discovered by Dimitar Sasselov's team
(OGLE-TR-56b), which orbits its parent star
every 29 hours. (Credit: David A. Aguilar,
Harvard-Smithsonian Center for Astrophysics)
Daytime highs of about 3,000 degrees Fahrenheit. An iron rain.
Hurricanes that would make Andrew or Hugo seem like a light breeze.
These are likely conditions on some of the gas-giant planets that have
been discovered orbiting extremely close to their parent stars, as
Harvard astrophysicist Dimitar Sasselov described in "Extrasolar Weather
Report," his October 27 lecture at the Hayden Planetarium. Sasselov heads
a team from the Harvard-Smithsonian Center for Astrophysics that found
its first extrasolar planet early this year.
We are fortunate to be witnessing the development of a new field within
of astronomy, one which Sasselov terms "planetary astrophysics": a merger
of planetary science and astrophysics to not only discover new worlds
orbiting other suns, but to begin to determine their characteristics. At
the time of the lecture, 117 extrasolar planets were known, all of them
giant worlds (Saturn-size or larger). Most are "close-in" planets, closer
to their stars than Mercury is to the Sun. Thirteen multiple planetary systems
containing 27 planets have been detected.
All but one of these planets were discovered spectroscopically, through
examining the Doppler shift of a star's light over time. A star with an
unseen companion will "wobble" slightly as it moves around their common
center of mass; spectral lines will be red-shifted when the star moves
away from us and blue-shifted when it approaches. By examining the
pattern and amplitude of such shifts, astronomers can not only deduce
the presence of planets but calculate their orbital period, mass and
radius of their orbits. This technique is limited to relatively nearby
stars that are bright enough to yield the precise spectroscopic data
that are needed (though larger telescopes will eventually bring more
such worlds within range).
Sasselov's team uses a different method, looking for minuscule (but
predictable) dips in a star's brightness as a planet passes in front
of it. In January, the group announced the discovery of the first planet
found by this "transit method." (The only other extrasolar planet
observed to transit a star--HD 209458--was first detected by the Doppler
method.) OGLE-TR-56b, about the size of Jupiter, lies closer to its
parent star than any known planet (only four stellar radii away) and
takes only 29 hours to circle its sun. During each transit, the light
of its primary dims by about 1%.
OGLE-TR-56b is about 5,000 light-years from Earth, about 10 times as
far as any planet found by the Doppler technique. Not only can the
transit technique detect planets at greater distances, it also yields
more accurate information about the planets' size and orbital characteristics t
han any other current method. Its downside is that it is hit or miss;
it can only detect planets whose orbital plane is edge-on to us so that
they pass in front of their stars. This favors planets in very close
orbits.
What sort of weather could we expect on these gas giants that
orbit very close to their suns? Rather forbidding, in short. OGLE-TR-56b
is hot enough that it might experience an "iron rain," a fine, opaque
mist formed of atomic iron droplets around 1 micron in diameter.
When the Hubble Space Telescope was used to observe HD 209458 when
its planet was in transit, enhancement of the star's sodium spectral
lines was noted. This was due to light from its star filtering through
the planet's atmosphere during the transit, and is indicative of the
planet's high temperature.
Using the same atmospheric modeling techniques used to model the
atmospheres of Earth, Jupiter and Saturn, Sasselov's team has proposed
that the planet’s atmosphere is extremely dynamic due to heating from
its star, with a powerful equatorial jet stream, strong polar systems
and hurricane-like systems with winds of 4,000-5,000 miles per hour
in the middle latitudes; strong convection keeps the night side
relatively hot. Other close-in “hot Jupiters” should experience
similar conditions.
The ultimate goal of the hunt for extrasolar planets is to discover worlds
with relatively tranquil weather which could support life. NASA's
planned Kepler mission will monitor thousands of stars in search
of transiting planets, but the discovery of Earth-sized worlds may
have to wait for even more sensitive instruments.
