Advances in astronomical technology have revealed that our Solar System, with its eight major planets, is not unique in hosting a large family of worlds. Modern astronomy now focuses on identifying planetary systems orbiting other stars, known as exoplanets, and determining which stellar family has the greatest number of confirmed members. The answer to which solar system has the most planets currently lies in a tight contest between our own sun and a distant, compact star system discovered by the Kepler space telescope.
The Current Record Holder for Planet Count
The star system Kepler-90 hosts eight confirmed worlds orbiting an F-type star located about 2,545 light-years away in the constellation Draco. This star is slightly larger and hotter than our Sun, but its planetary arrangement is much more compressed. All eight known planets in the Kepler-90 system orbit their star at distances closer than Earth orbits the Sun, making the entire system remarkably compact.
The innermost six planets are categorized as super-Earths or mini-Neptunes, a type of world common in exoplanet surveys but not found in our Solar System. The two outermost planets, Kepler-90g and Kepler-90h, are gas giants, mirroring the general arrangement of small worlds near the star and large worlds farther out. Kepler-90h, the outermost planet, orbits its star at roughly the same distance as Earth orbits the Sun, completing a revolution in about 331 days.
The discovery of the eighth planet, Kepler-90i, tied the system with our own for planet count. Researchers used an artificial neural network, a form of machine learning, to re-analyze archived data from the Kepler telescope. This AI was trained to identify faint transit signals that human analysis had previously missed. The AI successfully located the weak signal of Kepler-90i, a rocky world roughly 30 percent larger than Earth, completing an orbit every 14.4 days.
Defining Planetary Systems and Counting Criteria
The official count of planets in any system is determined by a process that distinguishes true worlds from false alarms. An initial observation suggesting a planet is called a candidate, such as a Kepler Object of Interest (KOI) or a TESS Object of Interest (TOI). These candidates are identified using the transit method, where scientists look for a periodic dip in a star’s brightness as a planet passes in front of it.
However, these light dips can be caused by other celestial events, such as a background eclipsing binary star system, which mimics a planetary transit. To move from candidate status to a confirmed exoplanet, a world must be verified through additional evidence that rules out false positives. Confirmation often uses the radial velocity method, which measures the subtle gravitational wobble of the host star caused by the planet’s mass.
For smaller or more distant planets too faint for radial velocity measurements, scientists rely on statistical validation or confirmation by multiplicity. This involves calculating the probability that the transit signal is a false positive; if that probability is extremely low, the world is accepted as confirmed. The record count constantly shifts as candidates are confirmed, a process that can take years, especially for planets with longer orbital periods.
Notable Exoplanet Systems with High Counts
While Kepler-90 holds the record, other multi-planet systems are notable due to the nature of their worlds and host stars. One of the most famous is the TRAPPIST-1 system, which orbits an ultra-cool red dwarf star about 40 light-years away. This system contains seven planets, all roughly Earth-sized and rocky, the highest number of terrestrial worlds found in a single system to date.
The TRAPPIST-1 system is incredibly compact; all seven planets orbit closer to their star than Mercury orbits the Sun, and three are situated within the star’s habitable zone. Their close proximity causes them to be locked in an orbital resonance, where the orbital periods of neighboring planets form simple, repeating ratios. This gravitational interaction helps stabilize the system despite its crowded nature.
The Transiting Exoplanet Survey Satellite (TESS) has contributed greatly to the total number of known planetary systems. TESS focuses on nearby and brighter stars, which are better targets for detailed follow-up observations. The mission has detected thousands of planet candidates and confirmed hundreds of exoplanets. These multi-planet systems are used to understand how planetary architectures form and evolve.