Next time you want to be the life of the party—if you’re hanging out with cool nerds that is—just drop that phrase into the conversation. And when they look at you quizzically, just say that’s the eventual fate of the Solar System.
If a solar system is a family, then some planets leave home early. Whether they want to or not. Once they’ve left the gravitational embrace of their family, they’re pretty much destined to drift through interstellar space forever, unbound to any star.
Astronomers like to call these drifters “rogue planets,” and they’re getting better at finding them. A team of astronomers have found one of these drifting rogues that’s about the same mass as Mars or Earth.
Everything in space is moving. Galaxies collide and merge, massive clouds of gas migrate, and asteroids, comets, and rogue planets zip around and between it all. And in our own Solar System, the planets follow their ancient orbits.
Now a new data visualization shows us just how much our view from Earth changes in two years, as the orbits of the planets change the distance between us and our neighbours.
Scientists have learned a lot about the atmospheres on various worlds in our Solar System simply from planetary sunrises or sunsets. Sunlight streaming through the haze of an atmosphere can be separated into its component colors to create spectra, just as prisms do with sunlight. From the spectra, astronomers can interpret the measurements of light to reveal the chemical makeup of an atmosphere.
Whatever we grow up with, we think of as normal. Our single solitary yellow star seems normal to us, with planets orbiting on the same, aligned ecliptic. But most stars aren’t alone; most are in binary relationships. And some are in triple-star systems.
And the planet-forming disks around those three-star systems can exhibit some misshapen orbits.
Astronomers like observing distant young stars as they form. Stars are born out of a molecular cloud, and once enough of the matter in that cloud clumps together, fusion ignites and a star begins its life. The leftover material from the formation of the star is called a circumstellar disk.
As the material in the circumstellar disk swirls around the now-rotating star, it clumps up into individual planets. As planets form in it, they leave gaps in that disk. Or so we think.
Today we push our aging curiosity out into the Solar System to ask that simple question: how old is it and how do we know? What techniques do astronomers use to age various objects and regions in the Solar System?
The Solar System is a beautiful thing to behold. Between its four terrestrial planets, four gas giants, multiple minor planets composed of ice and rock, and countless moons and smaller objects, there is simply no shortage of things to study and be captivated by. Add to that our Sun, an Asteroid Belt, the Kuiper Belt, and many comets, and you’ve got enough to keep your busy for the rest of your life.
But why exactly is it that the larger bodies in the Solar System are round? Whether we are talking about moon like Titan, or the largest planet in the Solar System (Jupiter), large astronomical bodies seem to favor the shape of a sphere (though not a perfect one). The answer to this question has to do with how gravity works, not to mention how the Solar System came to be.
According to the most widely-accepted model of star and planet formation – aka. Nebular Hypothesis – our Solar System began as a cloud of swirling dust and gas (i.e. a nebula). According to this theory, about 4.57 billion years ago, something happened that caused the cloud to collapse. This could have been the result of a passing star, or shock waves from a supernova, but the end result was a gravitational collapse at the center of the cloud.
Due to this collapse, pockets of dust and gas began to collect into denser regions. As the denser regions pulled in more matter, conservation of momentum caused them to begin rotating while increasing pressure caused them to heat up. Most of the material ended up in a ball at the center to form the Sun while the rest of the matter flattened out into disk that circled around it – i.e. a protoplanetary disc.
The planets formed by accretion from this disc, in which dust and gas gravitated together and coalesced to form ever larger bodies. Due to their higher boiling points, only metals and silicates could exist in solid form closer to the Sun, and these would eventually form the terrestrial planets of Mercury, Venus, Earth, and Mars. Because metallic elements only comprised a very small fraction of the solar nebula, the terrestrial planets could not grow very large.
In contrast, the giant planets (Jupiter, Saturn, Uranus, and Neptune) formed beyond the point between the orbits of Mars and Jupiter where material is cool enough for volatile icy compounds to remain solid (i.e. the Frost Line). The ices that formed these planets were more plentiful than the metals and silicates that formed the terrestrial inner planets, allowing them to grow massive enough to capture large atmospheres of hydrogen and helium.
