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Birthdays on Other Planets

Here on Earth we usually measure our ages in years, because the yearly cycle of the seasons is significant to us. Each year we have the same sequence of conditions that signals a fresh start, whatever time of year your birthday might fall.

But what if we were living on another planet - Mars for example? A year on Mars is 687 days - nearly twice as long as on Earth! Therefore if you were 20 years old on Earth you would be only 11 years old on Mars.

Many modern cultures place significance on a point around the age of 18 when people are regarded as becoming adults. For example in most of the world you need to be 18 to vote, drive and buy alcohol. On Mars, it is likely that the age limit for this would be 10 Martian years.

On the other hand, years on Mercury are only 88 days, so a 20-year-old on Earth would be nearly 90 Mercury years old. In most countries on Earth the retirement age is between 60 and 65, but on Mercury it would be at least 250 years!

Let's compare the years on different planets:


On Mercury, 100 years might be celebrated as the time when most people have left university education or vocational training and are starting their professional life.

Year length: 87.97 Earth days
Become teenager: 50 Mercury years
Legal to drive: 75 Mercury years
Retirement age: 250 Mercury years


10 Venus years represents the age when most children start full-time school.

Year length: 224.70 Earth days
Become teenager: 20 Venus years
Legal to drive: 30 Venus years
Retirement age: 100 Venus years


Measured in Martian years, you could expect to live into your 40s, but only a few people would reach their 50th birthday.

Year length: 686.98 Earth days (1.88 Earth years)
Become teenager: 7 Mars years
Legal to drive: 10 Mars years
Retirement age: 35 Mars years


Many people would move out of their parents' home when they were around 2 years old on Jupiter.

Year length: 4332.82 Earth days (11.86 Earth years)
Become teenager: 1 or 1.1 Jupiter years
Legal to drive: 1.5 Jupiter years
Retirement age: 5 or 6 Jupiter years


Most mothers in Saturn would give birth around their first birthday, and so people would become grandparents at around 2 years and great-grandparents at around 3 Saturn years.

Year length: 10755.70 Earth days (29.45 Earth years)
Become teenager: 0.4 or 0.5 Saturn years
Legal to drive: 0.6 Saturn years
Retirement age: 2.0 or 2.2 Saturn years


The life expectancy on Uranus is about one year, for developed regions during peacetime. Almost everybody is under one year old on Uranus!

Year length: 30687.15 Earth days (84.02 Earth years)
Become teenager: 0.15 Uranus years
Legal to drive: 0.2 Uranus years
Retirement age: 0.75 Uranus years


The year is so long on Neptune that people would probably not use years as a common unit of measurement.

Year length: 60182 Earth days (164.8 years)
Become teenager: 0.08 Neptune years
Legal to drive: 0.1 Neptune years
Retirement age: 0.4 Neptune years

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