Astronotes Armagh Planetarium's Stellar Blog! ... Sun . ... There could be an eighth planet to discover! ... The orbits of the planets could be predicted perfectly- apart from Mercuryт??s. Planetary orbits are not circles, but ellipses and Mercuryт??s orbit was the least circular (or most т??eccentricт??) known. ... Vulcan, according to Le Verrier, orbited the Sun every 19.7 days at an average distance of 21 million km (0.14 au), a little over a third of Mercury?s average distance from the Sun. ...