How to count alien worlds

Study suggests up to half exoplanet listings may be wrong

If you think Florida's voting is messy, try counting alien worlds. Ballot checkers examine what they certify. Astronomers make do with long-range touchy-feely techniques to confirm that something they can't see really is a planet tugging on its parent star.

There's enough ambiguity in those methods for critics like David Black at the Lunar and Planetary Institute in Houston to request a recount. Last month, Dr. Black, George Gatewood from the University of Pittsburgh, and Inwoo Han of the Korea Observatory in Kyung-Book reported preliminary results of their own study. It suggests that up to half of the 50 alien planet listings could be in error.

The team says many of the 24 candidates it has analyzed so far seem too massive to be planets. They may be brown dwarfs - objects a little too small to ignite a stellar fire.

Or they may actually be small stars. In either case, the team claims that some binary stars are being mistaken for alien solar systems. If that turns out to be true, planet hunters would have to tone down their claim that planetary systems are quite common.

Most planet hunters, such as Neville Woolf at Arizona State University in Tucson, aren't ready to concede. He says that, at this point, "I don't take their criticism seriously." He notes that the methods critics such as Black use have serious statistical ambiguities of their own.

However, hunters and critics agree on one crucial point: They need better ways to identify what they call exoplanets. Some of these methods should begin to phase in over the next few years. For example, Dr. Woolf is on a team developing ways to use Earth-based telescopes to image Jupiter-size planets and dust around stars using infrared light. He hopes to begin work with the twin mirror Large Binocular Telescope under construction on nearby Mt. Graham in 2004. Stars don't shine as brightly in the infrared as they do in visible light. This makes accompanying planets, which also emit infrared radiation, easier to see.

Meanwhile, most planet hunts rely on interpreting the wobbles a star undergoes as it's tugged about by an orbiting body. Motion along our line of sight to the star shows up in its light. Motion toward us shifts the light toward blue frequencies. Motion away shifts it toward the red. That's enough to pin down the mass and orbit of a planet if we see the orbit edge-on.

But if the orbit is at a tilt to our line of sight, the unseen body also pulls the star from side to side. In that case, line of sight wobbles only yield a minimum mass for the object.

Astronomers need to get the side-to-side wobble as well to pin down the mass. That requires precision position measurements. Black and his colleagues are using such measurements taken by the European Space Agency's high-precision Hipparchus orbiting telescope in claiming that many exoplanet discoveries are faulty.

Woolf notes that astronomers are "absolutely certain that we have detected small, nearly Jupiter-size planets." The artist's rendering here illustrates a case in which astronomers watched a star's light dim as a planet with two-thirds Jupiter's mass moved across it. Woolf calls such discoveries "the tip of an iceberg." He adds that "for every one of those planets ... there are likely to be perhaps a dozen lower-mass objects."

Woolf believes that the only reason astronomers haven't found more planets is lack of "instruments sensitive enough to detect them." These are coming.

Hunters expect to find more transiting planets to complement wobble analyses. They will also look for planet trails in circumstellar dust clouds. And soon, they will begin trying to image alien planets directly.

(c) Copyright 2000. The Christian Science Publishing Society

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