Pluto and the
Pluto: Discovery and Orbit
- Discovery: Not
visible with the naked eye; Predicted in the 1800s on the basis of
inaccurate measurements of the position of Uranus and Neptune, then
observed (by chance) with a telescope in 1930 close to the position
predicted by Percival Lowell.
- Origin of name: The
Roman god of the dead and the underworld (or Percival Lowell!)
- Orbit: 40
AU on average, but sometimes closer than Neptune due to its eccentricity;
tilted 17.2°! Period 248 yr, locked in a 3:2 resonance with Neptune
(that prevents them from colliding).
is it? Classified
as a planet after its discovery, when no other Trans-Neptunian Object
was known; The status grew increasingly uncertain after the 1992
discovery of the Kuiper Belt and other large objects in it; In 1999
the IAU reconfirmed it as a planet, but after an object larger than
Pluto (2003 UB313, Eris) was discovered, in 2006 the IAU voted to
reclassify it as a "dwarf planet", and in 2008 the term "plutoid"
Our best images are blurred ones from the HST; Covered
with nitrogen frost, with markings from complex molecules.
- Size and rotation: 1/5 Earth
radii [about 2300 km across]; We know size and mass [0.0025 Earth masses
and 0.06 of its gravity] because of Charon eclipses; Very tilted,
- Physical properties: We know something
about its composition from the density and from images [density
2.3 times that of water; made mostly of water ice], and about
its atmosphere from eclipses; Similar to Triton.
Not reached yet, but the New
Horizons spacecraft, launched in 2006, has passed the
orbit of Uranus and should reach Pluto by
2015 [it is the fastest spacecraft
yet, at > 83,000km/h after its 2007 Jupiter encounter].
- Charon: By far the largest moon; Named
after the boatman who ferries souls across the river Styx, discovered
in 1978; It has 1/6 of Pluto's mass,
about 1/2 of Pluto's radius
[about 1200 km across], is only 20,000 km from Pluto, and has active geysers
on its surface.
- Orbit: The masses of Pluto and
Charon are close, so they both orbit around
the common center of mass [once every 6.4 days], with their rotations
locked so that they always show the same side to each other!
- Origin: Do Pluto's and Charon's
strange orbital characteristics hint at an impact? Simulations indicate
Charon may have formed like our Moon, but we need to understand Kuiper
Belt objects better, and why relatively many of them are binaries.
- Other moons: Nix and Hydra (45–160 km in diameter) were
discovered in 2005; Two more moons have been discovered in 2011 and 2012; There may be more.
- What is it?
A swarm of icy/rocky objects beyond the orbit of Neptune,
between 30 and 50 AU or so from the Sun, where many comets
come from (Kuiper Belt Objects or Trans-Neptunian Objects); As
of 2013, thousands of individual ones are known, but the total number is much larger.
- History: Predicted by Kenneth
Edgeworth in the 1940s and Gerard Kuiper in the 1950s; Discovered
in 1992 (until then, only Pluto was known); Currently studied
by the Spacewatch project and others from Earth.
- Origin and evolution: Probably
formed from leftover material beyond the orbit of Neptune after
planet formation; We don't know much about its evolution, but
looking at orbits of different types of objects in it will give
us clues on possible past events that affected the Solar System.
Kuiper Belt Objects
- Size and shape: We can estimate
the size from their brightness if we make a guess for their albedo,
like for asteroids; Shape can be estimated from their brightness
variations if they rotate.
- Large objects: The largest known Kuiper
Belt object is Eris (a.k.a. 2003 UB313, or "Xena" the "10th
2400 km across), then Pluto, 2003 VB12 "Sedna" (about
1600 km, in a very elliptic orbit that reaches 900 AU),
2002 LM60 "Quaoar" (about 1250 km, in an almost
circular orbit at 43 AU),
and there may be other large ones; Some are in binary pairs, probably
formed by gravitational attraction.
- Other special ones: The oddest
one is Haumea (2003 EL61), the fifth dwarf planet, football-shaped
and spinning once every 3.9 hr, with two known ice moons.
- Plutinos: The ones that, like
Pluto, are in a 2:3 resonance with Neptune.
- Centaurs: There are also ambiguous
cases, intermediate between comets or Kuiper Belt objects and
asteroids; Some have been found inside the orbit of Neptune;
Chiron, 170 km across, is between Saturn and Uranus.
"Pluto started out as the ninth planet, a supported fulfillment of
Percival Lowell's prediction of Planet X. Let's simply retain Pluto as the
ninth major planet. After all, there is no Planet X. For 14 years, I combed
of the entire sky down to 17th magnitude, and no more planets showed
up. I did the job thoroughly and correctly... Pluto was your last chance for
a major planet."
– Clyde Tombaugh, discoverer of Pluto,
in a 1994 letter to Sky & Telescope
"It's pretty clear, if we discovered Pluto
today, knowing what we know about other objects
in the Kuiper Belt, we wouldn't even consider it a planet"
– Michael Brown, California Institute of Technology.
page by luca bombelli <bombelli at olemiss.edu>,
modified 24 oct 2013