Astronomers may have [1]found the best candidate yet for the elusive Planet Nine : a mysterious object in infrared sky surveys taken 23 years apart that appears to be more massive than Neptune and about 700 times farther from the sun than Earth. Space.com reports:

> [A] team led by astronomer Terry Long Phan of the National Tsing Hua University in Taiwan has delved into the archives of two far-infrared all-sky surveys in search of Planet Nine -- and incredibly, they have found something that could possibly be Planet Nine. The Infrared Astronomy Satellite, IRAS, launched in 1983 and surveyed the universe for almost a year before being decommissioned. Then, in 2006, the Japanese Aerospace Exploration Agency (JAXA) launched AKARI, another infrared astronomy satellite that was active between 2006 and 2011. Phan's team were looking for objects that appeared in IRAS's database, then appeared to have moved by the time AKARI took a look. The amount of movement on the sky would be tiny -- about three arcminutes per year at a distance of approximately 700 astronomical units (AU). One arcminute is 1/60 of an angular degree.

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> But there's an extra motion that Phan's team had to account for. As the Earth orbits the sun, our view of the position of very distant objects changes slightly in an effect called parallax. It is the same phenomenon as when you hold your index finger up to your face, close one eye and look at your finger, and then switch eyes -- your finger appears to move as a result of you looking at it from a slightly different position. Planet Nine would appear to move on the sky because of parallax as Earth moves around the sun. On any particular day, it might seem to be in one position, then six months later when Earth is on the other side of the sun, it would shift to another position, perhaps by 10 to 15 arcminutes -- then, six months after that, it would seem to shift back to its original position. To remove the effects of parallax, Phan's team searched for Planet Nine on the same date every year in the AKARI data, because on any given date it would appear in the same place, with zero parallax shift, every year. They then also scrutinized each candidate object that their search threw up on an hourly basis. If a candidate is a fast-moving, nearby object, then its motion would be detectable from hour to hour, and could therefore be ruled out. This careful search led Phan's team to a single object, a tiny dot in the infrared data.

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> It appears in one position in IRAS's 1983 image, though it was not in that position when AKARI looked. However, there is an object seen by AKARI in a position 47.4 arcminutes away that isn't there in the IRAS imagery, and it is within the range that Planet Nine could have traveled in the intervening time. In other words, this object has moved a little further along its orbit around the sun in the 23 or more years between IRAS and AKARI. The knowledge of its motion in that intervening time is not sufficient to be able to extrapolate the object's full orbit, therefore it's not yet possible to say for certain whether this is Planet Nine. First, astronomers need to recover it in more up-to-date imagery. [...] Based on the candidate object's brightness in the IRAS and AKARI images, Phan estimates that the object, if it really is Planet Nine, must be more massive than Neptune. This came as a surprise, because he and his team were searching for a super-Earth-size body. Previous surveys by NASA's Wide-field Infrared Survey Explorer (WISE) have ruled out any Jupiter-size planets out to 256,000 AU, and any Saturn-size planets out to 10,000 AU, but a smaller Neptune or Uranus-size world could still have gone undetected. Phan told Space.com that he had searched for his candidate in the WISE data, "but no convincing counterpart was found because it has moved since the 2006 position," and without knowing its orbit more accurately, we can't say where it has moved to.

"Once we know the position of the candidate, a longer exposure with the current large optical telescopes can detect it," Phan told Space.com. "However, the follow-up observations with optical telescopes still need to cover about three square degrees because Planet Nine would have moved from the position where AKARI detected it in 2006. This is doable with a camera that has a large field of view, such as the Dark Energy Camera, which has a field of view of three square degrees on the Blanco four-meter telescope [in Chile]."



[1] https://www.space.com/astronomy/solar-system/evidence-of-controversial-planet-9-uncovered-in-sky-surveys-taken-23-years-apart