Illustration of the planet passing in front of a star HD149026 [(c) Lynette Cook] According to high surface temperature, the atmosphere of the planet may be evaporating to make a tail.(Larger Image)

A collaboration of astronomers from Japan, USA, and Chile have discovered an unusual extra-solar planet whose properties favors a formation process of gaseous planets to start from a solid core and accumulate gaseous material. However its excessive weight created another mystery to solve. The Subaru and Keck telescopes on Mauna Kea, Hawaii, contributed to this discovery.

While more than 150 extrasolar planets have been found by observing wobbles in the stars they orbit, it is difficult to glean direct information about the structure of the planet. "In extremely rare cases, like this one, the planet passes in front of its star and dims the starlight, an event called a transit," said Debra Fischer, at San Francisco State University and team leader for the consortium. "When that happens, we can calculate the physical size of the planet, whether it has a solid core, and even what the planet atmosphere is like. We surely didn't expect anything like this."

The planet, orbiting the sun-like star HD 149026, takes just 2.87 days to closely circle around its star. The planet has about the same mass as Saturn, but a significantly smaller diameter. Modeling of the planet structure by Peter Bodenheimer at UC Santa Cruz shows that the planet has a core that is 70 times heavier than the mass of the Earth. "For theorists, the discovery of a planet with such a large core is as important as the discovery of the first extrasolar planet around the star 51 Pegasi," said Shigeru Ida at Tokyo Institute of Technology.

Interior models of Jupiter and HD 149026 b [(c) Greg Laughlin] Dark grey regions express solid cores. White and medium grey regions are hydrogen/helium in gas and metallic phases, respectively.

There are two competing theories for giant planet formation: planets form from fragmentation of a contracting dense cloud, or planets start as small rock-ice cores and grow as they gravitationally acquire additional mass. The large core of this planet couldn't have formed by the first model. "This is the first “solid” evidence to confirm core accretion as a mechanism of planetary formation. It also implies that terrestrial-type planets should exist in abundance in other planetary systems," said Greg Henry, at Tennessee State University, who detected the dimming of the star by the planet with his robotic telescopes at Fairborn Observatory in Arizona.

"This discovery distinguishes which formation path planets take," said Greg Laughlin at UC Santa Cruz. "The N2K team members bounced some pretty wild hypotheses back and forth across the Pacific until we hit on a couple of formation mechanisms that might just work." In one of the most dramatic, the planet formed from the collision and merger of two conventional protoplanets, each about 35 times the mass of the Earth.

"All of us expected a much larger planet, similar to Jupiter, with a much smaller core. None of the models predicted that Nature could make a planet like this," said Bun'ei Sato at the National Astronomical Observatory of Japan. "We expect many more breakthrough discoveries from this collaboration of the Subaru, Keck, and Magellan telescopes, which would give important clues for the questions: how planetary systems form, what varieties planetary systems come in, and whether our Solar System is unique or common."

Illustration of close-up view of HD 149026 b when viewed in a crescent phase [(c) Greg Laughlin & James Cho] Even the "dark" side of the planet would have significant optical emission, because its surface temperature may be as high as 1200 C. The flow patterns on the darkside suggest a turbulent atmosphere with varying cloudiness and were sampled from simulations by Dr. James Cho (Carnegie Institute of Washington).

Details of the discovery will be published in the Astrophysical Journal.

• HD149026 is a solar-type star with spectral type G0IV, 1.3 times solar mass, visual magnitude 8.15. The planet has 0.36 times Jupiter mass and 0.72 times Jupiter size. The orbit is a circular one with an orbital radius of 0.046AU and inclination of 85 degrees from line of sight.

• The paper: title: "The N2K Consorsium. II. A Transiting Hot Saturn Around HD149026 With a Large Dense Core",
  Authors: Sato, B., D. Fischer, G. Henry, G. Laughlin, P., ..., Shigeru Ida, ..., Eri Toyota, ... (total 21 astronomers)
  Journal: Astrophysical Journal