|It's not rocket science, it's...|
|The Final Frontier|
|The abyss stares back|
Theory and observations
Claims about objects seen transiting in front of the Sun have existed since the invention of the telescope, back in the 17th century and reaching a maximum in the 19th century, even if most if not all of them are more than likely to have been just reports of the observations of sunspots if real at all.
Theories about an innermost planet to Mercury began circulating in such century and were followed by observations that hoped to catch such planet when it passed in front of the Sun, given that seeing it after sunset or before dawn would be very difficult or outright impossible getting lost in the Sun's glare. All of them, of course, came empty, suggesting that if such planet existed at all it would be very small and would orbit close to the Sun, as otherwise it would have been detected before -either transiting in front of the Sun or during a solar eclipse, as such closeness to the Sun means a lot of solar irradiation thus that it would have been quite bright, especially in the case of a large planet-.
Anomalies in Mercury's orbit that models of the latter failed to account for, causing astronomers to think the gravity of planet(s) orbiting closer to the Sun were responsible, were a mystery until 1915, when Einstein's Theory of Relativity showed they were caused by space-time being warped by the Sun, matching exactly the observations done in the previous century, with such prediction being confirmed during the solar eclipse of May 29, 1919 catapulting Einstein to fame and Vulcan's existence being no longer needed.
Earth-based searches have been supplemented by observations from space using telescopes such as SOHO, to spacecraft
that have boldly gone where no spacecraft has gone before positioned within Mercury's orbit, such as NASA's Parker Solar Probe, that have not only failed to find such a planet, but also revealed that the space within Mercury's orbit is devoid of bodies of significant size, with a proposed population of asteroids that would be there having also failed to materialize, besides the occasional comet or asteroid.
It should also be noted that many other planetary systems contain planets (sometimes a lot) within a radius equivalent to Mercury's orbit. Whether this is simply an observational bias, as such systems are easier to detect with our present technology, or the lack of them in our Solar System is a feature
and not a bug is a matter of research.
- Drobyshevskii, E. M. (1992). "Impact Avalanche Ejection of Silicates from Mercury and the Evolution of the Mercury / Venus System". Soviet Astr 36 (4): 436–443. Bibcode 1992SvA....36..436D.
- Blake, William (1796). "The Monthly magazine. v.45 (1818)" (in en). Monthly Magazine and Critical Register of Books. Retrieved 2017-07-04.
- Elger, T.G.E. (May 4, 1869). "The Supposed New Planet Vulcan". Astronomical Register 7: 164. Bibcode 1869AReg....7..164E.
- ,Baum, Richard P.; Sheehan, William (August 2003). In Search of Planet Vulcan: The Ghost in Newton's Clockwork. New York: Basic Books. ISBN 0738208892.
- Kurt Larson, Jan 2, 1860: The Discovery of Planet Vulcan, oddsalon.com
- David Baron (2017). American Eclipse. Liveright. ISBN 9781631490163.