What makes a planet a planet?
The IAU definition: it orbits the Sun, is round under its own gravity, and has cleared its orbital neighbourhood. That third rule is why Pluto was reclassified as a dwarf planet in 2006.
Just know.
17 facts — science, space, the body, the world. No fluff.How many planets are in the Solar System?
Eight: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. Pluto, Eris, Ceres and others are dwarf planets.
What is the Sun made of?
The Sun is about 73% hydrogen and 25% helium by mass, fusing roughly 600 million tonnes of hydrogen into helium every second. It holds 99.86% of the Solar System's mass.
What is a light-year?
A light-year is the distance light travels in one year — about 9.46 trillion kilometres. It measures distance, not time.
What is the Milky Way?
The Milky Way is the barred spiral galaxy that contains our Solar System — roughly 100,000 light-years across with 100–400 billion stars. The Sun takes about 230 million years to orbit it once.
What is a comet?
A comet is a "dirty snowball" of ice, dust and rock left over from the Solar System's birth. Near the Sun it sublimates, growing a glowing coma and tail that always points away from the Sun.
What is the asteroid belt?
The asteroid belt is a ring of rocky leftovers between Mars and Jupiter. Despite the movies, it is mostly empty space — total mass under 4% of the Moon's.
Meteor, meteorite — what's the difference?
A meteoroid is a space rock; a meteor is the streak of light as it burns in our atmosphere; a meteorite is what survives and lands on the ground.
Why is Mars red?
Martian dust is rich in iron oxide — literal rust. The same dust storms can wrap the whole planet and turn its sky butterscotch at noon and blue at sunset.
What is the ISS?
The International Space Station is a football-field-sized laboratory orbiting Earth every 90 minutes at 400 km up. Humans have lived aboard continuously since November 2000.
What is a nebula?
A nebula is a giant cloud of gas and dust in space. Some are stellar nurseries where new stars ignite; others are the glowing wreckage of dead ones.
What is the Big Bang?
The Big Bang is the model describing how the universe expanded from an extremely hot, dense state about 13.8 billion years ago. Its afterglow — the cosmic microwave background — still fills the sky.
How old is the universe?
About 13.8 billion years, measured from the cosmic microwave background. Earth is less than a third of that age at 4.54 billion years.
What is dark matter?
Dark matter is invisible stuff whose gravity shapes galaxies — we see its pull but not the thing itself. It makes up about 27% of the universe; ordinary matter only 5%.
How does a rocket work?
A rocket throws mass (hot exhaust) backwards at high speed, and by Newton's third law the rocket is pushed forward. Unlike jets, it carries its own oxidiser, so it works in vacuum.
Can astronauts cry in space?
Tears form but don't fall — no gravity pulls them down, so they pool as stinging blobs around the eye. Astronauts say crying in space is "the worst".
What is space junk?
Space debris is dead satellites, spent rocket stages and collision fragments orbiting Earth at up to 28,000 km/h. Even a paint fleck at that speed can crack a window.