Astronomer discovers hints of elusive Planet Nine in decades-old data

Astronomers have long speculated that our solar system may have a ninth planet orbiting at a vast distance from the sun. However, no one has ever been able to observe the so-called Planet Nine directly. Speculation has suggested that Planet Nine orbits at a significant distance farther out than the orbit of Pluto.

Astronomer Michael Rowan-Robinson from Imperial College London recently analyzed data collected in 1983 by the Infrared Astronomical Satellite (IRAS). During his research, Rowan-Robinson discovered a trio of point sources could be Planet Nine. However, the astronomer does say in his paper that it's unlikely the point sources are an actual detection of the planet, but the data could be used to model where the planet might be.

Gathering data on where Planet Nine might be could allow astronomers to conduct a more in-depth study using modern instruments that could detect the elusive planet once and for all. Rowan-Robinson wrote in the paper that considering the poor quality of IRAS detections, which in this case were at the limit of the survey and in a part of the sky that's difficult for infrared detections, the probability of the candidate being real is unlikely.

However, he says, given the interest in the Planet Night hypothesis, it's worthwhile to investigate further. Astronomers have been interested in determining if there is another distant planet orbiting in our solar system for many years. However, in 2016 a paper was published proposing new evidence that added fuel to the fire. In that paper, astronomers from Caltech found small objects in the Kuiper Belt with odd orbits hinting they were being pushed by the gravitational influence of something large.

While astronomers have certainly not confirmed anything about the speculated planet, if it is out there, it's estimated that it could be 5 to 10 times the mass of Earth. Speculation suggests that it might orbit at a distance between 400 and 800 astronomical units. For comparison, Pluto orbits at a distance of 40 astronomical units from the sun and was once considered the most distant planet (now a dwarf planet) from the sun.

The reason it's been so difficult to prove if Planet Nine exists at all is because it's very far away, comparatively small, very cold, and reflects little sunlight. Astronomers point out that the sky is massive, and we don't know where to look. IRAS is a project that few will remember. The satellite was in operation for only ten months, starting in January 1983. While operating, it conducted a far-infrared survey of 96 percent of the sky.

Astronomers believe in infrared wavelengths cool objects like Planet Nine may be detectable. The possibility of detection in that wavelength is what led Rowan-Robinson to take another look at the data. He found in June, July, and September 1983, he found that the satellite had discovered what appeared to be an object moving across the sky. However, the detections were made in a low galactic latitude close to the plane of the galaxy, and the satellite was strongly affected by galactic cirrus.

Galactic cirrus are filamentary clouds in space that glow in far-infrared, raising the possibility that the detections by IRAS were noise from those clouds. Tempering the excitement about the old data is that a much more sensitive survey called Panoramic Survey Telescope and Rapid Response System operating since 2008 hasn't discovered the candidate.

Rowan-Robinson says that if we assume the data gathered by IRAS is accurate, we can make some assumptions about Planet Nine. The data indicates it would be about three to five times the mass of Earth and orbits about 225 astronomical units from the sun. The data also gives us a good indication of where we should look now to discover the planet and where we can look in other data.