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Chapter 1: What are the characteristics of multi-star systems?
In our solar system, we like to keep things simple. Just the one star found in the centre, with everything else orbiting around it, as is the case for most planetary systems found in the universe. However, there are some planetary systems out there where things get a bit more complicated, specifically multi-star systems, where there are two or more stars that orbit each other.
In such configurations, what happens to any planets orbiting them? In fact, can planets orbit them at all with the gravitational tugs from different directions? To answer the latter question, the simple answer is yes, planets can orbit in such situations, although to answer the former question, there is no one-answer-fits-all rule about how a planetary system in a multistar system might look.
What we can do, however, is explore some of the possibilities out there. But before we look at planets, it would be good to understand how multistar systems work. For the most part, stable star systems have organised themselves into hierarchical systems. This is due to the proximity in which they formed with each other, which we will touch on a little later.
Binary star systems are generally simple enough. Binary stars orbit around a barycentre, or in other words, their centre of mass. If the masses of these two stars are similar, then nearly symmetrical elliptical orbits are often seen. Although, there can be occasions where they orbit in circles, in a similar fashion to Pluto and Charon.
In the case that one object is more massive than the other, then the more massive object's orbit doesn't take it as far out compared to the less massive object. Beyond binary systems, you can have 3, 4, 5, 6, 7 or more stars in the same system, and as you will see, there is a structure within these systems to keep them stable.
In the case of three stars, you'll have two stars orbiting each other in a binary configuration, with the third orbiting around a barycentre with the other two. This keeps the system stable, because if three stars had their orbits cross, one would certainly get ejected from the system at some point.
In a three star system, two of the stars are contained in their own enclosed little system, acting as one star in the grand scheme of the whole system itself. We group this binary configuration into a tier, with that tier acting together in its association with the single star.
In a way, once you have grouped the binary configuration in the system, this upper tier now acts like a two star system again, with the two stars and the one star orbiting each other.
In the case of four stars, you'll either have two binary configurations orbiting around a barycentre, or one binary system orbiting a barycentre with a third star, and all three of those stars orbiting around a barycentre with a fourth star. From here on is where the hierarchical system really comes in handy. With a chart like this, you can easily see how the system works.
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Chapter 2: How do planets orbit in binary star systems?
What a cool sight that would be too. I just can't help letting my imagination run with what that would be like. However, it probably wouldn't be so great for a nice consistent day and night cycle. So there we have it, how planets orbit in multi-star systems. Saturn is easily the most recognisable planet in our solar system, and some of its moons are the most famous too.
Titan, Enceladus and Rhea all move like clockwork in an elegant dance around this ringed giant. But out in the far reaches of Saturn's gravitational influence, millions of kilometres from the planet, prowls an object that does not belong. It is dark, scarred and solitary. It moves backwards, crashing against the flow of the rest of the system.
It is a time capsule from an era when the giant planets migrated and the solar system tore itself apart. This is a world of landslides, of frozen carbon dioxide, and a hidden ring of dust so massive it dwarfs Saturn itself. It is a moon that shouldn't be there, a visitor snatched by Saturn's gravity and held prisoner for 4 billion years. This is Phoebe.
I'm Alex McColgan and you're watching Astrum. Join me today as we journey to the edge of Saturn's domain to unravel the secrets of its darkest, most mysterious moon. The story of Phoebe begins with a quiet revolution. For thousands of years, astronomy was limited by what is possible to see with the human eye.
First astronomers used the naked eye, and then in the early 1600s, the invention of the telescope revolutionised what it was possible to see. But it wasn't until the 19th century that the dry plate photographic revolution changed everything.
For the first time, astronomers could leave a camera shutter open for hours, allowing light to accumulate on glass plates, revealing objects thousands of times fainter than what Galileo or Cassini could have ever dreamed of observing themselves. In 1898, a team from the Harvard College Observatory, led by William Henry Pickering, set up an outpost in the thin, dry air of Arequipa, Peru.
Using the 24-inch Bruce telescope, they began a deep photographic survey of the southern sky. The work was tedious. The glass plates were shipped back to Cambridge, Massachusetts, where they were poured over by computers, human analysts using magnifying loops to spot new objects on the plates. Then, in 1899, Pickering was examining plates taken the previous August when he found a speck.
It was faint, magnitude 15.5, roughly 4,000 times fainter than the limit of the naked eye. But what was most fascinating about this object was that it moved. Pickering traced its path across multiple nights. The stars stayed fixed. Saturn moved, but this speck moved with Saturn, yet not like the other moons we already knew of.
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Chapter 3: What happens in a three-star system?
On the 18th of March 1899, he announced the discovery of Phoebe. It was a landmark moment, the first natural satellite in history to be discovered not by direct observation, but by an image on a photographic plate. As astronomers tracked Phoebe in the early 20th century, the excitement of discovery turned into confusion. Phoebe wasn't just far away, it was wrong. The solar system has rules.