The leftover debris that never became planets congregated in regions such as the Asteroid Belt, the Kuiper Belt, and the Oort Cloud. So this is how and why the Solar System formed in the first place. Why is it that the larger objects formed as spheres instead of say, squares? The answer to this has to do with a concept known as hydrostatic equilibrium.
In astrophysical terms, hydrostatic equilibrium refers to the state where there is a balance between the outward thermal pressure from inside a planet and the weight of the material pressing inward. This state occurs once an object (a star, planet, or planetoid) becomes so massive that the force of gravity they exert causes them to collapse into the most efficient shape – a sphere.
Typically, objects reach this point once they exceed a diameter of 1,000 km (621 mi), though this depends on their density as well. This concept has also become an important factor in determining whether an astronomical object will be designated as a planet. This was based on the resolution adopted in 2006 by the 26th General Assembly for the International Astronomical Union.
In accordance with Resolution 5A, the definition of a planet is:
A “planet” is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighborhood around its orbit.
A “dwarf planet” is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape , (c) has not cleared the neighborhood around its orbit, and (d) is not a satellite.
All other objects, except satellites, orbiting the Sun shall be referred to collectively as “Small Solar-System Bodies”.
So why are planets round? Well, part of it is because when objects get particularly massive, nature favors that they assume the most efficient shape. On the other hand, we could say that planets are round because that is how we choose to define the word “planet”. But then again, “a rose by any other name”, right?
The Milky Way is an extremely big place. Measured from end to end, our galaxy in an estimated 100,000 to 180,000 light years (31,000 – 55,000 parsecs) in diameter. And it is extremely well-populated, with an estimated 100 to 400 million stars contained within. And according to recent estimates, it is believed that there are as many as 100 billion planets in the Milky Way. And our galaxy is merely one of trillions within the Universe.
So if we were to break it down, just how much matter would we find out there? Estimating how much there is overall would involve some serious math and incredible figures. But what about a single light year? As the most commonly-used unit for measuring the distances between stars and galaxies, determining how much stuff can be found within a single light year (on average) is a good way to get an idea of how stuff is out there.
Even though the name is a little confusing, you probably already know that a light year is the distance that light travels in the space of a year. Given that the speed of light has been measured to 299,792, 458 m/s (1080 million km/h; 671 million mph), the distance light travels in a single year is quite immense. All told, a single light year works out to 9,460,730,472,580.8 kilometers (5,878,625,373,183.6 mi).
So to determine how much stuff is in a light year, we need to take that distance and turn it into a cube, with each side measuring one light year in length. Imagine that giant volume of space (a little challenging for some of us to get our heads around) and imagine just how much “stuff” would be in there. And not just “stuff”, in the sense of dust, gas, stars or planets, either. How much nothing is in there, as in, the empty vacuum of space?
There is an answer, but it all depends on where you put your giant cube. Measure it at the core of the galaxy, and there are stars buzzing around all over the place. Perhaps in the heart of a globular cluster? In a star forming nebula? Or maybe out in the suburbs of the Milky Way? There’s also great voids that exist between galaxies, where there’s almost nothing.
Density of the Milky Way:
There’s no getting around the math in this one. First, let’s figure out an average density for the Milky Way and then go from there. Its about 100,000 to 180,000 light-years across and 1000 light-years thick. According to my buddy and famed astronomer Phil Plait (of Bad Astronomy), the total volume of the Milky Way is about 8 trillion cubic light-years.
And the total mass of the Milky Way is 6 x 1042 kilograms (that’s 6,000 trillion trillion trillion metric tons or 6,610 trillion trillion trillion US tons). Divide those together and you get 8 x 1029 kilograms (800 trillion trillion metric tons or 881.85 trillion trillion US tons) per light year. That’s an 8 followed by 29 zeros. This sounds like a lot, but its actually the equivalent of 0.4 Solar Masses – 40% of the mass of our Sun.
In other words, on average, across the Milky Way, there’s about 40% the mass of the Sun in every cubic light year. But in an average cubic meter, there’s only about 950 attograms, which is almost one femtogram (a quadrillionth of a gram of matter), which is pretty close to nothing. Compare this to air, which has more than a kilogram of mass per cubic meter.
To be fair, in the densest regions of the Milky Way – like inside globular clusters – you can get densities of stars with 100, or even 1000 times greater than our region of the galaxy. Stars can get as close together as the radius of the Solar System. But out in the vast interstellar gulfs between stars, the density drops significantly. There are only a few hundred individual atoms per cubic meter in interstellar space.
