The Kepler space observatory has been in orbit around the sun since 2009, collecting data on 150,000 stars and searching for Earth-like planets in the Milky Way galaxy. The mission has identified 8,826 objects of interest. Of them, 4,696 are considered candidates, or possible planets. And 1,030 of the candidates have been confirmed.

According to a study focused on gas giants, however, the total may include more false positives than previously thought. The study, announced this week, has been accepted for publication in the journal Astronomy & Astrophysics.

"It was thought that the reliability of the Kepler exoplanets detection was very good -- between 10 and 20 percent of them were not planets," researcher Alexandre Santerne of the Portugal-based Institute of Astrophysics and Space Sciences, who led the international team behind the study, said in a statement.

"Our extensive spectroscopic survey, of the largest exoplanets discovered by Kepler, shows that this percentage is much higher, even above 50 percent. This has strong implications in our understanding of the exoplanet population in the Kepler field."

NASA/JPL-Caltech

The Kepler observatory uses a technique called transit photometry to detect objects orbiting stars. This measures the periodic dimming of the star caused by another object passing in front of it. The amount the light dims and the duration of the dim period can help determine the size and orbit of the object.

This is no mean feat.

"Detecting and characterizing planets is usually a very subtle and difficult task," researcher Vardan Adibekyan of the Institute of Astrophysics and Space Sciences said in a statement. "In this work, we showed that even big, easy to detect planets are also difficult to deal with. In particular, it was shown that less than half of the detected big transiting planet candidates are actually there. The rest are false positives, due to different kind of astrophysical sources of light or noise."

The team began their survey in 2010, using the Spectrographe pour l'Observation des Phénomènes des Intérieurs stellaires et des Exoplanètes (SOPHIE) at the Observatory of Haute-Provence, France. This was so that the team could collect spectroscopic, rather than photometric, data. The team then narrowed the 8,826 objects of interest down to 129 objects on 125 stars. They ruled out stars too faint for SOPHIE to observe, known false positives, and objects with an orbit longer than 400 days. The latter was ruled out so that the team could observe at least three orbits for each object.

What they found is that 52.3 percent of the objects they examined were eclipsing binary stars, or pairs of stars locked in orbit around each other so that they pass in front of each other as we observe them. A further 2.3 percent were found to be brown dwarfs, gaseous objects that are too large to be considered planets, but too small to be considered stars.

This means there could be fewer gas giants in the galaxy than we believed. But the search for Earth-like exoplanets is as yet unaffected.

"This doesn't change our estimates of the most interesting planets, the smaller Earth-like worlds," said Australian astronomer Alan Duffy, who did not participate in the research. "Even after this recalculation of the number of planets in the Milky Way there are still billions of alien worlds out there. The overall result is that we need to follow up these exciting Kepler systems with upcoming, even more powerful telescopes, like the Giant Magellan Telescope."

The study also uncovered some mysteries surrounding gas giants. For instance, gas giants' atmospheres are inflated by the radiation of their stars. But the inflation of some of the giant planets the team surveyed was higher than could be explained with the normal processes. Moreover, some giants that were only moderately irradiated were not inflated at all.

The team hopes that further study of the internal structure of these planets will provide insight into the processes of planet formation and evolution.