How long is a year on other planets? A simple guide
A year on Mercury lasts 88 days; on Neptune it lasts almost 165 Earth years. Here’s why planet years differ and how your age on Mars is calculated.
Published: 3 min read
On Earth, a year is the time it takes our planet to travel once around the Sun. Every other planet has its own year too — and they are wildly different. A full orbit can take a few months or more than a human lifetime.
The length of a year on each planet
These are the planets’ orbital periods, measured in Earth days and Earth years (rounded):
| Planet | One year in Earth days | In Earth years |
|---|---|---|
| Mercury | 88 | 0.24 |
| Venus | 225 | 0.62 |
| Earth | 365.25 | 1 |
| Mars | 687 | 1.88 |
| Jupiter | 4,333 | 11.86 |
| Saturn | 10,759 | 29.46 |
| Uranus | 30,689 | 84 |
| Neptune | 60,182 | 164.8 |
Why the outer planets take so long
Two things make distant planets slower. First, their path around the Sun is much longer: Neptune’s orbit is about 30 times wider than Earth’s. Second, they move more slowly, because the Sun’s gravity is weaker far away.
In the early 1600s, Johannes Kepler discovered a neat rule connecting these facts: the square of a planet’s orbital period is proportional to the cube of its average distance from the Sun. Put simply, if one planet is four times farther out than another, its year is eight times longer.
Some fun consequences
- Neptune’s first birthday. Neptune was discovered in 1846. It only completed its first full orbit since discovery in 2011.
- A day longer than a year. Venus spins so slowly that one rotation takes about 243 Earth days — longer than its 225-day year.
- Mercury makes everyone older. Because Mercury orbits so quickly, a 20-year-old on Earth would be about 83 “Mercury years” old.
How to calculate your age on another planet
You don’t need a telescope, just a little arithmetic:
- Count the days you’ve been alive. For example, someone born exactly 25 years ago has lived about 9,131 days (including leap days).
- Divide by the planet’s year length in Earth days. On Mars: 9,131 ÷ 687 ≈ 13.3.
- That number is your age in that planet’s years — so this person would be about 13 on Mars.
You can also work out your next birthday on another planet: take the remainder of that division and subtract it from the planet’s year length. The answer is how many Earth days you have left until your next Martian (or Jovian) birthday.
Why does Earth have leap years?
Earth’s year isn’t exactly 365 days. Measured season to season, it lasts about 365.2422 days. That extra quarter day adds up, so the calendar adds a leap day almost every four years to keep the seasons in place. Without it, the calendar would drift by about one day every four years.
Try it yourself
Our planet age calculator does all of this automatically: enter your birthday and it shows how many days you’ve been alive, your age on all eight planets and how long until your next birthday on each one. Your birthday stays on your device — nothing is sent to a server.