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Title: On the Origin of HD149026b
Authors: M. Ikoma, T. Guillot, H. Genda, T. Takayuki, S. Ida

The high density of the close-in extrasolar planet HD149026b suggests the presence of a huge core in the planet, which challenges planet formation theory. Researchers first derive constraints on the amount of heavy elements and hydrogen/helium present in the planet: They find that preferred values of the core mass are between 50 and 80 earth masses. They then investigate the possibility of subcritical core accretion as envisioned for Uranus and Neptune and find that the subcritical accretion scenario is unlikely in the case of HD149026b for at least two reasons:
(i) Subcritical planets are such that the ratio of their core mass to their total mass is above ~0.7, in contradiction with constraints for all but the most extreme interior models of HD149026b;
(ii) High accretion rates and large isolation mass required for the formation of a subcritical core of 30 Earth masses are possible only at specific orbital distances in a disk with a surface density of dust equal to at least 10 times that of the minimum mass solar nebula. This value climbs to 30 when considering a 50 Earth mass core.
These facts point toward two main routes for the formation of this planet:
(i) Gas accretion that is limited by a slow viscous inflow of gas in an evaporating disk;
(ii) A significant modification of the composition of the planet after as accretion has stopped. These two routes are not mutually exclusive.
Illustrating the second route, the researchers show that for a wide range of impact parameters, giant impacts lead to a loss of the gas component of the planet and thus may lead to planets that are highly enriched in heavy elements. In the giant impact scenario, they expect an outer giant planet to be present. Observational studies by imaging, astrometry and long term interferometry of this system are needed to better narrow down the ensemble of possibilities.

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Title: Transit Photometry of the Core-Dominated Planet HD 149026b

Authors: David Charbonneau, Joshua N. Winn, David W. Latham, Gaspar Bakos,
Emilio E. Falco, Matthew J. Holman, Robert W. Noyes, Balazs Csak, Gilbert A.
Esquerdo, Mark E. Everett, and Francis T. O'Donovan

Researchers report g, V, and r photometric time series of HD 149026 spanning predicted times of transit of the Saturn-mass planetary companion, which was recently discovered by Sato and collaborators.
They present a joint analysis of their observations and the previously reported photometry and radial velocities of the central star.

They have refined the estimate of the transit ephemeris to Tc [HJD] =2453527.87455^{+0.00085}_{-0.00091} + N * 2.87598^{+0.00012}_{-0.00017}.

Assuming that the star has a radius of 1.45 ± 0.10 Solar radius and a mass of 1.30 ± 0.10 solar masses, they estimate the planet radius to be 0.726 ± 0.064 Jupiter Radius, which implies a mean density of 1.07^{+0.42}_{-0.30} g/cm^3.

This density is significantly greater than that predicted for models which include the effects of stellar insolation and for which the planet has only a small core of solid material. Thus they confirm that this planet likely contains a large core, and that the ratio of core mass to total planet mass is more akin to that of Uranus and Neptune than that of either Jupiter or Saturn.

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-- Edited by Blobrana at 17:11, 2005-08-03

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Two possible models of the new planet's interior are compared to interior models of Saturn and Neptune.
One model of the new planet assumes a core of rock only, the other of "ices" only.
(Saturn and Neptune contain some of each.)
In each case, the centre of the planet is at left, and its surface is at right.


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NASA researchers recently discovered the largest solid core ever found in an extrasolar planet, and their discovery confirms a planet formation theory.

"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 in 1995" - Shigeru Ida, theorist from the Tokyo Institute of Technology, Japan.


Position(2000): RA 16 30 29.6192 Dec +38 20 50.315

When a consortium of American, Japanese and Chilean astronomers first looked at this planet, they expected one similar to Jupiter. "None of our models predicted that nature could make a planet like the one we are studying" - Bun'ei Sato, consortium member and postdoctoral fellow at Okayama Astrophysical Observatory, Japan.

Scientists have rarely had opportunities like this to collect such solid evidence about planet formation. More than 150 extrasolar planets have been discovered by observing changes in the speed of a star, as it moves toward and away from Earth. The changes in speed are caused by the gravitational pull of planets.
This planet also passes in front of its star and dims the starlight.

"When that happens, we are able to calculate the physical size of the planet, whether it has a solid core, and even what its atmosphere is like" - Debra Fischer. She is consortium team leader and professor of astronomy at San Francisco State University, California.

The planet, orbiting the sun-like star HD 149026, is roughly equal in mass to Saturn, but it is significantly smaller in diameter. It takes just 2.87 days to circle its star, and the upper atmosphere temperature is approximately 2,000 degrees Fahrenheit. Modelling of the planet's structure shows it has a solid core approximately 70 times Earth's mass.

This is the first observational evidence that proves the "core accretion" theory about how planets are formed. Scientists have two competing but viable theories about planet formation.


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In the "gravitational instability" theory, planets form during a rapid collapse of a dense cloud. With the "core accretion" theory, planets start as small rock-ice cores that grow as they gravitationally acquire additional mass. Scientists believe the large, rocky core of this planet could not have formed by cloud collapse. They think it must have grown a core first, and then acquired gas.

"This is a confirmation of the core accretion theory for planet formation and evidence that planets of this kind should exist in abundance" - Greg Henry, an astronomer at Tennessee State University, Nashville. He detected the dimming of the star by the planet with his robotic telescopes at Fairborn Observatory in Mount Hopkins, Arizona.

HD 149026 is a G0 IV star. The Hipparcos parallax of 12:68 mas places the star at a distance of 78.9 pc with an absolute visual magnitude, MV = 3:66.

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