And in the intergalactic voids; the gulfs between galaxies, there are just a handful of atoms per meter. Like it or not, much of the Universe is pretty close to being empty space, with just trace amounts of dust or gas particles to be found between all the stars, galaxies, clusters and super clusters.
So how much stuff is there in a light year? It all depends on where you look, but if you spread all the matter around by shaking the Universe up like a snow globe, the answer is very close to nothing.
Beyond the Solar System’s Main Asteroid Belt lies the realm of the giants. It is here, staring with Jupiter and extending to Neptune, that the largest planets in the Solar System are located. Appropriately named “gas giants” because of their composition, these planets dwarf the rocky (terrestrial) planets of the inner Solar System many times over.
Just take a look at Saturn, the gas giant that takes its name from the Roman god of agriculture, and the second largest planet in the Solar System (behind Jupiter). In addition to its beautiful ring system and its large system of moons, this planet is renowned for its incredible size. Just how big is this planet? Well that depends on what your frame of reference is.
First let’s consider how large Saturn is from one end to the other – i.e. it’s diameter. The equatorial diameter of Saturn is 120,536 km ± 8 km (74,898 ± 5 mi) – or the equivalent of almost 9.5 Earths. However, as with all planets, their is a difference between the equatorial vs. the polar diameter. This difference is due to the flattening the planet experiences at the poles, which is caused by the planet’s rapid rotation.
The poles are about 5,904 km closer to the center of Saturn than points on the equator. As a result, Saturn’s polar radius is about 108,728 ± 20 km (67,560 ± 12 mi) – or the equivalent of 8.5 Earths. That’s a pretty big difference, and you can actually see that Saturn looks a little squashed in pictures. Just for comparison, the equatorial diameter of Saturn is 9.4 times bigger than Earth, and it’s about 84% the diameter of Jupiter.
Volume and Surface Area:
In terms of volume and surface area, the numbers get even more impressive! For starters, the surface area of Saturn is 42.7 billion km² (16.5 billion sq miles), which works out to about 83.7 times the surface area of Earth. That’s smaller than Jupiter though, being only 68.7% of Jupiter’s surface area. Still, that is pretty astounding when put into perspective.
On the other hand, Saturn has a volume of 827.13 trillion km³ (198.44 trillion cubic miles), which effectively means you could fit Earth inside of it 763 times over and still have room enough for about twenty Moons! Again, Jupiter has it beat since Saturn has only 57.8% the volume of Jupiter. It’s big, but Jupiter is just that much bigger.
Mass and Density:
What about mass? Of course Saturn is much, much, MUCH more massive than Earth. In fact, it’s mass has been estimated to be a whopping 568,360,000 trillion trillion kg (1,253,000,000 trillion trillion lbs) – which works out to 95 times the mass of Earth. Granted, that only works out to about 30% the mass of Jupiter, but that’s still a staggering amount of matter.
Looking at the numbers, you may notice that this seems like a bit of a discrepancy. If you could actually fit 763 Earth-sized planets inside Saturn with room to spare, how is it that it is only 95 times Earth’s mass? The answer to that has to do with density. Since Saturn is a gas giant, its matter is distributed less densely than a rocky planet’s.
Whereas Earth has a density of 5.514 g/cm³ (or 0.1992 lb per cubic inch), Saturn’s density is only 0.687 g/cm3 (0.0248 lb/cu in). Like all gas giants, Saturn’s is made up largely of gases that exist under varying degrees of pressure. Whereas the density increases considerably the deeper one goes into Saturn’s interior, the overall density is less than that of water – 1 g/cm³ (0.0361273 lb/cu in).
Yes, Saturn is quite the behemoth. And yet, ongoing investigations into extra-solar planets are revealing that even it and its big brother Jupiter can be beaten in the size department. In fact, thanks to the Kepler mission and other exoplanet surveys, astronomers have found a plethora of “Super-Jupiters” in the cosmos, which refers to planets that are up to 80 times the mass of Jupiter.
I guess the takeaway from this is that there’s always a bigger planet out there. So watch your step and remember not to throw your weight (or mass or volume) around!