Vulcan

This article is not about the planet from which Mr. Spock comes.

Hypothetical planet, Vulcan, from an 1846 lithograph.

It's not rocket science, it's... Astronomy

The Final Frontier

Gliese 581c Star Planet Extraterrestrial Heliocentrism Vatican Observatory Andromeda Galaxy Earth Comet Elenin Water Carl Sagan Buzz Aldrin

The abyss stares back

Carl Sagan Ptolemy Edwin Hubble Jason Lisle Fred Hoyle v - t - e

Vulcan is a hypothetical planet that was thought to orbit the Sun in an orbit inner to Mercury before the 20^th century, named after the Roman god of the same name equivalent to the Greek Hephaestus.

Theory and observations

Claims about objects seen transiting in front of the Sun have existed since the invention of the telescope, back in the 17^th century and reaching a maximum in the 19^th 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 or were impossible to replicate, 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-.

Enter Einstein

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 almost 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.

Today

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 as an alternative to the existence of such planet having also failed to materialize, besides the occasional comet or asteroid.

Many other star systems contain planets (sometimes very comfortably) within a radius equivalent to Mercury's orbit. Whether this is simply an observational bias (as such kind of planetary systems are easier to detect with our current technology), or the lack of them in our Solar System is actually a feature and not a bug is a mystery.

The name "Vulcan" being still today reserved for a planet that turned out not to exist is the reason why he is the only Olympian not to have an object in the solar system named after him.