Category: Extrasolar Planets

[/caption]
If any creature lives on COROT-7b, the recently confirmed rocky exoplanet, they might think the sky is falling. This planet is close enough to its star that its “day-face” is hot enough to melt rock, and according to models by scientists at Washington University in St. Louis, COROT-7b’s atmosphere is made up of the ingredients of rocks and when “a front moves in,” pebbles condense out of the air and rain into lakes of molten lava below. Yikes!

This unusual rocky world was the first planet found orbiting the star COROT-7, an orange dwarf in the constellation Monoceros, or the Unicorn. COROT-7b is less than twice the size of Earth and only five times its mass. But this place is nothing like Earth.

“The only atmosphere this object has is produced from vapor arising from hot molten silicates in a lava lake or lava ocean,” said Bruce Fegley Jr., Ph.D., professor at Wash U, who created models of COROT-7b along with research assistant Laura Schaefer. Their paper appears in the Oct. 1 issue of The Astrophysical Journal.

This star-facing side has a temperature of about 2600 degrees Kelvin (4220 degrees Fahrenheit). That’s infernally hot—hot enough to vaporize rocks. The global average temperature of Earth’s surface, in contrast, is only about 288 degrees Kelvin (59 degrees Fahrenheit).

The side in perpetual shadow, on the other hand, is positively chilly at 50 degrees Kelvin (-369 degrees Fahrenheit).

So, what might the planet’s atmosphere be like? To find out Schaefer and Fegley used thermochemical equilibrium calculations with a special computer program called MAGMA that was used to study high-temperature volcanism on Io, Jupiter’s innermost Galilean satellite.

Because the scientists didn’t know the exact composition of the planet, they ran the program with four different starting compositions. “We got essentially the same result in all four cases,” says Fegley.
Perhaps because they were cooked off, COROT-7b’s atmosphere has none of the volatile elements or compounds that make up Earth’s atmosphere, such as water, nitrogen and carbon dioxide.

“Sodium, potassium, silicon monoxide and then oxygen — either atomic or molecular oxygen — make up most of the atmosphere.” But there are also smaller amounts of the other elements found in silicate rock, such as magnesium, aluminum, calcium and iron.

Why is there oxygen on a dead planet, when it didn’t show up in Earth’s atmosphere until 2.4 billion years ago, when plants started to produce it?

“Oxygen is the most abundant element in rock,” says Fegley, “so when you vaporize rock what you end up doing is producing a lot of oxygen.”

The peculiar atmosphere has its own singular weather. “As you go higher the atmosphere gets cooler and eventually you get saturated with different types of ‘rock’ the way you get saturated with water in the atmosphere of Earth,” explains Fegley. “But instead of a water cloud forming and then raining water droplets, you get a ‘rock cloud’ forming and it starts raining out little pebbles of different types of rock.”

Even more strangely, the kind of rock condensing out of the cloud depends on the altitude. The atmosphere works the same way as fractionating columns, the tall knobby columns that make petrochemical plants recognizable from afar. In a fractionating column, crude oil is boiled and its components condense out on a series of trays, with the heaviest one (with the highest boiling point) sulking at the bottom, and the lightest (and most volatile) rising to the top.

Instead of condensing out hydrocarbons such as asphalt, petroleum jelly, kerosene and gasoline, the exoplanet’s atmosphere condenses out minerals such as enstatite, corundum, spinel, and wollastonite. In both cases the fractions fall out in order of boiling point.

The atmosphere of COROT-7b may not be breathable, but it is certainly amusing.

Source: Washington University

[/caption]
Planet hunters from the UK have discovered the largest exoplanet yet, and its uniqueness doesn’t end there. Dubbed WASP-17, this extra large world is twice the size of Jupiter but is super-lightweight, “as dense as expanded polystyrene” one astronomer said. Plus it is going the wrong way around its home sun, making it the first exoplanet known to have a retrograde orbit. As a likely a victim of planetary billiards, astronomers say this unusual planet casts new light on how planetary systems form and evolve.

