A newly discovered planet 4,000 light-years away is just too dense.

Dubbed CoRoT-20b, the planet is thought to be a gas giant about four-fifths the size of Jupiter and orbits close to a sunlike star.

Despite the new planet's relatively diminutive size, this world has four times Jupiter's mass, making CoRoT-20b one of the densest known planets, a new study says. (Related: "'Backward' Planet Has Density of Foam Coffee Cups.")

That poses a problem for astronomers: If CoRoT-20b is structured like a traditional gas giant, with a solid core surrounded by a gassy atmosphere, the planet's core would have to make up 50 to 77 percent of the world's total mass.

By contrast, Jupiter's core is thought to represent just 15 percent of that planet's mass.

To have such a robust core, CoRoT-20b would defy current theories for how planets form. (Also see "New Model of Jupiter's Core Ignites Planet Birth Debate.")

Astronomers think planets are born from disks of debris that surround newborn stars. In our solar system, the sun's so-called protoplanetary disk gave rise to several worlds and still had leftovers from planet formation—what we call asteroids and comets.

However, the new study says that CoRoT-20b would have had to have sucked up every last atom of material heavier than helium from its star's protoplanetary disk to form the planet's massive core.

"That is something difficult to understand, and to admit," said study leader Magali Deleuil, of the Laboratory of Astrophysics of Marseilles in France.

Another possibility, Deleuil said, is that the planet's heavy elements are mixed throughout its atmosphere rather than embedded in a central core—but this would mean the world represents a completely new class of planet.

Dense World Has Stellar Synchronicity?

The ultradense planet was discovered last year by the European CoRoT mission, a space telescope that searches for the telltale dimming when a planet passes in front of its host star, as seen from Earth.

The team followed up with measurements from the HARPS instrument in Chile, which watches for the gravitational "wobble" of a star due to the tug of an orbiting planet.

The combined measurements allowed the astronomers to confirm the planet's size and mass as well as its orbital path—a highly elliptical one that brings the world a mere 8,366,029 miles (13,463,820 kilometers) from its host star at its farthest point.

The team considered whether CoRoT-20b could have been much bigger originally, and perhaps its lighter elements had been stripped away by its parent star.

But CoRoT-20b is in a fairly stable orbit that doesn't pass within the host star's Roche limit, the boundary beyond which a planet would be subjected to stellar stripping.

"If the planet has lost part of its globe, it appears smaller than you would expect because of the loss. But that happens if your planet is entering the Roche limit, and that's not the case for this planet."

In fact, Deleuil said, the planet is approaching what's known as Darwin stability—named not for the famed naturalist but for his son George, an astronomer.

Darwin stability is when a planet and star have settled into a triple-synchronized state: The star's spin about its axis is equal to the planet's spin, which in turn is equal to the planet's orbital speed.

Further planetary wobble measurements could reveal whether the CoRoT-20 system has more than one planet, which might offer new clues to the mystery.

But for now, the high density of CoRoT-20b remains a puzzle.