Pluto

Mark Showalter, Zolt Levay,
SETI, STScI, ESA, NASA



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On June 28, 2011, a fourth moon was found
around Pluto (more).

On July 20, 2011, astronomers working with the Hubble Space Telescope announced the discovery of a fourth moon around Pluto. Found in images taken on June 28, 2011, the object is currently designated "P4." This new satellite is the smallest yet found, with an estimated diameter of only 8 to 21 miles (13 to 34 km). By comparison, Pluto's largest moon Charon spans 648 miles (1,043 km) across, while the other two satellites discovered in 2003, Nix and Hydra, are 20 to 70 miles (32 to 113 km) wide. P4 moves between the orbits of Nix and Hydra and completes an orbit around Pluto roughly every 31 days. Its discovery supports the hypothesis that that all four satellites were formed during a massive collision 4.6 billion years ago. The astronomers found P4 while searching for rings or other potential hazards for NASA's New Horizons probe, which is due to pass through the Pluto system in 2015 (HubbleSite.org news release; NASA science news; Astronomy Picture of the Day; Alan Boyle, MSNBC, July 20, 2011; and David Shiga, New Scientist, July 20, 2011).

Ann Feild, STScI, ESA, NASA





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The new object moves around
Pluto within 31 days between
the orbits of Nix and Hydra,
which were discovered in
2003 (more).

NASA



Larger image.



Pluto probably
resembles
Neptune's
largest moon,
Triton.

On June 11, 2008, the International Astronomical Union (IAU) voted at the meeting of its Executive Committee to establish bright "dwarf planets beyond the orbit of Neptune as a new class of substellar objects in the Solar System called "plutoids" (IAU press release). According to the IAU, these plutoids "are celestial bodies in orbit around the Sun at a distance greater than that of Neptune that have sufficient mass for their self-gravity to overcome rigid body forces so that they assume a hydrostatic equilibrium (near-spherical) shape, and that have not cleared the neighbourhood around their orbit. Satellites of plutoids are not plutoids themselves, even if they are massive enough that their shape is dictated by self-gravity. As of July 14, 2008, the three known and named plutoids are Pluto, Eris, and Make-make. In addition, "it was decided that, for naming purposes, any Solar System body having (a) a semimajor axis greater than that of Neptune, and (b) an absolute magnitude brighter than H = +1 will, for the purpose of naming, be considered to be a plutoid," so that many large and relatively round Edgeworth-Kuiper ice bodies may eventually be designated as plutoids.

Unknown artist, ESO



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In addition to two recently
discovered small moons,
Pluto has a moon, Charon,
that is roughly half it's
size, with about an eighth
of its mass (more).

Pluto has a moon Charon that is so large relative to it size that it is often called a double planet, as Charon is about half Pluto's diameter while Earth's Moon is only one-quarter of Earth's diameter. Like the Earth's Moon, Charon may be the result of a collision between Pluto and another large planetary body, as Pluto’s axis of rotation and Charon’s orbit are highly inclined which could result from such an impact. (See an animation of the orbit of Charon around Pluto or of the orbit of Pluto around the Sun, with a table of basic orbital and physical characteristics.)

Although Pluto is usually farther away from the Sun than any other planet, its orbit is highly elliptical ("eccentric"), and so Pluto is closer than Neptune for 20 years out of its 249-year orbit. As Pluto approaches its close approach to the Sun (at perihelion), it also reaches its farthest distance from the plane of the planets ("ecliptic") due to its orbital inclination of 17 degree, which is much higher than those of the other planets. Thus, Pluto is always far above or below the plane of Neptune's orbit, and so the two planets will not collide because they do not approach any closer than 18 AUs (18 times the distance between the Earth and the Sun) to one another.

Pluto's period of rotation is 6.4 days, which is the same as its moon Charon. Although many other moons in the Solar System travel in a synchronous orbit with their planet, Pluto is the only planet to rotate synchronously with the orbit of its largest satellite. This means that they are tidally locked so that the two present the same face to each other as they travel through space.

