The retraction comes now that astronomers have focused the same telescope and instrument on two stars known not to have planetary disks around them. The astronomers were disappointed to get results that resembled the ostensibly positive finding of a disk last year.
"The conclusion is clear: the presumed dust disk around iota Horologii is an artifact, resulting from an underestimation of the calibration uncertainties in this type of delicate observation," according to a prepared statement released by the ESO.
"The observers and the ESO EPR Dept. regret the incorrect announcement."
Had the finding been correct, the alleged dust disk with a planet in an Earth-like orbit at iota Horologii would have been the first example of solar system formation resembling ours in the Universe.
Yet, despite the retraction on the iota Horologii findings, other research since the iota Horologii announcement in September 2000 has suggested that there are
like ours in the Universe. A light pattern assumption underlay the initial finding
The previous findings showed an elliptically shaped emission around iota Horologii when compared with two reference stars.
That emission, if real, was only explainable by the presence of a dust disk around this star. What made this mistake easy was the difficulties inherent in searching for faint dust disks around bright stars, according to the ESO.
The search involves blocking the starlight with a so-called coronographic mask. But even this is not sufficient -- it is also necessary to subtract the remaining "light-wings" from the star (i.e., the pattern that results from the stray light in the telescope and camera).
For this purpose, a reference star assumed to be without excess emission is always observed before and after each observation of the target star. The light pattern observed for the reference star (the standard Point-Spread-Function) is then used to remove numerically the pattern of the target star, thereby isolating any additional light that may come from a circumstellar disk.
For this analysis, it is crucial that the Point-Spread-Function remains unchanged during the observation of the reference and target stars. In fact, for each observation of the target star, at least two different reference stars are observed in order to verify this assumption.
Following this observational methodology carefully, observations of iota Horologii with the ADONIS adaptive optics instrument showed an emission which was interpreted as the signature of a circumstellar dust disk.
Blame ADONIS
The trouble came when the astronomers performed the observations again several months later, this time with different light filters and other reference stars. This time, they were unable to confirm the emission around iota Horologii that was previously interpreted to be the signature of a dust disk.
Knowing this was a problem, the team made observations to verify the basic assumption that the Point-Spread-Function remains unchanged for reference stars of slightly different brightness (within half a magnitude).
Unfortunately, they found substantial changes in the Point-Spread-Function of the ADONIS system can occur for reference stars in the brightness interval used for the iota Horologii observations.
Indeed, when they pointed the instrument and telescope at two reference stars with no circumstellar material and applied their standard analysis technique, they got a finding showing an emission pattern resembling that found around iota Horologii. In other words, the initial finding at iota Horologii was proven false.
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