Over the past several years, the Kepler Space Telescope has been staring at a section of the sky, waiting for the stars to get a little bit dimmer. A sudden drop of brightness could mean that a planet is passing in front, and Kepler has seen thousands of these brightness dips over the past decade, discovering a wealth of planets outside the solar system.
Kepler has transformed what astronomers know about the number of exoplanets in the galaxy, but some worlds thought to be discovered may not be there after all. A recent published in the Astronomical Journal scrutinizes one way that astronomers confirm the brightness dips that Kepler sees are actually planets.
In the first few years of operation, whenever Kepler spotted one of these brightness dips, that star would be examined by another telescope to determine if there really was a planet there. Sometimes, the dimming of the star is caused by noise in the data or by something else blocking the star’s light.
But as more and more potential planets were discovered, astronomers shifted to a different strategy for confirmation, simply calculating the statistical chance that a signal was in fact a planet. Any likelihood over 99 percent was considered a ‘confirmed’ planet. But according to the study, this analysis doesn’t take into account potential errors in the telescope’s instruments.
These potential errors are most important when looking at planets similar to ours: small and distant from their host stars. Signals from potentially Earth-like planets could just as easily be an instrumentation glitch, which is a problem when searching for worlds like our own.
To illustrate their point, the authors one exoplanet discovered in 2015, Kepler 452b. At the time, the planet was described as a potentially an ‘older cousin’ or ‘older brother’ to Earth, positioned in the perfect place for life. However, Kepler 452b was confirmed by statistical analysis rather than independent observation.
Taking into account instrument error, the researchers believe that the odds that Kepler 452b is a genuine planet range anywhere from 16 to 92 percent. The particular instrument that spotted Kepler 452b is generally pretty reliable, but even so, the data falls below the 99 percent threshold needed to ‘confirm’ a planet.
If every planet candidate is like Kepler 452b, then at best only 9 out of 10 of them actually exist, and potentially much fewer do. Revisiting the data and conducting follow-up observations will be necessary before astronomers study these planets in earnest with telescopes like the upcoming James Webb Space Telescope. Depending on how reliable Kepler’s instruments are, some of the targets for James Webb could turn out to be nothing.