The Sun’s big. Really big. And really grippy.

I was asked the other week, on Facebook, why Jupiter doesn’t just fly off into space. You know, it’s so enormous, surely the Sun can’t hold it.

Jupiter, Saturn, Uranus and Neptune to scale against the Sun. Drawing by Urhixidur, public domain, via Wikipedia.
Jupiter, Saturn, Uranus and Neptune to scale against the Sun. Drawing by Urhixidur, public domain, via Wikipedia.

Actually, it can. Jupiter is around 300 times the mass of Earth, sure – but it’s only 0.09 percent the mass of the Sun. Not only can the Sun hold Jupiter tightly within its gravitational grip, but it can also hold on to objects waaaaay further away.

How much further? As much as a light year, which is where we think the edge of the Oort cloud is. That’s the outermost reaches of the solar system – the loose cloud of detritus left over from its formation about 4.5 billion years ago, thought to be the source of long-period comets.

Diagram of the Oort cloud and Kuiper Belt. Public domain, NASA, via Wikipedia.
Diagram of the Oort cloud and Kuiper Belt. Public domain, NASA, via Wikipedia.

In point of fact, something as much as 2 light years away – even in the direction of the Alpha Centauri star systems – could still orbit the Sun. Actually, that’s less likely. Ongoing sky surveys have not revealed anything, and astronomers now think that there are likely to be a few more bodies, maybe up to the size of Pluto, out there in the Kuiper Belt (the detritus that sits inside the Oort cloud).

There has even been speculation that an Earth-mass body might orbit, somewhere out there. That doesn’t mean Earth-like, of course. Not at the sort of temperatures planets waaaaaay out there get to. (‘Excuse me, Sir, your glass seems a little low. Can I top up your liquid hydrogen for you?’) But as our instruments have become more sensitive, the number of places for such planets to hide is dropping.

That’s not to say there isn’t one, of course. We’ll see. But at this stage, it’s unlikely. And it’s also possible – perhaps – that the Sun has a brown dwarf companion orbiting even further out again, cool enough to be invisible (so far) to our instruments. But again – unlikely.

The other thing about the Sun’s far-spread system is that, every so often, it suffers an interloper. Or the Sun intrudes into another system, depending on your viewpoint. Stars orbit the galactic centre, and this means that all the stars we see around us are moving, relative to us. Every so often, a star comes close enough to interfere with the Oort cloud. We think this happened as recently as 70,000 years ago. The star in question is between 17 and 23 light years away now – it’s moving that fast – and is dubbed WISE J072003.20−084651.2. It’s also known as Scholz’s Star.

It’s from encounters like this that long-period comets are thought to come from – a ‘close’ stellar encounter tips Oort-cloud snowballs into orbits that drop them towards the Sun. There’s also been speculation that Sedna – a Kuiper Belt object with a particularly elongated orbit – was caught up in an earlier tug-of-war between the Sun and a passing star. The possibility of objects near the fringes of solar systems – ours and others – being dislocated or stolen by a near-miss always exists. And the Sun could do the same to a passing solar system.

Sedna's orbit (in red) with the other major planets and Kuiper Belt Objects in blue. A diagram I made with Celestia.
Sedna’s orbit (in red) with the other major planets and Kuiper Belt Objects in blue. See how “close” Pluto is to the Sun at this scale. Yet even Sedna at its 900-odd AU apehelion isn’t actually “far” by comparison with the Oort Cloud. A diagram I made with Celestia.

There’s also the possibility of a ‘deep’ encounter, where the intruding star gets close enough to dislodge major planets. This doesn’t seem to have happened to our solar system, though work has been done on the forces involved – here’s the science paper: http://arxiv.org/abs/1402.0077

But it’s not something to lose sleep over – we’d get tens of thousands of years warning if anything dangerous was on its way. A survey recently showed that just 14 stars are likely to come close within the next half-million years – but that’s all relative, and even the closest shave, HIP 85605, will only bring it within 0.13-0.65 light years, in 240,000 years at the earliest (there are uncertainties). That’s enough to disturb the Oort cloud, but not the planets.

What’s more, most stars are red dwarfs, which are wa-a-a-a-y smaller than the Sun. So even if a star was going to make a bulls-eye encounter through the solar system, our central star would likely win the gravity battle.

Copyright © Matthew Wright 2015


7 thoughts on “The Sun’s big. Really big. And really grippy.

  1. I’m sure I read somewhere that there’s a quirk in the properties of gravity that means its influence grows over increasing distances. I think the article was pointing out that objects, even small objects like human beings, can have a gravitational influence on an object in space billions of miles away. Unlike other forces, gravity doesn’t seem to diminish over distance.

    1. Um… sorry, but no. You might need to check your source. Gravity – which is a specific outcome of space-time distortion, the equations for which WordPress won’t let me paste as a graphic into this comment box (but which I could do as a blog post, if you’d like the details) doesn’t produce constant effect with distance. Einstein’s theory of General Relativity, which describes gravity, has been shown to be true every which way, including having to be calculated for the GPS satellites, so they can necessarily compensate for Einsteinian frame-dragging even at their orbital altitude and velocity. The solar system precisely follows these gravitational rules – it was proven empirically for the first time in 1919. This is the Wikipedia article: https://en.wikipedia.org/wiki/General_relativity

      1. Can’t remember where I read it, but a glance at Wikipedia (which I know wasn’t the source) states that gravity has negiligible effect at the microscale, ie sub-atomic, but is the dominant force at the macroscale, ie astronomical. So maybe that’s where my memory is confusing me. It has effect over great distances, but virtually no effect at the sub-atomic level.

        1. The bottom line here is that the strong and weak nuclear forces are short-ranged, and neglible outside of (as the name implies) nuclear distances. The electromagnetic force is many, many orders of magnitude more powerful than the gravitational force, but while electromagnetic attraction and repulsion exist in precise equipoise*, the puny gravitational force is universally attractive and so, ultimately, wins.

          *Graffito I recall from my university days: “The net electromagnetic charge of the universe is +3, and the Russianshave two of them.”

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