The Sun: General Properties

• Appearance: A 0.5°-wide bright disk; Apparently uniform, but shows spots and other features upon closer inspection...
• What is it? From its spectrum we know it is a star (a mid-sized star, the only one we can see from up close); A burning ball of gas (plasma); Heat pressure and gravity keep it in equilibrium (for 10 billion years – it is now about half way).
• Distance and size: 1 AU = 150 Mkm away from Earth, 8 minutes in terms of light travel time; Its radius is 700,000 km = 100 Earths; Its rotation is slow and differential (about 27 days at the equator), so the Sun is almost perfectly round.
• Temperature and luminosity: T = 6000 K (from the spectrum); The emitted power [4 × 10²⁶ W!] and length of existence are way more than could be explained if it was a regular burning fire.

The Sun: Activity and Effects

• Energy production: Core is at T = 15 M degrees, totally ionized; Nuclear reactions convert H nuclei (protons) into He nuclei, with a huge release of energy; High energy photons and motion of the plasma carry the energy to the surface.
• The surface: From here we see the bubbling photosphere, where the visible light comes from; Other, thinner layers extend further out.
• Solar activity: In addition to sunspots, due to twisting of magnetic field lines, we see flares and prominences; Coronal mass ejections send a solar wind made of photons, ions, and subatomic particles, towards the planets.
• Effects on Earth: We see its effects in the auroras and (especially in periods of intense activity) it can affect our telecommunications; Its effect on climate is less understood.

  Major Planets

• Two types: A total of 8 planets, four relatively "small", "closely spaced", rocky ones with few moons (if any) and no rings, in the inner Solar System; Four large, widely spaced, mostly gaseous ones, all with many moons and rings, in the outer Solar System.
• Terrestrial Planets: Mercury at 0.4 AU (seen always close to the Sun, the smallest of the 8 planets, heavily cratered); Venus at 0.7 AU (the "Morning Star" or "Evening Star", the Blue Planet, extremely hot under the atmosphere); Earth and its Moon (also a heavily cratered surface, the only celestial object visited by humans); Mars at 1.5 AU (the Red Planet, the most explored one – currently has both orbiters and rovers on the surface – with two small moons).
• Jovian Planets: Jupiter at 5 AU (the king of the planets, with at least 60 moons and a centuries-old storm, the Great Red Spot); Saturn at 10 AU (with the most complex set of rings and many moons – and the Cassini orbiter currently around it); Uranus at 20 AU and Neptune at 30 AU (two bluish giants).
• Why are they different? It mostly boils down to their different distances from the Sun

Minor Planets

• Asteroids: Thousands of asteroids, from 950-km Ceres to the smallest fragments, in the main belt between the orbits of Mars and Jupiter (we have landed a spacecraft on Eros); Others scattered in other places, including Apollo asteroids, Centaurs, ...
• Pluto, Charon and the Kuiper Belt: Beyond the orbit of Neptune there is a huge number of icy/rocky objects; The biggest known one is Eris, then Pluto (with its moon Charon), Sedna and Quaoar which are almost as big, and there may be others; Some of the smaller ones visit us periodically and are seen as comets.
• Effects on Earth: Stray asteroids, comets and residual dust are everywhere; Small meteoroids enter our atmosphere all the time; Larger ones have caused damage and craters; The ones that are potentially very destructive are being monitored.
• Further out: The Heliopause and the Oort cloud (the farthest, coldest Solar System region, not yet directly seen, from which we also get comets), and then open interstellar space.

  Life in the Solar System and Elsewhere

• In the Solar System: The most likely places in the Solar System are Mars (water? microorganisms?), Jupiter's moon Europa (water under the icy crust?), and Saturn's moon Titan (has an atmosphere).
• Are there other planetary systems? There must be lots of them; Starting in the early 1990's, more than 200 extrasolar planets have been found; They are all larger than the Earth, but that is probably just because those are easier to find.
• Life outside the Solar System? There is no specific place where we suspect there may be life, but with so many planets around other stars, the general feeling is that there must be life in many places; We can try to detect life-related chemicals and possibly radio signals, and we have sent some objects and signals ourselves.