Deducing the Weather on Extrasolar Worlds

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Artist's conception of the large planet
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.

E-mail to tonyhoffman [at] earthlink [dot] net