If the Sun and another star formed a binary star system, Earth and the life on it, as well as human civilization, would certainly be completely different from what it is now.
1. Binary Stars
In astronomy, optical binary stars are cases where two stars appear to be close to each other when observed from Earth. There are two types of optical binary stars: visual binaries – which are a true binary star system, and apparent optical binaries – where two stars seem close, but are actually far apart.
With the naked eye, binary stars will appear similar to a single star, but they can be clearly distinguished with a telescope. When looking at the universe from Earth, all stars are projected onto a two-dimensional surface called the celestial sphere, so stars that appear to be close together may actually be very far apart. Such binary stars are referred to as apparent optical binaries and have little astronomical significance.
However, what we will discuss in this article are visual binaries, also known as Binary Stars, which are true binary star systems that are close enough to interact gravitationally and orbit each other.
Accordingly, binary stars can also be divided into two forms:
- Visual binaries are gravitationally bound binary star systems that can be resolved into two stars. According to Kepler’s third law, these stars are estimated to have periods ranging from a few years to thousands of years. A visual binary system consists of two stars, usually with different brightness. In simple terms, this binary star system can be identified through a telescope.
- Non-visual binaries refer to binary star systems that cannot be determined through telescopes and require specialized research methods for identification.
Most binary stars are pairs that were born in the same primordial nebula, and only a few are separate objects that have been drawn together by gravitational attraction and moved close to each other.
2. Planets in Binary Star Systems
In reality, binary star systems will form their own planetary systems in different ways. If two stars are far apart, they may have their own planets that typically have little influence on each other, allowing us to consider them as independent evolving star systems.
If two stars are relatively close to each other, planets will orbit both stars. Astronomers are currently very interested in such planets, often referring to them as circumbinary planets – on such planets, we would witness the phenomenon of having two suns.
If two stars are relatively close to each other, planets will orbit both stars.
However, the main threat to circumbinary planets comes from the two stars at the center, meaning stable orbits can only be found in regions far from the stars. According to simulations by NASA’s Kepler team, a planet can only maintain a stable orbit if its orbital radius exceeds 3.5 times the distance between the binary stars.
The formation process of circumbinary planets is also very different from the formation process of planets in single star systems like our Solar System.
According to modern planetary evolution theory, the formation and evolution of the Solar System began about 4.6 billion years ago with the gravitational collapse of a small part of a giant molecular cloud. Most of the collapsing mass accumulated at the center, forming the Sun, while the rest flattened out to create a protoplanetary dust disk that gradually evolved into rocky planets (like Mercury, Venus, Earth, and Mars), moons, asteroids, and other small celestial bodies in the Solar System.
The Solar System has evolved significantly from its initial form. Many moons formed from gas and dust disks orbiting the planets, while others formed independently but were later captured into planetary orbits. Still, others, like Earth’s moon, may result from colossal impacts. Such celestial collisions occur frequently even today and play a central role in the evolution of the Solar System. The positions of the planets frequently change, and this planetary migration is thought to be essential in the early evolution of the Solar System.
In binary star systems, rocky planets like Earth will not exist.
Applying this principle to binary star systems, astronomers have drawn the conclusion that:
In binary star systems, rocky planets like Earth will not exist. This is because the creation of planets is a very intricate process. The formation of planets relies on collisions and follows their distinct trajectories. In single star systems, a large amount of dust orbits along the same path, gradually developing into a planet at a certain speed. Meanwhile, near binary stars, gravitational disturbances are frequent and uneven, making it very hard to accumulate cosmic dust. Even if clusters of dust do gather, they can easily be ejected by disturbances in the field.
This aligns perfectly with Kepler’s observations; the circumbinary planets observed today are indeed giant gas and ice planets, and most of them are located at the edges of stable orbital zones.
Thus, imagining Earth existing in a binary star system would be impossible, as we have no foundational premise to reason from. However, some people still believe that in binary star systems, protoplanetary cores will automatically find stable elliptical orbits to avoid violent collisions. This would create opportunities for circumbinary planets to gradually evolve into rocky planets.
Whether this novel idea is accepted or not will depend on whether Kepler can find rocky planets around binary stars in the future. Thus, the question we initially raised remains unanswered. Regardless of how we speculate, at the very least, we need to find foundational premises to develop an answer.
