Overview of the
and the Solar System
The Sun: General
- 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
- 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.
- 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
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
- 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,
- 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
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
Life in the Solar System
- 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.
page by luca bombelli <bombelli at olemiss.edu>,
modified 29 sep 2012