How long is a year on each planet? Years, days and your age across the solar system

Ask how old you are and you will give a number of years, but a year is not a fixed amount of time across the solar system. It is one trip around the Sun, and every planet makes that trip at a different speed. On Mercury you would have a birthday every three months. On Neptune, nobody alive today has yet reached their first. This guide explains what a year and a day really are, lists both for all eight planets, explains why the outer planets are so slow, looks at some genuinely strange days, and shows how to work out your own age on any planet.

What a year really is

A year is the time a planet takes to travel once around the Sun and return to the same position relative to the distant stars. Astronomers call this the sidereal period. For Earth it is about 365.256 days. The calendar year of 365 or 366 days is a practical rounding of a slightly different measure tied to the seasons, but for comparing planets the sidereal year is the one that matters, and it is the one NASA lists for each planet.

A day is a separate thing: the time a planet takes to spin. There are two ways to measure it. A sidereal day is one full spin relative to the stars. A solar day is the time from one noon to the next, which is what we normally mean by a day. On Earth the two differ by only about four minutes, but on slowly spinning planets they can be wildly different, as Mercury and Venus show below.

Years and days on every planet

The table lists each planet’s year in Earth days and Earth years, along with the length of its solar day. The last column shows the age, in that planet’s years, of someone who has been alive for 13,350 days, which is a person born on 12 April 1990 on 30 October 2026, the example we use across the site.

Mercury and Venus are the only planets with years shorter than Earth’s, because they are closer to the Sun. Everything beyond Earth has a longer year, and the jump grows fast. Mars takes not quite two Earth years, Jupiter almost twelve, Saturn nearly thirty, Uranus 84 and Neptune almost 165.

Why outer planets take so long

There are two reasons the outer planets have such long years, and they multiply each other. The first is distance: Neptune’s orbit is about thirty times wider than Earth’s, so the path it has to travel is about thirty times longer. The second is speed. The Sun’s gravity weakens with distance, so planets farther out move more slowly. Earth travels at an average of nearly 30 kilometres per second, while Neptune moves at only about 5.4.

The German astronomer Johannes Kepler described the relationship in the early 17th century. His third law says that the square of a planet’s year is proportional to the cube of its average distance from the Sun. Neptune is about 30 times farther out than Earth, and 30 cubed is 27,000, whose square root is roughly 164, which is why a Neptunian year lasts almost 165 Earth years. The same law works for moons around planets, and for planets around other stars, and astronomers use it to estimate the masses of distant stars.

Strange days: Mercury, Venus and Uranus

Mercury spins slowly, turning once every 58.6 Earth days relative to the stars, while circling the Sun every 88 days. The two motions combine so that a full solar day on Mercury, from one sunrise to the next, lasts about 176 Earth days. That is two whole Mercurian years. If you stood on Mercury, you would have two birthdays for every sunrise.

Venus is stranger still. It spins backwards compared with most planets, so the Sun rises in the west, and it spins so slowly that one rotation relative to the stars takes about 243 Earth days, longer than its 225-day year. Because it rotates backwards, though, its solar day is shorter than its spin, at about 117 Earth days. Venus is also the hottest planet, hotter than Mercury, because its thick carbon dioxide atmosphere traps heat.

Uranus is tipped on its side, with its axis tilted by about 98 degrees. As it travels around the Sun, each pole spends roughly a quarter of the 84-year orbit pointing towards the Sun, so parts of the planet have about 42 years of continuous daylight followed by 42 years of darkness. The giant planets, by contrast, spin very fast: Jupiter’s day lasts less than ten hours, even though it is eleven times wider than Earth.

How to work out your age on another planet

The calculation is a single division. Work out how many days you have been alive, then divide by the number of Earth days in the planet’s year. Counting the days is the only fiddly part, because it needs to include every leap day since your birth; our days alive calculator or the borrowing method in our guide to calculating your age will give it to you.

1. Count your days alive. For our example, someone born on 12 April 1990 has lived 13,350 days by 30 October 2026.
2. Look up the planet’s year. Use the Earth-days column in the table above. For Mars it is 686.98.
3. Divide. 13,350 ÷ 686.98 = 19.43, so the person is 19 in Mars years, and a little over two-fifths of the way to their 20th Martian birthday.

You can also go the other way, using Earth years rather than days, by dividing your age in years by the planet’s year in Earth years. It gives almost the same answer, but working in days is more precise, because it captures exactly how far you are through your current year.

Worked example. At 13,350 days old, the person in our example is about 151.8 in Mercury years, 59.4 in Venus years, 19.4 in Mars years, 3.1 in Jupiter years and 1.2 in Saturn years. They will not complete a single year on Uranus until they are about 84, and they would need to live to almost 165 to celebrate a first birthday on Neptune.

Birthdays across the solar system

Thinking in planet years makes some fun comparisons. A child turning one on Earth has already had four birthdays on Mercury. A person celebrating their 12th Earth birthday is just turning one on Jupiter, which makes Jupiter years a neat measure of the stages of life: one at the end of childhood, two around 24, three in the mid-thirties, and so on. Saturn birthdays come round about once every 29.5 years, so most people see two or three of them in a lifetime.

Mars birthdays fall roughly every 22 and a half months, which makes them the most practical planetary birthday to celebrate. They also move through the Earth calendar, because 687 days does not divide evenly into Earth years, so your Mars birthday lands in a different month each time. Our Mars birthday calculator works out the exact dates, and the planet age calculator shows your age and next birthday on every planet at once. If you would rather know how much you would weigh when you got there, try your weight on other worlds.

Questions people ask

Sources. NASA Goddard Space Flight Center, Planetary Fact Sheet (sidereal orbit periods, length of day, axial tilt); NASA Science, planet overviews; NASA Mars Exploration Program, Mars sol length.