On Earth, strange things, including frogs and fish,
sometimes fall from the sky, but on a distant extrasolar planet, the weather
could be even weirder: When a front moves in, small rocks rain down on the
surface, a new study suggests.
The exoplanet,
COROT-7b, was discovered in February by the COROT space telescope launched by
the French and European space agencies. Last month it became the first planet
outside our solar system to be confirmed
as a rocky body most other known exoplanets are gas giants.
The planet is nearly twice the size of Earth and about five
times the mass of our world. Calculations have indicated it has a density about
that of Earth's, which means it is likely made up of silicate rocks, just as
Earth's crust is.
The planet is likely much less hospitable to life though, as
it is only about 1.6 million miles (2.6 million km) away from its parent star
23 times closer than Mercury sits to the sun.
Locked and scorching
Because the planet is so close to the star, it is gravitationally
locked to it in the same way the Moon is locked to Earth. One side of the
planet always faces its star, just as one side of the Moon always faces Earth.
This star-facing side has a temperature of about 4,220
degrees Fahrenheit (2,326 degrees Celsius) hot enough to vaporize rock.
So unlike the much cooler Earth, COROT-7b has no volatile
gases (carbon dioxide, water vapor, nitrogen) in its atmosphere. Instead its
atmosphere consists of what might be called vaporized rock.
"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., of Washington University in St. Louis.
Rocky weather forecast
To find out what COROT-7b's atmosphere might be like, Fegley
and his colleagues modeled it. They found that 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.
"Sodium, potassium, silicon monoxide and then oxygen
either atomic or molecular oxygen make up most of the atmosphere,"
Fegley said. But there are also smaller amounts of the other elements found in
silicate rock, such as magnesium, aluminum, calcium and iron.
The rock rains form similarly to Earth's
watery 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," Fegley explained.
"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."
Menagerie of minerals
The exoplanet's atmosphere condenses out minerals such as
enstatite, corundum, spinel, and wollastonite.
Elemental sodium and potassium, which have very low boiling
points in comparison with rocks, do not rain out but would instead stay in the
atmosphere, where they would form high gas clouds buffeted by the stellar wind
from COROT-7.
These large clouds may be detectable by Earth-based
telescopes. The sodium, for example, should glow in the orange part of the
spectrum, like a giant but very faint sodium vapor streetlamp.
Observers have recently spotted sodium in the atmospheres of
two other exoplanets.
The new modeling finding is detailed in the Oct. 1 issue of
The Astrophysical Journal.
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