Stargazing in September: Neptune is a world in the twilight zone

Neptune: a world in the twilight zone

Neptune is the most distant planet in the Solar System (iStock)

Many people have told us that they’ve never spotted Mercury, the fleet-footed “messenger of the gods” that circles the Sun so rapidly. And we’re pretty sure that this is even more true for Neptune – now officially classed as the most distant planet in the Solar System.

It’s also the one planet you can’t see with the unaided eye – but if you have binoculars or a small telescope, you might like to add Neptune to your bucket-list this month as it swings closest to the Earth. For details of how to spot this remote world, see What’s Up – but don’t expect anything too sensational!

Neptune is the only planet to have been discovered by the power of mathematics. After William Herschel’s discovery of Uranus in 1781, astronomers noticed that the new world was being pulled off-course by something – possibly a large planet, further out.

Two men got down to their calculations. The French astronomer Urbain Le Verrier and Cambridge student John Couch Adams came up with almost identical predictions as to where the missing planet should be. Adams tried to convince the Astronomer Royal – Sir George Airy – where the undiscovered world was. But Airy wanted more detailed calculations, and Adams was known to be slow off the mark.

In the end, Le Verrier won. On 23 September 1846, German astronomer Johann Galle tracked down the missing planet very close to his predicted position. To be fair to history, both mathematicians are now credited with the discovery.

As befits its blue-green appearance, Neptune is named after the Roman god of the sea. Like Jupiter, Saturn and Uranus, it is a gas-giant – a world far enough away from the Sun to be able to cloak itself in our star’s nascent gases. Slightly smaller than Uranus, Neptune is 17 times heavier than Earth. It takes an incredible 165 years to orbit the Sun.

The planet has five thin rings and a family of 14 moons. Most spectacular is Triton – at 2,700km across, a big moon that – amazingly for a world so far from the Sun – is geologically active.

When we were at Nasa’s mission control for the encounter between the spacecraft Voyager 2 and Neptune in 1989, images of Triton would pop up on our TV screens, “What’s that?” we’d ask the researchers. “Don’t know,” was the reply. “Your guess is as good as ours.” It turns out that Triton has volcanic plumes that eject plumes of nitrogen and dust into space.

Neptune itself is no slouch, either. Compared to boring Uranus, this planet is positively frisky. Its core blazes at 5,000C, almost as hot as the Sun’s surface. This internal heat drives dramatic storm systems and dark spots. It also generates winds of 2,000km/hr: the fastest in the Solar System.

But Neptune is a world that combines fire and ice. Most of the planet is made of water, mixed with ammonia and methane. Scientists in California have modelled the conditions in Neptune’s interior, where the pressure is enormous. Their conclusion? The planet could be squeezing methane into lumps of solid carbon. So, deep within Neptune, it’s raining diamonds.

What’s up

Brilliant Jupiter, which has lit up the southern skies all summer, is now sinking down to the west, dropping below the horizon about 10.30pm (which is why it’s disappeared from our 11pm sky chart). Saturn, on view to the left of Jupiter, stays up rather longer. You can spot the ringed planet low in the southwest until after midnight.

The reddish star in the west, above Jupiter and Saturn, is Arcturus, the brightest star in the constellation Boötes, the Herdsman. In Greek, Arcturus means “the bear driver”, because this star follows the Great Bear (Ursa Major) as the Earth rotates.

The night sky at around 11pm this month (Heather Couper/Nigel Henbest)

High in the south, you’ll find the bright stars Vega, Deneb and Altair making up the giant Summer Triangle (which featured in last month’s column). To the left, over the east, a large square of stars marks the body of the flying horse, Pegasus. The two right-hand stars of the square point downwards to a distinct small pattern: three faint stars arranged around a fourth star. Looking like the badge on a Mercedes-Benz, astronomers call it the Water Jar, depicting the ewer of flowing liquid being dispensed by Aquarius, the Water Bearer.

And this brings us to September’s challenge: spot the most distant planet! Although Neptune is huge (see main story) and passes closest to Earth on 10 September, it lies so far away that the most distant planet is too faint to see with the naked eye. But you can pick it out with binoculars if you know exactly where to look. And this month we have the rare chance of using a moderately bright star to point the way.

To the lower left of the Water Jar, pick out the star phi Aquarii (marked “phi” on the chart). Until 9 September, point your binoculars at phi, and the fainter “star” nearby is the elusive planet Neptune. The planet’s bluish-green hue forms a lovely contrast to the red giant star. On 6 September, they’re so up close and personal that you may need the power of a telescope to separate the star and the planet.

Diary

5 September: Moon near Jupiter and Antares
6 September, 4.10am: Moon at First Quarter; Neptune very near phi Aquarii
8 September: Moon near Saturn
10 September: Neptune at opposition
14 September, 5.33am: Full Moon
19 September: Moon near Pleiades and Hyades star clusters
22 September, 3.41am: Moon at Last Quarter, occults the Hyades star cluster
23 September, 8.50am: Autumn Equinox
28 September, 7.26am: New Moon

Newly published! ‘Philip’s 2020 Stargazing’ (Philip’s £6.99) by Heather Couper and Nigel Henbest reveals everything that’s going on in the sky next year

Fully illustrated, ‘Heather and Nigel’s The Universe Explained’ (Firefly, £16.99) is packed with 185 of the questions that people ask about the Cosmos