Proposed exoplanet may not exist after all

If a new research is to be believed, a proposed exoplanet near a star some 6 parsecs from Earth may not exist after all.

Updated: Dec 09, 2009, 13:01 PM IST

London: If a new research is to be believed, a proposed exoplanet near a star some 6 parsecs from Earth may not exist after all.

Ground-based astrometry has been used for more than a century, but none of the extrasolar planets it has detected has been verified in subsequent studies.

In May, Steven Pravdo of the Jet Propulsion Laboratory in Pasadena, California, and his colleagues raised fresh hopes for the technique when they announced an exoplanet, six times more massive than Jupiter, orbiting VB10, a star about one-thirteenth the mass of the Sun, using a telescope at the Palomar Observatory in southern California.

But now, according to a report by Nature News, a group led by Jacob Bean at the Georg-August University in Gottingen, Germany, has used a different approach, and found nothing.

“The planet is not there,” said Bean.

Bean and his colleagues used a well-honed technique called radial velocity, which has found most of the extrasolar planets detected so far.

The method looks for shifts in the lines of a star’s absorption spectrum to track its motion towards and away from Earth, which would be caused by the influence of a planet.

Radial-velocity measurements typically exploit the visible bands of the electromagnetic spectrum.

But, VB10 is a very dim star and gives off most of its light as infrared radiation.

At the Very Large Telescope in Chile, Bean placed a gas cell filled with ammonia in the path of the starlight, enabling him to calibrate the instrument for the infrared.

“We would definitely have seen a significant amount of variation in our data if the planet was there,” said Bean.

“Unfortunately, astrometry is a very difficult business,” said Bean, explaining that Earth’s atmosphere can introduce distortions that affect the measurements.
Astrometrists rely on watching a field of stars about the same distance away as the target star to calibrate their measurements, and that can be tricky, according to Alessandro Sozzetti, an astrometry expert at the Turin Observatory in Italy.
“Even if we think we have selected a good set of reference stars, we may still be limited by atmospheric effects that cause an extra jitter” in the motion of those stars,” he said.