How Planets Work

Congratulations!

• You are now the proud owner of an Acme planet, the finest planets in the galaxy. Whether it's a rocky terrestrial planet, a gas giant, or an ice planet, your Acme planet is guaranteed to give you billions of years of satisfaction.

• Your planet comes with a durable factory finish. You can keep the finish pristine for eons by following a few simple rules. Protect the surface from harsh abrasives like large impacting asteroids. Avoid sources of excessive heat, like exploding supernovae. Melting or vaporizing your planet voids the warranty.

• If your planet is stored in a moderate temperature setting, it may become coated with a thin film of liquid water. This is normal and does not constitute a malfunction.

• Cooler parts of your planet may become coated with frozen water, this too is normal and does not constitute a malfunction.

• Under some circumstances, your planet may become covered with a thin greenish film called life. Connoisseurs consider this the best part.

Important Points

1. Bulk Density2. Shape of Planets3. Planetary Interiors4. Atmospheres5. Oceans6. Coordinates7. Names8. What is a Planet?

Bulk Density

• Bulk density = Mass/Volume• = gm/cc = kg/m3

• 1 gm/cc = 1000 kg/m3

• Volume determined from dimensions• Mass determined from gravitational pull– Effect on other bodies– Effects on spacecraft

1. Bulk Density

Bulk Density• <1 gm/cc (1000 kg/m3)– Small body: Probably ice with a lot of void space– Giant planet: Mostly gases. – Saturn, 0.7 gm/cc (700 kg/m3) is light enough to float.

• 1-3 gm/cc (1000 - 3000 kg/m3)– Small bodies: Mixtures of ice and rock– Large planets: Mostly gases with dense cores.

• 3-5 gm/cc (3000 - 5000 kg/m3)– Rock, possibly with a denser core of metal.

• >5 gm/cc (5000 kg/m3)– Rock, with a large dense core of metal.

1. Bulk Density

Why Are Planets Round?• Planets are round because of gravity. Gravity

pulls everything inward toward the center of a planet

• Large hills would flatten under their weight and material would flow into depressions

• Earth in proportion to its size is smoother than a billiard ball

• Depending on materials, non-round objects can be up to a few hundred kilometers across.

2. Shapes of Planets

How Gravity Rounds Planets

Equatorial Bulges• Rotating planets bulge at the equator due to

centrifugal force• The earth's equatorial bulge is 1/298 of its

diameter– Radius at the equator is about 11 kilometers more

than at the poles. – Chimborazo (Ecuador) and Huascaran (Peru) are

the farthest points from the center of the Earth, not Mount Everest.

• Jupiter and Saturn have 10% bulges2. Shapes of Planets

Planetary Interiors

• Gravity pulls dense materials toward centers• Satellites and close flybys allow detection of

deep masses of material• Moment of inertia is a measure of how

material is concentrated in a planet– Shows up in precession of planets– Affects exact size and shape of equatorial bulge

3. Planetary Interiors

Atmospheres• Atoms in space follow the same laws of

physics as planets• If they hit escape velocity, they can escape a

planet• Can a planet retain an atmosphere?– Escape velocity (Determined by mass)– Temperature (Determines speed of atoms)– Stripping

4. Atmospheres

Oceans• It’s never too cold for solids, never too hot for

gases• Liquids exist only in a restricted temperature

range• At low pressures, it’s easier for liquids to

evaporate– Oceans cannot exist without atmospheres

• Oceans, lakes and seas are probably rare in the Universe– Only known on Earth and Titan

5. Oceans

“Sea Level”• How do we know where sea level would be

under Mount Everest?– We use a mathematical description of earth’s

shape called the geoid– Now determined from satellite data

• On waterless planets, we use the average radius of the planet

• Earth’s average elevation is 2.5 km below sea level

6. Coordinates

Latitude and Longitude• Latitude defined by rotation axis• Free-rotating planets have arbitrary zero

longitude– Usually defined by longitude facing Earth at some

specified time

• Satellites locked to their parent planets– Zero longitude is the longitude facing the planet– Usually 0,0 is directly under the planet

• N,E are positive; S, W are negative6. Coordinates

Names• Set by a commission of the International

Astronomical Union • Names (so far) are recognized by the various

spacefaring nations.• Features cannot be named after living people or

people dead less than three years.• Features cannot be named for political figures

after 1800. • Features cannot be named for any religious

figures from Christianity, Judaism, Islam, Hinduism, Buddhism or Confucianism.

7. Names on Planets

Planetary Geographical Features

• Latin names. Latin is traditional, apolitical, and the closest thing to a universal language in history

• Linea (Line) Line or band• Macula (Spot) Dark spot• Mons (Mountain) Mountain, plural Montes• Planitia (Plain) Low plain, basin• Planum (Plain) Plateau• Regio (Region) Region• Rupes (Cliff) Cliff or scarp• Vallis (Valley) Valley

7. Names on Planets

What is a Planet?• Ancient: Sun, Moon, Mercury, Venus, Mars,

Jupiter, Saturn• Copernicus and Kepler: Earth in, Sun and

Moon out• Uranus (1781), Ceres (1801)…• New Class created: Minor Planets• Neptune (1846), Pluto (1930), Kuiper Belt

(1992)• 2007: New Class created: Dwarf Planets

8. What is a Planet?

What is a Planet?• Planet:– Round because of gravity– Massive enough to clear its neighborhood of other

objects

• Dwarf Planet:– Round because of gravity

• This is not Over Yet– How round is round enough?– What else is out there?

8. What is a Planet?

Clearing the

Vicinity

Important Points

1. Bulk Density2. Shape of Planets3. Planetary Interiors4. Atmospheres5. Oceans6. Coordinates7. Names8. What is a Planet?