Astronomers say the planet must have flipped direction after a near miss with another huge “big brother” planet swung it around like a slingshot. “Newly formed solar systems can be violent places,” said graduate student David Anderson, of Keele University. “Our own moon is thought to have been created when a Mars-sized planet collided with the recently formed Earth and threw up a cloud of debris that turned into the moon. A near collision during the early, violent stage of this planetary system could well have caused a gravitational slingshot, flinging WASP-17 into its backwards orbit.”

Though it is only half the mass of Jupiter it is bloated to nearly twice Jupiter’s size.

Astronomers have long wondered why some extra-solar planets are far bigger than expected, and WASP-17 points to the explanation. Scattered into a highly elliptical, retrograde orbit, it would have been subjected to intense tides. Tidal compression and stretching would have heated the gas-giant planet to its current, hugely bloated extent. “This planet is only as dense as expanded polystyrene, seventy times less dense than the planet we’re standing on”, said Coel Hellier, also of Keele University.

WASP-17 is the 17th new exoplanet found by the Wide Area Search for Planets (WASP) consortium of UK universities. The WASP team detected the planet using an array of cameras that monitor hundreds of thousands of stars, searching for small dips in their light when a planet transits in front of them. Geneva Observatory then measured the mass of WASP-17, showing that it was the right mass to be a planet. The WASP-South camera array that led to the discovery of WASP-17 is hosted by the South African Astronomical Observatory.

Read the team’s paper here.

Source: STFC

[/caption]
In what might be a evidence of planetary billiards, astronomers have found an exoplanet with an extremely odd orbit. The question is, was this planet the cue ball or the object ball? While most planets orbit around a star’s mid-section, this one – called XO-3b — is tilted about 37 degrees from the star’s equator. It’s also a massive planet, about 10 times the size of Jupiter. Such a misalignment must have occurred as a result of a disturbance, such as a collision with another object, sometime after the planet’s formation. But astronomers say they don’t yet know what caused the unusual orbit of XO-3b.

Detecting this oddball orbit required a combination of good luck, advanced technology and ingenious methodology. The planet was discovered back in 2007 using the transit method by measuring how the star is dimmed by the planet passing in through the line-of-sight between Earth and the star.

Using the Keck I telescope, detecting the planet itself was relatively easy, as it dimmed the star’s light by about 1 percent. But to go one step further and measure the angle of its orbit, meant that “we have to be sneaky about it,” said MIT physicist Joshua Winn, who led the team that measured the planet’s tilted orbit. It turns out that if a planet crosses the star’s disk at an angle to the star’s own rotation, it causes a distinctive pattern of change in the overall color of the star, as measured by a highly sensitive spectrograph, because of the Doppler shifts caused by the star’s rotation.

Hints of such a spectral signature were seen last year by another team, but that team acknowledged that they could not be confident of their result. The new observations, carried out by Winn and his team in February at the Keck I Observatory in Hawaii, provided a clear, solid measurement of the planet’s distinctive tilt, determining the angle of the orbit to be about 37 degrees from the star’s equator. The results are reported in a paper in the Astrophysical Journal, which was recently posted online and will be published in the journal’s August issue.

A majority of the exoplanet discovered so far are very large planets comparable to the gas giants in our solar system, but orbiting their stars much closer in (and thus faster). That’s because the method used to detect these planets makes it much easier to detect such close-in giants than smaller or more distant ones. In the case of XO-3b, it is about 13 times as massive as Jupiter, yet orbits its star with a period, or “year,” of just 3.5 days (Jupiter, by contrast, takes almost 12 years for an orbit). That size and closeness to its star are “unusual, even by the standards of exoplanets,” Winn says.

Such “hot Jupiters” – so named because they resemble the solar system’s largest planet, but would be much hotter because of their proximity to their parent stars – could not have formed in the places they are seen now, according to accepted planet-formation theory. They must have formed much further out from the star, then migrated inward to their present positions. Astronomers have come up with different mechanisms to account for the migration: the gravitational attraction of other planets as they passed close by, or the attraction of the disk of dust and gas from which the star and its planets formed.

Close encounters with other planets could greatly amplify a slight initial tilt, but attraction from the disk of material could not. Likely, a cataclysmic event occurred in this planet’s past.

Read the team’s paper.

Source: MIT