General orientation to the Solar System I
The Scientific Method
Despite the impression you may get from media and intro-level science courses, science is absolutely not a body of knowledge. Rather it is the method of inquiry that has yielded that knowledge.
The guiding principle of scientific inquiry was perhaps best summed up by a non-scientist, the novelist Arthur Conan Doyle, who placed into the mouth of his character Sherlock Holmes these words:
"Once you eliminate the impossible, whatever remains, no matter how improbable, must be the truth."
The modern practice of science is based on the hypothetico-deductive method of science (a.k.a. the "scientific method"), a method that employs that principle, with a little twist.
Key components of the hypothetico-deductive method of science:
- Observations: Any impression of the physical universe that comes to us directly through our senses or our indirectly through instruments. E.g. "I looked out my east window this morning and saw the Sun rise."
- Repeatability: The ability of two different individuals operating under the same circumstances independently to make the same observation. E.g. "My neighbor down the block also saw the Sun rise in the east." Some types of observations are inherently non-repeatable. E.g. "My wife is the most beautiful woman in the world." Sweet, but unlikely that all observers would have the same impression.
- Patterns: Tend to become apparant when we make and compare many observations. E.g. "The Sun appears to rise in the East every morning." Humans are compulsive pattern recognizers, both seeing real ones and confabulating ones that aren't really there. (The basic principle of Rorschach tests.) The scientist's task is to distinguish the two.
- Hypothesis: A formal statement about a pattern of observations whose truth or falsehood is initially unknown. E.g. "The Sun always rises in the East." Remember, a hypothesis is a statement about a pattern of events, not a single observation.
- Hypothesis falsification: the scientific method does not absolutely prove anything. It can, however, disprove things. We do this by looking for observations that are inconsistent with our hypothesis. When a such an observation is made and verified, we consider the hypothesis to be disproven or "falsified." E.g. If the Sun were to rise in the North one day, the example hypothesis given above would be falsified. Obviously, being falsifiable is different from being false.
- Falsification is definite - "proof" is unattainable: When an inconsistent observation is made, the hypothesis is definitely falsified - i.e. it's dead dead dead. No amount of consistent observations will bring it back to life. But even if we see ten, a hundred, or a gajillion observations that are consistent with our hypothesis, we can never be completely sure that the next observation won't falsify it. Thus, one never definitively "proves" a scientific hypothesis.
- Non-falsifiable hypotheses: Hypotheses that are inherently impossible to falsify, either because of technical limitations or because of subjectivity. E.g.:
- "Chocolate is always better than vanilla." [subjective].
- "There are living beings in the Andromeda galaxy." [beyond technical grasp] Another term for non-falsifiable hyoptheses is "speculation."
- A problem: Suppose you have a falsifiable hypothesis that consistently resists all attempts at falsification. At a certain point, it becomes sort of perverse to not provisionally accept it as "truth." Often, we see groups of such hypotheses dealing witht he same general issue.
For example, humans have been watching the sun rise for over 100,000 years, and is always rises in the East. Not only that, the moon, stars, and planets also rise in the East. Such an amazing congruence of provisional "truths" about celestial bodies gets us thinking about the underlying mechanisms governing them. From this we get theory.
- Theory: Set of universal rules that explain wide ranges of falsification-resistant hypotheses in terms of underlying processes. E.g. We now have theory that explains our earlier hypothesis about the Sun rising in the east: "Inertia causes the Earth rotate in the same direction on its axis once every day, causing observers on its surface to observe celestial bodies appear to rise in the East every day." These rules allow us to understand not only the sunrises we witness directly, but the rising of all celestial bodies everywhere throughout time.
WATCH OUT! In common speech, people often use the terms "theory" and "hypothesis" as synonyms. That's not the proper scientific way! When we refer to the "theory of the expanding universe," we are not suggesting that there's anything hypothetical about it. On the contrary, we are saying that the patterns of observations that lead us to conclude that the universe is expanding are so well attested that they merit the elucidation of the underlying processes governing them. In the exact same sense, when a music professor teaches "music theory," they aren't in any way suggesting that maybe music really doesn't exist.
- Overturning theories: Still, because hypotheses can be falsified, the theories to which they give rise can be overturned. The difference is one of scale. Hypotheses are falsified every day - trivial events. The falsification of a scientific theory is a big deal that gets into newspapers and history books.
Two ways to overturn a theory:
- Observation: New observations are made which are inconsistent with the theory. E.g. Einstein's theory of general relativity superceded Newton's theory of universal gravitation because it better explained several astronomical observations.
- Parsimony: A simpler theory is preferred by the principle of parsimony (= simplicity). When two or more posssible answers exist to a question, the simpler one is generally preferred. E.G. Copernicus' heliocentric astronomy superceded Ptolomey's geocentric astronomy because it was simpler. Actual observational rejection of Ptolemey's astronomy didn't come until a century later.
Introducing the Solar System
From The Solstice Blog
Orbits of the planets (and Pluto) to scale. Not shown are the minor bodies (asteroids and comets) and the icy worlds of the Kuiper Belt (except Pluto). Key concepts:
- Ecliptic: All major bodies' orbits lie in or near a single plane - the ecliptic. By convention, the ecliptic is the plane of Earth's orbit. Viewed from inside the plane (E.G. from Earth) the ecliptic appears as a line through the background constellations of the zodiac.
- Orbital inclination: The angle between the plane of a planet's orbit and the ecliptic. (Earth's orbital plane is the ecliptic, so by definition, Earth's orbital inclination is 0.)
