Planet X and SPHEREx
Could SPHEREx see Planet X?
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Why one spot of sky cannot show it
SPHEREx comes back to the same sky every half year, and by then it looks from the other side of the Earth's orbit. A body a few hundred AU away therefore appears to jump against the stars (parallax), far more than it moves along its own orbit. We measured the size of the jump from the real exposure times of our five strips.
How faint we can find it, against how bright it should be
Curves: the visual brightness a planet of 2.07 Earth radii would have at each distance, for three assumed reflectivities. Lines with dots: the faintest brightness at which our search still finds half, and nine in ten, of the test bodies we added to the real images. Above our lines we would have found it; below them we would not.
Where we looked
The predicted orbit on the sky, the band that holds 95 % of the predicted positions, and our five strips. The places were fixed and written down before any image was read.
What the search found
The test behind the limit
We added artificial bodies to the real images of the same five strips, on exact orbits and with the real image sharpness, and searched again with the same code. A body counts as found only if the search links it in three seasons and reports it as a candidate.