火神星，除了是《星际迷航》中虚构的星球，也是19世纪许多天文学家认为可能存在的一颗假想行星。 这颗行星应该质量不大，比水星更靠近太阳，近到当时的望远镜由于太阳本身的亮度过大而无法分辨。 火神星如果存在，其表面温度可能比水星的温度还要高，水星的峰值为700度（华氏801度），也许超过一千度。

The reason astronomers inferred that Planet Vulcan was likely to exist was due to discrepancies in the orbit of Mercury, detected by the French mathematician Urbain Jean Joseph Le Verrier in 1840-1843 when he was trying to predict the planet's motion based on Newton's theories. The procession of its perehelion (the point at which it is closest to the Sun) around its orbit was about 43 arcseconds per century off from what Newton's theories would predict. Considering that every other planet in the Solar System moved in ways precisely predicted by Newton's theories, this was puzzling, and Planet Vulcan was invoked as the cause of the discrepancy. In 1846, Le Verrier discovered the planet of Neptune based on the same principle, perturbations in the orbit of Uranus. This galvanized astronomers to look for the planet Le Verrier predicted.

从1859年开始，天文学家开始报告有小黑点在太阳表面过境，这些小黑点被认为是火神星。 在接下来的六年里，大约有十几次据称是对黑点过境太阳的观测，但根据过去的观测结果，这些过境总是不能重新出现。 1866年，对火神星的观测开始减少，但先前的观测被认为是该行星存在的 "证据"，因为Le Verrier在1860年宣布了他的发现。 1867年，两位可靠的天文学家声称在一次日食中看到一颗类似火神的行星靠近太阳，但事后看来这可能是一颗星星。 1877年Le Verrier去世，仍然相信他已经发现两颗新行星。

In 1915, it became clear that there never was a Planet Vulcan. Einstein's new theory of relativity precisely explained the anomaly as a byproduct of the Sun's gravitational field. The new numbers his theory predicted matched observations exactly. In more modern times, astronomers have intensely searched the region around the Sun for any rocky bodies, such as Vulcanoid asteroids, which could orbit in a stable gravitational region right next to the Sun. However, observations have ruled out anything larger than about 60 km (37 mi) in width. Today, the search for Vulcanoid objects continues. Many scientists are skeptical, arguing that the Yarkovsky effect, which alters orbits based on the emission of high-energy photons on an asteroid right next to a star, would cause any Vulcanoid asteroid orbits to become unstable, either sinking into the Sun or impacting Mercury.