To us, a year lasts 365 days.
Aliens, if they exist, may have quite different attitudes to time. That's because some planets have much shorter years than Earth's, while others are vastly longer.
Why the drastic difference in year lengths? The mass of the star has some effect, because more massive stars swing their planets around faster than less massive ones, but orbital distance plays the main role. "The year of a planet is basically dictated by how far away from the star it is," says David Kipping of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, US.
In our solar system, the planet closest to the Sun is Mercury, with a year lasting just 88 Earth days. That's probably still too long to stay entirely on the straight and narrow resolution-wise, but curiously it's shorter than the length of the planet's day. Thanks to Mercury's leisurely rotation rate, a day there lasts 176 Earth days – twice as long as a year.
At the other end of the spectrum is Neptune, our solar system's most distant planet, now that Pluto has been demoted to the status of dwarf planet. Neptune lies about 30 times as far from the Sun as the Earth and takes 165 Earth years to make one orbit.
So when is the next New Year on those planets? In other words, which moment in Neptune's orbit should be designated as midnight on 31 December?
On Earth, our choice of 1 January as the first day of the year is pretty much arbitrary, but there is a way to link it more closely to the planet's orbit. That's because the orbits of the planets are not perfectly circular, but rather are slightly elongated. That means there is a point in each planet's orbit where it comes closest to the Sun. We could make this point, called perihelion, the marker for the New Year.
That means Neptunians (if there were any) would celebrate their next annual bacchanalia in 2042. Mercurians would only have to wait until 22 January 2015. We Earthlings just have to hold tight until 4 January: the date changes from year to year.
But just because Neptune is the planet with the longest year in our solar system, it is not the galaxy's record holder by any stretch of the imagination. That title holder, however, is a matter of some debate. "The question of the longest year quickly gets into arguments about exactly what counts as a planet," says Adam Kraus of the University of Texas at Austin, US.
Nearly 2000 exoplanets have been confirmed, with another 3000 candidates found. Still, "there is no formal definition of an extrasolar planet yet," says Marie-Eve Naud of the University of Montreal in Canada.
Nevertheless, we can narrow it down a bit. First, we rule out worlds that weigh more than 13 times the mass of Jupiter, because such weighty objects are typically thought to be pseudo-stars, called brown dwarfs. Next, we insist that our planets all orbit bona fide stars, or at least the dead remains of stars, rather than brown dwarfs.
PSR 1719-14b goes all the way around in just over two Earth hours
In that case, the crown goes to GU Piscium b, says Naud. With an orbit nearly 70 times as wide as Neptune's, GU Piscium b has a year estimated to last 163,000 Earth years, says Naud, who led the team that reported the discovery earlier this year.
At the other end of the scale, New Year's resolutions would be easy to keep on PSR 1719-14b. With a little willpower, you could be virtuous for an entire year, and probably even longer. That's because the planet is 250 times closer to its host star, a whirling stellar corpse called a pulsar, than Earth is to the Sun. As a result, PSR 1719-14b goes all the way around in just over two Earth hours, making it the alien world with the shortest year known.
These aren't just trivial factoids. "The year of a planet is one of the most important parameters we measure in exoplanets," says Kipping. "It tells us how far away a planet is from a star, and that reveals how hot a planet is. For Mercury, you can tell it's going to be way too hot to have life on it, and for Jupiter, you can tell it's far too cold."
In theory, a planet could orbit even farther out than GU Piscium B, even in our own solar system, says Kipping. That's because the only thing that limits a planet's orbit is other stars that can pull it away with their gravity.
Stars move around, but currently the nearest star to the Sun is Proxima Centauri, about 4 light-years away. So anything closer than about half that distance –about 4000 times Neptune's orbit – could circle the Sun, Kipping says. The solar system is actually surrounded by a shell of icy bodies that extends to about this distance.
Planets could theoretically exist within Mercury's orbit, but they can only get so close to the Sun. Rock starts to evaporate at about 1200 degrees Celsius, so Mercury could start to vaporise if it were four times closer in, says Scott Kenyon of the Harvard-Smithsonian Center for Astrophysics.
At even tighter orbits, the Sun's gravitational tugs will destroy the planet. "There's only so much stress the rock can take before it will break apart," says Kipping. This distance is called the Roche limit, and for a rocky planet like Mercury it's about 1% Mercury's orbital distance from the Sun.
"We do think we're seeing some exoplanets that are right on the edge of having this effect happen," says Kipping. For example, Kepler-78b sprints around its host in about 8.5 hours, orbiting its star at about 1% Earth's orbital distance. It may be the exoplanet with the shortest year around an ordinary, living star, says William Borucki, leader of NASA's planet-hunting probe Kepler.
One last thing. If you can't keep New Year's resolutions now, remember that the Earth's year could be twice as long in the future. That's because, in 5 billion years or so, the Sun will bloat up into a red giant star. Earth might well get swallowed, but it could escape if, as some predict, the ageing Sun sheds half its mass in winds before expanding.
If that happens, "Earth will end up orbiting somewhere out beyond Mars's current orbit," says Kraus. It would take roughly two years to go around the Sun. You may feel that's too long to go without chocolate.