A team of astronomers led by Mark Showalter of the SETI Institute have used the Hubble Space Telescope (HST) to find a tiny, previously undiscovered moon orbiting around Pluto. This discovery of the fifth moon comes a little more than a year after the fourth moon was discovered. Both these moons are so new that they have yet to be named and are simply referred to as P4 and P5.
P5 is incredibly small, with a diameter of between 10 and 25 kilometers. If that figure seems a little imprecise, it’s because it’s almost a miracle that something so small and distant can be seen at all. Even to the HST, it just appears as a point of light, like a faint 27th magnitude star. This means that the only way to estimate its size is to guess what it’s made of, and then calculate how big it would need to be to reflect that amount of sunlight.
Incidentally, 27th magnitude is very very faint. How faint? Consider that the brightest stars in the sky (apart from the Sun) are all around magnitude zero, and the faintest that can be seen in the darkest city suburbs are probably about magnitude five. P5 is about a six-hundred-millionth as bright as that. That’s pretty impressive, even for the Hubble!