Illustration of the hypothetical Planet Nine. Credit: ZME Science. For years, astronomers have wondered whether another enormous planet lurks unseen in the cold darkness beyond Neptune. It sounds silly seeing how astronomers have found thousands of alien planets in the night sky, orbiting planets even thousands of light years away. But the Solar System is a vast place, and its edges are so devoid of light that it is actually harder to detect a planetary body at the edge of the Solar System than in distant star systems. However, the signs are hard to ignore. Several extremely distant objects appear to have clustered, unusually inclined orbits, as if an invisible planet has been slowly guiding them with its gravity. That hypothetical world became known as Planet Nine. Now astronomers may have found the first direct clue that there really is something there. Researchers searching for decades-old infrared observations have identified a faint object that appears to have moved across the sky between two surveys conducted 23 years apart. Its brightness and slow motion fit what the team expected from a large planet hundreds of times farther from the Sun than Earth. Researchers emphasize that they have not discovered Planet Nine, or at least not definitively. But the object was chosen after a search among millions of infrared sources and emerged as its best candidate. The result, described in a study published in Publications of the Astronomical Society of Australia, offers an intriguing new target in one of astronomy’s oldest planetary hunts. Hunting for Planet Nine Finding Planet Nine wouldn’t be like spotting Jupiter with a telescope. If the planet exists hundreds of astronomical units away (one astronomical unit is the Earth-Sun distance), very little sunlight would reach it and it would bounce back to Earth. That makes it extraordinarily dim in visible light. ×Thank you! One more thing… Please check your inbox and confirm your subscription. That’s where infrared comes into play. A planet also emits heat, and its infrared thermal radiation fades with distance more slowly than reflected sunlight. Previous searches therefore targeted spacecraft that mapped the sky at wavelengths invisible to the human eye. Terry Long Phan of National Tsing Hua University in Taiwan and colleagues compared observations from two such missions. IRAS, the infrared astronomical satellite, surveyed the sky in 1983. The Japanese AKARI spacecraft repeated an infrared study in 2006 and 2007. The 23-year gap gave researchers a particularly long baseline: A very distant planet barely appears to move from night to night, yet it drifts far enough across the sky over decades to stand out. The team looked for objects that appeared in both surveys in positions consistent with such slow motion. They focused on a possible planet between 500 and 700 astronomical units from the Sun and approximately 7 to 17 times the mass of the Earth. After filtering more than a million sources from each study for brightness, location and infrared characteristics, the researchers produced 22 possible pairings. Additional checks reduced that number to 13. Visual inspection left only one. The apparent source had moved 47.46 arcminutes between the IRAS and AKARI observations, about one and a half times the apparent width of the full Moon. So have we found Planet Nine? Not yet. The researchers found a pair of infrared detections consistent with a slow-moving object. AKARI’s observations also suggest that the source changed position for six months, as a distant body in the Solar System should do. But two positions separated by decades cannot reveal a reliable orbit. TPlanet Nine was proposed not simply as another distant planet, but as an explanation for the unusual trajectories of objects in the outer Solar System. Therefore, a candidate must move on the right kind of path to exert the gravitational influence that astronomers are trying to explain, which is why determining the orbit is crucial. There are other uncertainties. One of the candidate’s AKARI infrared measurements appears unusually weak and may be unreliable. And the search itself depended on assumptions about Planet Nine’s mass, temperature, density and distance. A substantially smaller, colder, or more distant planet might have escaped these studies entirely. The next step is, in principle, simple: look again. Phan and his colleagues suggest using the Dark Energy Camera on the 4-meter Blanco Telescope in Chile. More observations could reveal whether the source continues to move along a planetary orbit or disappears as another red herring. Until then, Planet Nine remains hypothetical. But after years of inferring the planet from the gravitational fingerprints it might leave behind, astronomers finally have something much more tangible to chase: a particular weak spot in the sky.
