For every planet that orbits a star, like our Earth orbits the Sun, there are probably thousands of “orphan planets” wandering the galaxy alone.
In our Solar System, we can confidently observe the orbit of at least eight planets around our home star. But during the existence of the Solar System-4.5 billion years — we can not know how many planets there were in total. We can only be sure of which planets have survived to our time. Perhaps there were planets that formed around our Sun at an early stage, and then they were thrown out of the Solar System under the influence of some gravitational force. Over the past few years, we have begun to find similar orphan planets, sometimes called rogue planets, in interstellar space. Based on what we know about stars, gravity, and cosmic evolution, we can make a rough estimate of the total number of planets in the universe, and it probably exceeds the number of stars anywhere from 100 to 100,000. Space is full of planets, and most of them don’t even have home stars.
Recently, we have begun to realize that solar systems like ours are the rule in the universe, not the exception. Studies of exoplanets have shown us that not only do many (if not all) stars probably have planets around them, most of them may have worlds with different masses, sizes, and orbital periods of rotation. Stars can have gas giants in the inner parts of their planetary systems, have many worlds in inner orbits, such as Mercury, or have planets much farther away than even Neptune around the Sun.
There is probably much more diversity among the planets that orbit other stars than we ever imagined when looking at our Solar System. There are probably even stars with dozens of planets orbiting them; we hope to discover them in the future, when we can increase the resolution of our space telescopes.
On average, we can say that there are probably 10 planets for every star in our Milky Way galaxy, meaning that this is an estimate based on incomplete information. The true average may be a smaller number, such as 3, or a larger one, such as 30, but 10 is a reasonable approximate number based on what we currently know. However, as we mentioned earlier, this number only reflects the number of planets available today. During the life of the Solar System, there were many worlds that were created, but did not survive to this day. Some of them collided and merged with others, forming larger planets. Others lost their kinetic energy under the influence of gravity and fell on the central star.
Over time, the planets gravitationally pull each other and by migration create the most stable configurations, like the current Solar System. This usually means that the largest, most massive planets occupy their position in orbits at the expense of other, smaller, lighter planets. In the cosmic battle for planetary permanence, the most common outcome should be to push the losing planets out of the star system and into interstellar space.
Perhaps a few trillion of them wander the Milky Way and the vast majority of rogue planets have never had a parent star. To understand why, we need to go back to how stars form.
Whenever you have a large, cool molecular cloud of gas, it will split and collapse into several clumps as gravity pulls the mass in and radiation pushes it out. If your cloud of gas is cool enough and massive enough, it can reach enough temperatures and densities in the cores of its densest clumps to ignite nuclear fusion and form stars. There is a huge race going on within the star-forming region: between gravity, which tries to form as many stars as possible of as much mass as possible, and between radiation, which literally blows away the gas and prevents gravitational growth. When we look at a cluster of newborn stars, our eyes tell us that gravity has won, as we see a huge number of massive stars.
But this conclusion is deceptive. For every hot, blue, massive star we see, there are usually hundreds or even thousands of smaller, low-mass stars that are hard to spot due to being dimmer and fainter. But just because they’re invisible doesn’t mean they don’t exist yet! Three out of every four stars in the universe are red dwarfs: stars with a low mass of 8% to 40% of the mass of the Sun, but the stars that are easiest to see are many tens or even hundreds of times larger than the Sun. As these massive stars glow brightly, they blow away the gas that would otherwise form new stars. Not only do they prevent the further development of these low-mass stars, they stop the gravitational growth of potential stars.
If you look at all the mass in a molecular gas cloud before the stars are formed, you will find that 90% of its matter returns to the interstellar medium; only about 10% of the nebula’s mass becomes stars or planets. The most massive stars form very quickly, and then blow away the remaining gas with their radiation over millions of years, stopping the possibility of further star formation. This leaves many low-and intermediate-mass stars in the cluster, but it also creates a large number of failed stars: clumps of matter that have never crossed the mass threshold to become a star. These clusters, despite never forming around stars, are large enough and massive enough to meet the geophysical definition of a planet.
According to a 2012 study, for every newly formed star, there are between 100 and 100,000 nomad planets that have also formed and are wandering starless through interstellar space.
Our Solar System contains hundreds or even thousands of objects that potentially meet the geophysical definition of a planet, but are astronomically excluded only by virtue of their orbital location. For every star like our Sun, there are most likely hundreds of failed stars that just didn’t gain enough mass to flare up. These are homeless planets — or rogue planets, which are far more numerous than planets like our own orbiting a star. These rogue planets are extremely common, but because they are very far away and do not emit light, they are extremely difficult to detect.
So far, only four possible candidates for orphan planets have been found. In the vastness of space, these bodies, which do not emit their own visible light, can be seen either in the reflection of starlight, or if they emit their own infrared light, or against the background of the star’s light, when the planet blocks this light while moving.
Our Galaxy contains about 400 billion stars, and there are about two trillion galaxies in the universe , so the realization that there are about ten planets for each star is staggering. But there are between 100 and 100,000 times more orphan planets wandering in interstellar space than there are stars themselves.
Only a small percentage of them have been ejected from their solar system, and the vast majority have never known the heat of a home star. Many of them are gas giants, but even more are likely to be rocky and icy, with many containing all the ingredients necessary for life to arise. Maybe someday, they’ll get their chance. Until then, they will continue to travel across the galaxy and across the universe, far surpassing the dizzying array of star lights illuminating the cosmos.