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Planets Formed Close to Their Stars Are Named for Vulcan, the Roman God of Fire

Closer to their stars than Mercury is to the Sun, these hot worlds deserve an explanation

An artist’s interpretation of an alien planet and its star (NASA/X00653/Reuters/Corbis)
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NASA’s planet-hunting Kepler space telescope has brought us many unimaginably fascinating alien worlds: A mega-Earth so huge it shouldn’t be rocky (but apparently is), diamond planets and waterworlds, plus Earth-like exoplanets and super-Earths bigger than Earth but smaller than Neptune—more of them that might be capable of supporting life than we every believed possible. It's inspiring.

How some of those Earth-like planets and super-Earths form, though, is a mystery. Researchers call these Vulcan planets, and they are almost 100 times closer to their stars than we are to the Sun.

The name doesn’t come from the Star Trek universe but from the Roman god Vulcan, who is associated with fire, metal-working, forges and volcanoes. And it has a bit of history behind it: a 19th century mathematician, Urbain Le Verrier, proposed that a small planet inside Mercury’s orbit could explain percularities in Mercury's transit—it didn't move around the sun exactly in the manner predicted based on Newton’s laws. Le Verrier named the proposed object Vulcan, given its proximity to the Sun. An amateur astronomer even thought he saw Vulcan transit. (He didn’t: Einstein later explained Mercury’s unexpected movements.)  

These newly discovered Vulcan planets, which are discussed in a recent Astrophysical Journal Letters paper, are a class, not a single planet. The lead scientist of the Kepler mission, Natalie Batalha, has referred specifically to at least one of them, Kepler-10b, as Vulcan, though. Orbiting 23 times closer to its star than Mercury is to the sun, this planet carries surface temperatures so extreme that iron would melt. "An entire hemisphere is an ocean of lava, not of water," she told PBS.

The problem is that these Vulcan planets couldn’t have formed the normal way. Typically, researchers think that planets are born out of the disk of debris circling young stars. But Vulcan planets require a lot of material very close to the star—thick, massive disks that just don’t match up with the models.

Researchers Sourav Chatterjee, of Northwestern University in Illinois, and Jonathan C. Tan, of the University of Florida, Gainesville, proposed that, instead, Vulcan planets come from "Inside-Out Planet Formation." Their theory is that the planets did form in the scorching close orbits they occupy now, but that a stream of pebbles and small rocks delivered from farther away spiraled in to create them. This theory keeps most of the mass farther out in the planetary disk, as predicted, but still accounts for the existence of Vulcan planets. 

Thinking creatively about planet formation is probably necessary, a press statement explains. It's the only way we’ll be able to understand the "diversity of worlds that are now being discovered by planet hunters."

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