At least three of the objects considered likely planets orbiting other stars are in fact small stars themselves, a reassessment has shown. These may not be the only mistakes among the exoplanet catalogs. Still, out of 5,000 discoveries considered confirmed, it's a failure rate of less than 0.1 percent.

The great thing about science is that it corrects its errors, albeit sometimes quite slowly. For that, however, you have to get things wrong in the first place, and it seems that is what happened with observations made by the Kepler Space Telescope. 

Once classified as planets, three objects discovered in the course of Kepler's mission have now been shown in the Astronomical Journal to be small stars. Another may have been misclassified the same way.

The fault here lies not in our stars, nor even in Kepler, but in the confirmation program. Kepler recorded the brightness of hundreds of thousands of stars repeatedly, and the data was searched for dips that might indicate a planet blocking a little of the star's light. However, even where regular dips consistent with a likely planet were found they were only considered “planetary candidates”. More than 5,000 such candidates have been identified in Kepler and TESS data without being added to catalogs of known exoplanets.

Only after follow-up to rule out alternative explanations were these candidates listed as official exoplanets. Initially, this involved looking for movements of the star under the supposed planet's gravitational influence. Unfortunately, this is a slow process requiring time on some of the world's largest telescopes, so statistical validation techniques have taken over.

However, it seems that in the case of Kepler-854b, Kepler-840b, and Kepler-699b (not to be confused with GJ 699b, better known as the planet orbiting Barnard's star) these confirmations slipped up.

The problem, according to the authors, is these three objects are simply too large to be planets.

“Most exoplanets are Jupiter-sized or much smaller,” said MIT graduate student Prajwal Niraula in a statement. “Twice [the size of] Jupiter is already suspicious. Larger than that cannot be a planet, which is what we found.” Niraula is referring to planetary radii: Twice Jupiter would mean eight times larger by volume.

At 1.8 times Jupiter's radius, a fourth "planet", Kepler-747b, is in the star/planet borderlands.

None of these ex-planets (as opposed to exoplanets) attracted any fame previously. They were clearly too large to sustain life, and none had been identified as having exceptional features such as raining iron, so some might question why the correction matters.

However, Dr Avi Shporer, an author on the study, noted: “People rely on this list to study the population of planets as a whole. If you use a sample with a few interlopers your results may be inaccurate. So it's important that the list of planets is not contaminated.”

The discovery was made when Niraula applied data from the Gaia space telescope to the parental star Kepler-854, showing it was considerably more distant and therefore larger than less accurate measurements made at the time of the “planet's” discovery. Using the updated evidence the authors concluded its companion is three times the size of Jupiter.

“There's no way the universe can make a planet of that size,” Shporer said. “It just doesn't exist.”

Having confirmed Kepler-854b is actually Kepler-854B – the capital letter indicating a star – the authors decided to check for similar cases, which led to the discovery of the other two, possibly three, ex-planets.