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Title: Evidence of Controversial Planet 9 Uncovered In Sky Surveys Taken 23
Years Apart

Link: https://science.slashdot.org/story/25/05/03/0...

Astronomers may have 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. 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.
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]."

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