70,000 years ago, a pair of brown dwarfs known as Scholz's star, located just on the cusp of hydrogen fusion in their cores, passed through the Solar System's Oort cloud. Unlike the stars in this illustration, they were not visible to the human eye.
We are used to thinking of our solar system as a stable, peaceful place. Of course, from time to time we learn that planets and other celestial bodies kicked some comet or asteroid, but for the most part, everything remains constant. Even a rare interstellar visitor does not carry much risk, at least not for the integrity of a world like ours. But our entire solar system is orbiting through the galaxy, which means that it has hundreds of billions of chances for close interaction with another star. How often does this actually happen and what are the potential consequences of this? Our reader asks a question:
Opportunities range from routine incidents in which several objects in the Oort cloud go out of their way to catastrophic collisions with a planet or its ejection from the system. Let's see what actually happens.
A density map of the Milky Way and the surrounding sky, which clearly shows the Milky Way, the Large and Small Magellanic Clouds, and if you look closely, NGC 104 to the left of the Small Cloud, NGC 6205 just above and to the left of the galactic core, and NGC 7078 just below. In total, the Milky Way contains about 200 billion stars.
Our best estimate is that the Milky Way contains 200 billion to 400 billion stars. And although stars come in very different sizes and masses, most of them (3 out of every 4) are red dwarfs: from 8% to 40% of the mass of the Sun. The size of these stars is smaller than the sun: on average, about 25% of the Sun's diameter. We also roughly know the size of the Milky Way: it is a disk about 2,000 light years thick and 100,000 light years in diameter, with a central bulge with a radius of 5,000-8,000 light years.
Finally, relative to the Sun, a typical star moves at a speed of 20 km / s: about 1/10 of the speed with which the Sun (and all stars) orbit the Milky Way.
Although the Sun moves in the plane of the Milky Way at a distance of 25,000 to 27,000 light-years from the center, the directions of the orbits of the planets of the Solar System are not aligned with the plane of the galaxy.
This is the statistics for the stars in our Galaxy. There are many details, nuances and tricks that we will ignore - such as the change in density depending on whether we are in the spiral arm or not; the fact that more stars are located closer to the center than closer to the edge (and our Sun is halfway to the edge); the inclination of the solar system's orbits in relation to the galactic disk; small changes, depending on whether we are in the middle of the galactic plane or not … But we can ignore them because only using the above quantities allows us to calculate how often the stars of the Galaxy come within a certain distance to our Sun, and therefore how often close encounters or various clashes can be expected.
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Distances between the Sun and many of the nearby stars are accurate, but each star - even the largest of them - would be less than one millionth of a pixel in diameter to scale.
We calculate this value very simply - we calculate the density of stars, the cross section of interest to us (determined by how close you want the star to come to ours), and the speed at which the stars move relative to each other, and then we multiply all this to get the number of collisions per unit of time. This method of counting the number of collisions is suitable for everything from particle physics to condensed matter physics (for experts, this is essentially the Drude model), and just as well applies to astrophysics. If we assume that there are 200 billion stars in the Milky Way, that the stars are evenly distributed over the disk (ignoring the bulge), and that the stars move relative to each other at a speed of 20 km / s, then, by plotting the dependence of the number of interactions on the distance to the Sun, we get following:
A graph showing how often the stars in the Milky Way will pass a certain distance from the Sun. The graph is logarithmic on both axes, the y-axis is the distance, and the x-axis - typical expectation of this event in years.
He says that, on average, for the entire history of the Universe, one can expect that the closest distance to which another star approaches the Sun will be 500 AU, or about ten times farther than the distance from the Sun to Pluto. He also suggests that once every billion years, a star can be expected to approach us at a distance of 1500 AU, which is close to the edge of the scattered Kuiper belt. And more often, about once every 300,000 years, a star will pass at a distance of the order of a light year from us.
The logarithmic representation of the solar system, extending to the nearest stars, shows how far the Kuiper belt and Oort clouds extend.
This is definitely good for the long-term stability of the planets in our solar system. It follows from this that over 4.5 billion years of existence of our solar system, the chances that a star will approach any of our planets at a distance equal to the distance from the Sun to Pluto are about 1 in 10,000; the chances that a star will approach the Sun at a distance equal to the distance from the Sun to the Earth (which would greatly disrupt the orbit and lead to an ejection from the system) is less than 1 in 1,000,000,000. This means that the probability of passing by us another star from the galaxy, which could cause us serious inconvenience, is terribly low. We will not lose in the space lottery - it is very unlikely that, since nothing has happened yet, something will happen in the foreseeable future.
Orbits of inner and outer planets obeying Kepler's laws. The chances that the star will pass at some small distance from us, and even at a distance comparable to the distance to Pluto, are extremely small.
But the cases of the passage of a star through the Oort cloud (located 1.9 light years from the Sun), as a result of which the orbits of a huge number of ice bodies were disrupted, during this time about 40,000 should have accumulated. With such a passage of a star through the solar system, many interesting things happen., since two factors converge here:
Oort cloud objects are very weakly connected to the solar system, so even a very small gravitational push can significantly change their orbit.
Stars are very massive, so even if a star travels at a distance from an object equal to the distance from it to the Sun, it can kick it hard enough for its orbit to change.
It follows from this that every time we come close to a passing star, the risk increases that for, say, several million years after that we may collide with an object from the Oort cloud.
The Kuiper belt contains the largest number of objects in the solar system, but the farther and fainter Oort cloud not only contains more objects - it is also more susceptible to disturbances from a passing mass, such as another star. All Kuiper belt and Oort cloud objects move at extremely low speeds relative to the Sun.
In other words, we will not see the results of the impact of a passing star on icy comet-like bodies, which, possibly, will fall into the solar system, until about 20 successive stars have passed close enough to ours! This is a problem, since the last star system, Scholz's star (which passed 70,000 years ago) is already 20 light years away. However, an optimistic conclusion can be drawn from this analysis: the better our map of stars and their movements, located 500 light years from us, the better we can predict where and when the uncontrolled objects of the Oort cloud will appear. And if we are concerned about protecting the planet from objects thrown into our system by passing stars, then the acquisition of such knowledge is the obvious next step.
WISEPC J045853.90 + 643451.9, the green dot is the first ultracold brown dwarf discovered by the Wide-Field Infrared Survey Explorer, or WISE (Wide-Field Infrared Survey Explorer). This star is located 20 light years from us. To study the entire sky and find all the stars that could pass near the Sun and bring storms to the Oort Cloud, it would take a look at 500 light years.
This will require building wide-angle telescopes capable of seeing faint stars at great distances. The WISE mission became the prototype for such a technique, but the distance at which it is able to see the faintest stars, that is, the stars of the most common type, is greatly limited by its size and observation time. An infrared space telescope that observes the entire sky could mark our surroundings, tell us about what can come to us, how long it takes, from what directions, and what stars have caused disturbances among the objects of the Oort cloud. Gravitational interactions occur constantly, even despite the huge distances between the stars in space; the Oort cloud is huge, and we have a very long time for objects from there to fly past us and somehow influence us. Everything will happen in a long enough timewhat you can imagine.
Alexander Kolesnik