March 9, 2007

Distant Planets Spark Interest

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Extra-solar planets are the new kids on the astronomy block and have been receiving much attention, raising interest in the possibility of extra-terrestrial life and bringing greater understanding of how solar systems such as ours might have developed. Phil Arras, Ph.D ’99, now an assistant professor at the University of Virginia, spoke at the astronomy colloquium yesterday to cast some light on his research and also on the theory forming from observations made over the last decade.

Over 200 hundred planets have been observed orbiting stars other than the sun. These planets vary from being 50 times farther from the sun than the Earth to orbiting just a few solar radii. So far the various techniques to discover extra-solar planets are focused toward finding those with period proportional to the length of the survey that found them and planets with larger mass.

The most popular technique is Doppler Spectroscopy, also known as radial velocity, which is responsible for over 150 of those planets found. Gravity binds planets into orbits about stars, but the gravity from the planet also has an effect on the star, pulling it in a small circle or ellipse. The star’s motion is very small compared to its own size but astronomers have learned how to measure it using spectroscopy. The technique also uses the Doppler concept that light is blue-shifted as the star moves toward the observer on the Earth and then red-shifted as it moves away. These observations can provide a minimum mass for the planet and a radius of the orbit.

A smaller portion of planets observed using Doppler Spectroscopy, about ten percent, have transiting orbits. A transiting orbit is similar to a solar eclipse in that the planet passes between the observer and its sun. When a planet passes in front of its sun — a primary eclipse — it will cause a decrease in the brightness of the sun. If we were to observe Jupiter eclipsing our sun at an equivalent distance, there would be a one percent decrease in the sun’s brightness. The length of the change in brightness and the frequency provide a much more accurate estimate of the mass and a measure of the planet’s radius.

A secondary eclipse occurs when the planet passes behind the star. This is also a decrease in brightness but on a much smaller scale, about one thousandth of a percent. Astronomers have been able to measure this decrease, which provides information about the surface temperature of the planet.

Arras spoke on a group of transiting planets that orbit their suns in a matter of days only a few stellar radii distant These stars are, on average, the mass of Jupiter but have radii sometimes twice that of Jupiter.
After flashing a New York Times piece on the overhead, he said, “When you see such exciting new observations like these you get theorists appearing in the New York Times going, ‘Interesting, I have no idea.’”

Arras has performed calculations and modeling in an attempt to understand the thermodynamics necessary to support such large planets.

He explained that current theory suggests that gas planets are all born hot and puffy (with large temperatures and radii) but planets like Jupiter and Saturn quickly contract, heat up and then cool slowly over millions and even billions of years. In fact, Jupiter still emits more energy than it receives from the sun and this energy is thought to come mostly from its creation over four billion years ago. The extra-solar planets, though, have not collapsed to provide that energy and so Arras, and others around the world are beginning to develop an understanding as to why. Tidal heating may provide the missing energy. Since the planets orbit so close to their stars, their tides can often be hundreds of kilometers high compared to the meter-scale tides seen on the Earth from the Moon. These tides can create viscous heating (layers of fluid moving past each other and frictionally heating) that according to Arras may provide the energy needed to support these very large and puffy planets.