Question: Is it possible for a planet of an established solar system orbiting a star to be in a decaying orbit and if so how do astronomers confirm this as the star and its solar system can be vast numbers of light years away? — David
Answer: Yes, it is possible to see an exoplanet’s orbit decaying, but this would require measurements of its orbit over a period long enough to measure the decay, or change, in its orbital parameters (i.e. the orbital period). Also, this would be a measurement referenced to the state of this planet’s orbit when the light we receive was emitted by the planet, which might be many light years away.