New research suggests that there could be many more wandering planets in our galaxy than there are stars. Planets could be torn from their parent suns and slung into space. Estimates suggest that a galaxy like ours could have billions of them.
Removed from their warmth of their parent stars such worlds would become very cold very quickly, but according to some scientists there are ways that life could survive on them. If the planet had an active internal geology providing geothermal energy and an atmosphere that was hydrogen rich then it is possible that its surface would not become too cold even if there were no star to warm it. Technological life could adapt, and it's conceivable that life could even begin on these dark worlds.
The vastness of space means that such nomad planets are very unlikely to come wandering close enough to our solar system to detect. Drifting through our galaxy there must be many dead worlds carrying the remains of extinct civilisations with deserted libraries telling their ancient stories. Most never to be read by anyone in whatever aeons the universe has left.
It's not just orphan planets that wander space. Stars do it as well.
Go out on a starry night as far from artificial light as you can, and let your eyes become accustomed to the dark. On a good night you can see a couple of thousand stars bejewel the sky; stars of all brightness's, of varying colours and characters, many of which will have planets circling them. This is the view looking out from within our star-strewn galaxy. On a clear night you might just see an object that does not belong to our galaxy - a faint smudge on the outskirts of the constellation of Andromeda. This is M31, our companion galaxy. Now imagine removing all the stars so that dim M31 stands alone in a devoid sky. The cosmos is made of many vast starless regions with lonely nomad stars drifting through these intergalactic voids. This then is the view of the universe experienced by many life-forms - one of no stars and no hope.
One of the Hubble Space Telescope's most intriguing images is of a 300 million light-year distant pair of galaxies called NGC 4676. These galaxies have passed close to each other and will eventually merge. Their last encounter - a hundred million years ago - resulted in a burst of new stars. But look closer and you can see that as they swept past each other they produced streamers of stars some of which will go into wide orbits taking them away from their parent galaxy for billions of years, but others will be lost forever, more nomads.
It's a common story. The Chandra X-ray observatory has seen stars forming in hydrogen streamers billowing behind the galaxy ESO 137-001 as it ploughs through galactic cluster Abel 3627. It's also occurring in the cluster called Stephan's Quintet, the "tadpole" galaxy UGC 10214 as well as in NGC 4388. It happens everywhere. The Universe is full of nomad stars. A 1997 observation found a swarm of them over 300,000 light years from their parent galaxy, all travelling outward. Some estimate that ten per cent of all stars could have been lost from their home galaxies.
Life in the Dark
What interests me is how intelligent life would develop on such worlds? How would the absence of stars in sky have affected their creation myths? Lacking constellations would they see no sign of the gods we once imagined lived in the night sky, or would they come to believe that the gods had not yet born who could bring light to the Cosmos and show them their place within it? But whatever their myths, when such civilisations develop science they may find such isolation hard to bear.
We hope to find intelligence out in space, many expect it. We scan the sky for a radio beacon, or a laser flash, to show us that we are not alone. But there will be none of these quests for those civilisations on lone planets or circling nomad stars. They are just too far away, not just a few light years from similar stars as we are, but millions of light years - surely for most an uncrossable gulf.
No doubt they will build telescopes to peer across the void to see on the other side of the sky inviting galaxies each one a relatively short hop from each other. Then they will know just how unlucky they are. Nothing they could do, no probe or beacon they could ever devise will tell the rest of the Cosmos that there are, or were, there. Whatever advances or insights they as a species may make in terms of science or art will never be appreciated by others.
The M31 galaxy is heading towards our galaxy at 300 km a second and we may collide in three billion years during which stars from our galaxy will be scattered into the void. Among them may be our own. Perhaps our sun's ultimate fate is also one of isolation and loneliness, as, carrying the earth in tow, it leaves the Milky Way heading into the uncharted darkness where it will wander almost for eternity.