Download DEFINE A PLANET YOUR ASSIGNMENT: Make your own definition

DEFINE A PLANET
To the ancients, the planets were simple. There were five 'wanderers' (Mercury, Venus, Mars, Jupiter
and Saturn) that orbited the Earth, and two special cases of the Moon and the Sun that ruled the day and the
night. Since then, Copernicus and Kepler corrected our view of the Solar System and planetary orbits, and
we've added two new planets: Uranus and Neptune, and Sir Edmond Halley showed us there were comets.
Then things got more complicated.
In 1801, Giovanni Piazzi discovered Ceres between Mars and Jupiter. It was called the fifth planet.
Astronomers didn't like that because it was smaller than our Moon and only appeared as a dot, even through a
telescope. (http://aa.usno.navy.mil/faq/docs/minorplanets.php)
By 1846, when Adams and LeVerrier discovered Neptune (at the time, the thirteenth planet), four more
had been found. By the time they numbered in the hundreds, astronomical catalogs had separated them out as
'asteroids'. There are now more than 200,000 known, according to the Minor Planet Center:
http://www.cfa.harvard.edu/iau/lists/Apollos.html
In 1930, Clyde Tombaugh discovered Pluto, which was immediately called the ninth planet. Pluto was
also suspiciously small (smaller than our Moon), but it was unique so astronomers let it slide. When Pluto's
moon Charon was discovered, Pluto turned out to be even smaller than they originally thought. The
International Astronomical Union again decided to keep Pluto.
In 1992, the object 1992 QB1 was discovered, and since then many other Trans-Neptunian Objects have
been discovered, many of them quite large. Finally, in 2005 the object “Eris” was discovered, and shown to be
larger than Pluto. It was immediately called the Tenth planet. Pluto was not unique any more.
Additionally, since the discovery of PSR 1257+12 b in 1992 (or 51 Pegasi b in 1995), 342 planet-mass
objects have been discovered in orbit of other stars. (number as of March 11th, 2009)
When the International Astronomical Union met in 2006, it had a few choices:
● Planets are the current nine planets, because astronomers don't care what things are called.
○ This ignores the problem entirely.
● Planets are the “classical eight”.
○ Two definite planets were discovered in the last 300 years, this doesn't explain what a planet IS.
● A planet is an object the size of Pluto and larger, whose principal orbit is around the Sun.
○ This makes Eris the tenth planet, but ignores all the stuff that's the same size, composition and orbit.
● A Planet is an object large enough for gravity to make it spherical.
○ Mike Brown notes that this would mean there are 53 planets.
http://web.gps.caltech.edu/~mbrown/eightplanets/
● A Planet is “A celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its selfgravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape,
and (c) has cleared the neighbourhood around its orbit.” (from http://www.iau.org/public_press/themes/
pluto/)
○ This is the one the IAU went with. Pluto, Eris, Ceres, Haumea, and Makemake are now “Dwarf
planets”, they satisfy parts (a) and (b), but not (c). Pluto now has the minor planet number 134340.
There are problems with this definition:
● Their requirement that it orbit 'the Sun' means none of the 342 extrasolar planets are really planets. Also,
the Moon's orbit around the Earth never loops back on itself as seen from the Sun.
● “Cleared the neighbourhood” is vague. There are many asteroids that cross the orbits of the planets; one
of them killed the dinosaurs. Pluto crosses Neptune's orbit. Comets cross the orbits of ALL the planets.
There are even asteroids captured or gravitationally controlled by the planets: Jupiter (and Mars, and
Neptune) has two giant clouds of asteroids (named after characters from the Trojan Wars) trapped in
gravitationally stable points in its own orbit called Lagrange Points, 1/6 of the way ahead and behind the
planet.
● Demoting Pluto irritated many astronomers, and most of the general public.
YOUR ASSIGNMENT: Make your own definition of what a planet is.
Things to consider when you make your definition:
RESONANCE: ex, Every three times Neptune orbits the Sun, Pluto orbits twice (3:2). If it didn't, Neptune
would pull it out of its orbit. When using a swingset, if you push at the wrong time you can
actually slow a person down; gravitational pull at the wrong time can actually push things away.
Other popular resonances are 2:1 and 1:1, where a planet or moon holds an asteroid in a stable
point in its own orbit (usually 1/6 of an orbit ahead or behind).
