[LINK] Two links, two odd planets

[rfmcdonald]

In Andrew Fazekas' National Geographic "New Planet Found; Star's Fourth World Stumps Astronomers", the fourth planet of the very young and pretty bright young star HR 8799 turns out to be a very odd one indeed. How could it have formed where it did?

[T]he fourth world challenges current theories of planet formation, according to the study authors.

"This is the first multiplanet system directly imaged so far, so it's quite a feat," said lead study author Christian Marois, an astronomer at the Herzberg Institute of Astrophysics in Canada.

"But we are now stuck with four planets [and] we cannot explain their formation and their current locations by any of our models."

The previously known planets around HR 8799 are about five, seven, and ten times Jupiter's mass. They orbit between 2.2 billion miles (3.5 billion kilometers) from the star—roughly the same as Neptune's distance from the sun—and 6.3 billion miles (10.1 billion kilometers), or almost twice Pluto's distance.

[. . .]

Images from Keck II show that the newfound planet is also a gas giant, about seven times Jupiter's mass. But it orbits closer to the star, at a mere 1.4 billion miles (2.2 billion kilometers), equivalent to between the orbits of Saturn and Uranus.

[. . .]

It's thought planets in general form out of the disks of gas and debris that surround many young stars. One model for how gas giants form is called gravitational instability, in which a perturbation in the disk causes a clump of material to suddenly coalesce into a gassy planet.

The other model is core accretion, in which material first clumps into a rocky core, and the core then gravitationally gathers nearby gases. (See "New Model of Jupiter's Core Ignites Planet Birth Debate.")

According to Marois's team, the problem with such widespread gas giants is that they all couldn't have formed the same way. Too far from the star and there's not enough gas for core accretion to work, but too close and it's too hot for debris to become gravitationally unstable.

"The system is either just too young and the planets are just too far away and don't have time to form before the gas in the disk is depleted, or they are too close and the disk is too warm to form planets," Marois said.

Gas giant planets have been found in orbits close to their parent stars, sometimes even closer than Mercury is to the sun. In these cases, many experts believe the so-called hot Jupiters may have formed farther away from their stars and then migrated closer over time.

Meanwhile, the CBC reports in "Carbon-rich planet may have diamond core" that the first carbon planet--a planet developed in a solar nebula carbon-rich and oxygen-poor--may have been found in the extrasolar planet WASP-12b.

Reporting in the journal Nature, Nikku Madhusudhan and colleagues from Princeton University say the planet WASP-12B has an atmosphere with equal parts carbon and oxygen.

Madhusudhan said that's double the ratio of carbon to oxygen seen in planets in our solar system, and has implications for the planet's internal structure.

"The high carbon-to-oxygen ratio indicates a carbide or diamond interior rather than the silicate geology of the Earth," he said.

Madhusudhan said the planet's atmosphere is abundant in carbon monoxide, and has considerably more methane and less water than would be expected for a planet of this temperature in our solar system.

"If WASP-12B's host star has a carbon-to-oxygen ratio similar to the sun, then it's difficult to explain the ratio in the planet's atmosphere," he said.

Because the planet passes in front of and behind its star as viewed from Earth, astronomers can use multi-wavelength spectroscopy to directly view the planet's atmospheric chemical composition and thermal structure.

Madhusudhan said these observations have revealed the absence of a strong thermal inversion — the difference between night and day temperatures.

"The ability to also see the planet's daytime side just before it passes behind its host star as seen from Earth allows us to take accurate atmospheric temperature readings at different locations," he said.

According to Madhusudhan, WASP-12B, which is 871 light years from Earth, orbits 40 times closer to its host star than our planet does to the sun.

Being so close, the star's tidal forces are distorting the planet into an egg shape, which generates tidal heating.

The star is also slightly hotter than the sun, giving the planet a surface temperature of more than 2,200 C.