Stargazing in July: How Saturn saved the Earth
The two giants of the solar system, Jupiter and Saturn, are lording it over the skies this month. Low in the south all night long (see the starchart), they both approach closest to Earth in July. Even though they lie hundreds of millions of kilometres away, just ponder this thought as you gaze at these distant worlds: our planet Earth owes its existence to the orbital dance of Jupiter and Saturn.
The solar system was born over four billion years ago, from a swirling disc of gas and fine dust surrounding the early sun. With powerful new telescopes like the Alma array of radio telescopes in Chile, astronomers can now see discs like this around newly formed stars, and make out clumps where the gas and dust is starting to coagulate into planets.
Around the nascent sun, two huge blobs condensed, and grew ever bigger as their gravity pulled in the surrounding gas. One of them ended up 318 times more massive than the Earth, the other 95 times heavier: the planets we now call Jupiter and Saturn.
There’s a striking difference between the arrangement of the solar system and most of the other planetary systems that astronomers have discovered over the past 25 years. We have no really large planets right out to Jupiter’s orbit, five times the Earth’s distance from the sun: just Earth and its smaller siblings Mercury, Venus and Mars. Yet the cosmic norm seems to be several “super-Earths” – planets typically five times heavier than our own – close to the star.
Many other systems contain a “hot Jupiter”: a planet as massive as Jupiter, but circling its star more closely than Mercury orbits the sun. That’s odd because there shouldn’t have been enough planet-forming gas and dust so near the star. Most astronomers think that a hot Jupiter was actually born further out. As it ploughed through the remaining material in the disc, the planet lost orbital energy and gradually spiralled inwards. Crashing through the inner region of the disc, it either drove the material – including nascent planets – into the star to be incinerated, or tossed it away out into space.
From a cosmic perspective, it’s our solar system that is the oddball, with four small planets close to the sun and the giant of our system lying a long way out. To explain this conundrum, let me introduce the grand tack theory.
Formulated by an international team of astronomers, the theory says that Jupiter first moved towards the sun, then changed its mind and headed back outwards again – like a sailing boat tacking against the wind.
If we backtrack to the start of the solar system, this theory says Jupiter was born roughly where the asteroids are now (a bit inside its current orbit). Like the giants in other planetary systems, it felt friction from the surrounding gas in the disc and started to spiral inwards. As it went, Jupiter’s gravity cleared away most – but not all – of the gas and dust closer to the sun. That’s why there are no super-Earths in this region.
So the regular tug of Saturn on Jupiter grew out of all the proportion to the fact it was the smaller world
Just when it seemed that Jupiter was destined to sweep the inner solar system totally clean – with no chance of the Earth ever forming – and end up as a close companion to the sun, another planet leapt to the rescue.
Saturn had formed outside Jupiter’s orbit, and was slowly migrating too. When Jupiter was just beyond where Mars lies now, something magical happened. The two planets reached a point where Saturn was orbiting the sun twice in exactly the same time as Jupiter took to make three orbits. The planets now kept meeting up regularly, and their gravitational tug hit a steady rhythm.
It’s like pushing a child on a swing: do it regularly, at the right time, and the swing oscillates higher and higher. So the regular tug of Saturn on Jupiter grew out of all the proportion to the fact it was the smaller world.
When the team ran through all the calculations, they found that Jupiter and Saturn – now locked in their gravitational rhythm – would then both migrate out again. As time passed, they eventually reached their current orbits, about five and ten times further from the sun than the Earth lies.
Safe from marauding Jupiter, the remaining sparse material closer to the sun accumulated to create just four small worlds. And one of them was just the right size, and the right distance from the sun, for life to evolve. We call it Earth.
What’s Up
So soon after Midsummer’s Day, the nights are inconveniently short for stargazing: indeed, from the north of Scotland it hardly grows dark, and even the bright lights of Jupiter and Saturn (see main story) barely skim the horizon.
On the other hand, the balmy weather makes it ideal for spending a long leisurely night under the stars, and we are treated to some planetary delights if you’re prepared to be a night owl.
Giant Jupiter and his sidekick Saturn are visible from dusk right through to dawn, with the moon below Jupiter on 5 July and near Saturn the following night.
Jupiter is opposite to the sun in the sky on 14 July, though because the planets’ orbits aren’t circular, it is closest to us the following night. “Close” is relative, though, with the king of the planets lying 619 million kilometres away. Point a pair of binoculars at Jupiter, and you’ll spot its four largest moons: the biggest, Ganymede, is larger than the planet Mercury.
Through a telescope, Saturn is even more stunning, with its famous rings girdling the pale yellow globe. Saturn is also closest to us the day after its opposition on 20 July, over twice Jupiter’s distance at 1346 million kilometres from us.
After midnight, look to the lower left of Saturn to spot Mars rising, a little brighter than the ringworld. If you are still partying at 2.30am (or if you’re an early riser), you can’t miss brilliant Venus rising in the east, outshining even Jupiter. There’s a lovely sight on the morning of 17 July when the thin crescent moon lies directly above Venus,
Finally, during the last week of the month, scour the horizon well to the lower left of Venus, just after 4am, and you may catch a glimpse of the remaining bright member of the sun’s family, little Mercury.
Diary
2 July: moon near Antares
4 July: Earth at aphelion (furthest from the sun)
5 July, 5.44am: full moon near Jupiter
6 July: moon near Saturn
11 July (am): Venus near Aldebaran
12 July (am): moon near Mars; Venus near Aldebaran
13 July, 0.29am: last quarter moon
14 July: Jupiter at opposition
17 July (am): moon near Venus, Aldebaran and the Pleiades
20 July, 6.33pm: new moon; Saturn at opposition
22 July: moon near Regulus; Mercury at greatest western elongation
26 July: moon near Spica
27 July, 1.33pm: first quarter moon
29 July: moon near Antares
Philip’s 2020 Stargazing (Philip’s £6.99) by Heather Couper and Nigel Henbest reveals everything that’s going on in the sky this 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.
