Solar System vs. Stellar Systems: A Comparison of Planetary Systems

Comparison of planetary systems

The solitary Solar System as a reference

Our Solar System is often used as a model for studying planetary systems around distant stars. The general structure includes a set of rocky planets close to the Sun and gas giant planets further away. The orbits are mostly circular and coplanar, and orbital distances approximately follow the empirical Titius-Bode law. The total mass of the planets represents less than 0.2% of the solar mass.

Diversity of stellar systems

Stellar systems containing exoplanets show a wide variety of orbital configurations and compositions. Some have gas giant planets very close to their star, called hot Jupiters, which are absent in our Solar System. Orbits can be highly eccentric, and the number of detected planets varies from two to more than a dozen depending on the system.

Major physical differences

• The Solar System contains 8 main planets with nearly circular orbits and orbital periods ranging from 88 days (Mercury) to 165 years (Neptune).
• Stellar systems can have highly eccentric orbits, sometimes inclined relative to the star's equatorial plane.
• The total mass of the planets varies greatly; some giant exoplanets exceed several hundred Earth masses.
• Some systems contain hot Jupiters, causing significant tidal effects and orbital migrations.

Exoplanets around red dwarf stars

Some recent discoveries highlight multi-planetary systems around red dwarf stars. In these systems, Earth-sized planets orbit very close to their star, increasing the probability of detectable transits by high-precision photometry. Spectroscopic analysis of their atmospheres allows direct assessment of potential habitability and the possible presence of liquid water.

Binary and multiple stellar systems

Prevalence in the Milky Way

Astronomical observations show that nearly 50 to 60% of solar-type stars in our Galaxy belong to binary or multiple systems. These systems contain two or more gravitationally bound stars orbiting around a common center of mass. Their frequency is particularly high for massive stars (>1.5 M☉), where more than 70% belong to multiple systems.

Types of binary systems

• Visual systems: both stars can be resolved separately with a telescope.
• Spectroscopic systems: stars are too close to be visually separated, but their orbital motion is detectable via Doppler shifts in their spectra.
• Eclipsing systems: the orbit is aligned with the line of sight, causing periodic variations in brightness when one star passes in front of the other.

Consequences for planetary systems

The presence of a second star strongly influences the formation and stability of planets. In close binary systems, only orbits very close to one or the other star are stable (<1 AU). In wide binary systems (>50 AU), planets can exist around each star or in circumbinary orbits, orbiting around both stars.

Known examples

• Alpha Centauri: a triple system composed of Alpha Centauri A and Alpha Centauri B (close binaries) and Proxima Centauri, a distant red dwarf. Gravitational interactions limit the habitable zone around each star.
• Kepler-16: a binary system with a circumbinary planet, demonstrating that a planet can orbit around two stars simultaneously.
• Kepler-47: a multiple system with two stars and at least three detected circumbinary planets, illustrating the complexity of stable orbits in multiple systems.
• 61 Cygni: a close binary composed of two K dwarfs separated by 84 AU, each theoretically capable of hosting stable planets around it.
• Fomalhaut: a triple system with a main star surrounded by a debris disk and candidate exoplanets, accompanied by two distant companion stars.
• HD 98800: a quadruple system with two tightly linked binaries, known for its protoplanetary debris disk, showing that planetary formation can exist even in very dynamic configurations.
• Castor: a sextuple system consisting of three binaries, illustrating the extreme diversity of multiple systems in our galactic neighborhood.

In summary, although our Solar System is solitary, the majority of stars in the Galaxy evolve in binary or multiple systems.