The host planet’s orbit must be nearly circular to avoid extremes of temperature throughout the year. If the star has more than one planet, the orbits must nest neatly to prevent collisions. All the orbits must be nearly circular. Many extrasolar systems have one large planet in a highly elliptical orbit. Imagine Jupiter in a highly elliptical orbit that overlaps the Earth’s orbit. It would run around in our solar system like a bull in a china shop. The Earth would sooner or later suffer a fatal collision that would wipe out all known life.