IN INTER-PLANETARY TERMS, today is the first birthday of the planet Neptune.

The most outlying planet from our sun (now that Pluto has been robbed of its planetary status) was first observed from Earth on 24 September 1846 – becoming the first planet to be ‘deliberately’ discovered based on an expected sighting.

As the BBC News Magazine recounts, the planet Uranus had been classified over 60 years earlier – but its staggered orbit led astronomers to believe that another body must have been exerting a gravitational pull on it.

So they set looking for a body that may have been skewing Uranus’ path – and having spent decades working on a mathematical plan to predict where such a body might have been, it took only an hour to spot a planet where no planet had previously been known.

But how does the date of Neptune’s first observation* make today its birthday? Well, Neptune takes the equivalent of 164.79 Earth years to complete its orbit around the sun – one Neptunian year.

Today, Neptune will occupy exactly the same spot (relative to the sun) that it did when Earthlings discovered it – as exactly one Neptunian year has passed since that date, making today the planet’s first birthday.

Of course, it’s not technically the planet’s birthday – if you’re going by Neptunian years, the planet is 27,307,482 years old; by Earth terms, it’s around 4.5 billion.

Either way, we won’t be sending a cake – because even at the speed of light, it would take four hours to arrive, and when it got there it would instantly be made slightly less appetising by the local temperature: an approximate -218°C.

Still, it gives you an excuse to skip the birthday party…

* No offence to Galileo Galilei, who is credited with observing Neptune twice in 1612 and 1613, but who mistook it for a fixed star in the distance.