Last week saw Neptune Day, the first anniversary - in Neptune years - of the planet's discovery on September 23, 1846. That got us thinking: what are the "birthdays" for all the other planets? Here's a handy, mildly insane guide.
As you might recall, a Neptune year is equivalent to 164.79 Earth years, which is the length of time between September 23, 1846 and July 11, 2011. Ignoring the minor detail that all these planets existed for billions of years before humans set eyes on them (minor detail, I say!), we can call these anniversaries the planets' birthdays. And, with thanks to one ludicrously helpful website, here's a list of birthdays for all major and minor objects in the solar system.
Now, an exact calculation for Neptune's birthday isn't really possible - we can't be 100% sure that's the exact date the planet was discovered, and defining a Neptune orbit isn't as straightforward as you might think. (There's also the little matter that Galileo might have discovered Neptune in the 1600s, but let's keep this simple.) Anyway, we can say this - this last week, Neptune was in pretty much the same spot in our solar system that it was when it was first spotted.
Neptune is one of only three planets (now two, thanks to Pluto's demotion) that were discovered during the historical era. Uranus was the first of these to be spotted. William Herschel gets the credit for being the first astronomer to realize he was looking at a planet instead of a star when he spotted it, and the discovery date is generally given as March 13, 1781. The Uranus year is equivalent to almost exactly 30,799 Earth days, which we'll round to 84 years and 119 days. That means the first anniversary of the planet's discovery in Uranus years was July 10, 1865, the second was November 6, 1949, and you can set your calendars now for Uranus's third birthday, on March 5, 2034.
You'll probably need to get your great-grandchildren to commemorate Pluto's birthday (or invest heavily in immortality research - they're both great options, really). Clyde Tombaugh first discovered it on February 18, 1930, and the Pluto year is about 90,613 days, or 248 years and 33 days. That means Pluto's first anniversary won't be until March 23, 2178. That's a Monday, so you should probably give your boss the heads-up now that you'll be skipping out on work to go celebrate.
It's harder to pin down good dates for any of the other major planets, so let's take a quick look at Pluto's fellow minor planets instead. The asteroid belt's lone planet Ceres was discovered by Giuseppe Piazzi on January 1, 1801. Ceres's orbit is much more similar to ours, as you might expect, so its year is significantly shorter at only 1680.5 Earth days, or 4 years and 219 days. Obviously, we've had plenty of Ceres birthday - the 46th is the next one, on August 1, 2012 - and it hits 50 years old on December 25, 2030. I guess that really means we have to get Ceres Christmas presents, doesn't it? Lousy freeloading minor planets...
As for the other, more recently discovered minor planets...well, we'll be waiting a while for them to come around again. In order of discovery, Haumea was first spotted on December 28, 2004, Eris on January 5, 2005, and Makemake on March 31 later that year. Their orbital periods are 283 years and 104 days, 557 years and 160 days, and 309 years and 323 days, respectively. So then, Haumea will reach its first birthday on April 10, 2288, Makemake on February 17, 2315, and Eris is stuck waiting all the way until June 14, 2562.
Like I said earlier, since all the remaining major planets were visible to the ancients, they don't really have obvious birthdays. But since it's probably not a good idea to leave something as big as Jupiter out of these festivities, we can come up with some decent proxies.
For Saturn, the choice is fairly obvious: the discovery of its magnificent rings. While Galileo was the first to observe them (he was the first to observe a lot of things), but it was Christiaan Huygens in 1655 who first came up with the possibility that Saturn was surrounded by rings. The Saturn year is just 29 years and 167 days, which means we just passed its 12th birthday in 2008 and are due for the 13th in 2038.
Galileo can take credit for the discovery of Jupiter's moons, which he first discussed in a letter written on January 7, 1610. Since those are the first moons ever observed outside that of Earth, that seems as good an anniversary date as any. The Jupiter year is 11 years and 314 days, so Jupiter's 34th birthday will be March 31, 2013.
Speaking of moons, the most crucial Mars-related discovery of modern history was the observation of its pint-sized satellites Phobos and Deimos in 1877. American astronomer Asaph Hall spotted Deimos on August 12 and Phobos on August 18 of that year - we'll use Deimos's discovery date for Mars's birthday. Mars's year is almost the same as our own, at 1 year and 322 days. That means its 71st birthday March 17, 2011, while its 72nd will be February 1, 2013. The Martian centennial is set for October 9, 2065, which I presume everyone will be using as a nice warm-up for the American tricentennial in 2076.
Venus and Mercury are trickier, if only because they don't have moons. (Moons are very helpful in coming up with birthdays.) For Venus, we'll go with the discovery of its atmosphere, which the Russian polymath Mikhail Lomonosov theorized following his observation of the planet on June 7, 1761. Venus's orbit is just 224.7 days, meaning its 406th birthday was this last March 17, its next birthday is this October 28, and it hits the big 5-0-0 on January 13, 2069.
As for Mercury, the most obvious milestone to use is the discovery of the planet's orbital period - in other words, how long the year actually is. In the March 29, 1890 issue of Publications of the Astronomical Society of the Pacific, Italian astronomer Giovanni Schiaparelli announced that Mercury's orbit was 87.969 days. At the time, Schiaparelli thought the planet was tidally locked, meaning it also took 87.969 days to complete one rotation, so that a Mercury year and a Mercury day were one and the same.
In 1965, it was discovered the planet actually rotated in a 3:2 resonance, meaning the planet's rotational day was actually 2/3 the length of a year, or 58.646 days. Anyway, using March 29, 1890 as Mercury's birth date, that actually means we just passed Mercury's 500th birthday on August 31, 2010, and its 504th is a month from now on August 18.
And what about the Sun? Well, it obviously doesn't orbit itself, so we can't use that as a stand-in for its birthday. Thankfully, astronomers have actually come up with a solution to this little problem: the galactic year. It's equal to how long it takes the Sun to complete one circuit around the galactic center, which is equivalent to about 225 to 250 million years. Currently, we're in the 20th galactic year, and it is going to be a long, long time until the Sun turns 21. So, I don't know, it might want to keep some fake IDs handy...