How this instrument works
Your age is really just elapsed time divided by however long a 'year' lasts where you're counting from. On Earth that's 365.25 days, the time our planet takes to complete one orbit of the Sun. On another planet, a year is that planet's own orbital period — the time it takes to complete one full circuit — which can be dramatically shorter or longer than Earth's, depending on how close it sits to the Sun.
Mercury, the closest planet, completes an orbit in just 87.97 Earth days, so a person's age in Mercury years is a much bigger number than their Earth age. Neptune, the farthest planet, takes 60,182 Earth days — about 165 Earth years — to complete one lap, so almost nobody alive has reached even a single Neptune year. The pattern follows Kepler's laws of orbital motion: the farther a planet sits from the Sun, the slower it moves and the longer its year.
This instrument uses each planet's real, measured sidereal orbital period — the time to return to the same position relative to the distant stars — sourced from NASA's planetary data. It's a straightforward division, not a claim about how a birthday would actually be celebrated on another world; some planets don't even complete a whole number of their own daily rotations in one orbit.
- Enter your age in Earth days into 'Age (Earth days)' — multiply your age in years by 365.25 if you're starting from years instead.
- Choose a planet from the dropdown; each option already carries that planet's real orbital period in Earth days.
- Read 'Age in that planet's years' — the number of times that planet has orbited the Sun since the day you entered.
- Switch planets without changing the day count to compare instantly — a Mercury age looks startling large, a Neptune age startlingly small.
Worked example — 30 Earth years, measured on Mars
Enter 10950 into Age (Earth days) — that's 30 Earth years, since 365 × 30 = 10,950 — and choose Mars, whose orbital period is 686.98 Earth days. Age in that planet's years reads 15.9393: this person has lived through just under 16 Martian years, because Mars takes almost twice as long as Earth to circle the Sun.
Keep the day count at 13,000 Earth days (about 35.6 Earth years) but switch the planet to Mercury, whose orbital period is only 87.97 Earth days, and the reading jumps to 147.7777 Mercury years — because Mercury completes an orbit in under three Earth months, the same stretch of life counts many more trips around the Sun there.
Questions
Why does my age change so much depending on the planet?
Because a 'year' is defined by orbital period, and planets take wildly different amounts of time to circle the Sun. Mercury orbits in 87.97 Earth days, so ages in Mercury years look large; Neptune takes 60,182 Earth days, so ages in Neptune years look tiny. The number of Earth days you've been alive never changes — only the length of the yardstick used to measure it does.
What exactly counts as one 'year' on another planet?
It's the planet's sidereal orbital period: the real time it takes to complete one full trip around the Sun, measured against the fixed background of distant stars. This is a physical, measured quantity for every planet in our solar system, published by NASA, and it's what this calculator divides your age in Earth days by.
Does this account for a planet's own day length, like Mars sols?
No — this calculator only converts orbital years, not day length. A planet's rotation period (its 'day') is a separate quantity from its orbital period (its 'year'), and the two aren't related by any fixed ratio. Mars, for example, has a day only slightly longer than Earth's but a year almost twice as long, because rotation speed and orbital distance are independent.
Where do the orbital period numbers come from?
From NASA's published planetary data — the sidereal orbital periods used here match the figures on NASA's own planet fact resources, which are measured astronomically rather than estimated. Mercury's 87.97 days, Venus's 224.7 days, and Mars's 686.98 days are all real, cited values, not rounded approximations.
How do I find my age in Earth days if I only know my birth date?
Subtract your birth date from today's date to get the number of days you've been alive, then enter that figure directly. As a shortcut, multiplying your age in years by 365.25 (to account for leap years) gets you close enough for this calculator, which is meant for exploring the scale of planetary years rather than tracking a precise birthday.
Which planet gives the smallest 'age' number?
Neptune, by a wide margin. Its 60,182-Earth-day orbital period is the longest of any planet in our solar system, so dividing any human lifespan's worth of Earth days by that figure gives a result under one — nobody has lived long enough to complete a single Neptune year since Neptune was discovered in 1846.