Features

Other-worldly pursuit

"Other-worldly pursuit" Continued...

Issue: "Ghost streets," Feb. 27, 2010

The total number of exoplanets in space is a risky guess, but if each star in the Milky Way were orbited by just one planet, the galaxy would contain over 100 billion.

Popular interest in the question of life beyond our solar system is helping fuel the hunt for smaller and smaller exoplanets. Astronomers are looking for Earth-size planets that may lie within the "habitable zone" of their stars-an orbital distance where water can physically exist in its liquid form.

Jason Lisle, an astrophysicist who works as a research scientist for Answers in Genesis, a creationist organization, sounded upbeat about the hunt for exoplanets and said he expected improving technology to discover smaller planets. But, "I would not expect there to be another planet out there that's like Earth in the sense of having all the right ingredients for life, and having life on its surface. And so far, observations have borne out that prediction."

Lisle noted that some planetary systems-such as those with Jupiter-size planets orbiting close to their stars-or one discovered last year with a planet orbiting in the opposite direction of its star's rotation-throw a wrench in current theories about the formation of stars and planets.

Researchers have yet to find an exoplanet that can properly be called "Earth-like." Or hospitable. The smallest exoplanet discovered so far, CoRoT-7b, is 70 percent larger than Earth and has a density similar to Earth or Mercury, most likely making it a "rocky" planet. But with surface temperatures reaching perhaps 3,600°F, "lava" planet is a more realistic description.

Kepler has found even warmer and stranger worlds: "We've got a companion that is planet-sized, but is much hotter than the star it orbits," said Borucki. The star is about 16,500°F and the companion is 21,000°F, more than double the temperature of our sun. Borucki speculates the companion could be something like a white dwarf star, but he says, "We're a little bit puzzled about some of these things too."

But the Kepler mission itself is proof that perseverance precedes discovery. Borucki pioneered the idea of measuring star brightness to detect exoplanets in the '80s, and throughout the '90s tried to convince NASA to put a special telescope in space for that purpose. NASA rejected the idea as technologically impractical for several years. "And we were laughed at for a very long time," says Borucki. But by 2000 his team had overcome the technical challenges and sold the space agency on Kepler.

"What we're talking about is a major effort by humankind. You do it one step at a time," said Borucki of the extrasolar planet search. "The technology simply didn't exist 10, 15 years ago to do these things."

Detecting an exoplanet

If a planet passes directly between a star and an observer's line of sight, it blocks out a tiny portion of the star's light, thus reducing its apparent brightness. Sensitive instruments can detect this periodic dip in brightness. From the period and depth of the transits, the orbit and size of the planetary companions can be calculated. Smaller planets will produce a smaller effect, and vice versa. A terrestrial planet in an Earth-like orbit, for example, would produce a very small dip in stellar brightness that would last just a few hours. This is one of several ways in which astronomers can find exoplanets.

Daniel James Devine
Daniel James Devine

Daniel is a reporter for WORLD who covers science, technology, and other topics in the Midwest from his home base in Indiana. Follow Daniel on Twitter @DanJamDevine.

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