TIDALLY LOCKED: Like our moon, the rotation period is equal to the orbital period. Only one side ever faces
the planet. Mercury WOULD be tidally locked to the Sun if its orbit were more circular; several
exoplanets probably are. An eventual outcome of orbiting a massive object.
HYDROSTATIC EQUILIBRIUM: The shape is governed by gravity. Basically, hydrostatic equilibrium
happens when gravity gets large enough to pull mountains down into valleys.
The objects, by order of size (volume)
HD 80606 b A four Jupiter-mass object that has an elliptical orbit of 0.93; distance ranges between .84 AU
and .03 AU. Surface temperature ranges from 980 F to 2240 F. The star HD 80606 is a binary
system with another star HD 80607 (separation 1200 AU), but the planet only orbits HD 80606.
Jupiter
Currently the fifth planet. More massive than the rest of the Solar System put together. Thick
gas atmosphere with no solid core, rings, many moons.
Gliese 876 b 2.5 Jupiter-mass object orbiting the red dwarf star Gliese 876, 15 light years away. Due to a
quirk of how planets behave when you add more mass, it's probably the same size or smaller
than Jupiter. 60 day circular orbit (in a 2:1 resonance with Gliese 876 c).
Gliese 876 c 0.7 Jupiter-mass object in orbit of Gliese 876, 15 light years away. 30 day circular orbit (in a 2:1
resonance with Gliese 876 b).
Saturn
Currently the sixth planet. Thick gas atmosphere with no solid core, rings, many moons. It's
less dense than water, and spins fast enough that it's slightly flattened, almost a sphere.
Uranus
Currently the seventh planet. Thick gas and ice atmosphere, possibly a solid core. Spins on its
side (technically backwards), rings, many moons.
Neptune
Currently the eighth planet. Thick gas and ice atmosphere, possibly a solid core. Slightly more
massive (17 Earth masses) but smaller than Uranus. Rings, several moons.
Gliese 876 d 7.6 Earth-mass object in orbit of Gliese 876, 15 light years away. 2-day circular orbit.
Earth
Currently the third planet. Orbited by a satellite 1/80th of its mass, which makes it the closest to
a “double planet”. Substantial atmosphere, strong magnetic field, one moon, life.
Venus
Currently the second planet. Slightly smaller and less massive than the Earth, spins backwards
slowly. 93x thicker atmosphere than Earth, no moons.
Mars
Currently the fourth planet. Smaller than Earth and Venus. Thin atmosphere, two moons,
probably both captured asteroids. 1/10 the mass of the Earth, ½ the radius.
Ganymede
Less massive than Mercury. Tidally locked to Jupiter. In a 4:2:1 resonance with Europa and Io.
Has its own magnetic field, tidal forces probably keep its core liquid. 7 day orbit. 1/40th of the
mass of Earth.
Titan
Less massive than Mercury. Tidally locked to Saturn. Titan has methane rivers, possibly
methane oceans, and methane rain. It also has a substantial atmosphere (Earth-like pressure).
Mercury
Currently the first planet. No atmosphere, no moons. Elliptical orbit (0.21). Partially tidally
locked (3:2, three “days” every two “years”) to the Sun. 1/20th of the mass of Earth.
Callisto
Tidally locked to Jupiter. A frozen ice ball, coated with darker material.
Io
Tidally locked to Jupiter. In a 4:2:1 resonance with Ganymede and Europa. Tidal forces due to
Jupiter make it continually volcanic. Very weak atmosphere of Sulfur. 1.7 day orbit.
The Moon
Tidally locked to Earth. 1/80th the mass of the Earth. As seen from the Sun, the orbit seems to
be petal shaped, always pointing toward the Sun and never looping on itself.
Europa
Tidally locked to Jupiter. In a 4:2:1 resonance with Ganymede and Io. Tidal forces heat the
interior enough to keep an ocean of liquid water underneath its thick icy exterior. 3.5 day orbit.
Triton
Tidally locked to Neptune, but orbits backwards (“retrograde”). Has a weak atmosphere,
geysers, and possible clouds. Probably a captured Trans-Neptunian Object, given its orbit.
1/280th the mass of the Earth, 1/5 the radius of the Earth.
