Flame Breathers, The by Ray Cummings (1907 - 1957)

Did the planet Vulcan exist? It supposedly orbited our sun on the opposite side from the Earth but it was only theoretical speculation. One expedition had mysteriously disappeared, and now another was following in its path—searching for the truth about this planet's existence. Was there really an unknown menace that stalked Vulcan's shadowed gorges." This was first published in Planet Stories March 1943. (Summary by blurb)