Small 'scope spots big planet

500 light-years hence

A new extra-solar planet has been identified orbiting a star 500 light years from our solar system. The discovery was made by a team using a four-inch diameter telescope to make their observations - a size of 'scope readily available and much used by amateur observers.

The research was conducted at the Harvard-Smithsonian Center for Astrophysics (CfA), and suggests we are on the brink of a new age of planet discovery, the organisation says. Guillermo Torres, co-author of the study, says the results prove that "even humble telescopes can make huge contributions to planet searches".

Like all the other identified extra-solar planets, the body found orbiting the star in the constellation Lyra is a giant. Also in keeping with other identified planets, it orbits very close to the star: in this case only four million miles away - sixteen times the distance from the Earth to the moon. Its orbit is just 3.03 days long.

The planet was found during a survey of thousands of bright stars spread across the sky. The team hunted for planets using the transit method: watching a star for a dip in its brightness, indicating that something passed between the star and the observer. A planet the size of Jupiter, for example, would produce a one per cent drop in the amount of light reaching Earth - enough to be detectable.

Looking for planets using this method is a bit hit-and-miss, and so requires that many, many stars be surveyed. This is because a planet will only cause a dip in brightness if is orbiting a star at exactly the right angle, from our perspective. If it is in the wrong plane, it will pass undetected.

To survey so much of the sky, the team at the CfA used a network of small telescopes. David Charbonneau, who co-led the research, explains that although the equipment is easily available, the technique is cutting edge: "It took several Ph.D. scientists working full-time to develop the data analysis methods for this search program," he said.

Despite these limitations, the transit method provides more precise information about the planet than the Doppler method by which most other planets have been found.

The CfA explains that the Doppler method "detects a planet's gravitational effect on its star spectroscopically by breaking the star's light into its component colours".

However, because the Doppler method provides no way of working out the angle of incline of a given system, it can only provide a lower limit to the mass of a planet. The signal produced by a high mass brown dwarf orbiting at a very high angle to our line of sight will perfectly match that from a smaller planet orbiting edge-on.

"When astronomers find a transiting planet, we know that its orbit is essentially edge-on, so we can calculate its exact mass. From the amount of light it blocks, we learn its physical size. In one instance, we've even been able to detect and study a giant planet's atmosphere," Charbonneau commented.

Once the initial discovery was made, the team passed the stellar details to the W. M. Keck observatory. This facility operates two of the biggest telescope on Hawaii, and these follow up observations were essential to confirm the find. ®

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