Discovery of the First Exomoon by Columbia University Research Team Faces Skepticism from Other Astronomers.
Astronomers have long understood that discovering moons around exoplanets would be a significant achievement, but a current debate in the field of planetary science highlights just how challenging it is to detect exomoons, according to Live Science. The story began in 2018, when a research team, including David Kipping, an assistant professor of astronomy at Columbia University, was confident they had discovered the first exomoon. The object orbits the exoplanet Kepler-1625b, a Jupiter-like world located about 8,000 light-years from Earth. Initially, the object was detected using the Kepler Space Telescope.
Simulation of an exomoon orbiting a planet outside the Solar System. (Image: NASA GSFC/Jay Friedlander and Britt Griswold)
Upon discovery, the moon of Kepler-1625b was named “Kepler-1625 b I.” It was later confirmed with additional data from the Hubble Space Telescope. In 2022, another research team, which included Kipping, seemingly discovered a second exomoon, this time only with the Kepler telescope. This object orbits Kepler-1708 b, a gas giant located 5,400 light-years from Earth, with a mass 4.6 times that of Jupiter. The potential second exomoon was also named “Kepler-1708 b I.”
The technique used to discover these two exoplanets is similar to the transit method, which has added over 5,000 planets to the catalog of exoplanets to date. The transit method relies on detecting slight dips in the light emitted by the host star, occurring when a planet moves in front of the star from Earth’s viewpoint. A similar principle applies to exomoons, albeit on a much smaller scale. If these moons are positioned correctly around their host planets during transit, they too will cause a slight decrease in light.
However, such a small decrease in light serves as a clue proving the existence of Kepler-1625 b I and Kepler-1708 b I for the supporting team of exomoons. Nevertheless, the light reduction caused by these small exomoons is too subtle to be directly observed. Instead, researchers need to use powerful computer algorithms to extract them from telescope data.
According to Kipping, both his team and the opposing team led by René Heller used the same dataset from the same telescope, but the absence of Kepler-1625 b I and Kepler-1708 b I could be due to how the research teams processed the data through their algorithms. Kipping believes they may have overlooked Kepler-1708 b I due to the software they chose to analyze data from Hubble and Kepler. While related to the software used by Kipping’s team, Heller’s software is somewhat different. Kipping also suggested that Heller’s team use their software since it is often very reliable outside the default mode and sensitive to certain steps used in data processing. This could explain why the exomoons were missed in the calculations.
For Kepler-1625 b I, Heller and colleagues proposed using the “stellar limb darkening” effect, meaning the edges of a star are darker than its center, which affects the exomoon’s signal. Heller’s team argued that this effect better explains observations of the host star than the light dip caused by the exomoon. Kipping maintains that this approach is inappropriate for the potential exomoon because his team already considered the stellar limb darkening effect when describing the existence of Kepler-1625 b I. Heller and his team do not believe Kepler-1625 b I and Kepler-1708 b I exist.
At the very least, both Heller and Kipping agree on the need for continued research. The reason exomoons appear in the transit method is that they are massive bodies, comparable in size to minor Neptune-sized planets, with diameters 1.6 to 4 times that of Earth. If they do exist, they are quite large. Kipping believes this is partly why they are too unusual to be accepted as the first exomoon discovery. He plans to use the James Webb Space Telescope (JWST) to search for additional exomoons that resemble those in our Solar System.
