How old would you be on Mars, and how much would you weigh there? The arithmetic is short, and the reasons behind it are some of the oldest discoveries in physics. Here is one made-up family of five, worked through the solar system with NASA’s numbers as of January 1, 2027.
Grandma Rose (born November 30, 1956), Marcus and Lena (born 1987 and 1989), and their kids Ava (2015) and Leo (2020). They are invented for this example; the planets are real. To check your own numbers, use the age on other planets calculator and the weight on other planets calculator.
A planet’s year is one trip around the Sun. Your age there is the number of days you have been alive divided by the length of that year in Earth days: 88 for Mercury, 687 for Mars, 4,331 for Jupiter and 10,747 for Saturn, according to NASA’s Planetary Fact Sheet.
| Person | Born | Earth years | Mercury years | Mars years | Jupiter years | Saturn years |
|---|---|---|---|---|---|---|
| Grandma Rose | Nov 30, 1956 | 70.1 | 291.0 | 37.3 | 5.91 | 2.38 |
| Dad (Marcus) | Mar 12, 1987 | 39.8 | 165.3 | 21.2 | 3.36 | 1.35 |
| Mom (Lena) | Sep 2, 1989 | 37.3 | 155.0 | 19.8 | 3.15 | 1.27 |
| Ava | May 20, 2015 | 11.6 | 48.2 | 6.2 | 0.98 | 0.39 |
| Leo | Aug 8, 2020 | 6.4 | 26.6 | 3.4 | 0.54 | 0.22 |
Leo is 3.40 Mars years old, and Grandma Rose has been around Saturn 2 times. She has had 5 Jupiter birthdays; Ava’s first arrives on March 28, 2027.
A planet birthday falls every time your days alive reach a whole multiple of that planet’s year. Mercury birthdays come so often (every 88 days) that each person has 4 or 5 of them in 2027, so they are left out of this list. Everything else that lands in 2027:
| Date | Who | Birthday |
|---|---|---|
| January 17, 2027 | Grandma Rose | 114th Venus birthday |
| January 26, 2027 | Ava | 19th Venus birthday |
| March 7, 2027 | Dad (Marcus) | 65th Venus birthday |
| March 13, 2027 | Mom (Lena) | 61st Venus birthday |
| March 28, 2027 | Ava | 1st Jupiter birthday |
| April 15, 2027 | Mom (Lena) | 20th Mars birthday |
| May 15, 2027 | Leo | 11th Venus birthday |
| August 29, 2027 | Grandma Rose | 115th Venus birthday |
| September 7, 2027 | Ava | 20th Venus birthday |
| October 17, 2027 | Dad (Marcus) | 66th Venus birthday |
| October 24, 2027 | Mom (Lena) | 62nd Venus birthday |
| December 26, 2027 | Leo | 12th Venus birthday |
Two things stretch a planet’s year: its path around the Sun is longer, and it moves along it more slowly because the Sun’s pull is weaker far away. Johannes Kepler found the combined rule four centuries ago: the square of a planet’s year is proportional to the cube of its distance from the Sun. In plain numbers, a year in Earth years equals the distance in Earth–Sun distances raised to the power 1.5. NASA’s fact sheet values fit it to within about 1.5%:
| Planet | Distance from Sun (Earth = 1) | Distance1.5 | NASA year (Earth years) |
|---|---|---|---|
| Mercury | 0.387 | 0.24 | 0.241 |
| Venus | 0.723 | 0.61 | 0.615 |
| Mars | 1.52 | 1.87 | 1.88 |
| Jupiter | 5.2 | 11.86 | 11.9 |
| Saturn | 9.57 | 29.61 | 29.4 |
| Uranus | 19.2 | 83.93 | 83.7 |
| Neptune | 30.2 | 165.80 | 163.7 |
Weight is the pull of gravity on your body. Multiply your Earth weight by the planet’s gravity as a fraction of Earth’s: 0.166 on the Moon, 0.377 on Mars, 2.36 on Jupiter (at cloud level near the equator) and 0.071 on Pluto, from NASA’s fact sheet.
| Person | Earth | Moon | Mars | Jupiter | Pluto |
|---|---|---|---|---|---|
| Grandma Rose | 132 lb | 21.9 lb | 49.8 lb | 311.5 lb | 9.4 lb |
| Dad (Marcus) | 184 lb | 30.5 lb | 69.4 lb | 434.2 lb | 13.1 lb |
| Mom (Lena) | 141 lb | 23.4 lb | 53.2 lb | 332.8 lb | 10.0 lb |
| Ava | 68 lb | 11.3 lb | 25.6 lb | 160.5 lb | 4.8 lb |
| Leo | 41 lb | 6.8 lb | 15.5 lb | 96.8 lb | 2.9 lb |
Together the family weighs 566 lb on Earth, 213.5 lb on Mars and 1,335.8 lb on Jupiter. Nobody’s body changes: their mass, the amount of stuff they are made of, is the same everywhere. Only the scale reading moves.
A planet’s surface gravity rises with its mass and falls with the square of its radius, since standing on a bigger planet puts you farther from its center. Mars has about twice Mercury’s mass but is wider, so the two have nearly the same gravity (0.377 and 0.378). Saturn has 95 times Earth’s mass yet is so wide, and spins so fast, that at its equator you would weigh a little less than at home. Jupiter has no ground to stand on; NASA gives its gravity where the air pressure equals Earth’s at sea level.
The same mass-and-radius rule sets how fast something must go to coast away for good: escape velocity = √(2GM/r). Using NASA’s values for each body’s GM and radius, it is about 11.2 km/s from Earth, 2.4 km/s from the Moon, 5.0 km/s from Mars and 59.5 km/s from Jupiter’s 1-bar cloud level. Your weight doesn’t enter into it: little Leo and Grandma Rose would need the same speed. Try any body, or a made-up planet, in the escape velocity calculator.
Divide your age in days by 687, the length of a Mars year in Earth days; that is about 0.53 times your Earth age.
Take your Mars age, round it up to the next whole number, multiply by 686.97 days and count that many days from your birth date.
Mars has about a tenth of Earth’s mass and about half its diameter, giving it 0.377 of Earth’s surface gravity, so the same body presses down with less force.
Their orbits are longer and they move more slowly, because the Sun’s gravity is weaker farther out. Kepler’s third law ties the year to the distance: years squared are proportional to distance cubed.
No. Mass is how much matter you contain; weight is gravity’s pull on that mass, which is what changes.