Because everything formed from the same spinning disk of gas, the planets and their moons almost universally spin and orbit in the same direction, counterclockwise or prograde. Phoebe breaks this law. It orbits Saturn clockwise, in the opposite direction to the planet's rotation and the other moons. This is known as a retrograde orbit.
Now, when we find a moon orbiting backwards, we know one thing with absolute certainty. It did not form there. If Phoebe had formed from the dust surrounding the infant Saturn, the drag from the gas cloud would have forced it into a prograde orbit. A retrograde orbit is a smoking gun, pointing to a very violent history.
It means Phoebe is an immigrant, a captured object that formed elsewhere and was ensnared by Saturn's gravity. And the distance of this moon is staggering. Phoebe orbits at a mean distance of nearly 13 million kilometers from Saturn. That is nearly four times further out than Iapetus, and almost a quarter of the distance from the Sun to Mercury.
It takes 550 days, about 18 months, for Phoebe to complete a single, lonely lap around the ringed planet. It's so far out that, from the surface of Phoebe, you would not be able to see the rings of Saturn with the naked eye. All you would see is the glare of a tiny but bright planet in the night sky. Because it is so far from its host planet, Phoebe went uninvestigated for nearly a century.
Even Voyager 2, which flew through the system in 1981, only saw it as a jagged, dark blob from 2.2 million kilometres away. We knew it was there. We knew it was weird, but we didn't know what it was. That changed with Cassini. When mission planners were designing the trajectory for the Cassini spacecraft, they realised they had a unique problem. To enter orbit around Saturn, Cassini had to break.
This meant approaching the planet from the outside in. They realised that, on this arrival leg, before the critical engine burn that would trap the probe in Saturn's gravity, they could pass by Phoebe. It was a one-shot chance to get a close-up look at this elusive moon.
The encounter had to happen on the 11th of June 2004, 19 days before orbital insertion, because once Cassini fired its engines and settled into the inner system, that was it. It would never have the fuel to go back to Phoebe or match its backward speed. For a few frantic hours, the dark moon filled the cameras, transforming from a dot into a complex world.
The relative velocity of the flyby was a staggering 5.8 km per second. And then, Phoebe was gone, receding into the darkness. The images Cassini beamed back revealed a world that looked nothing like its siblings. Phoebe is roughly spherical, about 213km across, but it looks absolutely beaten. It is dark, with an albedo of just 10%, making it as black as asphalt.
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Chapter 4: Can planets exist in systems with multiple suns?
The Abhelia grouping that Oort noticed, where the long-period comets reached their farthest orbital distance from the Sun, was about 50,000 AU. to help picture the orbits of these long period comets. Keep in mind that the outermost planet in our solar system, Neptune, is around 30 AU from the Sun, or about 4.5 billion kilometres.
The main region of the Kuiper belt extends from Neptune's orbit at 30 AU out to around 50 AU, But recent evidence from NASA's New Horizons spacecraft suggests a second region of the Kuiper belt, called the Scattered Disk, which continues to around 1000 AU.
It's with these key findings from observed comets that they didn't come from far out in interstellar space, the orbital distances clustered around 50,000 AU, and the fact that they arrived from any direction and orbital inclination, that Oort theorised a special spherical swarm of icy debris that he believed to be the origin of long-period comets.
In the years since then, mathematical models have shown agreement with the Oort cloud theory, and astronomers have further theorised various mechanics by which the Oort cloud came to be in its current state. The leading idea is that the Oort cloud formed from ancient debris, leftovers from when our planet formed 4.6 billion years ago.
After the planets formed, the surrounding area was still rich with these smaller, leftover chunks of material called planetesimals. The gravity from these early planets then scattered the leftover material in every direction. Some material was flung out of the solar system entirely, but a significant portion was sent into seemingly random, eccentric orbits around the Sun.
These scattered planetesimals had eccentric enough orbits that they were influenced by gravitational forces outside of our solar system, while still remaining captured in our Sun's orbit. and it's believed that this is how these billions or trillions of icy chunks came to be part of the Oort cloud.
Gravitational perturbations can force Kuiper belt objects out of place, creating short period comets. We think that similar forces are what send Oort cloud objects into elliptical orbits with the Sun, thereby creating long period comets. These perturbations could be caused by passing stars, or molecular clouds, or tidal forces from the Milky Way itself.
In fact, about 70,000 years ago, Schultz's star gained the title of the star that came closest to our solar system, actually grazing the outer region of the Oort cloud. But luckily for us, it didn't cause any catastrophic disruptions to the Oort cloud or our solar system at large. Schultz's star is a low-mass binary system made up of a red dwarf and a brown dwarf companion.
So, 10 millennia ago, even at a much closer distance to the Oort cloud, the gravitational influence of this binary star was significantly weaker than that of our much more massive Sun. However, while most objects experience little to no impact during the low-mass star's brief encounter with the edge of the Oort cloud,
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