Neptune's birthday and a beautiful piece of maths


Neptune is about to celebrate its first birthday. On 12 July it will be exactly one Neptunian year - or 164.79 Earth years - since its discovery on 24 September 1846. But why do we still know so little about the distant planet?

About 4.4 billion kilometres away from Earth lies Neptune, the first planet in the solar system to be discovered deliberately.

After the classification of the planet Uranus in the 1780s, astronomers had been perplexed by its strange orbit. Scientists came to the conclusion that either Isaac Newton's laws were fundamentally flawed or that something else - another planet - was pulling Uranus from its expected orbit.

And so the search for the eighth planet began.

"It was such an incredible mathematical business, it makes searching for a needle in a haystack look like a 10-minute job for a child," says Dr Alan Chapman, author of the Victorian Amateur Astronomer.

Find out more

Happy Birthday Neptune is on BBC Radio 4, Monday 11 July, 1100 BST.

While mathematical predictions had been made over the previous decades, it was not until French mathematician Urbain le Verrier's theories were tested at the Berlin observatory by Johann Gottfried Galle that the planet was first seen.

After only an hour or so of searching, Neptune was observed for the first time on the night of 23 September 1846. It was found almost exactly where le Verrier had predicted it to be.

Independently, British scientist John Couch Adams also produced similar results, and now he and Verrier are given joint credit for the discovery.

The Solar System Following Pluto's declassification in 2006, Neptune is now the outermost planet in the solar system

But many claim it was not Galle who documented the planet first, but the famous astronomer and mathematician Galileo. In his famous work The Starry Messenger, some evidence points to his discovery.

"If you look at the drawings for January 1613, you can see a fantastic drawing of Jupiter and its moons," says Dr Robert Massey of the Royal Astronomical Society.

"It even includes an object labelled as 'fixed star' which is the first telescopic drawing of the planet Neptune."

Controversy aside, comparatively little is still known about the planet.

Unfriendly lump

Part of the problem is that there is no way for the planet to be viewed with the naked eye and until the Hubble telescope, scientific observation was very difficult.

So what is Neptune like?

More about Neptune

  • Named after Roman god of the sea, it's the Solar System's outermost planet
  • Cannot be seen from Earth without a telescope or binoculars
  • Covered by bright blue methane clouds that whip around the globe at speeds measuring more than 1,600km/h (994mph)
  • Though its diameter is four times that of the Earth's and a mass 17 times as great, it is less dense and doesn't have a solid surface
  • It is, on average, about 4.5 billion km (2.8 billion miles) from the Sun
  • The distance between Neptune and the Sun varies by 101 million km (63 million miles) depending on where the planet is in its orbit
  • Its atmosphere is made up of 80% hydrogen, 19% helium and traces of methane
  • There are 13 known moons which orbit Neptune, the largest of which is Triton

"It's a frozen lump of frozen gases and I suppose not a terribly friendly place," says Dr Chapman.

"Let's wish it a happy birthday but perhaps let's keep as far away from it as we can as it won't give you a welcome."

One of the things most interesting to scientists about Neptune is the weather.

"Cloudy with a chance of methane" is how planetary scientist Heidi Hammel, of the Association of Universities for Research in Astronomy (Aura), describes it.

Winds can reach 1,930km/h (1,200mph), creating storms unimaginable on Earth. These huge storms are seen as dark spots in a similar way to the Great Red Spot on Jupiter.

The reason astronomers know so little is because the planet has only been photographed once from close range - on the Voyager 2 mission in 1989. And because its seasons last 40 Earth years, only Neptune's spring and early summer have been closely documented.

"Every time we go to a telescope and look at this planet, it's doing something new, it's doing something we hadn't thought of before," says Dr Hammel.

What Dr Hammel found was that storms were appearing and forming and changing much more quickly than had previously been thought. She was looking at a planet very different from the photos taken by Voyager 2.

"We really have only been observing Neptune with big telescopes since shortly before 1989," she says.

"We haven't been looking long enough. This planet is not for the impatient."

Pluto's place

The chance to find out more about the planet close-up still seems a long way away - more than just the billions of kilometres in distance.

Nasa missions to discover more about the planet have been sidelined for the moment due to budget constraints. The Neptune Orbiter mission, once planned to launch sometime in 2016, no longer features on Nasa's proposed mission list.

"We've never had a mission that's been dedicated to Neptune," says Dr Robin Catchpole, of Cambridge University's Institute of Astronomy.

Galileo Galileo is credited with observing Neptune more than 200 years before its existence was confirmed

"We know how it fits into the sequence of planets as far as composition goes but we don't know a lot."

Even the New Horizons mission to discover more about Pluto and the outer reaches of the solar system - due to pass through the orbit path of Neptune on 24 August 2014 - has not been organised to closely monitor Neptune.

Instead, photos are being taken of it and its moon to test the imaging equipment rather than for any major scientific purposes.

And this mission is allowing some to question whether Pluto could be reclassified as the ninth planet in the solar system after its primary planet title was taken away in 2006.

If it was reinstated, Neptune would lose the honour of being the furthest planet from the Sun.

"Whether Pluto is called a planet or not is a matter of semantics," says Dr Catchpole.

"The situation with the classifications is that Pluto doesn't fit into the [current] system very well. I don't think it's ever going to change again."

So happy birthday Neptune. Though lighting any candles on a birthday cake might be tricky in those high winds.


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  • rate this

    Comment number 53.


    Whatever Pluto's classification it is still of importance and fascination. I look forward to seeing close up pics from future missions.

    Happy birthday Neptune of course. Nice article.

  • rate this

    Comment number 32.

    Neptune, blue giant, the coldest planet, the outermost relative surely needed more than NH! And NH as rightly pointed out is not going to help in anyway to better understand the distant planet! However, many happy returns of the Neptune day to our Earthly cousin! Have a cool day out!

  • rate this

    Comment number 19.

    @Richard #2
    When New Horizons was launched in 2006 Pluto was still classified as a planet...
    Changing a whole mission's objectives and agenda isn't as simple as changing the classification of a planet.

  • rate this

    Comment number 11.

    The reason why only spring/early summer have been observed is, as the article states in the same sentence, the planet's seasons last 40 Earth years each. Remeber that a year is the time it takes for a planet to orbit the Sun, and as it is further away from the Sun, a Neptune year takes roughly 160 Earth years.

  • rate this

    Comment number 2.

    It seems like an incredible oversight by NASA to not study Neptune when the New Horizons mission passes it. It seems inconceivable that it is just being disregarded and used as a photographic rehearsal. With such a lack of knowledge, no opportunity to further study planet in our solar system should ever be passed up.
    It makes no sense considering Pluto is not even considered a planet anymore.



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