So accustomed are we to the sunshine, rain, fog and snow of our
home planet that we find it next to impossible to imagine a
different atmosphere and other forms of precipitation.
To be sure, Dr. Seuss came up with a green gluey substance
called oobleck that fell from the skies and gummed up the Kingdom
of Didd, but it had to be conjured up by wizards and was clearly a
thing of magic.
Not so the atmosphere of COROT-7b, an exoplanet discovered last
February by the COROT space telescope launched by the French and
European space agencies.
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.
The work, by Laura Schaefer, research assistant in the Planetary
Chemistry Laboratory, and Bruce Fegley Jr., Ph.D., professor of
earth and planetary sciences in Arts & Sciences, appears in the
Oct. 1 issue of the Astrophysical Journal.
Astronomers have found nearly 400 extra-solar planets, or
exoplanets, in the past 20 years. But because of the limitations of
the indirect means by which they are discovered, most are Hot
Jupiters, chubby gas giants orbiting close to their parent stars.
(More than 1,300 Earths could be packed inside Jupiter, which has
300 times the mass of Earth.)
COROT-7b, on the other hand, is less than twice the size of
Earth and only five times its mass.
It was the first planet found orbiting the star COROT-7, an
orange dwarf in the constellation Monoceros, or the Unicorn. (This
priority is designated by the letter b.)
Solid as a Rock
In August 2009 a consortium of European observatories led by the
Swiss reported the discovery of COROT-7c, a second planet orbiting
COROT-7.
Using the data from both planets, they were able to calculate
that COROT-7b has an average density about the same as Earth's.
This means it is almost certainly a rocky planet made up of
silicate rocks like those in Earth's crust, says Fegley.
Not that anyone would call it Earth-like, much less hospitable
to life. The planet and its star are separated by only 1.6 million
miles, 23 times less than the distance between the parboiled planet
Mercury and our Sun.
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 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).
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.
"The only atmosphere this object has is produced from vapor
arising from hot molten silicates in a lava lake or lava ocean,"
Fegley says.
What might that atmosphere be like? To find out Schaefer and
Fegley have used thermochemical equilibrium calculations to model
COROT-7b's atmosphere.
The calculations, which reveal which mineral assemblages are
stable under different conditions, were carried out with MAGMA, a
computer program Fegley developed in 1986 with the late A. G. W.
Cameron, a professor of astrophysics at Harvard University.
Schaefer and Fegley modified the MAGMA program in 2004 in order
to study high-temperature volcanism on Io, Jupiter's innermost
Galilean satellite. This modified version was used in their present
work.
Raining Rocks
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.
"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.
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 atmosphere of COROT-7b may not be breathable, but it is
certainly amusing.
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