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FAQ - Planets

1. How did the planets form?

When we look at the night sky, we see what astronomers call nebula, such as the Orion Nebula. These are regions where stars are being formed. The nebula are formed when older massive stars exploded (supernova), creating huge regions of dust and gas. Something then happens. If you drop a rock in water, you send a wave through the water. If you have a nearby supernova, you send a shockwave through the nebula. At that point, gravity takes over and the cloud of dust and gas begins to collapse and stars are formed. Often, when stars form, they leave behind enough material in orbit around them to form planets. The gas and dust form a disk around the star. The dust particles hit each other, stick, and make bigger particles, eventually making what are called protoplanetary bodies and eventually planets. The stuff that does not make it into planets (and their moons) is what we now see as asteroids and comets. Astronomers have seen this happening (see the images below)!

Orion Nebulae
Orion Nebula
Eagle Nebulae
Eagle Nebula star formation
Protoplanet Disk
Planet in dust disk around star

2. What are the lengths of time it takes for Venus, Mars, and Mercury to orbit the Sun? How far away are Venus, Mars, and Mercury?

In the early 1600s, Johannes Kepler used observations of the motions of the planets (made by others) and formulated what we now call Kepler’s Laws. For elementary school, it is not necessary to get into the details. The closer a planet is to the Sun, the less time it takes for it to go around the Sun. It takes less time because the length of the orbit is shorter (a smaller orbit), but it also moves faster in its orbit. Thanks to gravity, it has to move faster in its orbit to stay in orbit! Below are the distances of the terrestrial planets from the Sun and the length of their years.

However, since the planets are very rarely lined up, their distance from Earth will change. For example, Mars can be as close as 78 million kilometers when both planets are on the same side of the Sun (228 million kilometers-150 million kilometers). But when they are on opposite sides of the Sun, they can be a far as 378 million kilometers apart. In reality, the closest and farthest distances are only approximations. The orbits are not quite circular; they are what we call elliptical. Because of this, it is possible for Mars, for example, to be much closer. In August 2003, a rare event occurred. Earth was at its farthest distance from the Sun, Mars was at its least distance from the Sun, and both planets were on the same side of the Sun. At that time, Mars was "only" 56 million kilometers from the Earth. This was the closest it had been in 60,000 years!

PlanetDistance from SunTime to Orbit SunOrbital Speed
 Millions of kmAU*Earth Dayskm/sec
* AU = Astronomical Unit-- the average distance of the Earth from the Sun

3. What is obliquity in regards to the rotation of planets? Is Mars the only planet that has obliquity? Where do the terrestrial planets fall on the axis in relation to the Earth and Moon?

Obliquity is simply a term for the tilt of the rotation axis of a planet, moon, etc. So, it applies to all objects as they all spin on an axis. It is an angle measured in degrees relative to the plane of its orbit around the Sun (for a planet or asteroid) or a planet for a moon. Some values: Mercury: 0.01 degrees Venus: -177.4 degrees, Earth: 23.44 degrees, Moon: 6.688 degrees, Mars: 25.19 degrees.

4. In the model of Solar System formation, the closer to the Sun, the denser the material. Why are the planets closer to the Sun not larger and why is the composition of the gas/rock planets different as you move away from the Sun?

There is less volume in the inner solar system compared to the outer solar system, so there was less material present in the protoplanetary disk to form planets much larger than the terrestrial planets. Some computer simulations show terrestrial planets a couple to a few times more massive than Earth, but not much beyond that if they formed in the inner part of the disk. Farther from the Sun, in the protoplanetary disk, the temperature was low enough that solid ices could form from the gas (it was too hot in the inner part of the disk for ices). Thus, at the distance where Jupiter is (and beyond) there was both more solid rocky material and more solid icy material for planets to form out of. This may have allowed the planets to grow much larger and eventually reach a mass that was so large that their gravity could begin capturing hydrogen and helium gas from the disk. This may be how the giant planets formed, although there is still a fair amount of debate.

5. Are all weather/rock cycles similar on the various planets?

Each planet, thanks to size, is different. On Earth, magma is brought to the surface by volcanic activity (heat generated in the interior being brought to the surface), these rocks cool to form igneous rocks. These rocks can react with the atmosphere (weathering and erosion) and form sedimentary rocks. All of these rocks can get reburied and create metamorphic rocks. Much of the volcanic activity and processes that lead to reburying rocks are the result of plate tectonics. We only see this on Earth. On Venus, which is about the same size as the Earth, we do not see evidence for plate tectonics, but we do see evidence for volcanism. The atmosphere likely reacts with the rocks, but there probably isnt any mechanism to create metamorphic rocks and there is no water to create that kind of erosion or sedimentation (though other things could rain out, like sulfuric acid). Mars does not have plate tectonics, but does have past volcanism. It has a thin atmosphere, so there can be erosion and transport by wind (great dust storms). There is evidence that the atmosphere used to be thicker, thick enough to have liquid water on the surface that would then lead to erosion and sedimentation, but not metamorphism. We are still learning about Mercury. It is a relatively dead object but does show evidence of past volcanism. Because it is much smaller than the Earth or Venus, it cooled off and formed a fairly thick crust long ago.

