Alien calendars would probably include leap year

Today is a leap day, a calendar oddity that helps align our timekeeping with the orbit of the Earth around the sun. But leap day may not be restricted to the Earth — it could occur on planets around other stars as well, as long as there are beings living there to mark the days, scientists say.

The extra day of Feb. 29 is added to the second month of the year every four years, making that year a leap year, according to our Gregorian calendar. This extra day is necessary to keep our calendar in line with the seasons, which depend on Earth's revolution around the sun.

That's because the year — the time it takes Earth to make one circuit around the sun — can't be evenly divided into days — the time it takes Earth to make one full rotation.

"We have a leap year because the spin rate of the Earth, which is 23 hours and 56 minutes, doesn't divide completely evenly into the length of the year, which is 365.242374 days," said astronomer Greg Laughlin of the University of California, Santa Cruz. "You can't fit an exact number of spins into one trip around the sun without having some left over."

And the same is probably true for most alien planets as well, said Laughlin, who is co-investigator of the Lick Carnegie Exoplanet Survey, which hunts for planets around other stars using the Keck telescope in Hawaii. Between this study and others, astronomers have now found more than 700 planets around other stars. 

Alien calendars

Most extrasolar planets "will all have spin periods that don't fit neatly into their orbital periods," Laughlin told SPACE.com. "So one could image you would have to design leap year-like systems for them. The degree to which they mismatch would be random." [Gallery: The Strangest Alien Planets]

An exception are those planets found orbiting extremely close to their stars, which have become tidally locked, bound by their star's gravity to spin exactly once a year. These planets only ever show one face to the star, the other side permanently turning away, just as the moon orbits Earth, never showing us its far side.

For all other planets, though, there shouldn't be any relationship between the planet's spin and its orbital period.

"If tidal locking hasn't occurred then there's nothing to inform the planet about how fast it's spinning relative to its year," Laughlin said. "The spin of the planet depends on the gory details of its history."

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