If you saw them side by side from a distance, you might mistake one for the other. They're almost exactly the same size. They're made of the same kind of rock and metal. They formed at the same time, from the same swirling cloud of dust, about 4.6 billion years ago.
But up close? Earth is the planet of forests, oceans, polar bears and pizza. Venus is a poison atmosphere over rocks that are hot enough to melt lead. Same family. Very different lives.
So how did this happen? Let's compare them properly, starting with the bits where they really do look like twins.
How Venus and Earth Are Similar
There's a reason scientists call Venus our sister planet. Almost all the basic stuff lines up.
- Size. Earth is 12,742 km across. Venus is 12,104 km across. Venus is about 95% the size of Earth.
- Mass. Venus weighs roughly 81% of what Earth weighs.
- What they're made of. Both planets are rocky, with similar materials all the way through.
- Gravity. Venus's gravity is 91% as strong as Earth's. If you weigh 100 pounds on Earth, you'd weigh 91 pounds there.
- Inside layout. Both planets have an iron core, then a hot rock mantle, then a thin rocky crust on the outside. Like a planet-sized boiled egg.
- Birthday. They both formed about 4.6 billion years ago, basically at the same time, right next door to each other.
Some scientists even reckon Venus had oceans of liquid water in its early years. So originally, the two planets really might have been close to identical.
How Venus and Earth Are Different
Then, somewhere between then and now, things went very wrong on Venus.
Here's the side-by-side:
| Feature | Earth | Venus |
|---|---|---|
| Distance from the Sun | 93 million miles | 67 million miles |
| Average surface temperature | 14 °C | 462 °C |
| Atmosphere mostly | Nitrogen, with 21% oxygen | 96% carbon dioxide |
| Surface pressure | 1 (you're feeling it now) | 92 times Earth's |
| Clouds made of | Water | Sulfuric acid |
| One day | 24 hours | 243 Earth days |
| One year | 365 days | 225 Earth days |
| Spin direction | Anticlockwise | Clockwise (backwards!) |
| Number of moons | 1 (the Moon) | 0 |
| Magnetic field | Strong | Very weak |
| Liquid water | Loads | None |
| Life | Yes (you, for example) | Probably not |
Earth's average temperature is 14 °C. Venus's is 462 °C. The same family, separated by 448 °C. That's a lot of degrees.
Why Did Our Sister Planets Turn Out So Different?
Three big reasons.
First, Venus is closer to the Sun. Not by a lot, but enough. The extra solar heat slowly evaporated Venus's water over millions of years. Water vapour is itself a greenhouse gas, so it trapped more heat, which evaporated more water, and so on.
Second, Venus has no plate tectonics. Earth's surface is broken into giant plates that slowly shift around. As they move, they pull old rock down into the planet (carrying carbon dioxide with it) and push fresh rock up. This is one of the ways Earth quietly recycles its CO₂ and keeps the air balanced. Venus's surface doesn't move like that. Once carbon dioxide built up in its air, there was nothing to take it back out.
Third, Venus's volcanoes pumped out a lot of CO₂. With no oceans to absorb it, no plants to breathe it in, and no plate tectonics to bury it, the gas just kept piling up.
Add those three things together and you get a planet wrapped in a CO₂ blanket so thick it traps every bit of solar heat. The runaway greenhouse effect locked Venus into its current state, and it's been stuck there for hundreds of millions of years.
→ More on that here: Why Is Venus So Hot? The Greenhouse Effect Explained for Kids
The "Evil Twin" Nickname
NASA scientists sometimes call Venus Earth's evil twin. It's a nickname rather than a science term, but it's caught on, and it makes a serious point.
Earth has all sorts of natural systems quietly working to keep our atmosphere balanced. Oceans that absorb carbon dioxide. Forests that breathe it in. Plate tectonics that bury it. Polar ice that reflects sunlight. Venus had a lot of those systems too, billions of years ago. Then it lost them, one by one.
That's why studying Venus matters so much. It's not just a nearby planet, it's a giant warning sign about how planet climates can flip from one state to another, and how hard it can be to flip them back.
Could Humans Live on Venus Instead of Earth?
The surface of Venus? A definite no. The heat alone would finish you in seconds, never mind the acid clouds and the crushing air pressure.
But about 50 km up in Venus's atmosphere, things get surprisingly comfortable. The temperature is between 30 and 70 °C, the air pressure is similar to Earth's, and there's plenty of sunshine. NASA has even drawn up plans for floating cities, giant balloons drifting through the upper clouds, with humans living comfortably inside.
So Venus might never be a holiday destination. But cloud-based homes there? Possibly, one day.
→ Read about that here: Can Humans Live on Venus?
Frequently Asked Questions
Why is Venus called Earth's sister planet?
Because the two planets are almost the same size and weight, are both made of rock and metal, and formed at the same time from the same materials. They're also next-door neighbours in the solar system.
How big is Venus compared to Earth?
Venus is about 95% the size of Earth. Its diameter is 12,104 km, compared to Earth's 12,742 km. Venus is also about 81% as heavy as Earth, and gravity there is 91% as strong as ours.
Why is Venus so much hotter than Earth?
Because of its thick atmosphere of carbon dioxide. CO₂ is excellent at trapping heat, and Venus has so much of it that the heat keeps building up. This is called the runaway greenhouse effect. Earth's atmosphere is mostly nitrogen with a tiny amount of CO₂ (about 0.04%), so we don't have the same problem.
Did Venus ever have water like Earth?
Probably yes. Many scientists think Venus had oceans of liquid water for hundreds of millions of years in its early life. Eventually it got too hot, the oceans evaporated into the atmosphere, and the water vapour was lost to space.
Keep Exploring Venus
- Venus Facts for Kids — The Complete Guide
- Why Is Venus So Hot? The Greenhouse Effect Explained for Kids
- Can Humans Live on Venus?
- Venus's Atmosphere — Acid Clouds and Crushing Pressure