Planet Discovered with a Massive Core

helioquake writes "A collaboration of astronomers discovers possible a 'Rossetta Stone' of planetary formation study, reported by San Francisco State Univerity and Subaru Observatory. This new planet, orbiting around G-star like our Sun (HD 149026), weighs roughly equal to that of Saturn, while its size is significantly smaller in diameter. Planetary modeling suggests that the core of the planet alone must have 70 times more mass than Earth, indicating the possible existence of a metallic solid core inside the planet. Just like the rocky planet discovered earlier, the finding of this dense-core planet may lead to better understading of the formation of rockey planets in the Universe."

Comments

Re:weight

[rsynnott]

Its influence on the star's wobble, AFAIR.

Re:weight

[Foolomon]

Undoubtedly they measure it by the effect it has on its surroundings. Mass equates to gravitational pull, which can manifest itself in the curvature of light as it passes by it.

Re:weight

[InternationalCow]

It's not too difficult, conceptually. The star's mass is a function of its brightness. So, you already know the mass of the star. The orbiting planet causes the star to wobble a bit. The more massive the planet, the more the star wobbles. Weight is not the same as mass, by the way. Weight is what you get when you place a mass in a gravitational field. More info on this: http://ethel.as.arizona.edu/~collins/astro/subject s/srchplanet5.html

Correction

[benhocking]

Actually, we do.

Re:weight

[TripMaster Monkey]

1. They have the gravitational wobble effect on the star, which gives them mass.
   
2. They have the silhouette of the planet as it transited the star, which gives them size.
   
3. Assuming a nearly spherical body, they can calculate volume.
   
4. Mass/Volume=Density
   
5. Given the planet's density, together with our prior knowledge of how elements tend to shake out as a planet cools (heaver ones sink, lighter ones float), the scientists can determine with a fair degree of certainty the size of the core.

Re:weight

[helioquake]

Let me add to that. We know that a G0 star has roughly the same mass as that of our Sun (*). Once you have some handle on its mass, you can do the following:

(1) examine the wobble pattern of the main star,
(2) then examine the effect of occultation (eclipse) by the planet (i.e., when the planet goes in front of the star, the brightness of the star decreases...which gives you a sense on how big this planet is with respect to the star's apparent disc),
(3) then use Kepler's third law to derive the size of its orbit,

Now you have two unique information: the orbital radius and apparent size of the planet. Unlike the earlier finding of the rocky planet, this study can provide you a quantitative estimate on how physically big this planet must be. And that turns out to be quite smallar than Saturn. You can also derive the mass of the planet from the scale of the wobble in the main star. Combining that with the physical size of the planet, you can derive the density of the planet.

(*) Kepler's law goes like this:

(2*pi/Period)^2 * (size)^3 = G * Mass

where G = gravitational constant.

If you plug in the Period (==2.87days) and size (0.046AU...circular logic, I know) of the planet, then you'd get the total mass of the star system to be about twice the mass of the Sun, roughly what we expect to be for a G0 main sequence star.