Life in our solar system is relatively simple; We have a star named the 'Sun,' and our planet Earth just revolves around it, completing a cycle in 365.242 days. In this system with eight planets, only one planet has the conditions to support life-- including water, oxygen, and protection from external radiation. Now, imagine a system where instead of one sun, there are two Suns present. They revolve around each other, and an external planet also revolves around the same, forming a weird shape of orbit influenced by gravitational factors. Sci-fi enthusiasts might immediately relate this with the infamous Tatooine from Star Wars, where the natives could observe binary sunsets (From the movie A New Hope). This fictional world had the presence of two stars, Tatoo I, and Tatoo II, resulting in the lone planet having extremely harsh and hot environments. However, this piece of fantasy tells us that even though there are two bright stars, the possibility of life remains.
Binary Star Systems
A binary star system is when two stars are gravitationally bound and orbit a common center of mass. The Binary Star System is the most common multi-star system that can be observed, and it can often appear as a single star to the naked eye. Many Binary Star Systems have been discovered by scientists across the globe, however, finding a Binary system with an exoplanet is comparatively rarer.
The reason for the same is that binary star systems introduce some complexities to a planet formation. Compared to a single-star system, the gravitational influence of the companion star can disturb the formation of a planet by influencing the protoplanetary disc. This disc is mainly composed of hydrogen, helium, dust, and ices. As per the core accretion theory, this disc surrounds a star and is the birthplace of planets. Slowly, dust collides and accumulates in these discs, forming planetesimals. These are kilometer-sized solid objects that can survive collisions. The theory tells us that the coagulation resulting from collisions has to be of sufficient mass to form a planet's core. After the core is formed, it can attract gas or other substances, eventually forming into a gas planet like Jupiter or a rocky planet like Earth. This is the case in a single-star system, where planets can form. However various complications arise, mainly due to gravitational factors, that lead to the disruption of the protoplanetary.
Disc. As a result, the collision speed of the planetesimals increases, making it difficult for a planet to be formed. In simpler terms, this is known as the eggbeater effect, which can lead to destructive collisions rather than gentle collisions required to nurture and grow a planet.
NASA has discovered several exoplanets in such systems, showcasing that while rare, exoplanets in binary systems are present. The only requirements are the perfect conditions for the growth of a planet. If the initial planetesimals formed after repeated collisions are at least ten kilometers in diameter and the protoplanetary disc maintains a near-circular structure, then the destructive collision speeds can be kept under control. These conditions allow the planetesimals to stick together and grow into larger bodies, forming planets. This observation offers a credible reason for the presence of "Tatooine" planets that have been detected by space telescopes.
Recently, Dr Trifon Trifonov, head of the Bulgarian exoplanet research group within the “EXO-RESTART” project at Sofia University’s Department of Astronomy, discovered an exoplanet 73 light-years from Earth that orbits a binary system (Nu Octantis ν Octantis) in a retrograde fashion - moving opposite to the direction of its companion star. This is a unique orbital pattern observed for the first time, which offers crucial insights into how planets form and evolve in tightly bound binary systems. Another unique fact about this system is that the star ν Oct B is a white dwarf. This contains 60% of the Sun's mass squeezed into a size as small as the Earth, revealing an extremely dense structure with a large gravitational field. This indicates that the star has already passed through its Red Giant Phase, a phase wherein the star explodes and then condenses, leading to other theories about the planet's formation. Some scientists theorize that the planet formed on ν Oct A's accretion disc after ν Oct B became a white dwarf, while others argue that the planet was formed elsewhere and is captured into the orbit from the unusual binary system's environment.
Sky and Seasons
After understanding the formation and presence of planets in the binary systems, it is time to enquire about what the planet's environment would be like.
Imagine looking up and seeing two suns traversing your sky. At specific times based on the orbit, both would be visible at the same time, resulting in an extraordinary view of the sky. You might see the sun rise and set at the same time, enveloping the feeling of dusk and dawn, experiencing twilight at the same time. If you find yourself near the sea, you may witness two disks rising and falling on the horizon, potentially displaying varying colors.
Depending on the star’s life stage. One might emit an ethereal blue glow, while the other would emit an orangish-yellow glow like our Sun. The sun could create unusual patterns, create dramatic or double shadows, or perhaps create colorful skies that change over time.
However, there is a reason why we haven't discovered life until now. These godly sceneries might exist, but the seasons would be very unpredictable. If a planet orbits both stars, known as circumbinary orbits, then the planet might experience intense heat when both stars are visible in the sky. Instead of a neat four-season cycle, the planet's seasons may fluctuate chaotically, depending on its orbit and the nearby presence of stars. If the planet orbits only one of the stars, known as an S-type orbit, then that star would largely govern the planet's climate, while the other star would provide additional 'add-ons' to the process. The seasons would be relatively ordered, but the varying influence of the companion star would be just as unpredictable.
Life forms?
The most important question of all - is whether there is a possibility of life existing on such a planet. Could there be evolved intelligent beings, or even simple single-celled organisms, capable of withstanding the atmosphere and unpredictable weather? Once again, it all depends on whether the system meets the habitable requirements. A stable orbit needs to be maintained by the planet - it has to be enough to keep in the habitable zone. The heat should exceed the boiling point of water because as per current theories, liquid water is essential for life to exist. Moreover, the atmosphere has to be capable of shielding life from harmful radiation. The UV rays would increase due to the presence of two stars; hence a stronger magnetic field and a protective atmosphere would be required for the presence of life.
Furthermore, the adaptability of these life forms should be of utmost importance. On Earth, the bodies of all organisms have synchronized with the day-night cycle, creating a proper rest and work cycle for all beings. However, the life of the organisms on such a planet would need unique adaptations, where their rhythm could be based on the complexity of the light intervals, including rapid shifts from high-intensity light to low-intensity light and burning periods to instant cool periods.
Clark and Mason simulated several close binary systems and presented their findings. The findings indicate that life in binary systems could be equally viable as in single-star systems,
As long as the system features low-mass twin stars—each about 80% of the Sun's mass—situated relatively close to one another. The close stars would create a much more stable habitable zone, reducing the radiation effects. The low-mass stars are more common than Sun-like stars, thus increasing the likelihood of such a binary system being present in the Universe. Stable orbits are also essential, to avoid the experience of unstable climates and unfavorable conditions for life. Lastly, life would depend on the 'cloud cover' it receives. This cover could help to both insulate the planet and shelter it from ultraviolet radiation. It can also help protect the planet from the changes it would encounter as it orbits closer first to one star and then to the other. The temperature fluctuation is also dependent on the same, however, the possibility of increased adaptability on lifeforms could eliminate the negative effect caused by drastic temperature changes.
Therefore, there is a possibility of life existing on an exoplanet present in a binary star system. All that is left is to bring the theories and simulations a step further and improve the observation of such systems to evaluate the possibility of life existing. With the universe being so unfathomably vast, the possibility of life existing is not slim. We just have to push forward in the right direction and hope that one day, we will discover other sentient beings like us, waiting to find companions in the universe bestowed to us by unknown entities.
References:
- https://www.space.com/19962-habitable-planets-binary-stars.html
- https://www.novinite.com/articles/232448/Bulgarian+Scientists+Confirm+First+Retrograde+Exoplanet+in+Close+Binary+Star+System
- https://www.cam.ac.uk/research/news/astronomers-show-how-planets-form-in-binary-systems-without-getting-crushed