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/m 3 )(g/cm 3 ) Mercury Venus Earth Moon—— — Mars Ceres (asteroid) Jupiter Saturn Uranus Neptune Pluto Comet Hale- Bopp x — Sun——332,000109—

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 System Planet 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.

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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.

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Solar System The Terrestrial Planets TERRESTRIAL PLANETS close to the Sunhigh density closely spaced orbits slower rotation small masses weak magnetic fields small radiifew moons predominantly rockyno rings solid surface

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Solar System Gas Giants JOVIAN PLANETS far from the Sunno solid surface widely spaced orbitslow density large massesfaster rotation large radiistrong magnetic fields predominantly gaseousmany 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.”