Pluto?

1
Pluto?

• Pluto is still included in your book as a planet,
  but has recently been demoted to a dwarf planet
• We will discuss some of the characteristics of
  Pluto today and compare them to the other planets
• With your current knowledge of the planets, you
  can also judge whether Pluto is 'worthy' of
  planetary status

2
Properties of Pluto
Semimajor Axis 39.5 A.U.
Orbital Eccentricity 0.249
Orbital Period 248 years
Rotational Period -6.4 days
Mass 1.27 x 1022 kg 0.0021 x Earth
Radius 1137 km 0.18 x Earth
Average Density 2.06 g/cm3 0.374 x Earth
Surface Gravity 0.66 m/s2 0.067 x Earth
Surface Temperature 40 - 60 K 300K for Earth
Atmosphere Methane?
3
Discovery of Pluto

• As was the case with Neptune, it was believed
  that an unseen planet was still tugging on the
  others
• The lead searcher for this undiscovered planet
  was Percival Lowell, who spent over a decade
  looking
• _
• Surprisingly, Pluto is too small to have any
  gravitational effects on Uranus or Neptune, and
  was found essentially by chance

4
Pluto's Orbit

• _
• At perihelion, Pluto is only 29.66 A.U. from the
  Sun, while at aphelion, Pluto is 49.31 A.U. away
• This orbit actually brings Pluto inside of
  Neptune's orbit for a period of time
• The two will never collide, however, because the
  planets share a 32 resonance

Pluto orbits the Sun 3 times in the time it take
for 2 revolutions of Neptune
5
Observations of Pluto
Ground-based image

• Pluto's huge distance from the Sun and its very
  small size make it hard to observe
• _
• Ground-based measurements of the Pluto-Charon
  system require the best telescopes on Earth
• Hubble Space Telescope has provided the most
  detailed images of Pluto and Charon to date

HST
6
Pluto and Charon

• Pluto and Charon are very similar in size
• Pluto diameter 2270 km
• Charon diameter 1300 km
• The two bodies are tidally locked, so that the
  same side constantly faces the other body
• _
• Recently, Pluto and Charon's orbit was aligned
  with Earth so that many eclipses between the two
  were visible

7
More Moons for Pluto

• In 2005, astronomers using the Hubble Space
  Telescope discovered two more moons in orbit
  around Pluto
• The two small bodies are named Nix and Hydra
• _
• _

8
Pluto's Surface

• Direct measurements of Pluto's surface have not
  been made, but it assumed to be an ice/rock
  mixture
• Spectroscopic studies have revealed the presence
  of methane
• _
• The 'image' at right is an HST model based on
  multiple observations

9
Visiting Pluto

• The first probe to visit Pluto and Charon will be
  the New Horizons spacecraft
• New Horizons was launched in January of this year
• _
• This craft will provide the first real images of
  Pluto and Charon's surface

10
What is Pluto?

• Pluto is far from the Sun, has a very odd orbit,
  and exhibits characteristic more like a moon than
  a planet
• A combination of factors has led astronomers to
  believe that Pluto may be a member of the Kuiper
  belt
• No matter what Pluto actually is, its title as a
  planet has been stripped by the International
  Astronomical Union

?
11
What Defines a Planet?

• _

• _

12
The Troublemakers
Note All images are artist conceptions
13
The IAU Definition
The definition of a planet as of August 2006 is

1. _
2. _
3. _

A dwarf planet fulfills conditions 12. All
other objects are small Solar System bodies.