Pluto: From Planet to Dwarf Planet to…Comet?

May 30, 2018: Actually, as this story explains, information from the New Horizons probe suggests that Pluto might be the result of many, many ancient comets! I wonder if they were like the one humanity visited via Rosetta in 2016. Perhaps, before the end of this century, we will land on Pluto and find out.

By the way, I’m adding a much better image (from Wikimedia) to this update in 2018, now that we have visited the dwarf planet.


Pluto is way out there. We only found it in the 20th century, while looking for Planet X (which, as it turns out, doesn’t exist).

Then in the 21st century we demoted Pluto to dwarf planet status. Now we have sent a spacecraft to see, among other things, if it is actually a comet.

Where is Pluto?

Scientists have divided the Solar System into three general zones.

  • We’re part of the inner zone, along with the asteroid belt and the other rocky planets – Mercury, Venus, and Mars
  • Then come the gas giants – Jupiter, Saturn, Uranus, and Neptune
  • Finally, there is the outer zone, which contains the Kuiper Belt – a vast collection of icy rocky bodies that orbit the Sun in roughly the same plane as the planets

    Note Pluto's inclined orbit.  Image source
    Note Pluto’s inclined orbit. Image source

    Pluto and its moons are in the Kuiper Belt. So are less massive objects, like comets.

    NASA gives a nice size comparison:

    If the sun were as tall as a typical front door, Earth would be the size of a nickel, dwarf planet Pluto would be the size of a head of a pin and the largest Kuiper Belt comet (about 100 km across, which is about one twentieth the size of Pluto) would only be about the size of a grain of dust.

    Why is Pluto a dwarf planet?

    Early telescopes couldn’t directly see Solar System objects beyond Uranus because of the distances involved. Astronomers had to use indirect methods, like math.

    In the 1840s, a French scientist detected changes in the orbit of Uranus that could only be due to another planet. That turned out to be Neptune, discovered in 1846.

    Pluto wasn't exactly Captain Obvious.  Source
    Pluto wasn’t exactly Captain Obvious. Source

    After that, observations of Neptune still didn’t completely explain the changes in Uranus’s orbit.

    There had to be something else out there – Planet X.

    Astronomers – particularly Percival Lowell – set out to find Planet X. The search was in vain until 1930, when young Clyde Tombaugh, working at the Lowell Observatory, identified the ninth planet, Pluto, beyond Neptune.

    That’s right. The Solar System only had eight planets in 1929, just like it does now. It kind of makes you wonder what all today’s fuss over Pluto is about.

    Our system’s planet count has been as high as 10 at times when Ceres, Pallas, Juno, or Vesta were being counted as planets.

    Once Tombaugh found the planet Pluto, observations of it began, of course. These were difficult, so it wasn’t until 1978 that the planet’s size was determined accurately enough to show that Pluto is too small to affect any of the gas giants.

    Another search for Planet X began. Then, in the 1990s, Voyager 2 took measurements that showed a slight overestimation of Neptune’s mass had caused the discrepancies in the orbit of Uranus.

    Today, most scientists believe there is no Planet X, as originally suggested, in the outer Solar System. (This is not to say that there aren’t other dwarf planets beyond Pluto.)

    There are, however, some sizable trans-Neptunian objects out there.

    Eris is more massive than Pluto but may be about the same size.  It sits further out, in the scattered disc.  Source
    Eris is more massive than Pluto but may be about the same size. It sits further out, in the scattered disk. Source

    Starting in 1992, scientists began finding small icy objects beyond Neptune that were similar to Pluto in size, composition and orbit. Neptune’s moon Triton, larger than Pluto, was recognized by many as a captured Kuiper Belt object.

    The outer Solar System started seeming pretty crowded, so the International Astronomical Union in 2006 officially defined a planet as an object:

    • That has an orbit around the Sun
    • That has enough mass to be a sphere by its own gravitational force
    • That has cleared the neighborhood around its orbit

    Pluto fails the test because of the last requirement. Its mass is very close to that of other objects in its orbit. The IAU therefore created the “dwarf planet” classification for Pluto.

    The media presence of the late 20th century/early 21st century ensured this change would become a public circus, compared to earlier downgrades of Ceres and the other former planets.

    Artist's impression of Ceres Source
    Artist’s impression of Ceres Source

    Is Pluto A Comet?

    As we saw last week, the line between asteroids and comets can be a blurry one. For example, the dwarf planet Ceres – the largest object in the asteroid belt – recently developed water vapor jets…like a comet.

    Based on measurements from Earth, scientists believe that Pluto has lots of ice in its structure…somewhat like a comet.

    Pluto’s atmosphere, discovered in the 1980s, is believed to be a thin layer of nitrogen, methane and carbon monoxide that freezes out when Pluto is farthest from the Sun and then sublimates back into gas on the dwarf planet’s closest approach to the Sun.

    Is that like a comet or not?

    Well, it depends, says the New Horizons mission team:

    Pluto’s very small gravitational acceleration (approximately 1/16 of Earth’s gravity) leads scientists to think that carbon monoxide, methane and molecular nitrogen are escaping from Pluto’s atmosphere at a rate of approximately 100 to 1,000 pounds (about 50-500 kilograms or 2-20 x 1027 molecules) per second.

    The behavior is similar to that of a comet, though Pluto is more than 100 times larger than a typical comet nucleus and materials blow away from comets at rates 100 to 1,000 times faster.

    If Pluto’s atmospheric escape rate is significantly greater than 100 pounds (about 50 kilograms, or 2 x 1027 molecules) per second then Pluto will resemble a comet. If the escape flux is significantly less, scientists expect the solar wind to be at least partially deflected around Pluto’s electrically-conducting ionosphere, similar to the situation at Mars or Venus.

    We will know more about it in 2015, when the New Horizons spacecraft visits Pluto.

    Maybe Planet X, er, planet, er, dwarf planet Pluto will turn out to be a comet’s cousin. That doesn’t mean that it might some day change paths and suddenly streak sunward to scare the blazing daylights out of us puny humans.

    No, Pluto is more massive than the Kuiper Belt objects that sometimes get knocked out of kilter by gravity changes from the gas giants. It isn’t going to come to us.

    Instead, we are going to it, curious about little Pluto, what it looks like, what its true nature is, and what it can teach us.

    There will be surprises. There may be confirmation of some of our notions about Pluto. And certainly, starting in May 2015 and lasting through August, there will be a spectacular show.

    New Horizons will arrive at Pluto in July 2015.  NASA
    New Horizons will arrive at Pluto in July 2015. NASA


    Front page image: NASA, ESA and G. Bacon (STScI)

    More information:

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