The Solar System
Durante la Noche de las Estrellas, thousands of people in Mexico look at the sky through telescopes and see Jupiter as a bright little dot. What is that dot, why doesn't it fall toward the Sun, and how far away is it really? On this page you'll answer those questions.
The Sun, Center of the System
We feel the Sun's light and heat every day, even across empty space. Where does so much energy come from if there is no fire out there like the one on a stove?
The solar system is made up of the Sun, the eight planets, the dwarf planets, the moons, and the smaller bodies (asteroids and comets). It formed about 4,600 million years ago from a cloud of gas and dust that flattened into a spinning disk.
List the components of the Solar System and explain in one sentence what each type of body is.
List the components of the Solar System and explain in one sentence what each type of body is.
The Planets: Rocky and Gaseous
Earth has a density of $5.5\ \text{g/cm}^3$ and Saturn only $0.7\ \text{g/cm}^3$: it would float on water. Why are the four planets near the Sun rocks and the four far away huge spheres of gas?
Close to the Sun it was so hot that only rocks and metals could condense. Far away, where it was cold, the planets gathered a lot of gas and ice.
The diagram shows the Solar System on a qualitative scale (the orbits and sizes are not realistic). Name the planets labelled with numbers from 1 to 8, from closest to…
The diagram shows the Solar System on a qualitative scale (the orbits and sizes are not realistic). Name the planets labelled with numbers from 1 to 8, from closest to furthest from the Sun.
Explain the difference between a rocky planet and a gas planet, and classify the eight planets of the Solar System into one of these two groups.
Explain the difference between a rocky planet and a gas planet, and classify the eight planets of the Solar System into one of these two groups.
Dwarf Planets and Smaller Bodies
For decades it was taught that there were nine planets. Then hundreds of icy worlds similar to Pluto were discovered beyond Neptune. How did astronomers decide what is a planet and what isn't?
In 2006, Pluto came to be considered a dwarf planet. Explain which requirement of the definition of a planet it does not meet and name another dwarf planet in the Solar…
In 2006, Pluto came to be considered a dwarf planet. Explain which requirement of the definition of a planet it does not meet and name another dwarf planet in the Solar System.
What are the asteroid belt and the Kuiper belt? State where each one is located and what type of bodies it contains.
What are the asteroid belt and the Kuiper belt? State where each one is located and what type of bodies it contains.
Gravitation Keeps Orbits in Place
If the Sun attracts the Earth, why don't we fall toward it? And why doesn't the Moon crash into us?
A planet moves sideways at high speed and, at the same time, the Sun pulls on it. It's like a ball on a string that you swing around: the pull bends its path and keeps it going in circles. If the pull disappeared, the planet would fly off in a straight line (Newton's first law). That is why the closer planets, which are pulled harder, move faster and have shorter years: Mercury goes around in 88 days and Neptune takes more than 164 Earth years.
Earth is currently the only planet with known life. Explain three characteristics of Earth that make it suitable for life, such as its distance from the Sun, the…
Earth is currently the only planet with known life. Explain three characteristics of Earth that make it suitable for life, such as its distance from the Sun, the presence of liquid water or its atmosphere.
The Earth and the Moon
During the total eclipse of April 8, 2024, day turned to night for a few minutes in Mazatlán. How does that happen, and why does the Moon change shape every week?
The Moon is the Earth's natural satellite. Earth's gravity pulls on it, and it goes around us once every 27 and a third days or so. We always see the same side. The Moon has no light of its own: it reflects the Sun's light, and half of it, the half facing the Sun, is always lit. Depending on where it is in its orbit, we see more or less of that lit half. This is how the phases are produced: new moon, first quarter, full moon, and last quarter, in a cycle of about 29.5 days.
Eclipses. If the Sun, the Moon, and the Earth line up almost exactly:
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In a solar eclipse, the Moon is between the Sun and the Earth and its shadow falls on a strip of the Earth. It happens at new moon.
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In a lunar eclipse, the Earth is between the Sun and the Moon and casts its shadow on it. It happens at full moon.
There is no eclipse every month because the Moon's orbit is tilted a little and the shadow almost always passes by.
Tides. The Moon's gravity (and, to a lesser extent, the Sun's) pulls on the ocean water and makes it rise and fall twice a day along the coasts, as in Veracruz or Cancún. That energy can be harnessed in tidal power plants.
Habitable Zone
Besides distance, other things help: an atmosphere that retains heat, a magnetic field that protects against radiation, and enough mass to hold on to the air. Cooler, smaller stars have their habitable zone much closer to them.
A space telescope has detected three rocky exoplanets orbiting a red dwarf star that is cooler and smaller than the Sun, whose habitable zone lies between…
A space telescope has detected three rocky exoplanets orbiting a red dwarf star that is cooler and smaller than the Sun, whose habitable zone lies between $0.15\ \text{AU}$ and $0.30\ \text{AU}$:
• Planet A: Distance to the star: $0.06\ \text{AU}$. Mass: $1.1$ Earth masses. Dense atmosphere rich in water vapor.
• Planet B: Distance to the star: $0.22\ \text{AU}$. Mass: $0.9$ Earth masses. Detectable magnetic field and atmosphere with $N_2$, $CO_2$, and water vapor.
• Planet C: Distance to the star: $0.25\ \text{AU}$. Mass: $0.02$ Earth masses (similar to the Moon). No noticeable atmosphere.
- a) Analyze with reasoning which of them is the ideal candidate to harbor liquid water and life, justifying why you rule out the other two.
- b) Explain why being located within the "habitable zone" is a necessary but not sufficient condition for a planet to be habitable.
Real-World Scales
Drawings of the solar system are misleading: the planets look close together and similar in size. What would a model with the correct scale look like?
To find out how long a signal takes, convert the distance to km and divide by the speed: $t = d / v$.
You are going to design a scale model of the Solar System in the hallway of your school, which is $50\ \text{m}$ long. You decide to represent the Sun with a basketball…
You are going to design a scale model of the Solar System in the hallway of your school, which is $50\ \text{m}$ long. You decide to represent the Sun with a basketball of $24\ \text{cm}$ in diameter (the actual diameter of the Sun is approximately $1\,390\,000\ \text{km}$).
- a) At that same scale, calculate what diameter the Earth should have (actual diameter: $12\,742\ \text{km}$) and compare it with a small everyday object (such as a pinhead or a grain of sand).
- b) Knowing that the average Earth-Sun distance is $1.5 \times 10^8\ \text{km}$ and Neptune's distance to the Sun is about $4.5 \times 10^9\ \text{km}$, calculate at what distance from the ball you would have to place both planets.
- c) Based on the results, justify whether the complete model fits in the hallway while simultaneously maintaining the scale of sizes and the scale of distances, and explain why Solar System diagrams in textbooks are often misleading.
An astronomical unit (AU) is the average distance from Earth to the Sun, about $150$ million kilometers, and Neptune is located about $30\ \text{UA}$ from the Sun.…
An astronomical unit (AU) is the average distance from Earth to the Sun, about $150$ million kilometers, and Neptune is located about $30\ \text{UA}$ from the Sun. Calculate how long it takes sunlight to reach Neptune, knowing that light travels at about $300\,000\ \text{km/s}$, and express the result in hours.
Back to the beginning: the bright little dot on the Noche de las Estrellas is Jupiter, a gaseous planet that doesn't fall into the Sun because its speed and the Sun's gravity keep it in orbit, at about $5$ AU, that is, about $750$ million km.