Vulcan, the planet

Cause of death

Killed by a successor

General relativity explained Mercury's orbit without needing an extra planet to explain it.

Mercury's orbit did not behave the way Newton said it should, so Urbain Le Verrier proposed a planet between Mercury and the Sun to account for the discrepancy. This was a reasonable bet: he had found Neptune the same way. Astronomers reported sightings for decades. Then Einstein published general relativity in 1915, the anomaly dissolved into the mathematics, and a planet that many people had personally observed stopped existing.