Like Venus and Uranus, Pluto actually rotates in the opposite direction (retrograde rotation) from most of the other planets. Pluto is locked in a 3:2 resonance orbit with Neptune (where its orbital period is exactly 1.5 times longer than Neptune's). Like Uranus, the plane of Pluto's equator is at almost right angles to the plane of its orbit.

Pluto's surface temperature varies ranges from -228 to -238 °C (-378 to -396 °F) during the course of its orbit, as the planet gets as close to the Sun as 30 AU before swinging as far away as 50 AU. In January 2006, astronomers announced that Pluto is "colder than it should be" at 43 K (-382 degrees F) in recent observations -- significantly colder than its largest moon, Charon (Smithsonian press release). The finding agrees with the current model that the low temperature of Pluto results for the equilibrium between the planet's surface ice and its thin nitrogen atmosphere, not just from the weak sunlight received. Sunlight reaching Pluto's surface converts some of the surface nitrogen ice to gas with a cooling effect, rather than heat the surface, through a process similar to the way evaporation of sweat cools Human skin.

Ron Miller, NASA



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The dwarf planet may have
sundogs and eruptions of
geysers when it orbits
closer to the Sun (more).

The planet's icy surface is virtually all (98 percent) nitrogen, with some methane and traces of carbon monoxide. A bright highlight on Pluto suggests that it has a smoothly reflecting surface layer, and that there is a bright area parallel to the equator on Pluto. The bright areas seem to be covered with ices of nitrogen with smaller amounts of solid methane, ethane, and carbon monoxide. Darker areas may primordial organic material or the result of photochemical reactions driven by cosmic rays. In general, Pluto is somewhat brighter and pinker than its moon Charon. While Pluto's internal composition is unknown, Hubble Space Telescope observations suggest that the planet's density lies between 1.8 and 2.1 gm/cm³ which indicates that the planet is probably a mixture of 50 to 70 percent rock with the remainder being water and other ices much like Nepture's moon Triton.

Luis Calçada, ESO



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Pluto and Eris share a very similar planetary
surface, that is mostly covered in nitrogen ice
with a small amount of methane ice (more).

The planet probably has a thin atmosphere that freezes and falls to the surface as Pluto moves away from the Sun. Given the composition of its surface materials, Pluto's atmosphere probably consists primarily of nitrogen with some carbon monoxide and methane. It must be extremely tenuous since surface pressure is only a few microbars. When Pluto is closest to the Sun (near its perihelion), some of the atmosphere's substance may escape into space and perhaps interact with its large moon, Charon. For most of Pluto's long orbital year, however, the atmospheric gases freeze into ice.

Gemini Observatory/NOAO/NSF (Pluto and Charon in near infrared -- larger image)
© Association of Universities for Research in Astronomy, Inc. (AURA). All Rights Reserved.

With a diameter between 750 and 753 miles (1,207 and 1,212 kilometers), Charon is the largest moon with respect to its primary planet in the Solar System. Its density is 1.71 times that of water, which suggests its icy contents are just more than 50 percent rock, and about 10 percent less rock by mass than Pluto. The moon probably does not have a significant atmosphere (ESO press release).

Charon's color is like neutral gray, and Hubble observations indicate that Charon is bluer than Pluto, implying that both worlds have different surface composition and structure. Indeed, Charon's surface seems to be rich with water ice, rather than the nitrogen ice found on Pluto. It's possible that lighter molecules on Charon's surface, such as methane, have escaped the moon's weaker gravity, and that a substantial amount eventually fell on Pluto and enriched its surface over time. (See an animation of Charon's orbit around Pluto, with a table of basic orbital and physical characteristics.)

© Tomislav Stimac

Artwork from Digital Eye
(used with permission).

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Charon viewed from Pluto
in a "naked eye simulation"
with extended light exposure
to compensate for their
actual dimness, as imagined
by Stimac.

