There are more homeless planets in the universe than ordinary ones.
For us, planets are gas giants or solid worlds orbiting the parent star. And as the stars fade away, the Milky Way is littered with hundreds of billions of such planets, including our own, the only and so far unique Earth. And each planet, in principle, has its own and also unique history of birth and life. Some of them are massive and bright, others are small and dim; some were born a couple of million years ago, others can compete with the age of the Universe itself. But there is one thing we share with all these planets: the solar system. As shown by the Kepler mission and other searches for exoplanets, if you want to find planets-just point your finger at a star and look around it: around it you will find not one, but a whole system of planets.
And yet-in addition to the stars and all the bodies that revolve around them — there must be a great many planets that are not tied to the central star at all: rogue planets. Scientists believe that this is true for any place in the universe, from small star clusters and interstellar space to the cores of giant galaxies. As far as we know, there are as many starless planets in space as there are stars themselves — maybe more. It follows that for every point of light you see, there are many more massive points that you don’t see because they don’t emit visible light.
Thanks to observations, we have discovered a number of possible candidates for wandering planets. “Candidate” is an important word; we cannot be sure that these planets are true, because we do not have a good technique for confirming this fact. Even with our best modern equipment, they are so difficult to detect that we must assume that there are many more worlds than we have already found. But we have already found something and can draw conclusions. Where do these wandering planets come from?
One of the most convincing sources of all these planets is close to us, and we cherish it very much.
We know how solar systems form: after a gravitational collapse creates a region of space in which fusion is ignited, a protoplanetary disk gathers around the central star. Gravitational perturbations regularly appear in this disk, drawing more and more matter from its surroundings, while the heat of the newly formed central star slowly blows the lightest gas into the interstellar medium. Over time, gravitational disturbances develop into asteroids, solid planets, and gas giants.
But the fact is that these worlds not only rotate around their star, but also gravitationally pull each other together. Over time, these planets migrate to the most stable configurations they can achieve: the most massive worlds take their most stable places, often sacrificing other smaller worlds. What happens to the “losers” in the space battle for planetary advantage? They are absorbed in the process of merging, fall on the Sun or are thrown out of the solar system into interstellar space.
Recent simulations have shown that for every planet-rich solar system like our own (with gas giants), at least one gas giant will be ejected — into the interstellar medium, where it will be doomed to wander around the galaxy as an errant rogue planet. At the same time, the number of smaller solid worlds ejected from the system can reach 5-10. This is, in principle, the largest source of rogue planets, and there are probably hundreds of billions of them in our own galaxy.
It is particularly funny that when scientists conduct theoretical calculations, the ejected planets from young solar systems are half as many as the expected number of wandering planets. Where, then, do they come from? To understand where most starless planets come from, we need to look more broadly at one time: not just when our Solar system formed, but also at the cluster of stars (and star systems) that formed at the same time.
Star clusters are formed during the slow collapse of cold gas, mostly hydrogen, and usually originate in an existing galaxy. Deep in the collapsing clouds, gravitational instabilities form, and the first, most massive instabilities attract more and more matter. When enough matter is collected in a small region of space and the density and temperature in the core become high enough, nuclear fusion begins and stars are formed.
But it is not one star and a star system that is born, but many of them, because each cloud that collapses to form a new star contains enough matter to form many stars. Along with this, something happens. The largest star formed is also the hottest and bluest, meaning it emits the most ionizing, ultraviolet radiation. And this star begins one of the most active races to take its place in space.
If you look into the star-forming nebula, you can see two processes occurring simultaneously:
Gravity tries to pull matter in the direction of this young, growing gravitational superdense, the radiation burns out the neutral gas and pushes it back into the interstellar medium.
The answer depends on what counts as a victory. The largest gravitational super-densities form the largest, hottest, and bluest stars — but such stars are extremely rare. Smaller superdensities (still large) form other stars, but as their mass decreases, there are more and more of them. That is why when we look deep into a cluster of young stars, it is easy to see the brightest (blue or other) stars, but they are significantly outnumbered by yellow (and red) stars with a smaller mass.
If it were not for the radiation that young stars emit, these dim, red and yellow stars would continue to grow, become more massive and brighter, and flare up more strongly. Stars (in the main sequence) are of various types, from O-stars (the hottest, largest, and bluest) to M-stars (the smallest, coldest, red, and low-mass). And while the majority of stars – ¾ – are M-class stars, and less than 1% of all stars are O-or B-type stars, the total mass of the latter two types of stars is comparable to the total mass of M-type stars. It takes about 250 M-class stars to match the mass of an O-star.
As it turned out, about 90% of the original gas and dust that was in this star-forming nebula is blown into the interstellar medium and does not go into star formation. The most massive stars form faster and start blowing material out of the nebula. In just a couple of million years, there is less and less material left, and new stars stop forming. The remaining gas and dust are completely burned out.
And now the most interesting thing. Not only are M-class stars — with a mass between 8% and 40% of the sun’s-the most common type of star in the universe. There’s a lot more to what could have been M-class stars if the high-mass stars hadn’t burned out the excess material.
In other words, for every star that has formed, there are many more failed stars that simply haven’t gained critical mass: and there can be tens to hundreds of thousands of such stars for every star that has formed.
Just imagine: our own Solar System contains hundreds or even thousands of objects that potentially meet the geophysical definition of a planet, but were astronomically excluded only because of their orbital position. Now imagine that for every star like our Sun, there are hundreds of failed stars that simply haven’t gained enough mass to trigger fusion in the core. These are the homeless planets — or wandering planets-which are far more numerous than planets like our own orbiting stars. Orphan planets can be with or without an atmosphere, and they are extremely difficult to detect, especially the smallest ones. But think about it: for every planet like ours in the galaxy, there may be up to 100,000 planets that not only don’t orbit the star now, but never have. It is very difficult to find them.
So while we may have a few planets ejected from young solar systems, and even a handful of such worlds in the galaxy come from the Solar System, the vast majority of all the planets in the galaxy have never held on to stars. Rogue planets roam the galaxy, doomed to wander forever in the dark, and never know the warmth of their parent star. Their potential parents may never even have become stars. There may be a quadrillion such wandering worlds in the galaxy that we haven’t even really begun to discover yet.