Ever wondered where the boundary of our solar system lies?
Is it up until the orbit of Pluto?
The Kuiper Belt?
Beyond?
You may have heard that the Voyager spacecrafts have already left the solar system, their
current position putting them well beyond the orbit of Pluto.
But in reality, to say they have left the solar system is not really true.
So how do we define the edge of a solar system, and following on from that, how big can solar
systems get?
Let’s look first of all at where the Voyagers are, and why some say they have left the solar
system.
Astonishingly, the Voyagers, which launched in 1977, are still operational today, although
at reduced capacities.
Their power sources are radioisotope thermoelectric generators, which are great at providing power
without the need of solar panels, however, with every passing year, the radioactive material
degrades, producing less power.
They are currently operating with about half the power they had when they started, meaning
a few of their instruments have had to be turned off.
What they are currently monitoring is the magnetic field environment of deep space,
and its interactions with the solar wind.
Of particular interest to scientists is the boundary where the magnetic field and solar
wind of the Sun are overcome by interstellar wind, this boundary known as the heliopause.
Scientists think Voyager 1 crossed this boundary in 2012, and Voyager 2 crossed it in 2018,
both being the only spacecraft to make it to interstellar space so far.
What they found out on their journey is that this boundary is not a spherical ball around
the Sun, but it’s rather bubbly and asymmetrical.
And much like planets facing into the Sun with their magnetic fields trailing behind
them because of interactions with the solar wind, the Sun’s magnetic field also trails
behind against the interstellar wind.
As the solar system orbits the centre of the Milky Way, it travels at over 800,000km/h.
As it moves, it passes through what is known as the interstellar medium at relative speeds
of 80,000km/h.
This creates a kind of wind effect on the magnetic field of the Sun, leaving a tail
in the interstellar wind’s wake.
But as fantastic as the Voyager mission is, reaching interstellar space is not the same
as reaching the edge of the solar system.
The edge of the solar system is defined by gravitational influences, a boundary known
as a Hill Sphere (although it should be noted that this is not really a sphere either, I’ll
come to that in a bit).
A Hill Sphere is the region around an object where the gravitational pull of that object
overcomes the gravitational pull of anything else.
For instance, the Moon orbits the Earth because it is within the Earth’s Hill Sphere, it’s
close enough that the Earth’s gravitational exertion on the Moon dominates the Sun’s.
Satellites orbiting the Moon are within the Moon’s Hill Sphere, where the Moon’s gravity
at that distance has a greater pull than that of the Earth and the Sun.
And everything orbiting the Sun is within the Sun’s Hill Sphere, where the Sun’s
gravity overcomes that of nearby stars.
This is the true boundary of the solar system.
Now, looking at our local neighbourhood of stars, we find that they are unevenly distributed
around us, this means that the Sun’s Hill Sphere isn’t really spherical.
Take the Alpha Centauri system, for instance.
It is roughly 4 light years away from us, yet Alpha Centauri contains two Sun-like stars,
so its total gravitational influence will be greater than the Sun’s, pushing the boundary
of that part of the Sun’s Hill Sphere closer to the Sun.
On the other hand, looking the opposite direction to Barnard’s Star, we have a red dwarf star
that is 6 light years away and much less massive than our star, so the Sun’s gravitational
influence would stretch much further in this direction.
And even if an object is orbiting within the Sun’s Hill Sphere near the boundary, its
orbit is very precarious.
The gravity of the Sun here is incredibly weak, the objects orbit would be very slow,
and the slightest perturbation might knock this object out of orbit.
That’s why astronomers currently think the furthest objects can safely orbit the Sun
is up to a maximum distance of 1 light year, even though the Sun’s Hill Sphere might
stretch out for 2 to 3 light years.
So, what’s out there at that distance?
Anything at all?
Well, way beyond the Kuiper belt, which is the asteroid belt beyond the orbit of Neptune,
up to a distance of about 1 - 3 light years is a region known as the Oort Cloud.
We believe the Oort cloud is a sparsely populated region surrounding the Sun containing billions
or even trillions of icy bodies.
We believe that’s where a small percentage of the long period comets come from, comets
which have orbits of tens to hundreds of thousands of years long.
To give you some perspective about the size of this cloud, it will still take Voyager
1 300 years to reach just the beginning of the Oort cloud, and potentially 30,000 - 150,000
thousand years to pass through it.
Astronomical distances are simply mind-blowing, and remember, at this distance we are considered
to still be within our own solar system.
Now imagine how many solar systems there are out there.
Which brings me onto the final point of this video.
How big can solar systems get?
Well, it’s something we can’t know for sure, although we can have an educated guess
about what it would look like.
We’ll stick strictly to stars for this thought experiment, to include black holes would be
to include entire galaxies.
The theoretical limit of a star is around 150 solar masses, or 150 times the mass of
our Sun.
Although, there are some stars out there that test this theory, like the most massive star
that we know of, R136a1, which is apparently 300 solar masses.
Now, this star resides in a very densely packed cluster, so although it is so massive, its
gravity competes with a lot of nearby stars.
This reduces the size of its Hill Sphere considerably.
Let’s say a star of this size was somehow ejected from its local cluster, even its local
galaxy, and it finds itself completely alone in space.
With no other gravitational forces for many light years in any direction, the only other
competing mass is other galaxies themselves.
That means, with nothing competing against it, such a solar system could be hundreds
to thousands of light years across.
Comparable in size to a galaxy depending on the distances involved.
In a universe where only this star and a second smaller object exist, that second object could
be billions of light years away and still be in orbit, because gravity doesn’t care
how far away the objects are, it will still have an influence on them.
In such a universe, the only escape for the second object from the gravitational pull
of the larger object would be to have an infinite distance between the two, which isn’t possible.
So, there we have it!
Solar systems are already mind bogglingly large, and under the right circumstances,
they can be far, far larger still.
The thought that sticks with me after researching all this is that the universe truly is a fascinating
place.
Thanks for watching!
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