6. Which planet is really close to Earth in terms of similarities?

While Venus is about the same size as Earth, Mars is closer to Earth if the focus is on where life might exist elsewhere and where we might establish human colonies. The thin atmosphere does not make for ideal living conditions, but it is tolerable. There is also evidence of water at the poles and ice trapped below the surface over much of the planet.

7. Do planets with heavier cores tend to be closer to the Sun?

Not really. Mercury, Venus, and Earth have iron cores. Mercury’s is thought to be relatively larger due to the loss of its crust. Mars probably has a smaller core because it is thought to contain less iron and may not have completely differentiated. However, once you get to Jupiter and Saturn, their cores are dense just by the sheer pressure (due to their size). It is thought that, in their interiors, Jupiter and Saturn have cores that are larger than the Earth (maybe 10 times the size of the Earth for Jupiter). It is thought that the pressure in the interior of Jupiter is about 40 million atmospheres. So whatever goes down there is going to be crushed to a fairly good density.

8. What proof is there that the cores of planets are made of iron?

Based on our understanding of planet formation, you can estimate of how much of each element you would expect. For the Earth, there is not much iron on its surface. However, if you look at its density, its interior "profile" from studying earthquakes, and the fact that it has a magnetic field, you can determine that the iron is in the core-- it sank to the core when the Earth was molten. While our knowledge of the other terrestrial planets is not as good, one would expect that their early histories were similar to Earth's. Again, by looking at things such as surface composition, density, etc, one can come up with interior profiles that require iron cores.

9. How do scientists measure temperatures on other planets?

There are two ways to estimate the surface temperatures of the planets. You can make an initial guess based on how far they are from the Sun and by how much sunlight they appear to be absorbing (closer to the Sun, hotter). You can also measure their temperature with infrared cameras. By seeing how much heat they give off, you can determine their temperature.

10. Is Earth more similar to Venus or Mars?

Venus and Mars have both similarities to the Earth. Venus is about the same size and it might be closer in geologic activity than Mars. Mars is colder than Earth, but closer to Earth in temperature. Mars has water, but presently, this water is frozen. Mars may have been more similar to Earth in the past and appears to have had flowing water and maybe oceans (or at least lakes).

11. How do scientists test for water on different planets?

You can measure the light reflected from a planet, moon, asteroid, or comet; its spectrum. Different minerals have different colors (i.e. spectra) and thus, one can uniquely identify the mineral. This is the way asteroids are studied. Water ice has also been detected on the Moon. The exact amount is unknown, but may be in the millions of tons. Regions near the Moon’s north pole never see the Sun, so it is always cold there. This water was detected by crashing a spacecraft onto the surface and measuring the water vapor in the resulting impact plume.

12. How do you know if a planet has a moon and that it is not just another planet?

By definition, a planet must orbit the Sun. Even if you include planets in other star systems, they must orbit a star. A moon (also called a natural satellite), by definition, orbits a planet or an asteroid. Some moons are bigger than Mercury and may even have atmospheres, but they are still defined as moons/satellites.

13. Do all terrestrial planets have an equal chance of being hit by objects?

The short answer is no. If you look at the distribution of objects that could potentially hit the terrestrial planets—Mercury, Venus, Earth, and Mars—the closer one is to the asteroid belt, the ultimate source of the Near Earth Objects (NEOs), the more often one will get hit by one of them. It gets more complicated when one looks at the satellites of the outer planets. Beyond the asteroid belt, there are fewer asteroids, but there are more comets. So, it is believed that comets are the dominant impactors of these satellites.

14. How does size impact the gravity of a planet/moon?

The gravitational pull of a body is dependent on the mass (m) of a body. Mass is volume times density (ρ) and so is proportional to r3. Gravity is proportional to mass and falls off at 1/r2. You can assume that all of the mass is concentrated at the center of the body, the center of mass, so, if you are standing on the body, you are r away from the center of mass. Therefore, gravity on the surface of a body is proportional to radius and density (proportional to r3 times 1/r2 times density = r times density). If you double the radius, there is 8 times more mass, but you are twice as far away from the center of mass, so gravity is 2 times stronger.

Therefore, if the Earth and the Moon had the same density, the Earth should have a gravity that is 3.67 times the Moon's since its diameter is 3.67 times the Moon's. However, as we know the Earth’s gravity is actually close to 6 times stronger, the Earth must be made of heavier stuff than the Moon. In fact, while the Earth is 49.5 times the volume of the Moon, it is 81.2 times the Moon's mass.

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