The idea of a planet orbiting two suns, famously depicted in science fiction, prompts a question about our own solar system. While Earth orbits a single star, systems with two or more stars are common throughout the Milky Way galaxy. Our planet does not have two suns, but these stellar pairings, known as binary or multiple star systems, are the norm for a significant portion of stars.
The Prevalence of Binary and Multiple Star Systems
A binary star system consists of two stars gravitationally bound to each other, orbiting around a common center of mass. Studies suggest that between one-third and half of all star systems in the Milky Way are composed of two or more stars. The likelihood of a star being part of a multiple system generally increases with the star’s mass.
Our nearest stellar neighbor, Alpha Centauri, is a well-known example of a multiple star system, containing the binary pair Alpha Centauri A and B, plus the distant Proxima Centauri. Astronomers categorize these pairings based on how they are observed, since many cannot be visually separated even with powerful telescopes.
Detecting Binary Systems
A visual binary is one where the two stars can be seen distinctly from Earth. Other types are detected indirectly. A spectroscopic binary is revealed by the periodic shifting of spectral lines caused by the stars’ orbital motion. An eclipsing binary is detected when one star passes in front of the other, causing a measurable dip in the system’s overall brightness.
Why Our Solar System Contains Only One Star
The formation of our Sun began about 4.6 billion years ago with the gravitational collapse of a dense region within a giant cloud of gas and dust, known as the solar nebula. As the cloud contracted, most of the mass accumulated at the center, forming the Sun, while the remaining material flattened into a rotating protoplanetary disc. This process resulted in a single, dominant central star that accounts for over 99% of the system’s mass.
The Sun’s solitary nature may be a consequence of its birth environment, as stars often form in clusters. A prevailing theory suggests the Sun may have initially had a low-mass companion star. This companion would have been gravitationally ejected very early in the system’s history due to interactions with other passing stars in the dense stellar nursery.
This theoretical early loss explains why the Sun is single today and offers a possible mechanism for the distribution of distant, icy bodies, like the Oort Cloud. The high frequency of binary systems suggests that the Sun’s current status as a single star may have been achieved through a dynamic early history.
Exoplanets Orbiting Two Stars
Planets discovered orbiting a pair of stars offer a direct answer to the popular question of what it would be like to see two suns in the sky. These exoplanets must contend with complex gravitational forces, which restrict the stability of their orbits to two main types.
Types of Orbits
The first is the S-type orbit, or “satellite-type,” where a planet orbits closely around only one of the two stars, with the second star being a distant gravitational influence. The second configuration is the P-type orbit, or “circumbinary” orbit, where the planet orbits around both stars, much like the fictional planet Tatooine.
The Kepler Space Telescope has confirmed several circumbinary worlds, including Kepler-16b, the first unambiguous example discovered. This Saturn-mass planet orbits its two stars every 229 days. Planets in P-type orbits experience a constantly fluctuating energy input, leading to complex and dynamic climate cycles. Other examples, like Kepler-34b and Kepler-35b, demonstrate that planets can successfully maintain stable orbits around a dual-star system.