Sky Matters: If we didn't orbit the sun we would live in an endless night

Four billion years ago, we crashed into an object the size of Mars and it changed our seasons forever

The Milky Way, Saturn and Jupiter rising over Church Bay Beach, Crosshaven, Co Cork. Picture: Cian O'Regan/www.cianoregan.com

September sees the transition from summer to autumn in the northern hemisphere. While meteorological autumn starts on September 1, astronomical autumn begins on September 21 at the autumnal equinox. On this day the Sun appears to be directly over the equator and the days and nights are of equal length across the globe. It hasn’t always been this way, however. The very early Earth, around 4.5bn years ago, is believed to have had its axis of rotation vertically upwards. But in a cataclysmic collision with a Mars-sized object, the axis was tilted by 23.5 degrees and it remains at this angle to this day. As the Earth settled into its “new normal” (to borrow a phrase from our present era) our seasons were born. A smaller collision would have resulted in a smaller tilt and less pronounced seasons. A larger collision could have resulted in the complete destruction of the Earth, but if our planet had survived, the tilt could have been larger and our seasonal variations would have been more pronounced. Plants and animals would have evolved to deal with greater extremes of heat and cold. Winters for much of the planet would have thrown life into an annual deep freeze the likes of which we have never seen. Given life’s capacity to survive in adversity, one can but wonder what fantastical creatures would have roamed this hypothetical tilted world.

Earth without the Sun 

But what if the Earth didn’t have a Sun to orbit? Evidently there would be no seasons, just a perpetual night accompanied by an unchanging temperature of minus-270 degrees Celsius. At such temperatures pretty much nothing of interest ever happens. Chemical reactions cease to take place in any meaningful way. Life has no chance to establish a foothold. Without a Sun there are no seasons. Yet this may be the fate of billions of so-called rogue planets that might even outnumber planets that are fortunate enough to orbit a star with all the warming benefits that brings. Rogue planets may once have been gravitationally bound to a star, much as the Earth is to the Sun. They may have been ripped from their star’s grasp by a close encounter with another planet. Or they may have formed directly out of massive clouds of dust in space without a star in sight and never experienced the warming glow of a parent Sun. Searching for roque planets will help us understand more about how planets form in our galaxy. Their very existence challenges long-held perceptions about how stars and planets are made and how they evolve. New research into roque planets is accelerating, but they are extremely hard to find. They are dark and they are cold, making them almost invisible. It’s only because they have mass that we can find them at all. If they pass between us and a distant star their gravity can bend the light from that star in a manner akin to a magnifying glass. The star appears to brighten for a few days to weeks. We don’t get to ever “see” the rogue planet itself.

Reappearing planets

Moving closer to home, the planet Jupiter will continue to shine very brightly in the southern sky after sunset, accompanied to its left by Saturn. If you look due east around 10pm you will also get a chance to see Mars making a reappearance after a period of absence as it was on the far side of the Sun to the earth. And in the morning sky, the cloud-covered planet Venus begins to show again. The ancient Greeks coined the word “planet” which means “wanderer”. To them, the term was appropriate because the five visible planets seem to wander amongst the fixed stars. In reality, we’re learning of the existence of planets which really do wander in the space between the stars, of no fixed abode, rogue outcasts which have remained completely hidden — until now.