Core finding: latest, oddest planet hints at how orbs form.

The 160 extrasolar planets discovered over the past decade constitute an odd menagerie. They include giant, Jupiter-like bodies with temperatures hot enough to melt metal, planets in orbits nearly as elongated as the paths of comets, and at least one distant cousin of Earth. But the planet announced last week seems the most bizarre of all. It displays the heaviest core of any planet yet detected.

With an orbit whose radius is only one-tenth that of Mercury's path around the sun, the planet has a searing surface temperature of 1,500 kelvins, and it whips around the sunlike star HD 149026 in just 2.88 days. It's likely to provide crucial new understanding of how planets form, says codiscoverer Bun'ei Sato of the Okayama Astrophysical Observatory in Japan.

"None of the models predicted that nature could make a planet like the one we are studying," says Sato. His team announced the discovery June 30, and details will appear in an upcoming Astrophysical Journal.

The scientists have studied the planet in two ways. First, Sato's team observed wobbles of the star HD 149026. Astronomers have detected nearly all other extrasolar planets by measuring such variations in velocity, which are caused by the tug of an orbiting object.

A second set of observations floored astronomers. An unusual alignment of the planet, its star, and Earth enabled the team to measure the starlight that the planet blocked each time that it passed between its star and Earth. The researchers found that although the planet is 20 percent more massive than Saturn, it blocks only about half the light that the giant planet would.

Coauthor Gregory W. Henry of Tennessee State University in Nashville calculated that the newfound orb's diameter is only 84 percent that of Saturn. That means that the planet packs into its core the equivalent of 70 Earths, or two-thirds of its total mass. In contrast, scientists estimate that Saturn has a rocky core of 15 to 20 Earth masses, about one-fifth of its total mass.

It's unclear whether the core of the newfound planet is liquid or solid. But in either ease, the planet's massive core appears to favor one of the two leading models of planet formation (SN: 3/26/05, p. 203). In this model, known as core accretion, a planet starts out as a small, rock-ice core. It then gravitationally captures large amounts of gas that surround the core. In the competing model, called gravitational instability, a planet forms wholesale when a cloud of gas, ice, and dust breaks into clumps.

Gravitational instability isn't likely to produce a dense core, comments Didier Saumon of the Los Alamos (N.M.) National Laboratory. However, be notes, core accretion might also have a hard time making a core as massive as that of the newfound planet. He speculates that two smaller planetary cores might have smacked together to form the planet.

"This is an exciting discovery and an exciting time for studying the formation of planetary systems," says Saumon.