Is the Universe Lively or Lonely?

Kepler with distant solar system Image courtesy of NASA

As the rate of discoveries in astronomy increases, so does the possibility that scientists are able to find planets that could potentially sustain life.

In a recent lecture sponsored by the AAAS Dialogue on Science, Ethics and Religion (DoSER) Dr. Carl Pilcher, senior scientist for Astrobiology in NASA's Astronomy and Physics Division, discussed ways to detect planets around other stars and to determine if they are habitable or even harbor life.

"Almost everything we thought we knew is wrong," is the alternate title Dr. Pilcher gave to his presentation on the topic of 'extrasolar' planets and their potential to harbor life.

Astronomers use different techniques to determine the size and distance of extrasolar planets from their stars. One method uses the Doppler shift - the same phenomenon that makes train whistles coming toward you sound higher pitched than when they travel away from you. When a planet goes around a star, the star moves slightly as well. The wavelengths of light from the star get slightly shorter (termed blue shift) when it swings toward the Earth, and slightly longer (red shift) as the star moves away from the Earth. Scientists measure the varying wavelengths of the star's light to determine if a planet is present and to determine its mass and proximity to its star.

Using this technique, scientists have determined that there are Jupiter-sized planets orbiting close to stars, a surprising fact that according to Dr. Pilcher, "stands our earlier thinking about how giant planets are formed on its head."

Scientists had assumed that larger planets, like Jupiter, must form at relatively large distances from their stars where solid ices such as water, carbon dioxide, and methane can condense. These ices would then form the cores around which gaseous hydrogen and helium could accrete to form the giant planets. The fact that they are finding giant planets very close to their stars (much less than the distance of Mercury from our sun) most likely means that planets can move great distances from where they're formed to their final orbits. The number and masses of the planets being found also imply that there could be Earth-sized planets scientists have yet to detect.

Another way to find the planets of a distant star is to measure the 'wobble' that a planets gravitational pull causes in the stars position. on the sky. Amazingly, astronomers are developing the ability to measure these tiny stellar wobbles. The goal of NASA's Space Interferometer Mission, scheduled for launch in 2009, is to determine the positions and distances of stars more accurately than before in the search for earth-like planets and life.

A third method is to look at the transits of planets that happen to pass in front of their star as viewed from Earth. When the planet is in front of the star, it blocks some of the star's light. By knowing the size of the star, astronomers can then calculate the radius of the planet.

Additionally, since scientists are able to measure the light passing through the outer rim of the planet, this technique also allows scientists to study a planet's atmosphere. By determining the chemical compounds present in the atmosphere, they can investigate what conditions on the planet are like.

A new NASA project set to launch in 2007 will use the transit method to detect planets. Over a period of four years the Kepler mission will monitor 100,000 stars every 15 minutes.

In principle, Kepler will help determine whether Earth-size planets are common or rare, and will boost our knowledge of planets from the hundreds to the thousands.

"If we find other planets like ours, we can begin determining whether there are other critters out there," Dr. Pilcher said.

Dr. Carl B. Pilcher has been the senior scientist for astrobiology in the Astronomy and Physics Division of the Office of Space Science, NASA since 2001. Prior to that, he served in a number of capacities in the Office of Space Science and the Office of Exploration related to human and robotic exploration of the solar system and to strategic and international planning.

Dr. Pilcher has advanced degrees in chemistry from the Polytechnic Institute of Brooklyn and MIT, respectively, and a masters degree in international relations from Princeton University. He was on the faculty of the Institute for Astronomy and the Department of Physics and Astronomy at the University of Hawaii for 12 years before joining NASA Headquarters in 1988.

—Carol Hoy

21 April 2004

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