The Discovery of Two New Satellites of Pluto

Pluto’s first known moon, Charon, was discovered in 19781 and has a diameter about half that of Pluto2–4, which makes it larger relative to its primary than any other moon in the Solar System. Previous searches for other satellites around Pluto have been unsuccessful5–7, but they were not sensitive to objects ∼ <150 km in diameter and there are no fundamental reasons why Pluto should not have more satellites6. Here we report the discovery of two additional moons around Pluto, provisionally designated S/2005 P1 (hereafter P1) and S/2005 P2 (hereafter P2), which makes Pluto the first Kuiper belt object (KBO) known to have multiple satellites. These new satellites are much smaller than Charon (diameter ∼1200 km), with P1 ranging in diameter from 60–165 km depending on the surface reflectivity, and P2 about 20% smaller than P1. Although definitive orbits cannot be derived, both new satellites appear to be moving in circular orbits in the same orbital plane as Charon, with orbital periods of ∼38 days (P1) and ∼25 days (P2). The implications of the discovery of P1 and P2 for the origin and evolution of the Pluto system, and for the satellite formation process in the Kuiper belt, are discussed in a companion paper8. We observed Pluto with the Hubble Space Telescope (HST) using the Wide-Field Channel (WFC) mode of the Advanced Camera for Surveys (ACS) on UT 2005 May 15 and May 18 (Fig. 1). The ACS/WFC consists of two 4096 × 2048 pixel CCDs (WFC1 and WFC2) butted together, effectively forming a single 4096 × 4096 pixel camera with a gap of ∼50 pixels between the two CCDs. The F606W (“Broad V”) filter, which has a center wavelength of 591.8 nm and a width of 67.2 nm, was used for all images. At the time of the observations, Pluto was 31.0 astronomical units (AU) from the sun, 30.1 AU from the Earth, and had a solar phase angle of 0.96 deg on May 15 and 0.88 deg on May 18. Identical strategies were employed on each observing date. First, a single short exposure (0.5 s) was taken to enable accurate positions of Pluto and Charon to be measured on unsaturated images. Then, two identical, long exposures (475 s) were taken at the same pointing to provide high sensitivity to faint objects. Finally, the telescope was moved by ∼5 pixels in one dimension and ∼60 pixels in the other dimension, and two identical, long exposures (475 s) were taken to provide data in the region of the sky falling in the inter-chip gap during the first two long exposures. The telescope was programmed to track the apparent motion of Pluto (∼3 arcsec hr) for all exposures. The two new satellites are detected with high signal-to-noise ratio (S/N ≥ 35) and have a spatial morphology consistent with the ACS point spread function (PSF; this is the spatial brightness