- Each orbit is a ellipse with the sun at one focus.
- Orbital eccentricity measures the extent to which the ellipse departs from being a circle.
- Astronomical unit: Earth's average distance from the sun (149.6 million km or 92.9 million miles) is the astronomical unit (AU).
Commit these approximate distances to memory right now:
- 1 AU: Sun to Earth
- 2.5 AU: Sun to mid-point of main asteroid belt
- 5 AU: Distance from Sun to Jupiter
- 30 AU: Distance from Sun to Neptune
- Prograde orbits: Looking "down" on the planets' north poles, the planets appear to orbit in a counterclockwise or prograde direction. Anything moving in the opposite direction is retrograde.
- Missing from this diagram:
- Asteroid belt inhabited by the dwarf planet Ceres and hundreds of minor bodies lies between the orbits of Mars and Jupiter.
- Trans-neptunian objects The region beyond the orbit of Neptune is inhabited by an unknown number of dwarf planets (including Pluto) and minor bodies. (We reserve speculation on "Planet 9" for later.)
The scale of the Solar System: On a 1/10 billion scale:
- the sun would be 13.8 cm across - the size of a grapefruit.
- Earth's diameter is 1.43 mm. - a sand grain
- Jupiter's diameter is 1.276 cm. - a marble.
- Earth would be roughly 15. m from the sun (roughly from the lectern to the sidewalk).
- Neptune would be 450 m away (roughly distance to the North Campus Diner).
- Most distant comets (by estimate) would be 185 km away (roughly distance to Trenton, NJ).
- Nearest star, Alpha Centauri, would be 3,990 km distant (the distance to Los Angeles)
- The sun is a star - a celestial object composed primarily of hydrogen and helium sufficiently massive to ignite prolonged nuclear fusion reactions in its core.
- The threshold where nuclear fusion becomes possible is at about 165 x 10 27 kg (about 87 * the mass of Jupiter). At 2 x 10 30 kg, the sun exceeds this threshold by a factor of roughly 12. Thus, the sun is a medium-sized star. An extremely massive hypergiant star might be up to 175 times as massive as the sun (But note Doran et al., 2013 on a 265 solar mass monster.).
- The sun's core is estimated to be 15 million K. (K = C+273) It's surface is 5770 K.
- As far as the planets are concerned, the sun is the the source of:
The Sun and Planets to scale
Solar System bodies: We see four general classes (!) distinguished by mass and composition:
- Terrestrial planets: Inhabit the inner solar system (i.e inside 2.5 AU). Composed mostly of silicate materials (rock) and metals. Includes:
- The Moon
Saturn from Wikipedia
- Jovian planets: Giant planets inhabiting the outer solar system (i.e outside 2.5 AU). Sufficiently massive to retain hydrogen and helium. Consequently they consist to a large extent of gasses and have relatively low densities. Their massive atmospheres extend down to regions experiencing such pressure that the concepts of "gas," "liquid," and "surface" began to lose their meaning. Include:
Saturn's moon Rhea from The Worlds of David Darling
- Icy worlds: Planetary bodies that formed over 3 AU from the sun, in regions where volatile substances such as water and carbon dioxide can exist as solid "ices." Contain significant amounts of ice in addition to silicate materials, so are less dense than terrestrial planets. Some have atmospheres, but they are too small to retain hydrogen and helium. Includes most of the larger moons of the giant planets (E.G. Saturn's moon Rhea, right), and the trans-neptunian worlds.
Note: The term "ice" encompasses numerous volatile substances with different freezing points. Potential ices include:
- Water - H2O
- Carbon dioxide - CO2
- Ammonia - NH3
- Methane - CH4
- Nitrogen - N2
The asteroid Eros from NASA
- Minor bodies: Planetary bodies too small to be able to pull themselves into a spherical shape by self-gravity. A.k.a. "potatoes." Includes small moons of the giant planets, smaller asteroids (E.G. Eros, right) and small trans-neptunian objects. Termed:
- Asteroids if composition is mostly silicate or metallic
- Comets if they contain much ice and occasionally cross into the inner Solar System.
Earth and Mars to scale from Wikipedia
Be careful! This taxonomy is crude and imperfect. Some categories are clearly arbitrary associations of unrelated things (minor bodies) and some planetary bodies don't fit neatly into any of them (E.G. Europa).
Key concepts and vocabulary. Understand these or you're toast:
- The hypothetico-deductive method of science
- Non-falsifiable hypothesis
- General familiarity with all objects described
- The Sun
- Solar System
- Orbital inclination
- Orbital eccentricity
- Astronomical unit
- Prograde / retrograde orbit
- Electro-magnetic spectrum
- Inverse-square relationship
- Terrestrial planet
- Giant planet
- Icy world
- Minor body
- Trans-neptunian object
- Doran, I, P. Crowther, A. de Koter, C. Evans, C. McEvoy, N. R. Walborn, N. Bastian, J. Bestenlehner, G. Gräfener, A. Herrero, K. Köhler, J. Maiz Apellaniz, F. Najarro, J. Puls, H. Sana, F. Schneider, W. Taylor, J. van Loon, and J. Vink. 2013. A census of the hot luminous stars and their feedback in 30 Doradus. Astronomy and Astrophysics 588.
- NASA. 2012. MESSENGER Provides New Look at Mercury's Surprising Core and Landscape Curiosities. News Releases. [accessed 2016]. http://www.nasa.gov/mission_pages/messenger/media/PressConf20120321.html.