This feature was first published in March 2025 in My Mensa Weekly, our exclusive newsletter for Mensa members. Find out more about becoming a Mensa member here.
Words: Jonathan McIntosh
Whether it’s super-slow orbits, incredibly dense stars or planets with mindboggling mountains, space is full of weird and wonderful things. Here, we investigate seven cosmic curiosities in our solar system.
1. A day is twice as long as a year on Mercury
A Mercurian solar day – one full day-night cycle – is equivalent to 176 days on Earth. This is twice the length of a year on Mercury, which lasts just 88 Earth days.
Mercury spins slowly on its axis, completing a single rotation every 59 days. When the planet reaches the speediest points of its elliptical orbit around the Sun, each rotation doesn’t bring with it a sunrise and sunset like other planets. Instead, during a Mercury morning the Sun appears to quickly rise and set before rising again soon after.
Ironically, despite its slow-paced days, Mercury is our solar system’s fastest-travelling planet, racing through space at speeds of up to 29 miles per second.
2. The Apollo astronauts’ footprints on the Moon will last for at least 100 million years
Because the Moon doesn’t have an atmosphere, there’s no wind or water available to wash away the marks made by the Apollo astronauts. This means that their vehicle tracks, spaceship indents, discarded materials and those small steps for mankind will likely remain in place for at least 100 million years.
However, this doesn’t mean these remnants will last for an eternity. The Moon’s environment is perpetually showered with micrometeorites, which cause surface erosion to take place – just at a much slower pace.
3. Mars houses the solar system’s highest mountain
Standing at a mighty 13 miles high, Mars’s Olympus Mons is the tallest mountain in our solar system. To put this into context, this volcanic crag is roughly two and a half times the height of Mount Everest.
But the Red Planet’s biggest mountain isn’t just impressively tall, it’s also incredibly broad. Olympus Mons is 20 times wider than it is high and has a 370-mile diameter. If you placed this impressive peak on Earth, it would completely cover France or Arizona.
4. The Sun is steadily shrinking
For the past century, scientists have been studying how the Sun is gradually getting smaller. This dynamic entity, on which our planet relies, consumes energy and loses 1.3 trillion particles every second as a result. That’s equivalent to 1 billion kilograms of matter per second.
Although these numbers sound huge, this shrinkage is a slow burner. If the Sun continues to lose mass at this rate over the next 100 years, it’ll be roughly 1,000 miles narrower than its current 860,000-mile diameter.
5. A teaspoon of neutron star weighs the same as the human population
When a medium-sized star reaches the end of its life, it explodes in a supernova. This burnout leaves behind an extremely dense core called a neutron star. These stars crush protons and electrons together to create more neutrons. A neutron star comprises about 90% neutrons, which are bundled together in a small radius.
Incredibly, just a single teaspoon of a neutron star’s make-up weighs over a trillion kilograms. This is heavier than the combined weight of humanity. To be equally as dense as a neutron star, all of humanity would need to be squeezed into a space the size of a sugar cube.
6. The universe is constantly expanding
Space objects have been slowly moving away from one another since the Big Bang due to the expansion of the Universe. The force behind this has been named dark energy. It’s thought this hypothetical energy force provides a negative pressure that fills the Universe and stretches spacetime’s fabric.
The rate of the Universe’s expansion is measured in a quantity known as the Hubble constant, which is roughly 73 kilometres per second per three million light years. This means that a galaxy located three million lightyears further away from another is receding 73 kilometres per second faster.
7. Neptune has only completed a single orbit around the Sun since its discovery
It takes 165 years for Neptune to complete one full orbit around the sun. This means that Neptune has only completed one full orbit since its official discovery on 23 September, 1846 – a hotly debated effort by Johann Gottfried Galle, Urbain Jean Joseph Le Verrier and John Couch Adams.
Neptune is 2.8 billion miles from the Sun, making it our solar system’s most outermost planet. The icy giant’s axis of rotation is tilted 28 degrees, which is similar to Earth’s. Because of this, Neptune also experiences four seasons, although each lasts for over 40 years.
Our Universe contains mysteries that would make even the strangest science fiction seem normal by comparison. And while we don’t know the answers to all the questions it raises, our fascination with space has inspired countless generations and led to groundbreaking discoveries. These have helped us better understand Earth’s place in the wider Universe.
From javelins to robots, read about the objects humanity has left behind on its trips to the Moon over the decades
NASA is journeying to Jupiter’s mysterious moon for the first time to determine whether it might be able to host alien life.
Image credit: Shutterstock
