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There are very large extinct volcanoes on Mars.
There is a very deep and long canyon that extends across Mars.
There are dried-up riverbeds on Mars.
Mars has polar caps made of "dry ice."
Mars has two small moons.
Rocky minerals and water, as on Earth
Hydrogen and helium
Ammonia and methane
Ammonia and water
Nitrogen and methane
Nuclear fusion occurring in the core of the protosun produced energy that heated the nebula.
As the cloud shrank, its gravitational potential energy was converted to thermal energy.
Radiation from other nearby stars that had formed earlier heated the nebula.
The shock wave from a nearby supernova heated the gas.
Collisions among planetesimals generated friction and heat.
The interstellar cloud from which the solar nebula formed was originally somewhat flat.
The force of gravity pulled the material downward into a flat disk.
As the nebula cooled, the gas and dust settled onto a disk.
It flattened as a natural consequence of collisions between particles in the nebula, changing random motions into more orderly ones.
The original solar nebula happened to be disk-shaped by chance.
Any planets that once orbited in the opposite direction or a different plane were ejected from the solar system.
The laws of conservation of energy and conservation of angular momentum ensure that any rotating, collapsing cloud will end up as a spinning disk.
The Sun formed first, and as it grew in size it spread into a disk, rather like the way a ball of dough can be flattened into a pizza by spinning it.
Luck explains it, as we would expect that most other solar systems would not have all their planets orbiting in such a pattern.
Molecules such as methane and ammonia
A collapsing nebula of gas
Flattened, spinning disks
Strong stellar winds
Kuiper belt comets
Oort cloud comets
All of the above
Our theory is not quite correct because it cannot explain these exceptions.
Most of the exceptions are the result of giant impacts or close gravitational encounters.
The exceptions probably represent objects that formed recently, rather than early in the history of the solar system.
The exceptions probably represent objects that were captured by our solar system from interstellar space.
The exceptions exist because, even though our theory is as correct as possible, nature never follows rules precisely.
Ice that condensed in the solar nebula in the region where Earth formed.
Chemical reactions that occurred in Earth's crust after Earth formed.
Chemical reactions that occurred in Earth's core after Earth formed.
Material left behind during the giant impact that formed the Moon.
Comets that impacted Earth.
Several million years.
Several tens of millions of years.
Several hundreds of millions of years.
About a billion years.
To the present time.
The oldest rocks on Earth.
The oldest rocks on the Moon.
The oldest meteorites.
The atmosphere of Mars.
It hasn't been done yet, but the age of the solar system could be obtained from a sample of Io's surface.
By Huygens, following his realization that other stars are Suns.
By Galileo following the invention of the telescope.
At the turn of last century.
About a decade ago.
At the turn of this century.
A few hundred
A few thousand
Tens of thousands
Measuring distances to stars
Searching for planets around stars
Measuring the positions of stars on the sky
Measuring the velocities of stars via the Doppler effect
Using metric units for distance (e.g. meters rather than light years)
Combining all the above
combining all the above