How space works
Orbits, gravity, and orbital resonance
An orbit is continuous falling with enough sideways motion to keep missing the object below. Its size, shape, and timing reveal how a planetary system works.
Updated
An orbit is not a fixed track
Gravity continually changes a moving body's velocity. In an ideal two-body system, a bound orbit is an ellipse, with the center of mass at one focus. A circle is a special case. Actual planets also feel the gravity of other planets, moons, and passing bodies, so their paths change gradually.
The orbital period measures how long one circuit takes. The semimajor axis measures the orbit's size. Eccentricity measures its departure from circularity: zero is circular, while values between zero and one describe increasingly elongated bound ellipses. Inclination needs a reference plane; an exoplanet inclination reported for transit work is not automatically comparable to a Solar System inclination measured relative to the ecliptic.
Why distant planets usually have longer years
For a small planet orbiting a much heavier star, the square of the orbital period is proportional to the cube of the semimajor axis and inversely proportional to the star's mass. In units of Earth years, AU, and solar masses, the approximate relation is P² = a³/M. A planet at 4 AU around a Sun-mass star therefore takes about eight years to orbit.
That example assumes a two-body approximation and a planet much less massive than its star. Moving the same orbit to a lower-mass star lengthens the period. A short year alone does not give the orbit's physical size without information about the host. In the Codex, the reported period and semimajor axis retain their own uncertainties rather than being silently replaced by a calculation.
Resonance means repeated timing
Two objects are near a mean-motion resonance when their orbital periods are close to a simple ratio. Jupiter's moons Io, Europa, and Ganymede provide a familiar chain. Repeated gravitational tugs can maintain eccentricity, organize orbital spacing, or help produce unstable regions. The outcome depends on the geometry and the system's history.
A period ratio near two is a useful clue, but it does not by itself prove that two planets are in resonance. Astronomers study how combinations of orbital angles behave over time. Calling every nearly tidy ratio a resonance skips that dynamical test. The TRAPPIST-1 system is valuable because repeated transits allow unusually detailed timing comparisons.
Orbit changes can reveal unseen mass
A planet can shift the time of another planet's transit. Those timing variations are measured against a model of regularly repeated events. Fitting the variations can constrain masses and interactions. Radial-velocity observations instead follow the host star's motion along our line of sight. Both techniques use gravity, but their data and uncertainties differ.
Even an orbit that looks circular on a screen may have an uncertain eccentricity. A tabulated zero can be a parameter fixed during fitting. Read the source paper before interpreting it as a precise measurement of perfect circularity. The explorer's circular visual paths make navigation simpler and do not replace the published orbital solution.