night sky, contemplating the existence of exoplanets alongside the myriad points of light above, remember that life might also possibly thrive amidst the gaps between stars.
As always, onwards and upwards
Image credit: Nasa/JPL-Caltech