The unusual nature of the orbits of Pluto and Triton and the similarity their physical properties suggest that there may be some historical connection between them. It was once thought that Pluto may have once been a satellite of Neptune's, but this now seems unlikely. A more popular idea is that Triton, like Pluto, once moved in an independent orbit around the Sun as a Edgeworth-Kuiper ice body and was later captured by Neptune. Perhaps Triton, Pluto, and Charon are the only remaining nearby members of relatively large Kuiper Belt objects of which most were ejected into the Oort Cloud.

Hal Weaver, Alan Stern, HST Pluto Companion Search Team, ESA,
NASA -- larger image

In 2003, two additional moons (Nix and Hydra) have been discovered
around Pluto (more).

On October 31, 2005, a team of astronomers (including Hal Weaver, Alan Stern, Max Mutchler, Andrew Steffl, William J. Merline, Marc Buie, Eliot Y. Young, John R. Spencer, and Leslie A. Young) using the Hubble Space Telescope announced their apparent discovery of two new moons orbiting Pluto, in addition to Charon (Hubble news release; latest news; and Stern et al, 2005). On June 22, 2006, astronomers associated with the discovery of the two new moons announced that the International Astronomical Union had approved the names of Nix for the inner satellite and Hydra for the outer one (press release and Astronomy Picture of the Day).

Ann Feild, STScI,
ESA, NASA





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The two new moons
are very small and
orbit in the same
plane as Charon
but at more than
twice the distance
from Pluto (more).

Estimated to be around 40 and 125 miles (64 and 200 kilometers) in diameter, the two small moons orbit about 27,000 miles (44,000 kilometers) from Pluto, more than twice as far as Charon, Pluto's other satellite. Provisionally designated S/2005 P1 and S/2005 P2, the moons are around 5,000 times dimmer than Charon. Initial observations suggest the new moons have circular orbits around Pluto in the same plane as Charon. The orbits of Charon and the two new moons appear to be in "resonance"; for every 12 times that Charon circles Pluto, P2 orbits three times and P1 twice. (More at hubblesite.org and at Astronomy Picture of the Day.) In February 2006, astronomers announced that they had confirmed the existence of the two new moons with the Hubble Space Telescope and speculated that the moons were created in the same collision that formed Charon (more from hubblesite.org and astrobio.net). On June 22, 2006, astronomers associated with the discovery of the two new moons announced that the International Astronomical Union had approved the names of Nix for the inner satellite and Hydra for the outer one (press release and Astronomy Picture of the Day).

Mark Showalter, Zolt Levay,
SETI, STScI, ESA, NASA



Larger and jumbo images.



On June 28, 2011, a fourth moon was found
around Pluto (more).

On July 20, 2011, astronomers working with the Hubble Space Telescope announced the discovery of a fourth moon around Pluto. Found in images taken on June 28, 2011, the object is currently designated "P4." This new satellite is the smallest yet found, with an estimated diameter of only 8 to 21 miles (13 to 34 km). By comparison, Pluto's largest moon Charon spans 648 miles (1,043 km) across, while the other two satellites discovered in 2003, Nix and Hydra, are 20 to 70 miles (32 to 113 km) wide. P4 moves between the orbits of Nix and Hydra and completes an orbit around Pluto roughly every 31 days. Its discovery supports the hypothesis that that all four satellites were formed during a massive collision 4.6 billion years ago. The astronomers found P4 while searching for rings or other potential hazards for NASA's New Horizons probe, which is due to pass through the Pluto system in 2015 (HubbleSite.org news release; NASA science news; Astronomy Picture of the Day; Alan Boyle, MSNBC, July 20, 2011; and David Shiga, New Scientist, July 20, 2011).

Ann Feild, STScI, ESA, NASA





Larger and jumbo illustrations.





The new object moves around
Pluto within 31 days between
the orbits of Nix and Hydra,
which were discovered in
2003 (more).