136199 Eris Formerly the 10th planet (2005-2006), now a Dwarf Planet. Codenamed “Xena”. One moon
(Dysomnia, codenamed “Gabrielle”). Very weak atmosphere. Elliptical orbit (0.43) well above
the plane of the Solar System. Kuiper Belt Object. 340 times less massive than Earth.
134340 Pluto Formerly the 9th planet (1930-2006), now a Dwarf Planet. One of many objects in a 3:2
resonance with Neptune. Tidally locked to Charon, 6 day orbit. Tilted on its side. Elliptical
orbit (0.25), out of the plane of the Solar System, crosses Neptune's orbit (from 1979-1999 it was
technically the 8th planet). Kuiper Belt object. Very weak atmosphere, which it may not have for
its entire orbit around the Sun, and which may extend to Charon. Three moons.
136108 Haumea
Currently a dwarf planet. Due to very fast rotation, this object is football shaped, but in
hydrostatic equlibrium anyway. It probably collided with something as there are many tiny
objects following it in its orbit. Haumea has two tiny moons. Haumea is in an increasingly
elliptical orbit with a 12:7 resonance with Neptune. This resonance is unstable and Neptune will
eventually turn it into an enormous comet. (http://www.gps.caltech.edu/~mbrown/2003EL61/)
Now, other smaller objects in random order:
Charon
Tidally locked to Pluto, 6 day orbit. 11% of Pluto's mass, the center of mass of their orbit is a
point between Pluto and Charon. Sometimes considered a 'double planet', in which case it shares
two moons with Pluto. Very weak atmosphere, which it may not have for its entire orbit around
the Sun, and which may extend to Pluto.
1 Ceres
Formerly the fifth planet (1801-1850s?), now Dwarf Planet. The only Main Belt object with
enough mass to be spherical. 1/3 the mass of the Main Belt, 4% the mass of The Moon.
Rhea
Orbits Saturn. Rhea is small ball of ice, with its own ring system.
243 Ida
A 50 km potato-shaped asteroid with a 1.2 km moon, Dactyl. Main Belt object.
624 Hektor One of Jupiter's “Trojan”s. Locked in a 1:1 resonance with Jupiter, held in Jupiter's L4 (Greek)
Lagrange point. Peanut shaped, with a moon of its own.
Halley's Comet
Eccentricity 0.97, above the plane of the Solar System, orbits the sun backwards
(retrograde). Maximum distance from the Sun ~36 AU. As it reaches its closest approach (0.6
AU) to the Sun, surface ices vaporize. Surface gravity is low enough the gas flies off into space
forming a giant tail, called a 'coma'. “Nucleus” is 16x8x8 km in size.
PSR 1257+12 b
A Mercury-mass object in a 25 day orbit around a pulsar (the remains of a supernova).
Probably formed after the supernova. We have no idea what it's made of.
Illustration 1: 243 Ida and Dactyl
http://www2.jpl.nasa.gov/webcast/galileo/a
rt/03_Ida_and_Dactyl_lg.jpg
Illustration 2: Lagrange Points. L4 and L5 are stable.
http://www.maa.org/editorial/mathgames/LagrangeP
oints.gif
Planetary Definition worksheet
Name: ___________________________________________________________________
1. Please list all sources you used to gather information (below are my sources):
http://web.gps.caltech.edu/~mbrown/whatsaplanet/IAU.html http://www.nineplanets.org/
http://web.gps.caltech.edu/~mbrown/eightplanets/
http://www.iau.org/public_press/news/release/iau
http://aa.usno.navy.mil/faq/docs/minorplanets.php
0603/questions_answers/
http://www.exoplanet.eu/
http://www.solarviews.com/eng/toc.htm
2. What is your final planetary definition? You may also define other types of object if you wish.
3. What are the following objects classified as, under your system: (The last 4 are optional, you don't need
to bother with them if you'd rather not)
Mercury
Venus
Earth
The Moon
Mars
1 Ceres
243 Ida
Jupiter
Ganymede
624 Hektor
Saturn
Titan
Rhea
Uranus
Neptune
Triton
134340 Pluto
Charon
136108 Haumea
136199 Eris
Gliese 876 c
Gliese 876 b
HD 80606 b
PSR 1257+12 b