The Solar System:

Pluto ...and Beyond

Statue of Pluto and Cerberus (2nd century CE).

Credit: 2nd century CE statue in the Heraklion Archaeological Museum; photo by Carole Raddato (Wikimedia Commons), CC BY SA 2.0 [link]

"Charon carries souls across the river Styx" by Alexander Litovchenko (1835-1890 CE), from 1861.

Credit: Alexander Litovchenko (1861); Russian Museum, St. Petersburg [link]

A new planet is discovered...

These photographic plates, taken 6 days apart in October 1930 with the 13 inch telescope at Lowell Observatory, enabled Clyde Tombaugh to discover Pluto. The arrows point to the position of Pluto in each photograph. Pluto's movement compared to the background of more distant stars identified it as an object orbiting the Sun within our Solar System.

Credit: Lowell Observatory Archives; Bruce Murray Space Image Library, the Planetary Society [link]

Pluto has an orbital period around the Sun of 248 years. It will complete its first orbit of the Sun since its discovery in the year 2178.

A moon for the new planet

While making observations of Pluto in 1978 to better determine its orbit, U.S. Naval Observatory (USNO) astronomer James W. Christy noticed that a number of the images of Pluto appeared elongated (left image here), but images of background stars on the same photographic plate did not. Other plates showed the planet as circular (right image here). Christy examined a number of Pluto images from the USNO archives, and he noticed the elongations again. Furthermore, the elongations appeared to change position with respect to the stars over time. After eliminating the possibility that the elongations were produced by plate defects and background stars, the only plausible explanation was that they were caused by a previously unknown moon orbiting Pluto at a distance of about 19,600 km (12,100 miles) with a period of just over six days. The discovery received the provisional designation "1978 P 1;" Christy proposed the name "Charon", after the mythological ferryman who carried souls across the rivers Styx and Acheron into Pluto's underworld.

By convention starting with Christy's usage, the name of Pluto's moon is not pronounced with an initial hard "k" sound (like the name of the mythical ferryman), but with a soft "sh" sound. In fact, the first syllable of the name of Charon the moon is pronounced like the first four letters of "Charlene", which is, coincidentally, the name of Christy’s wife.

Credit (image and some text): J. W. Christy (U.S. Naval Observatory) [link]

So many moons...

An additional pair of small moons of Pluto, later named Nix and Hydra, were discovered from observations made in 2005 with NASA's Hubble Space Telescope. Nix and Hydra are roughly 5,000 times fainter than Pluto and are about two to three times farther from Pluto than its large moon, Charon.

Credit (image and some text): NASA, ESA, H. Weaver (JHU/APL), A. Stern (SwRI), and the HST Pluto Companion Search Team [link]

Hubble Space Telescope image of Pluto and all five of its known moons, including Kerberos (discovered in 2011) and Styx (discovered in 2012).

Credit: NASA, ESA, and M. Showalter (SETI Institute) [link]

First visit to Pluto

New Horizons: "Shedding light on frontier worlds"

The New Horizons mission was launched in 2006, and flew by Pluto in 2015. In order to expedite the trip to the far outreaches of the Solar System, the spacecraft was traveling too fast to go into orbit around Pluto. (In fact, New Horizons is traveling so fast that it will eventually leave the Solar System altogether, joining Pioneer 10 & 11 and Voyager 1 & 2 as humanity's fleet of interstellar spacecraft).

After nine years and three billion miles, New Horizons flew past Pluto in mid-July 2015, with its closest approach within 12,500 km (7800 miles) from Pluto's surface, at a maximum velocity of almost 14 km/s (more than 30,000 mph). At a distance of more than 40 AU from Earth, it took another 15 months for New Horizons to transmit back the full complement of data collected during the Pluto flyby (amounting to a mere 6.25 Gb).

Logo of the New Horizons mission.

Credit: NASA, SwRI, JHU, USDOE, JPL [link]

Family portrait of Pluto's moons

Composite image from New Horizons showing a sliver of Pluto’s large moon, Charon, and all four of Pluto’s small moons. The moons are shown at the same relative brightness and size scales. Charon is by far the largest of Pluto’s moons, with a diameter of 1212 kilometers (751 miles). Nix and Hydra have comparable sizes, approximately 40 kilometers (25 miles) across in their longest dimension. Kerberos and Styx are much smaller and also have comparable sizes, roughly 10-12 kilometers (6-7 miles) across in their longest dimension. All four small moons have highly elongated shapes, a characteristic thought to be typical of small bodies in the Kuiper Belt.

Credit (image and some text): NASA/JHUAPL/SwRI [link]

New Horizons images of Pluto and Charon

(Shown at the correct relative size scale and in approximate true color.)


Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute/Alex Parker [link]


Credit: NASA/JHUAPL/SwRI [link]

Double planets?

  • Charon’s diameter (1212 km) is 51% of Pluto’s diameter (2377 km)

  • The Moon’s diameter (3474 km) is 27% of Earth’s diameter (12,742 km)

  • The Pluto-Charon orbital separation is less than 20,000 km, with an orbital period of 6.4 days

  • The Earth-Moon orbital separation is 385,000 km, with an orbital period of 27.3 days

Planet no more...

1930: Pluto is discovered, declared 9th planet of the Solar System.

2005: Eris is discovered in the outreaches of the Solar System.

      • Trans-Neptunian Object.

      • Orbital distance = 96 AU.

      • 27% more massive than Pluto (but 10 km smaller).

      • A handful of other large TNOs have been discovered since then.

      • There are potentially hundreds to thousands more out there.

          • That’s a lot of “planets”!

          • To make matters worse: discovery of new planets happens so rarely that there are no agreed-upon rules for naming planets, so who gets to do it if Eris is classified as a planet?

Animation showing the three discovery images for Eris. The images were obtained over 3 hours on 21 October 2003, but Eris was not discovered in the images until 2005.

Credit: Mike Brown (Caltech) [link]

2006: The International Astronomical Union formally defines “planet” with three criteria:

(1) A planet is in orbit around the Sun.

Got #1 only? Then you’re a “small solar system body”.

Pluto? Yes!

(2) A planet has sufficient mass to assume a nearly round shape.

Got #1 and #2 only? Then you’re a “dwarf planet”.

Pluto? Yes!

(3) A planet has "cleared the neighborhood" around its orbit.

Got all three? Congratulations, you’re a “planet”.

Pluto? No.

Pluto was demoted to a dwarf planet shortly after the launch of New Horizons. The Principal Investigator ("boss") of the New Horizons Mission was, mmmm, let's just say, "not pleased."

Current: Disagreement over criterion #3. Too vague and possibly Earth, Mars, Jupiter, and Neptune also do not qualify (e.g., there are approximately 10,000 known Near-Earth Asteroids - has Earth really "cleared its neighborhood"?).

    • Is it possible that Pluto might get promoted back to planet?

Dear Earth,

You suck.

Love, Pluto


New Horizons

  • Johns Hopkins University Applied Physics Laboratory, 2015 (July 14) - "NASA's Three-Billion-Mile Journey to Pluto Reaches Historic Encounter" [link]

  • Chang, K., 2016 (October 28), New York Times - "No More Data From Pluto" [link]