Solar System

Astronomers have always noticed planets, the stars, and the moons.

We will use the powerful and still emerging perspective of comparative planetology to understand better the conditions under which planets form and evolve.

Solar System

comparative planetology

The systematic study of the similarities and differences among the planets, with the goal of obtaining deeper insight into how the solar system formed and has evolved in time.

Solar System

solar system

The Sun and all the bodies that orbit it—Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, (and Pluto) their moons, the asteroids, and the comets.

Solar System

Comets appear as long, wispy strands of light in the night sky that remain visible for periods of up to several weeks, then slowly fade from view.

Solar System

Meteors, or "shooting stars" are sudden bright streaks of light that flash across the sky, usually vanishing less than a second after they first appear.

Asteroids or "minor planets" orbiting the Sun, mostly in a broad band (called the asteroid belt) lying between Mars and Jupiter.

Meteor

Solar System

OBJECT

ORBITAL SEMI-

MAJOR AXIS (A.U.)

ORBIT PERIOD (Earth years)

MASS (Earth masses)

RADIUS (Earth radii)

NUMBER OF KNOWN MOONS

ROTATION PERIOD*

(days)

AVERAGE DENSITY

(kg/m3) (g/cm3)

Mercury 0.39 0.24 0.055 0.38 0 59 5400 5.4

Venus 0.72 0.62 0.82 0.95 0 -243 5200 5.2

Earth 1 1 1 1 1 1 5500 5.5

Moon — — 0.012 0.27 — 27.3 3300  

Mars 1.5 1.9 0.11 0.53 2 1 3900 3.9

Ceres (asteroid)

2.8 4.7 0.00015 0.073 0 0.38 2700 2.7

Jupiter 5.2 11.9 318 11.2 16 0.41 1300 1.3

Saturn 9.5 29.4 95 9.5 18 0.44 700 0.7

Uranus 19.2 84 15 4 17 -0.72 1300 1.3

Neptune 30.1 164 17 3.9 8 -0.67 1600 1.6

Pluto 39.5 248 0.002 0.2 1 -6.4 2100 2.1

Comet Hale-Bopp

180 2400 1.0x 10-9 0.004 — 0.47 100 0.1

Sun — — 332,000 109 — 25.8 1400  

Solar System

Orbital Semi-Major Axis

The major axis of an ellipse:

Longest diameter, a line that runs through the widest points of the shape.

The semi-major axis is one half of the major axis, and thus runs from the centre, through a focus, and to the edge of the ellipse.

The distance of each planet from the Sun is known from Kepler's laws once the scale of the solar system is set by radar-ranging on Venus.

Solar System

Orbit Period

Length of time the object takes to orbit the sun.

A planet's (sidereal) orbital period is easily measurable from repeated observations of its location on the sky, so long as Earth's own motion around the Sun is properly taken into account.

Solar SystemPlanet Mass

The masses of planets with moons may be calculated by application of Newton's laws of motion and gravity, just by observing the moons' orbits around the planets.

The masses of Mercury and Venus (as well as those of our Moon) are a harder to determine because these bodies have no natural satellites

Solar System

Planet Mass

We observe their influence on other planets or nearby bodies.

Mercury and Venus produce small but measurable effects on each other's orbits, as well as that of Earth.

The Moon causes small "wobbles" in Earth's motion as the two bodies orbit their common center of mass.

Solar System

Rotation Period

Length of time for an object to rotate completely around its axis.

A planet's rotation period is determined simply by watching surface features appear and disappear again as the planet rotates. For some planets this is difficult to do, as their surfaces are hard to see or may even be nonexistent

Solar System

The planets’ paths are all ellipses, with the Sun at (or very near) one focus.

Most planetary orbits have low eccentricities.

The exceptions are the innermost and the outermost worlds, Mercury and Pluto.

High eccentricities indicate more oval and less circular shapes.

Accordingly, we can think of most planets' orbits as circles centered on the Sun.

Solar System

Maybe future space voyagers travel far enough from Earth to gain this perspective on our solar system

Except for Mercury and Pluto, the orbits of the planets lie nearly in the same plane. As we move out from the Sun, the distance between the orbits of the planets increases. The entire solar system spans nearly 80 A.U.

Solar System

AnAstronomicalRuler.MOV

Solar System

The Titius-Bode law seemed to "predict" the radii of the planetary orbits remarkably well.

Even the asteroid belt between Mars and Jupiter appeared to have a place in the scheme, which excited great interest among astronomers and numerologists alike.

There is apparently no simple explanation for this empirical "law."

Solar System

On large scales, the solar system presents us with a sense of orderly motion.

The planets move nearly in a plane, on almost concentric and nearly circular paths

They move in the same direction around the Sun, at steadily increasing orbital intervals.

However, the individual properties of the planets themselves are much less regular.

Solar System

•A clear distinction can be drawn between the inner and the outer members of our planetary system based on densities and other physical properties.

•The inner planets—Mercury, Venus, Earth, and Mars—are small, dense, and rocky in composition.

•The outer worlds—Jupiter, Saturn, Uranus, and Neptune (but not Pluto)—are large, of low density, and gaseous.

Solar System

Diagram, drawn to scale, of the relative sizes of the planets and our Sun. Notice how much larger the joviian planets are than Earth and the other terrestrials and how much larger still is the Sun.

Solar System•The terrestrial worlds lie close together, near the Sun

•the jovian worlds are widely spaced through the outer solar system.

•The terrestrial worlds are small, dense, and rocky;

•the jovian worlds are large and gaseous, being made up predominantly of hydrogen and helium (the lightest elements), which are rare on the inner planets.

•The terrestrial worlds have solid surfaces;

•the jovian worlds have none (their dense atmospheres thicken with depth, eventually merging with their liquid interiors).

Solar System

•The terrestrial worlds have weak magnetic fields, if any;

•the jovian worlds all have strong magnetic fields.

•The terrestrial worlds have only three moons among them;

•the jovian worlds have many moons each, no two of them alike and none of them like our own.

•Furthermore, all the jovian planets have rings, a feature unknown on the terrestrial planets.

•Despite their greater size, the jovian worlds all rotate much faster than any terrestrial planet.

Solar System

TheTerrestrialPlanetsI.MOV

Solar System

The Terrestrial Planets

TERRESTRIAL PLANETS

close to the Sun high density

closely spaced orbits

slower rotation

small massesweak magnetic

fields

small radii few moons

predominantly rocky no rings

solid surface

Solar System

TheGasGiantsI.MOV

Solar System

Gas GiantsJOVIAN PLANETS

far from the Sun no solid surface

widely spaced orbits low density

large masses faster rotation

large radii strong magnetic fields

predominantly gaseous many moons

  many rings

Solar System

•Beyond the outermost jovian planet, Neptune, lies one more small world, frozen and mysterious.

•Pluto doesn't fit well into either planetary category.

•“Indeed, there is debate among planetary scientists as to whether it should be classified as a planet at all. In both mass and composition, it has much more in common with the icy jovian moons than with any terrestrial or jovian planet. Astronomers speculate that it may in fact be the largest member of a newly recognized class of solar system objects that reside beyond the jovian worlds.”