There are eight planets in our solar system. Pluto was reclassified as a dwarf planet in 2006, joining Eris, Sedna, Quaoar, Ceres, and perhaps many more tiny worlds in the solar system. These are roughly described as bodies that circle the sun but are not large enough to remove material from their environs by gravitational dominance. However, astronomers wonder whether there isn’t a previously undiscovered ninth planet hiding in the solar system’s beyond reaches, maybe in the massive Oort cloud of debris that begins hundreds of astronomical units from the sun and stretches outward.
Recent data suggest that the orbital characteristics of several tiny things beyond Neptune appear to behave as if they had been impacted by the gravity of a big object in the outer solar system, reinforcing the idea that there may be a ninth massive planet in the outer solar system. Despite observational biases and statistical limitations, these findings have reignited interest in the possibility of an extraterrestrial presence.
Planet 9 would be roughly 5 to 10 Earth masses in size and orbit the sun at a distance of 400 to 800 au, according to estimations. Because of its faintness, a planet at this distance would be exceedingly difficult to discover in regular optical sky searches, even with telescopes like PanSTARRS and LSST. Most solar system objects were discovered using reflected sunlight, but the sunlight they receive diminishes due to the inverse square law for electromagnetic radiation, which states that measured light intensity is inversely proportional to the distance squared from the source of radiation; additionally, the reflected portion travels back to Earth’s telescopes and diminishes by a similar factor.
Although these objects are cold, they may release more infrared radiation than the optical light they reflect at the furthest reaches of the solar system, and astronomers have searched for them in the past using infrared surveys such as the Wide-field Infrared Explorer, but without success.
Benjamin Schmitt, a CfA astronomer, was part of a big team that used Chile’s 6-meter Atacama Cosmology Telescope (ACT) to look for Planet 9 at millimeter wavelengths. Despite the fact that ACT was created to research the cosmic microwave background radiation, its high angular resolution and sensitivity make it ideal for this sort of search.
Over a six-year period, the astronomers scanned around 87 percent of the sky visible from the Southern Hemisphere, then processed the millimeter pictures using a number of techniques, including binning and stacking approaches that may reveal faint sources but at the cost of losing positional information. They uncovered a lot of potential sources, but none of them could be confirmed, and there were no statistically significant findings.
The scientists, on the other hand, were able to eliminate with 95 percent certainty a planet 9 with the above-estimated attributes inside the investigated area, which is consistent with earlier null searches for Planet 9. The results only cover around 10 to 20% of the possibilities, but other sensitive millimeter facilities are coming up and should be able to finish the search for Planet 9 as predicted.
