The formation conundrum
In our Solar System, Moon-sized protoplanets are thought to have formed within a million years from slowly colliding dust particles in the disk of material orbiting the newborn Sun. Up to hundreds of these worlds collided and combined in the inner Solar System for about 100 million years until only four large bodies remained: Mercury, Venus, Earth, and Mars. But this kind of large-scale accretion may not function for planets around binary stars, which have a hard time forming in the first place.
The gravitational presence of an entire second star makes the disk particles orbit with much higher velocities. Instead of gently sticking on collision, these instead tend to break each other. Based on simulation, scientists think that even when baby planets a few hundred kilometers across form, their high velocities will likely fragment each other on collision.
We’ve already found several planets in binary and even multiple star systems. How do they exist? Are they rogue planets that the combined gravities of binaries capture into orbit when they pass by too closely? In 2021, scientists simulating the nature of protoplanetary disks around newborn binary stars published a more viable explanation: the disk’s gases, like wind, can drag and slow down high-velocity particles and baby planets enough in certain zones to enable them to grow into planets.
Gravity presents another steep barrier for binary star planets. Observations of disks surrounding newborn binary stars by the international ALMA radio telescope array suggest that young binary stars periodically attract more infalling material from their disks than single stars do. This triggers frequent energy outbursts from the stars, which heat up the gas and keep distorting the disk structures. As a result, only certain quieter parts of the disks may be favorable to giant planets accreting.
It gets worse. The pronounced periodic heating of planet-forming disks in binary systems could evaporate substantial water ice and dust on asteroids and comets, and alter the chemistry of their organic molecules — essential ingredients for life as we know it. Considering that scientists think comets and/or asteroids are what brought water and organic materials to early Earth, this would leave the hope for life on binary star planets literally deserted.
The silver lining
In 2017, based on extensive climate modeling, scientists published that an Earth-sized planet in the habitable zone around two Sun-like stars could retain its water for long, and thus presumably be conducive for life to emerge. They found that water vapor clouds in the atmospheres of such planets can persist and regulate temperature variations stemming from the combined gravitational effects of two stars swinging a planet in and out of a circular orbit. Since then, scientists have found more mechanisms that widen habitable zones in such systems.
